[chris (OP)]

Jack wants to know:

If you went faster than light, which i know is theoretically impossible - but if you could, would you see that light from before and thus see the past?

What do you think?

[jeffreyH]

We are always observing the past since the speed of light is finite. You cannot reverse causality so going faster than light would not take you into the past. You would see all the photons from the past in rapid succession so that remote objects would approach the present of the traveller heading toward them.

[PmbPhy]

Jack wants to know:

If you went faster than light, which i know is theoretically impossible - but if you could, would you see that light from before and thus see the past?

What do you think?

Yes.

Jeff - I believe the OP is asking about the seeing the past of the place that they're located, e.g. Earth. While we can see the past of Proxima Centari we can't see the past of Boston.

[dutch]

If you went faster than light, which i know is theoretically impossible - but if you could, would you see that light from before and thus see the past?

What do you think?

Going faster than light is not possible according to our current theories. No one knows what would happen if we could go faster than light. However, attempting to preserve the Principles of Relativity with v > c would require backwards time travel.

According to Special Relativity all inertial reference frames are treated equally. The Lorentz Time Transformation below is used:

t' = γ (t - v x / c²)

This transformation can be used to transform from one reference frame to another then back again. The important term to consider here is the - v x / c². Not only do clocks in another reference frame moving at v relative to us depend on our time t but they also depend on position x. I see their plane of simultaneity defined with t = 0 as having a slope -v / c² in the x t' plane.

What does this mean? Their plane of simultaneity (what they see as simultaneous) is sloped into our past if they are moving away from us (negative v) and into our future if moving towards us.

If reference frames are truly identical and we send a message instantaneously to another observer at rest relative to us at distance x and they send a signal straight back no backwards time travel occurs. The message is sent round trip instantly. However, if the other observer is moving away their plane of simultaneity is sloped into our past. According to Relativity they would receive our signal along our plane of simultaneity in our "present" and send their instantaneous signal back along their plane of simultaneity and into our past. If reference frames can be treated exactly equal then ANY faster than light transfer of information breaks causality for at least some subset of reference frames.

You can have any of the following two: Relativity, Causality, or Faster than Light travel 

Without some form of "background" to the universe the only known thing that preserves the ordering of events is the constant speed of light and nothing exceeding this speed. The only way I know of around this is to have all faster than light travel transferring off of one reference frame (an idea explored by physicist John Bell while struggling with an explanation of how entanglement might work). This would violate Relativity faster than the speed of light but this is fine because we haven't tested Relativity in that domain.

The safest answer is faster than the speed of light is impossible. Many thousands of experiments back this answer. If it is possible strictly using the principles of Relativity would demand backwards time travel otherwise all reference frames could not be treated equally. However, if FTL is possible relativity may be violated faster than c. This wouldn't mean relativity is wrong because it works perfectly in all domains it's been tested. This is why physicists continually try to push the boundaries to test theories. Always be skeptical of theories (only believe them in areas they've been well-tested) and always be even more skeptical of new claims saying they've proven theories wrong.

[PmbPhy]

Going faster than light is not possible according to our current theories.

That is incorrect. While its not possible to travel at a speed equal to or greater than the speed of light by accelerating a body of finite rest mass, it may be possible to travel from one point in space to another at a rate which would exceed that of light. One example is by using a wormhole (if one can be created that is). Another would be to use the Alcubierre drive aka Alcubierre warp drive . For details please see: https://en.wikipedia.org/wiki/Alcubierre_drive

As for time travel: At MIT they teach a method of time travel which uses two cosmic strings passing by each other. Its also possible by using a wormhole where the mouths are in relative motion.

So while its not yet know whether FTL is possible or not its still far too  premature to say that its not possible.

[dutch]

That is incorrect. While its not possible to travel at a speed equal to or greater than the speed of light by accelerating a body of finite rest mass, it may be possible to travel from one point in space to another at a rate which would exceed that of light. One example is by using a wormhole (if one can be created that is). Another would be to use the Alcubierre drive aka Alcubierre warp drive . For details please see: https://en.wikipedia.org/wiki/Alcubierre_drive

As for time travel: At MIT they teach a method of time travel which uses two cosmic strings passing by each other. Its also possible by using a wormhole where the mouths are in relative motion.

So while its not yet know whether FTL is possible or not its still far too  premature to say that its not possible.

Wormholes and warp drives require exotic forms of matter and have multiple issues. Even if they are possible these "FTL" methods would still break causality (unless relativity breaks). These solutions to GR are also technically not "faster than light" as nothing goes faster than light locally in wormholes, cosmic strings, or warp drives. Any "faster than light" loop according to Relativity breaks causality (for some subset of reference frames) even if light speed isn't broken locally and the method is a solution of GR.

Breaking light-speed is theoretically impossible. Bending "spacetime" to effectively move faster than light may be possible. 

[jeffreyH]

Good grief. A proper scientific debate on TNS. Am I in the right forum?

[puppypower]

Jack wants to know:

If you went faster than light, which i know is theoretically impossible - but if you could, would you see that light from before and thus see the past?

What do you think?

If you use the analogy of a jet traveling faster than the speed of sound; say Mach 1.1, the jet gets ahead of its own sound waves, therefore, the present of the jet will appear to occur in the future, in terms of the (sound) signal we observe. If we continue the analogy, the light speed signal, from someone going faster than the speed of light, would appear like a boom or a blast of energy when it reaches us in the future.   

[PmbPhy]

Wormholes and warp drives require exotic forms of matter and have multiple issues.

And as of yet there are no reasons so assume that exotic matter cannot exist.

Even if they are possible these "FTL" methods would still break causality (unless relativity breaks).

And yet there are solutions to Einstein's equations for which it can happen.

These solutions to GR are also technically not "faster than light" as nothing goes faster than light locally in wormholes, cosmic strings, or warp drives.

Yo! dutch! Nobody suggested that they did.

The OPs question is about an observer moving from Earth to a point in space r in as short enough time such that the observer arrives at r in time to observe that light signal. This requires a displacement which happens faster than light could reach r, i.e. FTL. That you took that to mean that an object or particle was moving at a speed v > c is beside the point. Then there's the question of the possible existence of tachyons.

Breaking light-speed is theoretically impossible.

That too is incorrect. Tachyons are theoretical particles which travel FTL and play an important part in physics.  Its been held that the most famous example of a tachyon is the Higgs boson. In its uncondensed phase, the square of the mass of the Higgs field is negative, and therefore, the associated particle is a tachyon.

See also: https://phys.org/news/2014-12-faster-than-light-particles.html

Folks - dutch is basing his assumptions of two things:

1) a particle with real and finite proper mass cannot be accelerated from a speed less than the speed of light to a speed equal to or greater than the speed of light. That would require infinite energy, which is a meaningless concept.

2) if FTL exists then there's problems with causality

It was long ago hypothesized by George Sudarshan that if particles were created which are already moving FTL  then no infinite energy would be necessary.

Problems with causality may not be a problem since the existence of tachyons does not imply the ability to violate causality. Using wormholes etc to move FTL (and I don't mean have a speed v > c) is problematic and not yet resolved.

Speaking as a moderator: The purpose of this forum is to inform visitors of all things that appear in the physics literature and community which includes those which may indeed be proven possible in the future. Please understand and respect that. This doesn't mean not to point out problems with current ideas. Just don't act like certain things have already been proven impossible.

[puppypower]

Many people work under the assumption that space-time has to always remain integrated. But as an intellectual exercise, consider separating the fabric of space-time into separated threads of space  and threads of time. In this case, each can act independently. This would allow affects what appears to be being able to travel faster that the speed of light.

If you could move in space; space thread, without restrictions of time, you can be anywhere is zero time. A wormhole could be explain with distance potential, dissociated from time. Such movement is not based on speed, since speed has the units of distance/time, whereas this does not use time. If you assume only space-time, then you would calculate movement faster than light. But if you assume this is pure distance potential, there is no violation of light speed, since this is not formally a velocity.

Proof that time and distance can separate can be seen with force and acceleration. Acceleration has the units of d/t/t or one part distance and two parts time. Which amounts to extra time potential embroidered into space-time.

[PmbPhy]

Many people work under the assumption that space-time has to always remain integrated.

I don't even know what that means if it were true.

Spacetime is merely a mathematical object called a manifold. The elements of this manifold are called events where an event is a place and a time, i.e. (r, t). There has never been any requirement or assumption that space-time has to always remain integrated. Spacetime is often used in relativity merely as a way to describe what happens in nature. The mathematical object we call spacetime has curvature described by Einstein's field equations. There is a correspondence between the mathematical object and what we observe in nature. That's all.

It must be kept in mind that space and time are physically very different things.

[dutch]

Breaking light-speed is theoretically impossible.

That too is incorrect. Tachyons are theoretical particles which travel FTL and play an important part in physics.  Its been held that the most famous example of a tachyon is the Higgs boson. In its uncondensed phase, the square of the mass of the Higgs field is negative, and therefore, the associated particle is a tachyon.

What? BREAKING LIGHT SPEED! Tachyons (if they exist in non-virtual form) don't break light-speed. They're already moving faster than light. You're misinterpreting and misreading what I said. Breaking light-speed means to transition from sub-light speed to faster than light-speed. This is similar to how breaking the sound barrier means to go from sub-sonic to supersonic. What other possible definition of the word breaking exists? Also were're talking about Jack not tachyons.

2) if FTL exists then there's problems with causality

How do tachyons go in faster than light loops? I CLEARLY stated and described faster than light round trips. I even discussed instantaneous faster than light round trips. If tachyons actually do go in FTL round trips they would break causality IF relativity doesn't break.

These solutions to GR are also technically not "faster than light" as nothing goes faster than light locally in wormholes, cosmic strings, or warp drives.
Yo! dutch! Nobody suggested that they did.

That's what I meant. Maybe you misinterpreted what I was saying and maybe I didn't write that part well enough.

Problems with causality may not be a problem since the existence of tachyons does not imply the ability to violate causality. Using wormholes etc to move FTL (and I don't mean have a speed v > c) is problematic and not yet resolved.

I wrote:

If reference frames are truly identical and we send a message instantaneously to another observer at rest relative to us at distance x and they send a signal straight back no backwards time travel occurs. The message is sent round trip instantly. However, if the other observer is moving away their plane of simultaneity is sloped into our past. According to Relativity they would receive our signal along our plane of simultaneity in our "present" and send their instantaneous signal back along their plane of simultaneity and into our past. If reference frames can be treated exactly equal then ANY faster than light transfer of information breaks causality for at least some subset of reference frames.

I describe round trips in the above from my original comment. Could you at least acknowledge this? I could have put round trip after "faster than light" again in the final sentence (however, read my last paragraph in this comment).

Jack wants to know:

If you went faster than light, which i know is theoretically impossible - but if you could, would you see that light from before and thus see the past?

What do you think?

That you took that to mean that an object or particle was moving at a speed v > c is beside the point. Then there's the question of the possible existence of tachyons.

The original poster was talking about himself or someone else moving faster than light not tachyons. Jack going faster than light from point a to b by any means also means he should have the ability to go from point b back to a FTL. To stipulate this is FTL in an area of roughly flat spacetime like a trip from Earth to Alpha Centauri. Just by Jack going from point a to b faster than light implies that FTL round trips would be possible because a trip from b to a would also be possible breaking causality (or relativity).

Even if they are possible these "FTL" methods would still break causality (unless relativity breaks).

And yet there are solutions to Einstein's equations for which it can happen.

What can happen? Please specify. What faster than light loop doesn't break causality (again if relativity doesn't break)? If I (I am not a tachyon and neither is Jack) can go from point a to b FTL then this implies I (or something else I hand the "baton" off to) should be able to go back FTL. This would violate causality (if relativity isn't violated).

[PmbPhy]

dutch - It's become cumbersome to keep correcting all the flawed assumptions that you keep making and I don't have the desire to keep correcting you. I'll merely explain to you your misconception that Einstein's field Equations don't allow for time travel this one last time.

There are indeed  solutions to Einstein's field equations which allow for time travel. I stated an example above and you chose to ignore it. It was first proposed by Richard Gott. It uses two cosmic strings in relative motion. For details start here:
http://www.dummies.com/education/science/physics/crossing-cosmic-strings-to-allow-time-travel/

In 1991, J. Richard Gott (who, with William Hiscock, solved Einstein’s field equations for cosmic strings in 1985) realized that two cosmic strings could actually allow time travel.

And there are ways around paradoxes of time travel. In fact they've been proposed by Gott as well. One such way has been used by Sci-Fi writers for a long time - Alternate time lines. This invokes the many worlds theory of quantum mechanics. Then there's the notion that all time travel remain self-consistent, i.e., one can visit the past but not change it, as in the Novikov self-consistency principle. The point is that there's nothing known to day which prevents time travel. However that doesn't mean that it won't be proven impossible in the future. Do us and yourself a favor and read more about it before making further erroneous assertions. E.g. start here:
https://en.wikipedia.org/wiki/J._Richard_Gott

And as I explained, OPs don't always know how to frame the question to get the answer that they're looking more. In this case the OP wanted to know about seeing back in time by moving to places which are so far away that one might have to travel FTL to get there. What he didn't appear to know is that there are ways around moving at speeds for which v > c. But that's why he came here. To learn and we have to keep in mind that the OP might not know how to ask a question to get to what he wants to know. You did him a disservice by taking him to mean, for example,  moving in a ship which is speeding at v > c. Don't make such assumptions in the future because you're doing the OP a disservice.

Please be careful. While I did place you in my ignore list it doesn't mean that I won't keep an ear open for your assertions. If you insist on posting deceiving comments I will talk to my fellow moderators and perhaps give you a warning. There's a person on the internet named david waite who posts in a manner like yours and he's a bad influence on people trying to learn physics. Try not to be like him.

[dutch]

I'll merely explain to you your misconception that Einstein's field Equations don't allow for time travel this one last time.

There are indeed solutions to Einstein's field equations which allow for time travel. I stated an example above and you chose to ignore it. It was first proposed by Richard Gott. It uses two cosmic strings in relative motion. For details start here:
http://www.dummies.com/education/science/physics/crossing-cosmic-strings-to-allow-time-travel/

Never disputed this. Where did I ignore what you wrote? What are you writing about? I did leave GR solutions out of my original comment and I acknowledged it.

And there are ways around paradoxes of time travel. In fact they've been proposed by Gott as well. One such way has been used by Sci-Fi writers for a long time - Alternate time lines. This invokes the many worlds theory of quantum mechanics. Then there's the notion that all time travel remain self-consistent, i.e., one can visit the past but not change it, as in the Novikov self-consistency principle. The point is that there's nothing known to day which prevents time travel. However that doesn't mean that it won't be proven impossible in the future. Do us and yourself a favor and read more about it before making further erroneous assertions. E.g. start here:
https://en.wikipedia.org/wiki/J._Richard_Gott

I never said there weren't. Where did I say there weren't? What are you possibly writing about? This paragraph doesn't relate to anything I wrote whatsoever. I said it would break causality and causality may be broken in nature. Or perhaps relativity breaks FTL instead. We haven't experimented in this domain even with countless attempts to get something moving faster than light (something I also said; this doesn't mean it's not impossible. I said it's probably a safe answer that it's not).

Cause → Effect. If this changes anywhere to Effect → Cause (retro-causality which occurs when time traveling to the past) then general causality breaks. A five-year-old child will understand that a movie rewinding does not look right. A broken coffee cup typically doesn't un-break. Causality reversing is a pretty clear occurrence in nearly all systems. I said the following:

You can have any of the following two: Relativity, Causality, or Faster than Light travel   (travel as in people traveling implying the possibility of round trips... writing about hypothetical a person called Jack or objects not tachyons that can't drop to sub-light speeds).

Hume expanded this to a list of eight ways of judging whether two things might be cause and effect. The first three:

1. "The cause and effect must be contiguous in space and time."
2. "The cause must be prior to the effect."
3. "There must be a constant union betwixt the cause and effect. 'Tis chiefly this quality, that constitutes the relation."

https://en.wikipedia.org/wiki/Causality#cite_note-44

Hypothetical superluminal particles called tachyons would have a spacelike trajectory, and thus move backward-in-time according to observers in some reference frames. Despite frequent depiction in science fiction as a method to send messages back in time, theories predicting tachyons do not permit them to interact with normal tardyonic matter in a way that would violate standard causality. Specifically, the Feinberg reinterpretation principle renders impossible construction of a tachyon detector capable of receiving information.[30]

And as I explained, OPs don't always know how to frame the question to get the answer that they're looking more. In this case the OP wanted to know about seeing back in time by moving to places which are so far away that one might have to travel FTL to get there. What he didn't appear to know is that there are ways around moving at speeds for which v > c. But that's why he came here. To learn and we have to keep in mind that the OP might not know how to ask a question to get to what he wants to know. You did him a disservice by taking him to mean, for example,  moving in a ship which is speeding at v > c. Don't make such assumptions in the future because you're doing the OP a disservice.

I think this answers it pretty well from my original comment: You can have any of the following two: Relativity, Causality, or Faster than Light travel This statement is commonly used by physicists (do a Google search of phrases similar to this).

Retro-causality occurs when you travel to the past. According to Relativity you wouldn't just see the past with FTL travel you could go there if performing FTL loops. Look into this series:

Problems with causality may not be a problem since the existence of tachyons does not imply the ability to violate causality. Using wormholes etc to move FTL (and I don't mean have a speed v > c) is problematic and not yet resolved.

Where are wormholes not problematic? If relativity is correct then round trip FTL with wormholes violates causality pure and simple. Not yet resolved implies that Relativity doesn't have clear predictions using it's principles but in this case it does.

If you insist on posting deceiving comments I will talk to my fellow moderators and perhaps give you a warning.

What is deceiving? Now you're not just saying I'm wrong but you're attacking my character. I stand my ground when moderators make errors if you and the forum can't accept that then the forum doesn't have integrity. You should keep moderators to the same level of scrutiny.

Also you didn't point out what was wrong in this moderator's comment below in this thread. You CAN reverse causality by taking someone like Jack (Jack is not a tachyon) faster than light IF Relativity is correct.

You cannot reverse causality so going faster than light would not take you into the past. You would see all the photons from the past in rapid succession so that remote objects would approach the present of the traveller heading toward them.

By your very standards your post below is also wrong. According to Relativity going in faster than light loops could have you see the past of Boston and actually be in the past of Boston. You could also go faster than light out to 50 light-years, stop, and look at the past of Boston (see the photons). It's quite bad when moderators are making so many mistakes in succession and can't take criticism.

Jeff - I believe the OP is asking about the seeing the past of the place that they're located, e.g. Earth. While we can see the past of Proxima Centari we can't see the past of Boston.

Please report me. I'd like to see if this forum is worth anyone's time. I think it's important to keep moderators honest and fair. If they aren't honest and fair then I don't want to deal with the forum. I'm not trying to offend but I will defend the what's right.

[jeffreyH]

Once a photon has left the vicinity of an event the event itself is in the past. To break causality is to undo the event. If the mathematics only work with time travel into the past then the mathematics are wrong.

[PmbPhy]

Once a photon has left the vicinity of an event the event itself is in the past. To break causality is to undo the event. If the mathematics only work with time travel into the past then the mathematics are wrong.

What do you mean when you say that the event is in the past? Do you mean that the time after the photon is emitted is greater than the time of the event in that frame? I assume so.

To break causality is to undo the event. If the mathematics only work with time travel into the past then the mathematics are wrong.

Not necessarily. It could mean that the theory is incomplete. E.g. we may need to take account the many worlds theory of QM.

[jeffreyH]

To the first part. Yes. To the second part maybe.

[dutch]

The parts quoted below are Relativity. Sure you can consider some parallel universe theory where new branches form. This may not have certain time paradoxes such as the Grandfather Paradox. OK. Fine I never said any different.  However, this does not change the fact that cause and effect can been seen as backwards by at least some subset of reference frames for faster than light travel. This breaks Einstein's causality conditions (a general causality condition). Even with MWI and parallel universes branching off cause and effect can still be seen as backwards FTL by at least some subset of reference frames. MWI does seem to offer ways out of many time paradoxes but this is a separate issue from breaking the causality condition outlined below. A subset of reference frames can still see "effects causing causes while others see causes causing effects." The ordering of events past to future is lost.

Consequently, the relativistic principle of causality says that the cause must precede its effect according to all inertial observers.
In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause

https://en.wikipedia.org/wiki/Causality_(physics)

In diagram 2 the interval AB is 'time-like'; i.e., there is a frame of reference in which events A and B occur at the same location in space, separated only by occurring at different times. If A precedes B in that frame, then A precedes B in all frames. It is hypothetically possible for matter (or information) to travel from A to B, so there can be a causal relationship (with A the cause and B the effect).

The interval AC in the diagram is 'space-like'; i.e., there is a frame of reference in which events A and C occur simultaneously, separated only in space. There are also frames in which A precedes C (as shown) and frames in which C precedes A. If it were possible for a cause-and-effect relationship to exist between events A and C, then paradoxes of causality would result (I’m adding here – if the Principles of Relativity hold greater than c) .  For example, if A was the cause, and C the effect, then there would be frames of reference in which the effect preceded the cause.

https://en.wikipedia.org/wiki/Special_relativity#cite_note-36

Causality is one of the most fundamental and essential notions of physics.[44] Causal efficacy cannot propagate faster than light. Otherwise, reference coordinate systems could be constructed (using the Lorentz transform of special relativity) in which an observer would see an effect precede its cause (i.e. the postulate of causality would be violated) . 

https://en.wikipedia.org/wiki/Causality#Physics

Certain reference frames would see cause and effect backwards and if you travel to the past you’d also overwhelmingly likely see the entropy of the universe decreased. "Causal relations" could still be maintained (even if running backwards for many reference frames). A "Causal relationship" (either q ≤ p and/or p ≤ q is true) does not care about maintaining the ordering (past to future) cause and effect it only demands a link between events p and q. It’s different than general causality ( If both p ≤ q and q ≤ p then p = q) where cause precedes effect for ALL frames of reference. If traveling faster than light cause would not precede effect for all reference frames. Perhaps there are no “causes and effects” in nature (no consistent ordering of cause and effect from past to future) but rather only links between events but then general causality would be broken.

https://en.wikipedia.org/wiki/Causality_conditions#Distinguishing

If someone still has issues with this then use the following (but it's a clumsier saying):

You can have any of the following two: Relativity, no time travel to the past, or faster than Light travel. 

(Again tachyons are not a Jack or a "You")

Once a photon has left the vicinity of an event the event itself is in the past. To break causality is to undo the event. If the mathematics only work with time travel into the past then the mathematics are wrong.

Don't shoot the messenger. I'm just conveying the conclusions of the Principles of Relativity should FTL occur.

To break causality is to undo the event.

No, according to Stephen Hawking (and others) on the clearest definition I could find (causal condition) breaking causality happens when the following is violated:

If both p ≤ q and q ≤ p then p = q   

where ≤ denotes a causal relation (cause ≤ effect) and p and q are events.

This does not mean events are undone as both p and q can still happen. I've seen this definition before and it makes sense. I have severe issues with PmbPhy definition of causation (whatever that is) and nothing I find corroborates it. Sure, he offers some ways to solve some time paradoxes but this doesn't solve the ordering issue. Observers can disagree on the order of events and maintain causality but they cannot disagree on the ordering of cause and effect.

If the mathematics only work with time travel into the past then the mathematics are wrong.

I would personally like to think this is right but your statement isn't true using current theory. Also time travel to the past is only predicted FTL (in anything mentioned in this thread).

[jeffreyH]

The OP was asking about the consequences of something impossible happening. Sometimes I just like to follow that course of thinking. It can be profitable sometimes.

[@@]

Surely, the fastest anything could get anywhere would be instantly.