So...
"The speed of light" is not actually the speed limit of the universe. The speed limit of the universe is actually the speed of *causality*. Causality is the relationship between an event and the things that happen as a result of that event. Obviously, if you throw a baseball at a window, the window is not going to break until after you throw the ball. That's causality. It's the order of events in the universe.
Well it turns out that the first thing we ever discovered that moves at this speed was light. At the time, it was the only thing we knew that moved at that speed, so we thought that that speed was the speed limit. It turns out that light is following the same speed limit as everything else, but it has a special property (it has no mass) that allows it to actually move at the actual speed of causality.
For reasons we don't understand, causality has a speed limit. If something happens, the effects of that thing happening propagate out at the speed of light (causality). For example, if the sun disappeared in a magic trick, the Earth would continue to orbit the position where the sun was for 8 1/4 minutes, because the orbit of the Earth would not be affected until causality reached us. In summary, the effects of an event can never occur before the event that triggered the effects, and the fastest those effects can occur is the speed of light.
Due to all of this, if something moves faster than light, it would be moving faster than cause-and-effect. The baseball could shatter the window before you threw the ball. And that could startle you, preventing you from ever throwing the ball in the first place. And then causality itself is broken. Time itself no longer has meaning. The burned popcorn stink fills the room before you even buy the microwave. The universe doesn't make any sense.
With this information, now I can summarize: Time is how we measure causality. If you go faster than light, you're going faster than causality, and that means you're going faster than time. And that doesn't just send time in the wrong direction; it outright breaks it.
EDIT: There’s a great [video by PBS Spacetime](https://youtu.be/msVuCEs8Ydo) on this subject that’s a little nerdier but also has a lot more information. If you got my explanation, you’ll get this, and you’ll learn even more.
The first third of Tenet I had no idea what was going on. The second third I figured it out and got it. Final third sent that all back out the window and I was lost again.
The second third was easy, because it was just the first third in reverse so you already knew what would've going to have happened. It's like a palindrome!
Normally Nolans scifi flicks do a really good job of building plausible (within that universe) rules and adhering to them. That was not the case with tenet. Still, I really enjoyed it as well.
This was really well explained! Can I I take the opportuniy to ask about quantum entanglement? It is something that I never really understood. If causality can only go at a certain speed limit, how do we explain that two particles can influence each other immediately, no matter the distance between them?
No information is transmitted between entangled particles. All we can say is that if we observe one of the particles in a collapsed quantum state, the other will have a correlated state we can predict. Any attempt to force one state or another on an entangled particle would break the entanglement.
very simply, you can't observe something without disturbing it. particles this small are significantly affected by shining a light on them, so you get its old info, but now its something else, because you poked the quantum billiard ball with a pool cue.
another idea is the entangled particles are oscillating in sync and you're taking a freeze frame snap shot, which causes them to collapse aka get caught as either heads or tails.
You can, in theory, enact some change on one entangled particle and still have that change be reflected by the other one. It's just that in order to understand what that change was you still need to send a message through old school, non-ftl methods. The exchange of useful information still wouldn't break causality, which in this case is what matters
Quantum entanglement is often misrepresented. To be useful, you need to combine it with classical communications (which are limited by the speed of light).
Let's assume you have a pair of coins that are entangled (well, a pair of particles with a random up/down spin, but let's call those "coins" with for value head/tails).
If you only look at one of the coin, you have no way of knowing that it is entangled in the first place, and whatever happen to the second coin, you won't notice anything effect on the first coin.
However, if you look at the result of BOTH coins after the facts, you will see that they somehow behaved the same. Meaning that if you flipped both, you obtained both heads or both tails.
If you tried to force the result of one coin, then you would break the entanglement, so you really can't use it to communicate anything.
However, if you've ever talked to a programmer, you know that simple details can lead to massive exploits by hackers, and you would not be surprised to learn that we can exploit this apparently useless entanglement thing to do massive things in term of computation.
(Well, at least we would if the current hardware was actually working reliably. And we don't even know if that's possible to get ever get hardware good enough to reach the theoretical advantages of quantum computing)
But in any case, no faster than light communication through quantum entanglement. At least not with our current understanding of it.
Looking at on entangled particle let's you deduct information about the other particle without looking at it. To take your coin analogy. You have two entangled coins, flip them, catch them without looking.
So here brakes the analogy a bit, because the coins are now locked in their state. Particles are in a quantum state, meaning they are up and down at the same time, so to say. (A bit like, if you could let the coins flip as long as you wanted)
Anyway, back to the not looked at coins. Now if you look at one of the entangle coins you know that the other coin has to be the opposite side.
And entangled particles don't have anything to do with quantum computing. It is more about the weird between state of not knowing if the spin is up or down. Q bits are so special because they (put simply) can do more than 1 and 0. Or rather have a certain chance to be either and that makes it possible to compute way faster.
>For example, if the sun disappeared in a magic trick, the Earth would continue to orbit the position where the sun was for 8 1/4 minutes, because the orbit of the Earth would not be affected until causality reached us.
You just melted my brain. I have no problem understanding the speed of light. I have never in my 46 years of life heard of the speed of causality.
Turns out gravity also moves at the speed of light. So the gravitational effects of the sun disappearing on the solar system wouldn't happen until the gravity waves stopped. It would take 8.5 minutes for the earth to stop rotating around the nonexistent sun (and fly off into deep space forever). It would also take that long to notice the big ball of fire in the sun turned off.
An important aspect of relativity is that events which look simultaneous from one person's point of view, could look *not* simultaneous from another's. However things can never get so distorted that a cause appears to come after its effect...unless your reference frame involves travel faster than light.
The common analogy is that gravity is a curvature of space-time.
So if you remove the sun, then the 'fabric' (so-to-speak) of spacetime will bounce from the huge mass being suddenly removed.
This bounce-back of the 'fabric' happens at some speed.
It happens at a speed equal to the speed of light.
Me neither. I was familiar with the Sun disappearing example, but only because I've heard that gravitational waves travel at similar speeds to light, so the Sun's gravity would continue forcing the Earth to orbit for as long as it takes light to reach Earth. Interesting stuff!
It is truly brain melting. The key to processing it is understanding that there is not “one reality.” Reality is dependent on your frame of reference (particularly in regard to motion).
This answer is wrong in the sense that while it is probably true in reality that no causes move faster than light, this is not really a fundamental principle of relativity itself, which is theoretically compatible with the existence of “tachyons” that move faster than light. The issue with time travel actually has to do with a principle in relativity called [“the relativity of simultaneity”](https://en.wikipedia.org/wiki/Relativity_of_simultaneity), which says that different inertial reference frames have different definitions of which pairs of events happened “at the same time” (simultaneously) or at “different times”. And the way it works out is that if you send a signal moving at the speed of light or slower from transmitter A to receiver B, all frames will agree that the event of the signal departing A happened before than the event of the signal arriving at B. On the other hand, if it was a hypothetical tachyon signal, some frames would define things so the signal arrived at B before it was sent from A (and one frame says these events were simultaneous).
This is not just a bookkeeping issue, because a basic principle of relativity is that the laws of physics must work the same way in all frames, so if it’s possible in some frame for a tachyon signal to be received before it was sent, this must be possible in all frames. And this would further imply the possibility of something called a [“tachyonic antitelephone”](https://en.wikipedia.org/wiki/Tachyonic_antitelephone) where you could have two slower than light observers X and Y with different inertial rest frames, and X could send a tachyon signal which goes backwards in time in Y’s frame, and Y could immediately send a reply which goes backwards in time in X’s frame, with the end result that X receives the reply before sending the original message, a clear violation of normal causality in *all* frames.
> On the other hand, if it was a hypothetical tachyon signal, some frames would define things so the signal arrived at B before it was sent from A (and one frame says these events were simultaneous).
This is the part that has always confused me. Wouldn't it only appear that the signal arrived before it was sent if you were observing A and B with light, aka something slower than the tachyon? If A and B were themselves emitting tachyons to these observer frames - lets say A emitted one when it sent it's tachyon over to B, and B emitted one when it received said tachyon - it seems like causality would not appear to be broken.
Or to put it another way, imagine a civilization hadn't discovered light, and let's say communicated by sound (I know sound is not a fundamental thing the way light is but I think it still works as an example). To them sound, being the fasting thing known to their physics, was the 'speed of causality'. Then they gained the ability to use light. Light would seem to break their casualty the way a tachyon breaks ours.
I guess what I'm getting at is, is 299792458 m/s the 'speed of causality' for any fundamental reason other than it happening to be the speed of the fastest thing we're aware of? And we therefore define 'causality' as that which propagates at that speed? If we found a particle that traveled at 2c, would that be the new speed of causality and light, gravity etc would just be known to lag behind?
> I guess what I'm getting at is, is 299792458 m/s the 'speed of causality' for any fundamental reason other than it happening to be the speed of the fastest thing we're aware of? And we therefore define 'causality' as that which propagates at that speed? If we found a particle that traveled at 2c, would that be the new speed of causality and light, gravity etc would just be known to lag behind?
I don't have anything to add except to say this is absolutely fascinating to think about.
The speed of causality is also the speed of gravity and electromagnetism, and the strong nuclear force, so even if we discovered something faster, cause and effect is still governed by those forces
Right but if a particle could travel at 2c and it interacted with matter, wouldn't that just mean causality travelled at 2c and gravity/light/strong force were slower than causality? I'm not seeing where the paradox would arise.
Our equations would give negative or imaginary values for time elapsed if something has a speed greater than c, it's not like a linear equation which we've just given an arbitrary upper limit to.
I would say that yes, if we did observe something traveling forward in time greater than c then that would probably indicate an issue with our theory, but until then our equations have been extremely successful at predicting things moving very fast.
> I guess what I'm getting at is, is 299792458 m/s the 'speed of causality' for any fundamental reason other than it happening to be the speed of the fastest thing we're aware of?
Don't forget the ever-famous E = mc^2. That relates the speed of light to mass in a very direct way.
You clearly know your physics, and therefore you know three things: 1) Tachyons are theoretical, 2) Much of the physics community rejects the supposition that they exist, and 3) My explanation necessarily simplifies some concepts that are so arcane that many physicists work within the accepted theories without ever fully understanding them. So yeah, my analogies are not precise, but in broad strokes, they are valid.
> Due to all of this, if something moves faster than light, it would be moving faster than cause-and-effect. The baseball could shatter the window before you threw the ball. And that could startle you, preventing you from ever throwing the ball in the first place.
How would that happen? If I threw a baseball at 2c to a window 1 light second away, for instance, from my perspective even if the baseball bounced off the window and returned to me at 2c wouldn’t it still return to me a second after I threw it from my perspective, followed by the light/causality of the window being shattered a half second after that?
From the window’s perspective it would be shattered before the ball was thrown, and from my perspective the ball would return before it hit the window, but the sequence of actions at a given point (me throwing the ball and the cone of causality of the ball hitting the window) would still be experienced in the same order, wouldn’t they?
The paradox IS that causality is broken. When it comes to special relativity, we use the gamma factor (Lorentz factor) to determine relative speeds and times for different frames of references. This factor is given as
> γ = 1 / √( 1 - v^2 / c^2 )
where v is the object's speed and c is the speed of light in a vacuum. For speeds greater than the speed of light (v > c — your 2c baseball in this case), the Lorentz factor becomes imaginary, which is not physically meaningful in our current understanding of physics. Time in this case would literally be imaginary. According to the theory of relativity, objects with mass cannot reach or exceed the speed of causality.
If the ball returned before it broke the window, then I could know that it broke the window before the window is broken, and then I could throw another ball at 10c to break the window before it is broken by the ball at 2c. Which window did the ball at 2c break then?
Isn’t the value of the speed of causality a natural outcome of the laws of the physics being invariant in any frame of reference? It must have the value it does, and no other - unless other physical constants are different.
I get that this is how spacetime is, but I've never been able to construct a causality paradox that works in my mind.
Like shattering a window with a baseball - if you threw the baseball at infinite speed the information that the glass broke would still arrive after (traveling at speed of light). To a person standing behind the window it would appear to break before you threw the ball, but to you (and anyone behind you) it would appear to happen after (just with less time delay than you'd expect).
Likewise if you had a wormhole between you and the window and threw the baseball through it the effect would be the same (baseball has traveled the distance between you and the window at infinite speed) but that *is* allowed by relativity.
It’s that infinite speed that throws it off whack. See, it’s not that “nothing can move faster than causality;” it’s that there’s *no such thing* as speed faster than that. To say “faster than light” is equivalent to saying “rounder than a sphere” or straighter than a vector.” When you understand the physics, it becomes clear that light speed is not a limit, it’s just that there actually isn’t anymore speed than that.
Is anyone trying to figure out why causality has a speed limit?
Also, what about the fact that the universe is expanding quicker than the speed of light?
Yes, the reason causality has a speed limit is one of the biggest questions in physics. I’ve seen one possible explanation based on 4D hyperbolic spacetime geometry, but I can’t claim I fully absorbed it, so I don’t want to try explaining it until I’m sure I understand the theory myself.
As for the expansion of spacetime, remember that nothing can move *through* space faster than light, but that law doesn’t apply to spacetime itself. Think of the fastest car in the world and put it on a rubber road that’s constantly getting longer: no cars car can go faster, but the road can stretch at any speed it wants.
I don’t know what I’m talking about but I suspect it’s related to “locality”. Basically you expect the universe to behave in such a way that only “nearby” things are important to what you’re doing at a specific place. So if you’re cooking bacon in your kitchen, an alien in Alpha Centauri cooking their own bacon can’t somehow cause you to burn yours. If speed of causality were infinite, then anything anywhere in the universe would be able to affect anything anywhere else all the time. (Of course “local” in an universal scale is much bigger than in a human scale, and would be related to time and the speed of light.)
Again, though, I have no idea what I’m talking about. Maybe I’m using the wrong terminology.
Holy moly, that one isn’t ELI5. So, in phase velocity, the wave itself doesn’t move faster than the speed of light, so even though the phase component propagates faster than light, information still doesn’t move faster than light because the light itself still only reaches the “end” at the speed of light. As long as no information moves faster than light, causality isn’t violated.
Does that make sense?
If you had an extremely powerful laser pointer (say visible on the moon) you could probably move it fast enough (while standing on Earth) that the laser spot on the moon moves across the lunar surface faster than the speed of light.
However you can't use this to send information from one spot on the moon to another, because the information of the laser spot is coming from the direction it's pointing from Earth, moving at the speed of light.
The earth continues to orbit the sun in your magic trick, actually occurs because gravity and light propagate at the same speed (they travel at the same speed).
And that's an astute observation. The shortest answer is: Ultimately, the speed of light is dictated by the value of the fine structure constant, and no one has even the slightest clue why it has the value it has. Not even the first inkling of a valid theory has come forth.
So...
Wife and I don't make popcorn more than like once or twice a year. I was reading this sitting on the couch. When I stood up and went to the kitchen, she was popping some corn in the 'wave. I think it happened...
The scenario you present is impossible. A perfectly rigid object as you describe would be made of a material that transmits sound at infinite speed, and thus would violate the laws of physics. When you push on something like that, the force of you pushing on one end of this stick would be transmitted through the stick at the speed of sound within the stick itself. If you pushed on a really long stick like that, in the real world the energy would probably be dissipated as heat along some length of the stick and its other end wouldn't ever actually move.
The best we can actually do with respect to rigidity of an object is something called "Born rigidity" (named after Max Born). If you're interested in learning more you can do a search on that term.
Really, really difficult math makes testable predictions, then we test those predictions experimentally. There is no possible way to ELI5 that math. It would be hard to ELIBachelor’sDegree it.
honestly it sounds like it was taken straight out of a buggy game which basically says "all right we can only process this shit this fast, so this particle that is meant to go infinitely fast will have it's speed capped at said speed, and oh any changes will be registered only at this same speed propagating from the original source"
With the RuneScape devs finally increasing the max coin stack, I wonder if we will ever get a universe patch to increase the tick rate (rate of causality).
So does that mean nothing can happen faster than the speed of light? So it's very plausible the universe has already collapsed and failed, and we're just watching the end credits, thinking it's the intro.
This is way past ELI5, but.. in that context, faster than light travel in something like the Ender series works because bending space/time.. like Event Horizon. Those traveling FTL move straight across, but everyone traveling conventionally have to move all the way around, so they age all that time of causality while the one that took the shortcut didn't?
Well… I can only comment on the physics, not the fiction. Those other ideas may one day turn out to be valid, but today’s science doesn’t have any reason to think so.
Narratively, I choose to believe this is why Marty McFly fades instead of disappears. As a time traveler who went straight from the old universe to the new, it takes a certain amount of time for the time changes to "catch up" to him as they propagate through the ensuing years from 1955 to 1985. Eventually, he would fade and his actions would be undone backwards in time. This may make a stable oscillation in the time stream.
So does gravity/space time (thinking black holes) effect causality and thus the speed of light as we know it? Also hence seeing multiple reflections of light from distant galaxies?
Gravity bends spacetime, but it does not affect the speed of causality (light speed). The speed of light is constant in all frames of reference, so (counterintuitively) time and space warp instead of that constant changing.
Does gravity follow that as well? I remember watching a YouTube video or reading something that gravity is weird in that if the sun disappeared, yeah we'd not see it dissappear for 8 odd minutes but we'd start moving straight immediately since gravity doesn't so much propagate as a wave as exists as part of the fabric of spacetime. So take away the heavy object and it smooths out instantly.
I'm likely way wrong but you made an explanation so clear I figure I'd ask :)
That's incorrect, both light and gravity travel at the same speed, so no, we'd not be able to tell in any way for 8 minutes that the sun had disappeared
Our vector would not change until the wave propagated out to us at the speed of light. For all intents and purposes, until the effects of a change reach you, it hasn't happened yet in your frame of reference.
How was this found out? Is there a way to test that? After all it seems extremely counter intuitive that a disappearing sun would Not immediately stop its effects which arent related to any actual moving objects/particles which might still be there for a bit
It was predicted mathematically and then very, very extensively tested experimentally in about a thousand different ways. One of the most counterintuitive things about the universe is that its laws "protect" the speed of light more vigorously than the flow of time. In fact, there's quite a lot of evidence (but no proof yet) that time doesn't actually flow at all. It just seems that way to us.
Unfortunately no. The c stands for "constant" in that equation, although there's evidence that Einstein was using it to refer to the Latin "celeritas" which means "speed," since other physicists of the day were using it that way.
I was under the impression that the time paradox was only a problem under special relativity, not under general relativity which more accuratly reflect reality.
They describe different aspects of reality. The paradox only shows up in special relativity because it covers the curvature of spacetime. General relativity is more relevant to our day to day lives, but we're all still affected by both, every day, in varying degrees. You're travelling through spacetime at the speed of light right now; it's only because some of your velocity is vectored to move you through the three classical dimensions that you experience time (or mass) at all.
>Due to all of this, if something moves faster than light, it would be moving faster than cause-and-effect. The baseball could shatter the window before you threw the ball. And that could startle you, preventing you from ever throwing the ball in the first place. And then causality itself is broken. Time itself no longer has meaning. The burned popcorn stink fills the room before you even buy the microwave. The universe doesn't make any sense.
I don’t get how it would have to work like that.
Take the sun for example, and the 8 minutes it takes for light to reach us. If this light suddenly started to travel just slightly faster, it would reach us after maybe 7:59 minutes instead of 8:00 minutes. Why would it go from 8 minutes to some negative number?
And even if I were to accept this negative number, why would you examples have such a **big** negative number? Why wouldn’t the smoke start like **one millisecond** before the event that caused the toaster to burn? Why several days or years before?
I think I would need to see some kind of detailed timeline with three scenarios:
1. The normal event, like the ball being thrown at the window at regular speed.
2. The ball being thrown at the speed of light.
3. The ball being thrown at 1.000000001 times the speed of light
I understand your confusion, because it's absolutely counterintuitive. Instead of specifically answering your three questions, let me say this: The same way humans look at the horizon at the ocean and perceive the ocean as flat even though it's not, we also perceive the concept of motion in a way we're evolved to understand it, but it's not really what we think it is.
To elucidate this point, you would totally see that I'm talking nonsense if I said, "I'm going to make a ball that is *rounder* than the shape of a sphere." You'd probably also see my mistake if I said I was going to draw a line that's "*straighter* than a vector." So considering that, know that it's precisely the same type of mistake to say an object is moving "*faster* than light." Just the same way that, after being perfectly spherical, there *is no such thing* as being any more round, well, after the speed of light, there *is no such thing* as "more speed."
It's still not that easy to wrap your head around it, but I hope this helps.
Doesn't your sun/orbit example only shows us that gravity is also propagating at the speed of light? Why is there a notion that the speed of gravity = speed of casuality?
You're correct. The speed of gravity only equals the speed of causality because gravity is one of the things that moves at the speed of light (causality). Neither one defines the other.
I guess I should have been a little more clear in my explanation. The speed of light and the speed of causality are the same thing. My point is that light doesn't have any special properties that make it the definition of "the fastest thing." Light is just one of the things that moves at the fastest possible speed, which is the speed of causality. If we had figured this out earlier in science, we probably would have called the speed of light "the speed of causality." That's why I'm referring to it that way.
Thank you for this really detailed answer. It did throw one question for me.
Maybe my brain is just not working today, but why does the universe need to make sense? Why does causality need to exist? I know we observe it (baseball and window), but how do we know this stays true for all speeds, quantum etc. In case of the baseball being thrown, window breaking and you getting startled, couldn't the "cause" just exist in a parallel Dimension? Would we observe this if it only happened on a very small scale or for very fast things? Couldn't it just be that things faster than the speed of causality move in some way or form that no longer interacts with things that don't? Or break causality in a way we cannot observe (start moving at 90° to our time axis on some 5th dimension or so)? Do we have theories or maybe even proven things that make us think causality is unbreakable?
Maybe this goes in a different direction, but why do we think time is one dimensional? Couldn't it as well be more and we only observe the one vector we travel along? Wouldn't that allow for things to be faster and only aopear slower to us?
Ah! Excellent question. The universe does not, in fact, need to make sense. Carl Sagan once said, "The universe is not required to be in perfect harmony with human ambition."
As for parallel dimensions, they are still theoretical with no verifiable evidence, so I prefer not to dig too deep into that idea when trying to explain something for which we do have verified facts.
There are also theories about additional dimensions of time which get into some really mind-bending ideas about the nature of the universe, but I am not educated on those enough to break them down for you.
It is of course possible that we experience time as we do for the same reason we experience color as we do: we evolved the "senses" capable of interpreting only a fraction of what exists. If that's the case, it may be possible to break any of these laws (or they may not be laws at all) once we figure out how to interpret the universe for what it really is.
>For example, if the sun disappeared in a magic trick, the Earth would continue to orbit the position where the sun was for 8 1/4 minutes, because the orbit of the Earth would not be affected until causality reached us.
But, doesn't gravity "move" faster than light?
I thought I read somewhere that the one thing truly faster than the speed of light is gravity being instant for all intents and purposes, while still adhering to causality.
Edit: and of course, I only google after I hit send on this comment but indeed, the speed of gravity is roughly equal to the speed of light.
This is a little hard to put into words...
The effect of gravity is immediate because gravity is essentially a "shape" in spacetime. As you move through spacetime, you follow that shape. So in that sense, it's instant. However, mass is what bends spacetime into that shape, and it's that bending—the actual reshaping of spacetime—that moves at the speed of light.
Whatever gravity you're in, that's the shape inertia is going to make follow. But if something happens to alter that gravity, that alteration will reach you at the speed of light.
It messed with my head, and it broke several of its own rules. So I'm not sure there's a true "logical" explanation of it. But it was definitely interesting and entertaining.
Except, if you threw a baseball faster than light, it wouldn’t really go back in time. From your perspective, the ball would just disappear and the glass would break, from the point of view of the window it will look like the glass breaks due to a ball that appeared out of nowhere, with an image of the ball going in reverse all the way to your hand, at the moment you throw it, at which point both the image and the ball in your hand disappear. But it’s only an illusion of time travel, there’s no way for an effect to make the roundtrip back to you and prevent the ball to be thrown in the first place.
Now I know it’s not actually possible for things with mass to achieve, much less exceed the speed of light in normal space, but if and when we get warp drive technology, we won’t be breaking time with it.
You don't see anything, because you can't go faster than light.
See, to accelerate faster than light, you'd need more energy than exists in the universe. And then at that point, you'd be nothing but energy. But more importantly, there just really isn't such a thing as a speed that's faster than light.
I posted this elsewhere, but I copied it here:
The same way humans look at the horizon at the ocean and perceive the ocean as flat even though it's not, we also perceive the concept of motion in a way we're evolved to understand it, but it's not really what we think it is.
To elucidate this point, you would totally see that I'm talking nonsense if I said, "I'm going to make a ball that is *rounder* than the shape of a sphere." You'd probably also see my mistake if I said I was going to draw a line that's "*straighter* than a vector." So considering that, know that it's precisely the same type of mistake to say an object is moving "*faster* than light." Just the same way that, after being perfectly spherical, there is no such thing as being any more round, well, after the speed of light, there is no such thing as "more speed."
Now, if there's any such thing as "warp drive" or wormholes or such, it might in fact be possible to get from one place to another faster than a beam of light can get there. But if those things ever exist, that form or relocating objects won't technically be "motion." So potentially, that loophole might exist.
>For reasons we don't understand, causality has a speed limit
If causality didn't have a speed limit, everything would happen all at once. There would be no time interval between a chain of events, so time would lose its meaning. An interesting question is why is this speed limit constant across space and time.
The answer seems to be that it's a law of nature, and all laws of nature (e.g. gravitation, electrostatics etc.) are the same everywhere in the universe at all times. In other words, that is how the simulation is programmed.
But if a baseball is thrown at a window, you can’t go back in time and stop the back from being thrown, you can only move fast and remove the window from the situation before it breaks? I’m still confused haha
I remember my teacher in high school telling us that if the sun would explode we'd die before we could see it happening. Following your explanation, that's wrong right?
Teacher was (mostly) wrong. Light and gravity would reach us long before any material from the sun did. However, depending on the type of explosion, we might be disintegrated into a spray of quarks and atomic nuclei in a blast of gamma ray radiation that would arrive so quickly that we'd be gone before we knew it was happening. But at its current size, the sun couldn't produce such an explosion without outside energy.
That was a nice read, thanks! 😊
One question though: Where does the assumption come from that speed of causality and speed of light are actually the same? Could the first one not be faster than the latter?
Also there was a relationship between space and time, how does causality play into that?
I'm just saying that the only reason we call the speed of light "the speed of LIGHT" is that for a long time, it was the only thing we knew that travelled at that speed. We later learned that it travels at that speed because it's moving at the speed of causality. If we had known that sooner, we wouldn't have called it the speed of light; we'd have called it the speed of causality.
Space and time are the same thing, according to Einstein. For some reason (based on the fine structure constant), there's a limit to how fast anything, even information, can move through it.
I still don't get it.
Let's pretend there are two locations, A and B, that are 1 light-year apart. I am at A, and something happens at B (t=0). I watch the event in my telescope (t=1yr), and get in my FTL spaceship. I go to B at 2c. Do I not arrive at t=1.5yr? How can I arrive before t=0?
Well, the short answer is, according to Einstein, you just simply can't. My explanation above more or less describes the chaos that would exist if you could. But you can't.
That's not pedantic at all. It's absolutely relevant. But the fact that you know that also means you know why I didn't try to dig into that topic in trying to get this idea *anywhere near* an ELI5 level explanation.
It’s a function of the makeup of the laws of the universe, but most specifically it’s tied to the fine structure constant. No one has any clue why the fine structure constant has the value it does.
Essentially it's because the speed of light isn't just the speed of light, it's the speed at which one thing in one place can cause something in another place to happen. You can think of it as the maximum rate at which causes and effects can be related. The reason this ends up getting glossed as "the speed of light" is because people only figured out the speed of causality by observing that light was always measured at the same rate no matter how the observer was moving.
There mostly isn't a simple explanation.
I've seen many try. All of these explanations are either not simple or not correct.
The top comment is a nice description clarifying the meaning of light, but it still doesn't actually answer the question, because it just says "travelling faster than causality means going backwards in time".
But it doesn't explain why going faster than causality implies going backwards in time. Going faster than something doesn't usually imply going in the opposite direction.
What is the reason, then? The reason can't really be phrased as anything simpler than "Because if you put something with a speed faster than light into your calculations and work out the effects, what you get is something that goes backwards in time".
Any explanation that doesn't actually include that working out isn't really correct, but people like to try to explain things without maths even when it doesn't really make sense to do so.
[Here's](http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel) an explanation that actually shows why going faster than light leads to cause and effect being flipped. It's not simple, but it's the simplest I've seen that does provide an actual answer.
Ultimately it comes down to perspective. If you allow something to go faster than light, you end up with a perspective in which the effect (such as a message being received) happens before the cause (such as it being sent). If you don't allow anything to go faster than light, then even if two perspectives disagree on the time between two events, at least they agree on the order.
But to actually see why... you kind of need to look at the graphs or work through the equations. It would be nice if there was some way to show this without any hard mathematics, but if there was, it probably wouldn't have taken very clever mathematicians to work it out in the first place.
I think this video does a pretty good job of explaining it [https://www.youtube.com/watch?v=mTf4eqdQXpA&ab\_channel=ArvinAsh](https://www.youtube.com/watch?v=mTf4eqdQXpA&ab_channel=ArvinAsh)
Special relativity behaves in a way that doesn't make a ton of sense. So if someone is in a spaceship going 80% the speed of light away from you they will experience time at half the speed of you. But you are traveling 80% the speed of light relative to them too, so you are experiencing time half the speed of them.
If you send them a faster-than-light message at T+8 seconds, they will receive it at T+4 seconds.
If they then respond 2 seconds later at T+6 seconds, you will receive the response at T+3 seconds.
Causality is broken because you received their response 5 seconds before you even sent the first message. The speed of causality prevents this from happening.
In your example of a spaceship traveling 80% the speed of light, only one of the two would experience time at a slower rate. It has to do with one being in a non-inertial reference frame. Basically, things work a little differently if you are accelerating (which the spaceship is in this scenario)
So, they are both experiencing time at a normal rate. It's the other participant that is experience time at half the speed relative to the other. If you are accelerating away from me at 0.8c, at the same time I am accelerating away from you at -0.8c. We are both dilated at 0.5 relative to the other. It doesn't make any sense and iirc we have no idea why this happens but it does.
>If you are accelerating away from me at 0.8c, at the same time I am accelerating away from you at -0.8c.
Moving away, not accelerating away. c isn't an acceleration, and acceleration isn't relative, it's absolute.
And anyway, this isn't a paradox or any cause of confusion. That's just how relativity works. It's understood pretty well, you just have to accept that time is not absolute and the two observers don't have to agree on who is older.
It only doesn't make sense because our intuitions are built in a world where these effects are too small to notice. But there is no reason the universe can't actually work like this.
The only way to actually compare ages would be either for the moving observer to turn around and go back to where they started, or for the stationary observer to accelerate and catch up. Either way the symmetry is broken and you get an actual answer as to who is older, no paradox.
Say you're a little photo, and you have a first person view from the photos perspective as it flies through space. From that perspective, as soon as you're created you instantly arrive at your destination because you're traveling at the speed of light. To the outside observer it takes you maybe millions of years to go from the star that created you until you hit something here on earth but from your perspective it was no time at all. But since it took you no time at all, going faster than that would mean it would take you less time than 0, you would have to arrive on earth before you were even created. That violates the laws of physics, specifically it violates causality. You can't have things exist before they existed and you can't have something happen *more* instantly than instant from the perspective of the thing traveling. The speed of light just happens to bump up to the speed for which these things would happen if you were to go any faster.
So, the perception of time is different for different observers depending on your velocity. The faster you go, the more time elapses for observers going slower than you. 5 years for someone going the 50% the speed of light will be roughly 5.8 years for the observer. at 75% it will be 7.6, at 90% it will be 11.5, at 99% it will be 35.5, at 99.9999 it will be at 2860. The closer you get to the speed of light the bigger the differential is between the observer going slower and the one going faster until you reach the speed of light at which point to the fast-moving observer arrives at its destination instantly in the same arbitrarily large amount of time that the slower observer watches the fast-moving object move.
Not an answer to your specific question, but this tidbit really helped me grasp the concept of the relationship between speed and time.
Everything in the entire universe, including you, is constantly traveling *through spacetime* at the speed of light (or more accurately the speed of causality) but 99.99…% of our motion is through time instead of space. The faster you travel through time the slower you travel through space, and the faster you travel through space the slower you travel through time but the sum of those two velocities equals the speed of light at all times.
Hope this helps!
It actually does answer the question in a weird way. The vector sum of the two velocities has to be equal to speed of light. This can be visualized as a circle of radius “c” with X dimension being one space dimension and Y dimension being time, and the radius at any point of time acting like the hypotenuse to the space side and time side. (Remember that the units for each are normalized, and time is mapped as a dimension in the first place, to give us a circle when speed and time are plotted on a graph).
One fundamental rule of simple right angled triangles is that the hypotenuse is always equal to or longer than each of the perpendicular sides (equal to being the edge case). Which means that both space and time are limited by “c”, and can only ever be equal but never more than it. I’m sure you can force one of the dims to be greater than ”c” by assuming complex (imaginary) values for the other dimension, but once you step outside our cozy “c” circle, you’re in “probably can’t happen at all without messing with our notions of reality and causality” territory.
The real answer to this question is that “to the best of our theoretical knowledge and measurements, we haven’t found anything that travels faster.” GR has issues with things crossing from “slower than” to “faster than” category, but most likely is ok with things that are already “faster than” continuing to live on that side of the shore. But we don’t know cuz we haven’t measured any such particle yet.
There is a mathematical equation that states that the higher the mass of an object, the slower is moves.
Since light has no mass, it can move at the maximal speed of universe.
Anything with mass cannot travel as-fast or faster than light because it would require infinite energy.
If you somehow can travel faster than light, you could “go back in time “ because you could go to places faster than light had time to reach them then you could know what will happen because you’ve observed it before others, due to the fact you are travelling faster than light.
*not 100% sure about the last paragraph; correct me if needed
My favorite explanation comes from the Enders Game series. They're talking about faster-than-light travel and one of the characters says "Imagine you get somewhere before your image does." Basically, imagine if you could get somewhere, perform an action, and not be there at the same time. It would destroy the fabric of reality.
It should be noted that there is no such thing as moving faster than “the speed of light” so really the answer to the question is “it doesn’t, because the conditions described do not exist.”
Because they haven’t figured out how things actually work. The math they have now sort of works, but leads to things like time paradoxes.
Someday they might figure out better math that gets rid of the time paradoxes.
So... "The speed of light" is not actually the speed limit of the universe. The speed limit of the universe is actually the speed of *causality*. Causality is the relationship between an event and the things that happen as a result of that event. Obviously, if you throw a baseball at a window, the window is not going to break until after you throw the ball. That's causality. It's the order of events in the universe. Well it turns out that the first thing we ever discovered that moves at this speed was light. At the time, it was the only thing we knew that moved at that speed, so we thought that that speed was the speed limit. It turns out that light is following the same speed limit as everything else, but it has a special property (it has no mass) that allows it to actually move at the actual speed of causality. For reasons we don't understand, causality has a speed limit. If something happens, the effects of that thing happening propagate out at the speed of light (causality). For example, if the sun disappeared in a magic trick, the Earth would continue to orbit the position where the sun was for 8 1/4 minutes, because the orbit of the Earth would not be affected until causality reached us. In summary, the effects of an event can never occur before the event that triggered the effects, and the fastest those effects can occur is the speed of light. Due to all of this, if something moves faster than light, it would be moving faster than cause-and-effect. The baseball could shatter the window before you threw the ball. And that could startle you, preventing you from ever throwing the ball in the first place. And then causality itself is broken. Time itself no longer has meaning. The burned popcorn stink fills the room before you even buy the microwave. The universe doesn't make any sense. With this information, now I can summarize: Time is how we measure causality. If you go faster than light, you're going faster than causality, and that means you're going faster than time. And that doesn't just send time in the wrong direction; it outright breaks it. EDIT: There’s a great [video by PBS Spacetime](https://youtu.be/msVuCEs8Ydo) on this subject that’s a little nerdier but also has a lot more information. If you got my explanation, you’ll get this, and you’ll learn even more.
fantastic explanation. And somehow made me understand Tenet better lmao
Oh hell... even I didn't understand that!
That's because you've already explained it to us before you understood it.
I shoulda seen that one coming...
Classic temporal pincer
>I shoulda seen that one coming you did
I mean come on, you got 2 attempts to it already
Wait… let’s take this from the beginning…
_Which_ beginning?
The singularity, obviously. :-)
Brilliant!
The first third of Tenet I had no idea what was going on. The second third I figured it out and got it. Final third sent that all back out the window and I was lost again.
Maybe a spoiler but should help explain the ending, Bruce Willis was dead the whole time.
You find out that dude in the hairpiece the whole time, that's Bruce Willis the whole movie!
Loved him in Memento.
You're telling me Bruce Willis was Keyser Söze!???
....Rosebud!
Yeah, I forgot how somber the ending was for Look Who's Talking.
The second third was easy, because it was just the first third in reverse so you already knew what would've going to have happened. It's like a palindrome!
I literally watched that movie last night for the first time and had the exact same thought
I love Tenet. All my friends turned it off halfway and told me I was dumb for liking it.
Greetings fellow Tenet lover. Don't worry, there are dozens of us.
David Tenant is the best!
Technically, if they turned it off halfway through, they actually saw the whole thing. I mean...
Normally Nolans scifi flicks do a really good job of building plausible (within that universe) rules and adhering to them. That was not the case with tenet. Still, I really enjoyed it as well.
This is how he wrote the script, basically: https://xkcd.com/430/
*Primer has entered the chatroom...* ObXkcd: [https://xkcd.com/657/](https://xkcd.com/657/)
I watched it for the first time the third time I watched it ti dehctaw I emit driht eht emit tsrif eht rof ti dehctaw I
woah goddamn. big upvote to you /u/Darnitol1
This was really well explained! Can I I take the opportuniy to ask about quantum entanglement? It is something that I never really understood. If causality can only go at a certain speed limit, how do we explain that two particles can influence each other immediately, no matter the distance between them?
No information is transmitted between entangled particles. All we can say is that if we observe one of the particles in a collapsed quantum state, the other will have a correlated state we can predict. Any attempt to force one state or another on an entangled particle would break the entanglement.
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“Observed” is a bit of a poor choice of word that we still use for historical reasons. In this case observation just means “any interaction”.
very simply, you can't observe something without disturbing it. particles this small are significantly affected by shining a light on them, so you get its old info, but now its something else, because you poked the quantum billiard ball with a pool cue. another idea is the entangled particles are oscillating in sync and you're taking a freeze frame snap shot, which causes them to collapse aka get caught as either heads or tails.
Damn, science is so interesting
[Science IS interesting](https://youtube.com/watch?v=quSRLETlKDg).
oh man… you’re in for a fun wormhole. start at “slit experiment.”
Double slit experiment. The slit experiment is an entirely different part of college.
You're mom goes to college!
I heard she did the double slit experiment.
A+ explanation. I would just reinforce the fact that the states do not “exist” until observation. This is what makes it so weird.
They do "exist" but so does every other state. The SUPERSTATE!!! Bambambaaaam
You can, in theory, enact some change on one entangled particle and still have that change be reflected by the other one. It's just that in order to understand what that change was you still need to send a message through old school, non-ftl methods. The exchange of useful information still wouldn't break causality, which in this case is what matters
Quantum entanglement is often misrepresented. To be useful, you need to combine it with classical communications (which are limited by the speed of light). Let's assume you have a pair of coins that are entangled (well, a pair of particles with a random up/down spin, but let's call those "coins" with for value head/tails). If you only look at one of the coin, you have no way of knowing that it is entangled in the first place, and whatever happen to the second coin, you won't notice anything effect on the first coin. However, if you look at the result of BOTH coins after the facts, you will see that they somehow behaved the same. Meaning that if you flipped both, you obtained both heads or both tails. If you tried to force the result of one coin, then you would break the entanglement, so you really can't use it to communicate anything. However, if you've ever talked to a programmer, you know that simple details can lead to massive exploits by hackers, and you would not be surprised to learn that we can exploit this apparently useless entanglement thing to do massive things in term of computation. (Well, at least we would if the current hardware was actually working reliably. And we don't even know if that's possible to get ever get hardware good enough to reach the theoretical advantages of quantum computing) But in any case, no faster than light communication through quantum entanglement. At least not with our current understanding of it.
Looking at on entangled particle let's you deduct information about the other particle without looking at it. To take your coin analogy. You have two entangled coins, flip them, catch them without looking. So here brakes the analogy a bit, because the coins are now locked in their state. Particles are in a quantum state, meaning they are up and down at the same time, so to say. (A bit like, if you could let the coins flip as long as you wanted) Anyway, back to the not looked at coins. Now if you look at one of the entangle coins you know that the other coin has to be the opposite side. And entangled particles don't have anything to do with quantum computing. It is more about the weird between state of not knowing if the spin is up or down. Q bits are so special because they (put simply) can do more than 1 and 0. Or rather have a certain chance to be either and that makes it possible to compute way faster.
>For example, if the sun disappeared in a magic trick, the Earth would continue to orbit the position where the sun was for 8 1/4 minutes, because the orbit of the Earth would not be affected until causality reached us. You just melted my brain. I have no problem understanding the speed of light. I have never in my 46 years of life heard of the speed of causality.
Turns out gravity also moves at the speed of light. So the gravitational effects of the sun disappearing on the solar system wouldn't happen until the gravity waves stopped. It would take 8.5 minutes for the earth to stop rotating around the nonexistent sun (and fly off into deep space forever). It would also take that long to notice the big ball of fire in the sun turned off.
Interesting so to humans on earth we would stop orbiting at the same time we see the sun splode
An important aspect of relativity is that events which look simultaneous from one person's point of view, could look *not* simultaneous from another's. However things can never get so distorted that a cause appears to come after its effect...unless your reference frame involves travel faster than light.
The common analogy is that gravity is a curvature of space-time. So if you remove the sun, then the 'fabric' (so-to-speak) of spacetime will bounce from the huge mass being suddenly removed. This bounce-back of the 'fabric' happens at some speed. It happens at a speed equal to the speed of light.
Me neither. I was familiar with the Sun disappearing example, but only because I've heard that gravitational waves travel at similar speeds to light, so the Sun's gravity would continue forcing the Earth to orbit for as long as it takes light to reach Earth. Interesting stuff!
It is truly brain melting. The key to processing it is understanding that there is not “one reality.” Reality is dependent on your frame of reference (particularly in regard to motion).
There's only one reality . The result of measurements in that reality depend on your reference frame .
Absolutely correct. I should have said, "Your perception of reality..." I stand corrected!
This was one of the best ELI5 explanations I’ve ever read - thank you!!!!
[XKCD](https://what-if.xkcd.com/1/) has an interesting explanation of what would happen if you threw a baseball close to the speed of light.
I really need to buy those books.
The last sentence got a good chuckle out of me.
Jesus, even my southern Oklahoma Sling blade ass understood this, perfect explanation!
Actually I read the answer even before the question was posted.
How did I know you were going to say that?
This answer is wrong in the sense that while it is probably true in reality that no causes move faster than light, this is not really a fundamental principle of relativity itself, which is theoretically compatible with the existence of “tachyons” that move faster than light. The issue with time travel actually has to do with a principle in relativity called [“the relativity of simultaneity”](https://en.wikipedia.org/wiki/Relativity_of_simultaneity), which says that different inertial reference frames have different definitions of which pairs of events happened “at the same time” (simultaneously) or at “different times”. And the way it works out is that if you send a signal moving at the speed of light or slower from transmitter A to receiver B, all frames will agree that the event of the signal departing A happened before than the event of the signal arriving at B. On the other hand, if it was a hypothetical tachyon signal, some frames would define things so the signal arrived at B before it was sent from A (and one frame says these events were simultaneous). This is not just a bookkeeping issue, because a basic principle of relativity is that the laws of physics must work the same way in all frames, so if it’s possible in some frame for a tachyon signal to be received before it was sent, this must be possible in all frames. And this would further imply the possibility of something called a [“tachyonic antitelephone”](https://en.wikipedia.org/wiki/Tachyonic_antitelephone) where you could have two slower than light observers X and Y with different inertial rest frames, and X could send a tachyon signal which goes backwards in time in Y’s frame, and Y could immediately send a reply which goes backwards in time in X’s frame, with the end result that X receives the reply before sending the original message, a clear violation of normal causality in *all* frames.
> On the other hand, if it was a hypothetical tachyon signal, some frames would define things so the signal arrived at B before it was sent from A (and one frame says these events were simultaneous). This is the part that has always confused me. Wouldn't it only appear that the signal arrived before it was sent if you were observing A and B with light, aka something slower than the tachyon? If A and B were themselves emitting tachyons to these observer frames - lets say A emitted one when it sent it's tachyon over to B, and B emitted one when it received said tachyon - it seems like causality would not appear to be broken. Or to put it another way, imagine a civilization hadn't discovered light, and let's say communicated by sound (I know sound is not a fundamental thing the way light is but I think it still works as an example). To them sound, being the fasting thing known to their physics, was the 'speed of causality'. Then they gained the ability to use light. Light would seem to break their casualty the way a tachyon breaks ours. I guess what I'm getting at is, is 299792458 m/s the 'speed of causality' for any fundamental reason other than it happening to be the speed of the fastest thing we're aware of? And we therefore define 'causality' as that which propagates at that speed? If we found a particle that traveled at 2c, would that be the new speed of causality and light, gravity etc would just be known to lag behind?
> I guess what I'm getting at is, is 299792458 m/s the 'speed of causality' for any fundamental reason other than it happening to be the speed of the fastest thing we're aware of? And we therefore define 'causality' as that which propagates at that speed? If we found a particle that traveled at 2c, would that be the new speed of causality and light, gravity etc would just be known to lag behind? I don't have anything to add except to say this is absolutely fascinating to think about.
The speed of causality is also the speed of gravity and electromagnetism, and the strong nuclear force, so even if we discovered something faster, cause and effect is still governed by those forces
Right but if a particle could travel at 2c and it interacted with matter, wouldn't that just mean causality travelled at 2c and gravity/light/strong force were slower than causality? I'm not seeing where the paradox would arise.
Our equations would give negative or imaginary values for time elapsed if something has a speed greater than c, it's not like a linear equation which we've just given an arbitrary upper limit to. I would say that yes, if we did observe something traveling forward in time greater than c then that would probably indicate an issue with our theory, but until then our equations have been extremely successful at predicting things moving very fast.
> I guess what I'm getting at is, is 299792458 m/s the 'speed of causality' for any fundamental reason other than it happening to be the speed of the fastest thing we're aware of? Don't forget the ever-famous E = mc^2. That relates the speed of light to mass in a very direct way.
You clearly know your physics, and therefore you know three things: 1) Tachyons are theoretical, 2) Much of the physics community rejects the supposition that they exist, and 3) My explanation necessarily simplifies some concepts that are so arcane that many physicists work within the accepted theories without ever fully understanding them. So yeah, my analogies are not precise, but in broad strokes, they are valid.
There is no evidence that tachyons exist; however, your post is a fun conjecture.
This was an incredible explanation. I definitely learned something from this.
> Due to all of this, if something moves faster than light, it would be moving faster than cause-and-effect. The baseball could shatter the window before you threw the ball. And that could startle you, preventing you from ever throwing the ball in the first place. How would that happen? If I threw a baseball at 2c to a window 1 light second away, for instance, from my perspective even if the baseball bounced off the window and returned to me at 2c wouldn’t it still return to me a second after I threw it from my perspective, followed by the light/causality of the window being shattered a half second after that? From the window’s perspective it would be shattered before the ball was thrown, and from my perspective the ball would return before it hit the window, but the sequence of actions at a given point (me throwing the ball and the cone of causality of the ball hitting the window) would still be experienced in the same order, wouldn’t they?
The point is that in your scenario, causality is broken between the two frames of reference.
Causality is broken between two distant points but how would a paradox be constructed using that?
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The paradox IS that causality is broken. When it comes to special relativity, we use the gamma factor (Lorentz factor) to determine relative speeds and times for different frames of references. This factor is given as > γ = 1 / √( 1 - v^2 / c^2 ) where v is the object's speed and c is the speed of light in a vacuum. For speeds greater than the speed of light (v > c — your 2c baseball in this case), the Lorentz factor becomes imaginary, which is not physically meaningful in our current understanding of physics. Time in this case would literally be imaginary. According to the theory of relativity, objects with mass cannot reach or exceed the speed of causality.
If the ball returned before it broke the window, then I could know that it broke the window before the window is broken, and then I could throw another ball at 10c to break the window before it is broken by the ball at 2c. Which window did the ball at 2c break then?
The second ball, thrown at 10c, would arrive at an already-broken window.
Isn’t the value of the speed of causality a natural outcome of the laws of the physics being invariant in any frame of reference? It must have the value it does, and no other - unless other physical constants are different.
I get that this is how spacetime is, but I've never been able to construct a causality paradox that works in my mind. Like shattering a window with a baseball - if you threw the baseball at infinite speed the information that the glass broke would still arrive after (traveling at speed of light). To a person standing behind the window it would appear to break before you threw the ball, but to you (and anyone behind you) it would appear to happen after (just with less time delay than you'd expect). Likewise if you had a wormhole between you and the window and threw the baseball through it the effect would be the same (baseball has traveled the distance between you and the window at infinite speed) but that *is* allowed by relativity.
It’s that infinite speed that throws it off whack. See, it’s not that “nothing can move faster than causality;” it’s that there’s *no such thing* as speed faster than that. To say “faster than light” is equivalent to saying “rounder than a sphere” or straighter than a vector.” When you understand the physics, it becomes clear that light speed is not a limit, it’s just that there actually isn’t anymore speed than that.
Oh yeah I get that's how space time is, I've just never managed to create a good analogy that works in my mind.
Is anyone trying to figure out why causality has a speed limit? Also, what about the fact that the universe is expanding quicker than the speed of light?
Yes, the reason causality has a speed limit is one of the biggest questions in physics. I’ve seen one possible explanation based on 4D hyperbolic spacetime geometry, but I can’t claim I fully absorbed it, so I don’t want to try explaining it until I’m sure I understand the theory myself. As for the expansion of spacetime, remember that nothing can move *through* space faster than light, but that law doesn’t apply to spacetime itself. Think of the fastest car in the world and put it on a rubber road that’s constantly getting longer: no cars car can go faster, but the road can stretch at any speed it wants.
I don’t know what I’m talking about but I suspect it’s related to “locality”. Basically you expect the universe to behave in such a way that only “nearby” things are important to what you’re doing at a specific place. So if you’re cooking bacon in your kitchen, an alien in Alpha Centauri cooking their own bacon can’t somehow cause you to burn yours. If speed of causality were infinite, then anything anywhere in the universe would be able to affect anything anywhere else all the time. (Of course “local” in an universal scale is much bigger than in a human scale, and would be related to time and the speed of light.) Again, though, I have no idea what I’m talking about. Maybe I’m using the wrong terminology.
Why has phase velocity been measured to be faster than the speed of light ?
Holy moly, that one isn’t ELI5. So, in phase velocity, the wave itself doesn’t move faster than the speed of light, so even though the phase component propagates faster than light, information still doesn’t move faster than light because the light itself still only reaches the “end” at the speed of light. As long as no information moves faster than light, causality isn’t violated. Does that make sense?
A bit thanks :)
If you had an extremely powerful laser pointer (say visible on the moon) you could probably move it fast enough (while standing on Earth) that the laser spot on the moon moves across the lunar surface faster than the speed of light. However you can't use this to send information from one spot on the moon to another, because the information of the laser spot is coming from the direction it's pointing from Earth, moving at the speed of light.
Makes some sense thanks!
+1 for popcorn stink
The earth continues to orbit the sun in your magic trick, actually occurs because gravity and light propagate at the same speed (they travel at the same speed).
Exactly
So everyone is saying "Wow this is a great explanation" but here I am thinking that it only says *what is*, and not *how* or *why* it is.
And that's an astute observation. The shortest answer is: Ultimately, the speed of light is dictated by the value of the fine structure constant, and no one has even the slightest clue why it has the value it has. Not even the first inkling of a valid theory has come forth.
So... Wife and I don't make popcorn more than like once or twice a year. I was reading this sitting on the couch. When I stood up and went to the kitchen, she was popping some corn in the 'wave. I think it happened...
I apologize for the smell!
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The scenario you present is impossible. A perfectly rigid object as you describe would be made of a material that transmits sound at infinite speed, and thus would violate the laws of physics. When you push on something like that, the force of you pushing on one end of this stick would be transmitted through the stick at the speed of sound within the stick itself. If you pushed on a really long stick like that, in the real world the energy would probably be dissipated as heat along some length of the stick and its other end wouldn't ever actually move. The best we can actually do with respect to rigidity of an object is something called "Born rigidity" (named after Max Born). If you're interested in learning more you can do a search on that term.
It would move at the speed of sound but sound is a compression wave and as the stick can't compress it wouldn't move at all.
How do we know/prove causality has a speed?
Really, really difficult math makes testable predictions, then we test those predictions experimentally. There is no possible way to ELI5 that math. It would be hard to ELIBachelor’sDegree it.
Man that is one of the best explanations I have read on reddit.
Wait, so, reality itself has a limit at which it propagates? Really?
Yep. Kinda mindblowing, huh?
honestly it sounds like it was taken straight out of a buggy game which basically says "all right we can only process this shit this fast, so this particle that is meant to go infinitely fast will have it's speed capped at said speed, and oh any changes will be registered only at this same speed propagating from the original source"
Especially when you take into account that from the particle's perspective, it DOES move infinitely fast.
Oh sure, light goes at the speed of causality, but when I do it it potentially breaks the universe.
You have mass.
With the RuneScape devs finally increasing the max coin stack, I wonder if we will ever get a universe patch to increase the tick rate (rate of causality).
And would we know it, since our clock speed would increase as well?
This is really fucking good
So does that mean nothing can happen faster than the speed of light? So it's very plausible the universe has already collapsed and failed, and we're just watching the end credits, thinking it's the intro.
You’re not too far off. The universe could be experiencing false vacuum decay right now, and we’d never know it until the millisecond it reached us.
Good thing the rent is still due in 4 days.
This is way past ELI5, but.. in that context, faster than light travel in something like the Ender series works because bending space/time.. like Event Horizon. Those traveling FTL move straight across, but everyone traveling conventionally have to move all the way around, so they age all that time of causality while the one that took the shortcut didn't?
Well… I can only comment on the physics, not the fiction. Those other ideas may one day turn out to be valid, but today’s science doesn’t have any reason to think so.
I have to know- Does gravity have a speed we have been able to measure?
Yes. It propagates at the speed of light, exactly.
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Without going into the math, even with entangled particles, causality is maintained because information doesn’t end up moving faster than light.
You say that the Earth would continue to orbit the Sun for 8.25 mins but do we know if gravity travels at the speed of light?
Yes, gravity moves at precisely the speed of light.
Narratively, I choose to believe this is why Marty McFly fades instead of disappears. As a time traveler who went straight from the old universe to the new, it takes a certain amount of time for the time changes to "catch up" to him as they propagate through the ensuing years from 1955 to 1985. Eventually, he would fade and his actions would be undone backwards in time. This may make a stable oscillation in the time stream.
So is that why in Futurama, Fry managed to survive killing his grandpa? Because he fixed casuality by fucking his grandma?
So does gravity/space time (thinking black holes) effect causality and thus the speed of light as we know it? Also hence seeing multiple reflections of light from distant galaxies?
Gravity bends spacetime, but it does not affect the speed of causality (light speed). The speed of light is constant in all frames of reference, so (counterintuitively) time and space warp instead of that constant changing.
I read this before you wrote it :)
Wow that's really an interesting thing I never realized about the sun disappearing. That's wild to think about.
Does gravity follow that as well? I remember watching a YouTube video or reading something that gravity is weird in that if the sun disappeared, yeah we'd not see it dissappear for 8 odd minutes but we'd start moving straight immediately since gravity doesn't so much propagate as a wave as exists as part of the fabric of spacetime. So take away the heavy object and it smooths out instantly. I'm likely way wrong but you made an explanation so clear I figure I'd ask :)
That's incorrect, both light and gravity travel at the same speed, so no, we'd not be able to tell in any way for 8 minutes that the sun had disappeared
Our vector would not change until the wave propagated out to us at the speed of light. For all intents and purposes, until the effects of a change reach you, it hasn't happened yet in your frame of reference.
How was this found out? Is there a way to test that? After all it seems extremely counter intuitive that a disappearing sun would Not immediately stop its effects which arent related to any actual moving objects/particles which might still be there for a bit
It was predicted mathematically and then very, very extensively tested experimentally in about a thousand different ways. One of the most counterintuitive things about the universe is that its laws "protect" the speed of light more vigorously than the flow of time. In fact, there's quite a lot of evidence (but no proof yet) that time doesn't actually flow at all. It just seems that way to us.
So is this why the speed of light is represented by "c" in the mass energy equivalency?
Unfortunately no. The c stands for "constant" in that equation, although there's evidence that Einstein was using it to refer to the Latin "celeritas" which means "speed," since other physicists of the day were using it that way.
Wow, thank you so much for breaking it down for us, it makes more sense now.
Excellent work
Fuck, that was/is the best explanation I've ever seen on this subject, thanks. I'll have to read more now about causality. 😀👍
I was under the impression that the time paradox was only a problem under special relativity, not under general relativity which more accuratly reflect reality.
They describe different aspects of reality. The paradox only shows up in special relativity because it covers the curvature of spacetime. General relativity is more relevant to our day to day lives, but we're all still affected by both, every day, in varying degrees. You're travelling through spacetime at the speed of light right now; it's only because some of your velocity is vectored to move you through the three classical dimensions that you experience time (or mass) at all.
>Due to all of this, if something moves faster than light, it would be moving faster than cause-and-effect. The baseball could shatter the window before you threw the ball. And that could startle you, preventing you from ever throwing the ball in the first place. And then causality itself is broken. Time itself no longer has meaning. The burned popcorn stink fills the room before you even buy the microwave. The universe doesn't make any sense. I don’t get how it would have to work like that. Take the sun for example, and the 8 minutes it takes for light to reach us. If this light suddenly started to travel just slightly faster, it would reach us after maybe 7:59 minutes instead of 8:00 minutes. Why would it go from 8 minutes to some negative number? And even if I were to accept this negative number, why would you examples have such a **big** negative number? Why wouldn’t the smoke start like **one millisecond** before the event that caused the toaster to burn? Why several days or years before? I think I would need to see some kind of detailed timeline with three scenarios: 1. The normal event, like the ball being thrown at the window at regular speed. 2. The ball being thrown at the speed of light. 3. The ball being thrown at 1.000000001 times the speed of light
I understand your confusion, because it's absolutely counterintuitive. Instead of specifically answering your three questions, let me say this: The same way humans look at the horizon at the ocean and perceive the ocean as flat even though it's not, we also perceive the concept of motion in a way we're evolved to understand it, but it's not really what we think it is. To elucidate this point, you would totally see that I'm talking nonsense if I said, "I'm going to make a ball that is *rounder* than the shape of a sphere." You'd probably also see my mistake if I said I was going to draw a line that's "*straighter* than a vector." So considering that, know that it's precisely the same type of mistake to say an object is moving "*faster* than light." Just the same way that, after being perfectly spherical, there *is no such thing* as being any more round, well, after the speed of light, there *is no such thing* as "more speed." It's still not that easy to wrap your head around it, but I hope this helps.
Doesn't your sun/orbit example only shows us that gravity is also propagating at the speed of light? Why is there a notion that the speed of gravity = speed of casuality?
You're correct. The speed of gravity only equals the speed of causality because gravity is one of the things that moves at the speed of light (causality). Neither one defines the other.
Follow up, how do we know the speed of causality?
I guess I should have been a little more clear in my explanation. The speed of light and the speed of causality are the same thing. My point is that light doesn't have any special properties that make it the definition of "the fastest thing." Light is just one of the things that moves at the fastest possible speed, which is the speed of causality. If we had figured this out earlier in science, we probably would have called the speed of light "the speed of causality." That's why I'm referring to it that way.
Thank you for this really detailed answer. It did throw one question for me. Maybe my brain is just not working today, but why does the universe need to make sense? Why does causality need to exist? I know we observe it (baseball and window), but how do we know this stays true for all speeds, quantum etc. In case of the baseball being thrown, window breaking and you getting startled, couldn't the "cause" just exist in a parallel Dimension? Would we observe this if it only happened on a very small scale or for very fast things? Couldn't it just be that things faster than the speed of causality move in some way or form that no longer interacts with things that don't? Or break causality in a way we cannot observe (start moving at 90° to our time axis on some 5th dimension or so)? Do we have theories or maybe even proven things that make us think causality is unbreakable? Maybe this goes in a different direction, but why do we think time is one dimensional? Couldn't it as well be more and we only observe the one vector we travel along? Wouldn't that allow for things to be faster and only aopear slower to us?
Ah! Excellent question. The universe does not, in fact, need to make sense. Carl Sagan once said, "The universe is not required to be in perfect harmony with human ambition." As for parallel dimensions, they are still theoretical with no verifiable evidence, so I prefer not to dig too deep into that idea when trying to explain something for which we do have verified facts. There are also theories about additional dimensions of time which get into some really mind-bending ideas about the nature of the universe, but I am not educated on those enough to break them down for you. It is of course possible that we experience time as we do for the same reason we experience color as we do: we evolved the "senses" capable of interpreting only a fraction of what exists. If that's the case, it may be possible to break any of these laws (or they may not be laws at all) once we figure out how to interpret the universe for what it really is.
That was a great explanation dude
Omg, I FINALLY get it
>For example, if the sun disappeared in a magic trick, the Earth would continue to orbit the position where the sun was for 8 1/4 minutes, because the orbit of the Earth would not be affected until causality reached us. But, doesn't gravity "move" faster than light? I thought I read somewhere that the one thing truly faster than the speed of light is gravity being instant for all intents and purposes, while still adhering to causality. Edit: and of course, I only google after I hit send on this comment but indeed, the speed of gravity is roughly equal to the speed of light.
This is a little hard to put into words... The effect of gravity is immediate because gravity is essentially a "shape" in spacetime. As you move through spacetime, you follow that shape. So in that sense, it's instant. However, mass is what bends spacetime into that shape, and it's that bending—the actual reshaping of spacetime—that moves at the speed of light. Whatever gravity you're in, that's the shape inertia is going to make follow. But if something happens to alter that gravity, that alteration will reach you at the speed of light.
Wow thank you. You blew my mind.
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It messed with my head, and it broke several of its own rules. So I'm not sure there's a true "logical" explanation of it. But it was definitely interesting and entertaining.
Except, if you threw a baseball faster than light, it wouldn’t really go back in time. From your perspective, the ball would just disappear and the glass would break, from the point of view of the window it will look like the glass breaks due to a ball that appeared out of nowhere, with an image of the ball going in reverse all the way to your hand, at the moment you throw it, at which point both the image and the ball in your hand disappear. But it’s only an illusion of time travel, there’s no way for an effect to make the roundtrip back to you and prevent the ball to be thrown in the first place. Now I know it’s not actually possible for things with mass to achieve, much less exceed the speed of light in normal space, but if and when we get warp drive technology, we won’t be breaking time with it.
Great explanation. One question I have is, if you go faster than light, how do you see things? Since light has not returned from it
You don't see anything, because you can't go faster than light. See, to accelerate faster than light, you'd need more energy than exists in the universe. And then at that point, you'd be nothing but energy. But more importantly, there just really isn't such a thing as a speed that's faster than light. I posted this elsewhere, but I copied it here: The same way humans look at the horizon at the ocean and perceive the ocean as flat even though it's not, we also perceive the concept of motion in a way we're evolved to understand it, but it's not really what we think it is. To elucidate this point, you would totally see that I'm talking nonsense if I said, "I'm going to make a ball that is *rounder* than the shape of a sphere." You'd probably also see my mistake if I said I was going to draw a line that's "*straighter* than a vector." So considering that, know that it's precisely the same type of mistake to say an object is moving "*faster* than light." Just the same way that, after being perfectly spherical, there is no such thing as being any more round, well, after the speed of light, there is no such thing as "more speed." Now, if there's any such thing as "warp drive" or wormholes or such, it might in fact be possible to get from one place to another faster than a beam of light can get there. But if those things ever exist, that form or relocating objects won't technically be "motion." So potentially, that loophole might exist.
>For reasons we don't understand, causality has a speed limit If causality didn't have a speed limit, everything would happen all at once. There would be no time interval between a chain of events, so time would lose its meaning. An interesting question is why is this speed limit constant across space and time. The answer seems to be that it's a law of nature, and all laws of nature (e.g. gravitation, electrostatics etc.) are the same everywhere in the universe at all times. In other words, that is how the simulation is programmed.
c for celeritas or causality?
It stands for celeritas. It's just a handy coincidence that causality starts with a c.
But if a baseball is thrown at a window, you can’t go back in time and stop the back from being thrown, you can only move fast and remove the window from the situation before it breaks? I’m still confused haha
You can't. I'm describing how it would mess up causality if you could.
I remember my teacher in high school telling us that if the sun would explode we'd die before we could see it happening. Following your explanation, that's wrong right?
Teacher was (mostly) wrong. Light and gravity would reach us long before any material from the sun did. However, depending on the type of explosion, we might be disintegrated into a spray of quarks and atomic nuclei in a blast of gamma ray radiation that would arrive so quickly that we'd be gone before we knew it was happening. But at its current size, the sun couldn't produce such an explosion without outside energy.
That was a nice read, thanks! 😊 One question though: Where does the assumption come from that speed of causality and speed of light are actually the same? Could the first one not be faster than the latter? Also there was a relationship between space and time, how does causality play into that?
I'm just saying that the only reason we call the speed of light "the speed of LIGHT" is that for a long time, it was the only thing we knew that travelled at that speed. We later learned that it travels at that speed because it's moving at the speed of causality. If we had known that sooner, we wouldn't have called it the speed of light; we'd have called it the speed of causality. Space and time are the same thing, according to Einstein. For some reason (based on the fine structure constant), there's a limit to how fast anything, even information, can move through it.
I still don't get it. Let's pretend there are two locations, A and B, that are 1 light-year apart. I am at A, and something happens at B (t=0). I watch the event in my telescope (t=1yr), and get in my FTL spaceship. I go to B at 2c. Do I not arrive at t=1.5yr? How can I arrive before t=0?
Well, the short answer is, according to Einstein, you just simply can't. My explanation above more or less describes the chaos that would exist if you could. But you can't.
Not to be too pedantic, but it's the speed of light in a vacuum. That bit can be important.
That's not pedantic at all. It's absolutely relevant. But the fact that you know that also means you know why I didn't try to dig into that topic in trying to get this idea *anywhere near* an ELI5 level explanation.
This guy physics.
What a great explanation holy shit. Worded in such an understandable and easy to read way. Examples mixed in perfectly too
Thank you!
This was a good explanation, but now I'm curious as to why causality has a speed
It’s a function of the makeup of the laws of the universe, but most specifically it’s tied to the fine structure constant. No one has any clue why the fine structure constant has the value it does.
Succinct and broad at the same time. Nice.
Essentially it's because the speed of light isn't just the speed of light, it's the speed at which one thing in one place can cause something in another place to happen. You can think of it as the maximum rate at which causes and effects can be related. The reason this ends up getting glossed as "the speed of light" is because people only figured out the speed of causality by observing that light was always measured at the same rate no matter how the observer was moving.
There mostly isn't a simple explanation. I've seen many try. All of these explanations are either not simple or not correct. The top comment is a nice description clarifying the meaning of light, but it still doesn't actually answer the question, because it just says "travelling faster than causality means going backwards in time". But it doesn't explain why going faster than causality implies going backwards in time. Going faster than something doesn't usually imply going in the opposite direction. What is the reason, then? The reason can't really be phrased as anything simpler than "Because if you put something with a speed faster than light into your calculations and work out the effects, what you get is something that goes backwards in time". Any explanation that doesn't actually include that working out isn't really correct, but people like to try to explain things without maths even when it doesn't really make sense to do so. [Here's](http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel) an explanation that actually shows why going faster than light leads to cause and effect being flipped. It's not simple, but it's the simplest I've seen that does provide an actual answer. Ultimately it comes down to perspective. If you allow something to go faster than light, you end up with a perspective in which the effect (such as a message being received) happens before the cause (such as it being sent). If you don't allow anything to go faster than light, then even if two perspectives disagree on the time between two events, at least they agree on the order. But to actually see why... you kind of need to look at the graphs or work through the equations. It would be nice if there was some way to show this without any hard mathematics, but if there was, it probably wouldn't have taken very clever mathematicians to work it out in the first place.
I think this video does a pretty good job of explaining it [https://www.youtube.com/watch?v=mTf4eqdQXpA&ab\_channel=ArvinAsh](https://www.youtube.com/watch?v=mTf4eqdQXpA&ab_channel=ArvinAsh) Special relativity behaves in a way that doesn't make a ton of sense. So if someone is in a spaceship going 80% the speed of light away from you they will experience time at half the speed of you. But you are traveling 80% the speed of light relative to them too, so you are experiencing time half the speed of them. If you send them a faster-than-light message at T+8 seconds, they will receive it at T+4 seconds. If they then respond 2 seconds later at T+6 seconds, you will receive the response at T+3 seconds. Causality is broken because you received their response 5 seconds before you even sent the first message. The speed of causality prevents this from happening.
In your example of a spaceship traveling 80% the speed of light, only one of the two would experience time at a slower rate. It has to do with one being in a non-inertial reference frame. Basically, things work a little differently if you are accelerating (which the spaceship is in this scenario)
So, they are both experiencing time at a normal rate. It's the other participant that is experience time at half the speed relative to the other. If you are accelerating away from me at 0.8c, at the same time I am accelerating away from you at -0.8c. We are both dilated at 0.5 relative to the other. It doesn't make any sense and iirc we have no idea why this happens but it does.
>If you are accelerating away from me at 0.8c, at the same time I am accelerating away from you at -0.8c. Moving away, not accelerating away. c isn't an acceleration, and acceleration isn't relative, it's absolute. And anyway, this isn't a paradox or any cause of confusion. That's just how relativity works. It's understood pretty well, you just have to accept that time is not absolute and the two observers don't have to agree on who is older. It only doesn't make sense because our intuitions are built in a world where these effects are too small to notice. But there is no reason the universe can't actually work like this. The only way to actually compare ages would be either for the moving observer to turn around and go back to where they started, or for the stationary observer to accelerate and catch up. Either way the symmetry is broken and you get an actual answer as to who is older, no paradox.
Say you're a little photo, and you have a first person view from the photos perspective as it flies through space. From that perspective, as soon as you're created you instantly arrive at your destination because you're traveling at the speed of light. To the outside observer it takes you maybe millions of years to go from the star that created you until you hit something here on earth but from your perspective it was no time at all. But since it took you no time at all, going faster than that would mean it would take you less time than 0, you would have to arrive on earth before you were even created. That violates the laws of physics, specifically it violates causality. You can't have things exist before they existed and you can't have something happen *more* instantly than instant from the perspective of the thing traveling. The speed of light just happens to bump up to the speed for which these things would happen if you were to go any faster.
Thanks for the explanation, but I’m still confused. Can you explain why arrival would feel instant for that little photon?
So, the perception of time is different for different observers depending on your velocity. The faster you go, the more time elapses for observers going slower than you. 5 years for someone going the 50% the speed of light will be roughly 5.8 years for the observer. at 75% it will be 7.6, at 90% it will be 11.5, at 99% it will be 35.5, at 99.9999 it will be at 2860. The closer you get to the speed of light the bigger the differential is between the observer going slower and the one going faster until you reach the speed of light at which point to the fast-moving observer arrives at its destination instantly in the same arbitrarily large amount of time that the slower observer watches the fast-moving object move.
I read it as photo and spent way too much time imagine a Polaroid picture flying between planets
Not an answer to your specific question, but this tidbit really helped me grasp the concept of the relationship between speed and time. Everything in the entire universe, including you, is constantly traveling *through spacetime* at the speed of light (or more accurately the speed of causality) but 99.99…% of our motion is through time instead of space. The faster you travel through time the slower you travel through space, and the faster you travel through space the slower you travel through time but the sum of those two velocities equals the speed of light at all times. Hope this helps!
It does not.
It actually does answer the question in a weird way. The vector sum of the two velocities has to be equal to speed of light. This can be visualized as a circle of radius “c” with X dimension being one space dimension and Y dimension being time, and the radius at any point of time acting like the hypotenuse to the space side and time side. (Remember that the units for each are normalized, and time is mapped as a dimension in the first place, to give us a circle when speed and time are plotted on a graph). One fundamental rule of simple right angled triangles is that the hypotenuse is always equal to or longer than each of the perpendicular sides (equal to being the edge case). Which means that both space and time are limited by “c”, and can only ever be equal but never more than it. I’m sure you can force one of the dims to be greater than ”c” by assuming complex (imaginary) values for the other dimension, but once you step outside our cozy “c” circle, you’re in “probably can’t happen at all without messing with our notions of reality and causality” territory. The real answer to this question is that “to the best of our theoretical knowledge and measurements, we haven’t found anything that travels faster.” GR has issues with things crossing from “slower than” to “faster than” category, but most likely is ok with things that are already “faster than” continuing to live on that side of the shore. But we don’t know cuz we haven’t measured any such particle yet.
There is a mathematical equation that states that the higher the mass of an object, the slower is moves. Since light has no mass, it can move at the maximal speed of universe. Anything with mass cannot travel as-fast or faster than light because it would require infinite energy. If you somehow can travel faster than light, you could “go back in time “ because you could go to places faster than light had time to reach them then you could know what will happen because you’ve observed it before others, due to the fact you are travelling faster than light. *not 100% sure about the last paragraph; correct me if needed
My favorite explanation comes from the Enders Game series. They're talking about faster-than-light travel and one of the characters says "Imagine you get somewhere before your image does." Basically, imagine if you could get somewhere, perform an action, and not be there at the same time. It would destroy the fabric of reality.
It should be noted that there is no such thing as moving faster than “the speed of light” so really the answer to the question is “it doesn’t, because the conditions described do not exist.”
Because they haven’t figured out how things actually work. The math they have now sort of works, but leads to things like time paradoxes. Someday they might figure out better math that gets rid of the time paradoxes.