# Why does FTL imply that BACKWARDS time travel is possible? - Very Detailed! [closed]

I know this question has been asked before and I have read and understand the answers however imo none of them still imply that backward time travel is possible.

I understand moving forward in time easily, it’s the backwards in time I don’t get; I just don’t think it is possible, even in theory. I think where I might be getting it wrong (if I am) is my perception of what time travel is so let me clear that up first.

I think what most non-scientific people (me) believe backward time travel is, is what we see in the movies – that is you can travel backwards in time to specific timeline and interact with it, potentially changing the course of some events. They key phrase here is “interacting with it” as in somehow change it or influence the future.

I am going to reference this question here and the answers proposed, I will build on them and their examples.

Every explanation so far I’ve read (inc link above) about backwards time travel always hinges on the speed of light, light cones and that you must travel faster than light to achieve it. However, from what I take away from these explanations is that it only “appears” as though (as in “observed”) backwards time travel was achieved, to me this not true time travel (or what my definition is of it). The best way to demonstrate my point is with an example so let’s expand on the great explanation already provided by the last poster from the link above (you don't need to read all answers just the last one right at the bottom by Andrew Steane).

In the last diagram he gives an example of us seeing someone or some event BEFORE they were born, then later seeing them born (read the whole thing first to get the context). But imo this is not time travel, the same events occurred in the correct order (A then B then C), it’s just that due to said object (the person or ship) traveling faster than light we OBSERVED event B first. So essentially we viewed these events out of order…but they STILL OCCURRED in the correct order of a,b,c. So my first question here is, is my understanding here correct or not?

Assume I am correct with the above. What this means is that, at best we can only ever observe past events, we cannot interact with them, influence them or change them. To illustrate this let’s use the last diagram but change it slightly. Let’s say said person did some terrible event at point B (which we “observe” first), then we view event A after (the birth of the person). We could (if we could travel fast enough) witness event B then travel to before event A using FTL to stop the birth/kill them and therefore prevent event B (according to the posters explanation). However, this statement is not true because event A already occurred, you just haven’t seen it yet. if we were to travel to where event A occurred in the hopes of stopping it (after seeing event B), we would find it still occurred no matter how fast we travelled. So, in other words we can’t go really go back in time – or at least in the concept of my understanding and what I believe is the concept of time travel (interacting with past event or changing it). So, in the last statement where he said:

“Now we have that someone or something born at A then went off to B and came back to C where they were present before their own birth.”

It isn’t true, you just observed it that way.

So to summarise, when we talk about “backward time travel” in the scientific definition of it - is it really just that we are observing events out of sync (in layman’s terms) if we use FTL or is it really possible (theoretically) to actually go back and interact with a past event? if yes, how?

Also, I know there are other things at play here like causality and possible other theories like multi-dimensional timelines. For the sake of keeping it as simple as possible I’d like to not discuss them at this point, maybe later after I grasp the above!

• The whole point of relativity is that notions like "simultaneous", "past" and "future" are relative to the observer uttering them. You need to be much more careful about these terms when talking about things like "past events". Commented Jun 27, 2021 at 10:02
• i get that. i always wonder why the term observer" is used, is to imply that you can only observe? To oberserve is to see/record some information or event. My question is simply, can you to interact with this event after it occured? if yes, how? Commented Jun 27, 2021 at 10:10
• Time dilation and space contraction would point to imaginary length and time scales, not negative ones. So travelling backwards in time never really made sense to begin with. But if you were to outrun light, by use of teleportation, you could indeed observe the past of an object you teleported away from. But this is a very simple issue. We also observe the past of stars, because their light still takes time to arrive here at earth. Commented Jun 27, 2021 at 17:56
• There are also notions of "absolute past" and "absolute future", which are not relative to the observer. It sounds like Mucker is wondering how FTL allows an observer to interact with their own absolute past.
– Mike
Commented Jun 27, 2021 at 17:56

However, from what I take away from these explanations is that it only “appears” as though (as in “observed”) backwards time travel was achieved. … So my first question here is, is my understanding here correct or not?

You have misunderstood the whole point of relativity. The Lorentz transform, which is what Andrew Steane used to produce those diagrams, does not describe when and where things appear to happen, but when and where they actually happen. It is about what is understood to have actually occurred after all of the optical effects due to the finite speed of light are corrected.

In other words, in relativity observers are aware of the finite speed of light and, when they see something happen 100 light years away they understand that it actually happened 100 years ago, not when they saw it.

Unfortunately, the way that relativity is often taught, with “observers” and “observers frames” is almost guaranteed to give this false impression to some students. But it is a false impression. The standard effects of relativity are what remain in our description of nature after accounting for the optical illusions.

Now, there are formulas that do describe the raw optical observations. These are important because the raw optical observations differ from classical physics. These include the relativistic Doppler effect, relativistic aberration, Terrell rotation, and others. However, the Lorentz transform that Andrew Steane used in his answer is not one of those.

In the spacetime diagram the worldline actually does go back to an earlier point on its own worldline. You have simply misunderstood the claim if you believe it to be one of appearances.

• I sitll don't get it :-( I understand that's likely i don't have a physics background. I guess I kinda see the two events playing out in the "Timezone" (for want of a better description). I even understand the spacetime diagram, it just seems to contradict how it would happen imo. To rationalise my point more, when you go back in time and can effect it, say you move an object - then what we are saying is that FTL allows you to change the matter of the past too. I just can't understand how traveling FTL somehow infers you can interact with matt that no longer exists (another way of looking atit Commented Jun 28, 2021 at 2:51
• Is the part i am nor grasping is that someone event a and B are in diff timelines/timezones somehow? If this is the case then does moving from point a to b change to diff timezone? even if you account for time dilation, doesn't this only slow time? Commented Jun 28, 2021 at 2:52
• @Mucker honestly, if you want to learn about relativity this is the wrong topic to start with. You should start with understanding what relativity predicts for physically possible scenarios and the experimental evidence confirming it. Once you have mastered that, then come back to examine the impossible scenarios and you will then understand both the scenario and the reason it is considered impossible. All I can tell you at this point is that your understanding is wrong. The clear prediction of relativity is that FTL implies physical causal loops, not optical illusions
– Dale
Commented Jun 28, 2021 at 3:03
• i thought about that. Just to understand relativity how long would you estimate it would take me to learn? I don't want to do a 4 year degree just to understand this part of physics. Just a rough guess please. I don't know calculus if that helps, I heard I'd need to learn that first at least. Commented Jun 28, 2021 at 3:10
• @Mucker it took me 7 years, but very inconsistent and unguided personal effort. What finally “did it” for me was (1) to start drawing accurate spacetime diagrams, including relevant objects, light pulses, and coordinate grids and (2) reading about all of the experimental evidence. I think that 7 months is more reasonable with guided effort, but start with real scenarios and actual evidence. Leave FTL until the end. Algebra is enough for the basics
– Dale
Commented Jun 28, 2021 at 4:45

Backwards time travel due to FTL would not merely be "seen", it would be a real effect. That is, if the principle of relativity is absolutely true, and if FTL travel is possible, then a person armed with both an FTL and STL (slower than light) drive could travel back in time and meet herself. Note that the STL part is necessary -- it's because of the relativity of simultaneity which says that moving observers have different notions of time.

However, we already know that slower than light travel is possible, so if time travel is to be ruled out then either faster than light travel is impossible, or else the FTL mechanism must somehow depend on an "absolute speed" (in violation of the principle of relativity -- not just Einstein's relativity, but Galileo's and Newton's, all of whom agreed that speed is relative).

Wikipedia has a good article on the tachyonic anti-telephone, a way to use FTL communication to communicate with the past. It's based on the physically observed fact of time dilation: if two observers are moving (slower than light) relative to one another, each would assert the other's clocks are running slow. Again, this is not an illusion, and it has been observed in experiment (e.g. the atomic clocks in GPS satellites have to be adjusted for their motion relative to the Earth's center).

I'm happy to have this request for further explication of the answer (by me) you referred to. I was about to provide this but I saw the answer here by Dale already does it: the Lorentz transformation and spacetime diagram set out not an appearance but the actual events: what actually occurs.

It may help to have a concrete example.

Suppose, for example, event A is "Alice sends 1 million gold bars to Bob" and B is "Bob then forwards them on to Charlie" and C is "Charlie receives 1 million gold bars." Now we have to consider what happens next. If Alice then behaves precisely as she did before, and sends the gold bars, then we have the situation that Charlie did not change the past. We just have a loop. (It is a bit like the situation called a closed timelike curve in general relativity, only here the curve is not timelike). Someone might want to claim that such a loop is possible. According to what we know of physics, in fact it is not possible, but your question is all about the scientific grounds on which it should be ruled in or out.

In quantum field theory we do contemplate loops of this kind, involving virtual particles. One should be cautious of this comparison because virtual particles are not things in their own right, they are a mathematical aid, a way to clarify and correctly account for the parts of a mathematical calculation involving complicated integrals. A spacelike loop involving virtual particles can contribute to a calculation of further events to which the virtual particles in the loop are connected. But if it is just a loop with no connection to anything else then it does not contribute to any calculation and can be ignored.

Now let's return to the consideration of things like people and gold bars. The logical paradox is sharpest when we consider that Charlie can opt to do something other than just hand the gold bars to Alice. Maybe in fact Alice sent off her grandchild to guard the gold bars, and the grandchild returns with them on the occasion of Alice's coming of age, where in the grip of a terrible rage, he kills her. Or does he? What can his experience of his ongoing life be? Is it all pre-ordained? Is it a loop after all, and all he can do is hand over the gold bars? What causes him to behave that way? Can a coherent account of cause and effect be constructed along these lines?

That last point has been discussed at length by scientists and philosophers. I do not need to answer it here. All I need to do is point out that the FTL scenario leads to precisely the same paradoxical issues as a travel backwards in time scenario. They are not essentially different in character.

Postscript

If you think the loop is ok, then how about if evil Alice makes and sends off a ticking bomb to kill poor Bob, but he immediately sends it back and it arrives at Alice before she made it in the first place. Could it now explode and kill Alice before she made it? If not then why not?

• thanks again buddy for a great explantion. I don't want to get into philosophical side of it yet, that's how other topic in itself lol. Just trying to keep it focuses on just on this example for now and the physical aspect of actually going into the past and interacting with matter if that's ok. See my comment to Dale above if you woulnd't mind providing some more info. I feel I am so close to understanding it!! thanks, Commented Jun 28, 2021 at 2:55

I think the other answers here miss a key point to this whole thing.

The core point of relativity is that there is no "absolute reference frame" at all. Each and every inertial observer has equivlaent stake to say that they have the correct physics.

Why does this matter here? Because if you have "event A causes event B, which causes event C", then you necessarily have, A coming before B, coming before C, or else the relationship doesn't make sense.

But, the thing is for any two events along a FTL line, there is some observer, who again, is equivalent to all other observers, where the two events are simultaneous, but separated in space. For this observer, there can be no talk of one causing the other, the concept doesn't make sense.

So, to make the whole thing work, you either have to come up with a way of disallowing this observer (and the whole class of observers that see the events all time-reversed), you need to come up with a way to say that the two events aren't really causally related, or you need to disallow faster than light travel. Nearly all physicists have decided that the last option does the least violence to the underlying physics.

FTL travel does indeed imply that an object would be able to return to a point in spacetime with a lower t coordinate. However, you need to make a very large assumption if you are to believe that FTL could somehow allow interactions with earlier events. Namely, you must assume that if one did travel back in time, one would find the universe still there as it then was.

To make the point less abstract, suppose you found a way to travel back to the region spacetime where New York was on Christmas Day 1993. According to the popular movie interpretation, you would be in New York as it then was. However, if all of the matter that comprised New York in 1993 has since moved forward through spacetime to the present, which is what our senses tell us, then the region of spacetime that corresponded to New York in 1993 is a strictly empty one.

• Why the downvotes??? Commented Jun 27, 2021 at 13:39
• I didnt but I should, the last paragraph is completely wrong
– user65081
Commented Jun 27, 2021 at 14:17
• @Wolphramjonny why? Commented Jun 27, 2021 at 14:37
• an object does not dissapear from the past as it evolves into the future. Think of the worldline of an object, which is a 4D surface in spacetime. You can take time slices of this object and wil find the starndard 3D object, at different time on each slice, Same with a movie, it does not get erased as you play it.
– user65081
Commented Jun 27, 2021 at 14:51
• This answer seems to imply that a particle disappears from the past as it travels along its worldline into the future. That's an interpretation that I don't think I've encountered before. I have always understood spacetime diagrams as a way of labeling events on a sort of tapestry which exists in both the future and the past. To say "the matter that comprised [the past] has since moved forward through spacetime to the present" seems to me to privilege "the present" in a way that's inconsistent with the axioms of special relativity.
– rob
Commented Jun 27, 2021 at 14:52

Suppose you have some event as a clock showing ten o'clock, eleven o'clock, noon, and that these images moves off at the speed of light.

Suppose at 1 pm you were to set off at a speed greater than the speed of light, then you would be able to catch up these images.

After some time later (depending on your speed) you would first view the image of the clock showing noon, then somewhat later you would catch up with the clock showing eleven o'clock and sometime later after that you would view the image of the clock showing ten o'clock.

In this fashion, you would be able to view the time on the clock as running backwards. This is obviously absurd and suggests the impossibility of FTL. It also links the importance of the speed of light with a maximum speed limit.

• I doubt that helps, because it's once again reinforcing the OP's misunderstanding that relativistic effects are "illusions" or somehow based solely on what one sees, rather than what one calculates after the effects of the finite speed of light are taken into account. The relativity of time is real, and persists even after adjusting for the speed of light. Commented Jun 27, 2021 at 11:36
• It's meant to suggest the impossibility of going faster than light,
– jim
Commented Jun 27, 2021 at 13:37
• @jim: and yet it does no such thing. Commented Jun 27, 2021 at 15:22