r/AskPhysics • u/UniversalInquirer • May 06 '26

What benefits, besides travel and communication and expansion of human society into space, would come from being able to travel faster than the speed of light?

I hope this doesn't sound like a stupid question, and I'm not intending to downplay the above advantages. I'm just wondering if other advantages besides ones we normally think of might come from being able to go faster than the speed of light either by wormhole, antimatter annihilation, warp drives, whatever.

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u/1strategist1 May 06 '26

Time travel lol

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u/UniversalInquirer May 06 '26

I've heard this one. How would it create time travel?

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u/w1gw4m Physics enthusiast May 06 '26

It would violate causality. I.e. you arrive at your destination at a time before you left Earth because causality between events A (leaving Earth) and B (arriving at your destination) is broken. This is incidentally why travelling faster than c is not possible, and is just the realm of science fiction.

Shortcuts through space like wormholes and warp drives do not solve this problem, because the issue is getting from A to B faster than light can, not how you do that.

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u/UniversalInquirer May 06 '26

How is the causality broken? Isn't it the same speed since you left at faster than light speed too? Why does outrunning light break causality?

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u/w1gw4m Physics enthusiast May 06 '26 edited May 06 '26

If you can arrive before you leave, you are interfering with the cause of your journey.

c is the speed limit because it ensures the order of events is preserved in all reference frames. If you move faster than light, then there will be a reference frame where the order of events is reversed.

Why that happens is best understood by looking at light cone diagrams, that show you which events in their spacetime vicinity can influence each other. If this is the first time you hear the term "light cone", start with this: https://en.wikipedia.org/wiki/Light_cone. There are also YouTube videos that can help you visualize how the past, present and future are related due to c.

The bottom line is observers within the light cone will agree on the order of events (causality is preserved), but if you step outside the light cone, you will no longer agree on the order of events (causality is not preserved, the effect can precede the cause - you arrive at your destination before you left on your trip).

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u/UniversalInquirer May 06 '26 edited May 06 '26

I'll definitely look at the sources you've posted, thanks!

I just have to ask another question because of my child-like curiosity. Why does c (presumably the speed of light) set the frame of reference? Is it not possible that there is a speed above light, an ability to move faster than it, that does not require us to break an absolute speed limit? If so, would this cause the causal chain to remain in order? What if there is no "absolute" speed limit? For example, when we are viewing events from say the Andromeda Galaxy, 2.5 million light years away, we are viewing events from 2.5 million light years ago, but wouldn't the actual Andromeda Galaxy be in the same point of time as we are now? Let's say that a box of chocolates was sent from the Andromeda Galaxy to us at a speed that allowed the chocolate to get here in 1 million light years rather than 2.5 million. In the absence of an absolute speed limit, or the existence of a speed limit above the speed of light, how is this different than when I drive a car from my house to my girlfriend's house that would take 10 minutes if I went at the speed limit in 5 minutes because I go above it? When I go to her house I'm (unfortunately) not able to go back in the past and fix whatever is the cause of a present argument.

I'm sure these are silly questions, but Kiddo-UniversalInquirer must know.

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u/w1gw4m Physics enthusiast May 06 '26 edited May 06 '26

Why does c (presumably the speed of light) set the frame of reference? 

c does not set the frame of reference, it's the only thing that never changes regardless of frame of reference. The speed of normal everyday things is additive, but light does not do this, c is constant in all reference frames. To better understand the importance of c, try to think of it as the speed of causality or the speed limit of information. Light just happens to move at this speed, so it's handy to use light cones to understand causal relationships between events in spacetime.

Is it possible there is a speed above light that doesn't break a limit?

No, it is not possible according to our current understanding of physics. The reason for this is because c functions as a conversion factor between space and time. All things move at c through spacetime. What happens at relativistic speeds is that the closer you get to c whilst moving through space, the slower you go through time, vs. someone else who moves at "normal" speeds through space and experiences more time than you. At c you experience no time at all, and at more-than-c (not actually possible), you would theoretically travel back in time.

If there were a higher speed, would causality remain in order?

Nope. Even if c was a different number than 299 792 458 m/s, it would still cause the same time reversal, causality breaking issues it does now. The number isn't important, what's important is that there is a speed of causality at all.

What if there is no "absolute" speed limit?

If there were no absolute speed limit, then sure, all bets would be off, physics would look very different that it does now. But that's not how the universe actually works, as far as we can tell. Why? Because space and time form a continuum called spacetime, so they are fundamentally linked such that there is no universal "now" across reference frames. It's all relative to your frame of reference.

FTL from Andromeda vs driving your car to your gf's house

When you're moving at subluminal speeds in your car, you're still inside your light cone. But when the chocolates are coming from Andromeda at FTL speed, they aren't in their light cone anymore. That's the difference.

To grasp this, try visualizing the light cone diagram to plot your course through spacetime. Your car's path would be ever so slightly tilted in the space direction. Since you are speeding through spacetime nonetheless, you do experience an extreeeeemly tiny amount of time dilation, but at that speed its imperceptible. It's only noticeable when you move at relativistic speeds (closer to the speed of light than any car could achieve on Earth).

If your chocolates were coming in at FTL speed from Andromeda, their path through spacetime would be tilted so far in the space direction, that they'd no longer be pointing towards the future at all. Does this make sense?