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On and off for a few years now I have been trying to puzzle together some basic understanding of relativity and there is one part that keeps tripping me up; FTL travel or communication is time-travel. Specifically, it seems like this is based on causality itself propagating at the speed of light, and I want to know if this is correct and why. To go with an example...

Jon decides to travel from earth to another planet 20 light years away using his magic FTL drive that gets him there in 1 year. He arrives and 19 years later is able to observe himself leaving earth. However, this doesn't violate causality because he is only witnessing the light arriving from an event which still happened in the past. On another trip, Jon arrives at the planet and realizes he forgot his telescope and must return to earth to get it. He arrives back on earth 2 years after he left, still not violating causality.

Now to my understanding the above is not how things (theoretically) would work, and I don't particularly understand why. I know that because of time dilation an individual moving at a higher relative velocity would experience less time going by, is it simply that if relative velocity exceeded the speed of light this effect would go into the negative and cause the individual to move backwards?

I think you are trying to confuse science fiction and science principles together which is always going to put some strain on the principles.

It's unlikely that there will ever be a technology that means you *actually* travel faster than light (given our current knowledge). Not only does time dilate but also as you approach the speed of light more and more energy gets put into your mass (because mass and photons are just different 'forms' of energy). There is some thinking that because space time might be 'bent' then you could potentially 'jump' between two sections of space-time and from a perspective you travel faster than light even though you only ever travelled at sub-light speeds (one example is the 'wormhole' and is best explained by folding a piece of paper over - travelling normally is like traveling over the surface; the wormhole is like 'punching' through the two surfaces). But this is all very hypothetical.

However I think you conundrum is that you are confusing the transfer of information with time. If we suppose 'jump FTL' is possible then just because you get there first doesn't mean that information is not still travelling. Take for example travelling between Paris and Beijing via either a train or plane. Suppose Jill goes on the plane and Jack the train. Jack carries a letter with some information on it. Jill would arrive much earlier, in fact she could go back and give George another letter and fly back to Beijing and still be ahead of both of them. That doesn't mean the transfer of information never happened, it's just that it is taking longer for that information to reach the destination, it will get there eventually - just because Jill is there waiting for them doesn't mean there has been any time travel.

As an aside there is nothing in physics that prevents time going backwards or 'time travel'. In some form it is an 'artificial' concept. There reason things move forward is that the universe is increasing in entropy (i.e. it naturally moves from an ordered state to a less ordered one, because for every ordered state there is a multitude of unordered ones a pile of bricks can be made into a house in one way, but there thousands of ways of making a pile of bricks). The reason we don't go 'backwards in time' is because that would result in the opposite and that breaks physical laws of the universe. Time is our natural concept of this process.

This might be helpful to see why FTL leads to time-travel and unsolvable paradoxes.

Have no one seen Event Horizon??

No using black hole tech to make a 'wormhole'.

Direct FTL is likely impossible, most merhods of FTL given remove the taveller from spacetime by one means other another. Either in a warp bubble, via hyperspace or via teleportation/jump drive.
All three bypass causality as linear time progression is not effected.


Automatically Appended Next Post:
To the OP.

 NinthMusketeer wrote:


Jon decides to travel from earth to another planet 20 light years away using his magic FTL drive that gets him there in 1 year. He arrives and 19 years later is able to observe himself leaving earth. However, this doesn't violate causality because he is only witnessing the light arriving from an event which still happened in the past. On another trip, Jon arrives at the planet and realizes he forgot his telescope and must return to earth to get it. He arrives back on earth 2 years after he left, still not violating causality.

This is how I see it also. The problems of 'paradox' exist when multiple objects travelling faster than c interact with each other. I am not even sure that these are in fact paradoxes as the one raw FTL experiment we have achieved that of accelerating particles at CERN to collide at combined velocities higher than c has odd by measurable effects.

 NinthMusketeer wrote:

Now to my understanding the above is not how things (theoretically) would work, and I don't particularly understand why. I know that because of time dilation an individual moving at a higher relative velocity would experience less time going by, is it simply that if relative velocity exceeded the speed of light this effect would go into the negative and cause the individual to move backwards?

Most causality problems occur by raw faster than light travel. Pure FTL is so discredited it is not used even in science fiction much anymore.
If raw FTL is bypassed then causality is bypassed because temporal compression doesn't occur, and any events are purely external.

It pays for me to at this point describe properly the three means of FTL travel, whether caused by psychic powers, magic spells, technology. This is important as a lot of franchises use incorrect terminology because it sounds cool to do so.

1. Teleportation
Normally either by a 'jump' engine that teleports the vessel, or a 'gate' which teleports things passing to or from it.
Some especially handwavium forms of teleportation such as wizard spells and Star Trek teleporters can completely seperate the gate from the two points of travel.

In any case teleportation involves instantaneous travel between two points, nominally this travel is stationary or near stationary at least relvant to the pojnt of arrival and departure. Some methods of teleportation involve transmission delays either from the point of view of the object travelling or an observer. Any distinction is purely poetic.

Correct examples of teleport drives:

Battletech KF drive - this is listed as a jump drive in universe and it is a jump drive.
Star Trek transporter beams - these are functional gate drives using teleporter pads, but can also teleport objects without pad interface at either end.


Incorrect examples of teleport drives:

Traveller jump drives - Listed by a jump rating however they are in fact functional hyperdrives (see below).
Stargate gates - These are gates but are hyperspace gates not teleport gates as travellers must pass though hyperspace to get between gates, the travel is also physical.


2. Hyperdrive
Objects enter hyperspace in order to travel a shorter difference than the coordinates in realspace, hyperspace is normally compressed space compared to realspace and allows vast distances to be traveled using more nominal engines in hyperspace.

Correct examples.

Star Wars hyperdrives - These are functional hyperdrives.

Incorrect examples:

Battlestar Galactica hyperdrives - These are jump drives, though the 'hyperdrive' is correctly used to 'jump' the ship.


3. Warp drives.
The only one of the three that makes a lick of sense outside of soft science fiction. It involves surrounding a vessel in a bublbe of realspace while compressing or expandinf surrounding vacuum to allow a vessel to distort spacetime and thus compress differences travelled.

Correct examples.

Star Trek - Properrly Alcubierres warp drive as opposed to Star Trek warp drive as Roddenbery deliberately sought out a plausible FTL technology to use for his show. This warpo dirve is as advertised.

Incorrect examples:

40K warp travel - The Imperium's (and Tau) warp drives are functional hyperdrives, however hyperspace is named 'the warp'; and is demon infested. By the way Event Horizon uses a demon infested jump drive method.

 Orlanth wrote:
I am not even sure that these are in fact paradoxes as the one raw FTL experiment we have achieved that of accelerating particles at CERN to collide at combined velocities higher than c has odd by measurable effects.

The relative velocity (i assume that is what you meant by combined velocities) of the colliding particle bunches at CERN or anywhere else is not higher than the vacuum speed of light, c. Relativistic velocities don't add up like slower speeds we are used to from day to day experience.

What you might be thinking of in terms of FTL and CERN is a publication about a neutrino beam from CERN to Gran Sasso that arrived earlier than expected. Unfortunately, that was an erroneous measurement, where IIRC one of the timing signal's fibre cables wasn't inserted properly and led to the wrong velocity measurement.

I mean combined velocities because one particle stream was moving in the opposite direction around the circuit from the other, and both were accelerated to above 50% c before the particle streams were channeled to impact head on. The impact was therefore at higher than c velocities, the particles themselves were not traveling that fast - nor can they under current understanding of physics.
CERN was primarily made for experiments such as this, and logically such collisions are as close to measuring the effects of particles travelling at relativistic velocities as we can manage.

I was unaware of the neutrino detection at Gran Sasso.

 Orlanth wrote:
I mean combined velocities because one particle stream was moving in the opposite direction around the circuit from the other, and both were accelerated to above 50% c before the particle streams were channeled to impact head on. The impact was therefore at higher than c velocities, the particles themselves were not traveling that fast

The particles in the LHC at CERN are traveling nearly at the speed of light and at least at relativistic speeds where you cannot just add up velocities. Two light beams don't hit each other at 2c but merely at c. Wikipedia has some of the relevant information.

 Minx wrote:

 Orlanth wrote:
I mean combined velocities because one particle stream was moving in the opposite direction around the circuit from the other, and both were accelerated to above 50% c before the particle streams were channeled to impact head on. The impact was therefore at higher than c velocities, the particles themselves were not traveling that fast

The particles in the LHC at CERN are traveling nearly at the speed of light and at least at relativistic speeds where you cannot just add up velocities. Two light beams don't hit each other at 2c but merely at c. Wikipedia has some of the relevant information.

Understood. The point remains that these werent light beams, they had mass, and accelerating particles that fast into a collision to combine speeds to achiever higher impact speeds was the whole idea of CERN.

The site, Project Rho, has a good amount of info on stuff like FTL.

http://www.projectrho.com/public_html/rocket/fasterlight.php

I don't believe that FTL communication does equal time travel. There are numerous thought experiments that suggest it does, but they often contain an ansible (a FTL communicator), which I believe is the source of most of the paradoxes, because the way it functions is usually quite poorly defined.

A typical thought experiment, is Jack and Jill decide to have a race to Alpha Centauri (about 4 light years away). Jill sets off right away in her space ship, which can travel close to the speed of light. Jack decides to wait, and work on a faster method of travel. Jill is travelling close enough to the speed of light that the journey will only feel like 4 hours for her, meanwhile on Earth 4 years will have passed. However, after two years on Earth, Jack succeeds in inventing instantaneous teleportation. He decides to teleport to Jill's ship and give her a lift the rest of the way. This might cause a paradox because it is no longer clear what reference frame he is in.

Here we can introduce the ansible, but instead of having it just send a short message (as it does in most of these thought experiments), I'd like you to imagine a continuous two way conversation between Jack and Jill as she accelerates away (like a Skype chat). Now there are two ways this can work, the first is that the ansible ignores time dilation, and allows the couple to speak normally, regardless of their reference frame. This leads to all kinds of paradoxes, as after four hours of chatting Jack will hear Jill arrive at her destination, before he's even started work on his machine. Worse, is that after two hours, Jack from the future will turn up on Jill's ship and interrupt their conversation.

The other system is that the ansible respects time dilation. As Jill accelerates away, Jack's voice becomes faster and incomprehensible to her, like mosquito buzzing. This system doesn't lead to any paradox or time travel.

As you can probably imagine, the first system is full of problems, but that is often how the ansible is assumed to work. It is the instantaneous communication between different moving reference frames that causes the time travel problems. These problems can be illustrated with light cone diagrams, which appear to prove that a message sent between three or four moving frames, can end up in its own past.

However, I believe that these problems are actually with the concept of the ansible, and not with the concept of FTL. If one supposes that the ansible is affected by time dilation, I imagine that a lot of the "time travel" issues would balance themselves out.

Just to be clear this isn't about any FTL travel in reality, just a fictional universe where FTL occurs. For me understanding this particular aspect is more about writing sci-fi settings than anything else.

Quoting this one in particular as it seems to be the best summary of where I am at:

 Minx wrote:
This might be helpful to see why FTL leads to time-travel and unsolvable paradoxes.

This seems to be supporting my initial idea that causality propagates at the speed of light. For the frame of reference concept to work it requires there to be a certain rate of travel, for example:

A star explodes. Jon on planet A is 10 light years away, and so witnesses the explosion 10 years after it occured. Jane is on planet B 15 light years away, and so witnesses the explosion 15 years after it occurred. From Jon's perspective, the event is witnessed 5 years earlier than from Jane's perspective. Now lets say Jon still has his magic FTL drive and travels to Jane's planet after witnessing the star explosion. He arrives 1 year later, 4 years before Jane would have witnessed explosion normally. But when Jon describes the explosion to Jane, he isn't telling her about an even that hasn't happened yet, he is simply telling her about an even before the light particles from said even reach her. The explosion still occurred in the past, 11 years before Jon told Jane about it.

My hang up is that it seems like the existence of the event itself has a speed; the speed of light. This means that 9 years after the star exploded, that event has still not happened yet on planet A. When Jon tells Jane about the event while on planet B, that event will not happen for another 4 years. If Jon then travels to the star's location, he will arrive before it has exploded. Ultimately it means there is a period where the event is in the past of planet A, but in the future of planet B. More conventional logic would assume if an event occurred then it's in the past wherever one is in the universe; while it may be witnessed at different times in different locations that doesn't change that the event is still in the past. But it seems that is not the case, and that the existence of an event does itself travel at the speed of light and therefore getting ahead of that is effectively time travel.

 NinthMusketeer wrote:
More conventional logic would assume if an event occurred then it's in the past wherever one is in the universe; while it may be witnessed at different times in different locations that doesn't change that the event is still in the past. But it seems that is not the case, and that the existence of an event does itself travel at the speed of light and therefore getting ahead of that is effectively time travel.

I think "conventional logic" is fine in that situation. Where things start to break down is with simultaneity. Special relativity says that there can never be agreement on the simultaneity of events, as events can happen at different times depending on your reference frame. However, all observers will always agree on the order of causally linked events, even if they don't agree on the exact timing. The problem arises when you have reference frames that see simultaneous events in a different order, and they are able to communicate those events FTL to an observer in their own reference frame who is still simultaneous for them, but skewed behind events in an outsiders frame. The 3rd person would then "hypothetically" be able to communicate the event to the outside observer (in a different reference frame) while he is still in the event's past.

You can actually build a theoretical loop, where you send a message, and after a series of "simultaneous" relays through different frames, you end up receiving your own message back before you've sent it.

 Orlanth wrote:

This is how I see it also. The problems of 'paradox' exist when multiple objects travelling faster than c interact with each other. I am not even sure that these are in fact paradoxes as the one raw FTL experiment we have achieved that of accelerating particles at CERN to collide at combined velocities higher than c has odd by measurable effects.

Special Relativity prevents any combined velocities from exceeding the speed of light. Even if you had 2 particles moving directly towards each other and both travelling at 99.9999999% of the the speed of light relative to the rest frame of the laboratory, neither particle would observe the other moving faster than the speed of light.

Sometimes it can appear that an object is travelling faster than light, but when you actually work out the physics of the problem the velocities always end up being below c.

Specifically, if two bodies are moving towards each other at 0.9c (relative to a third party observer) then each body will see the other approaching at 0.9944c, not 1.8c.

http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel2.html#c3

The spacetime diagrams in the links posted above are fairly straightforward for "continuous" FTL in "normal" spacetime*, but offhand I'm not sure about teleportation - that would give you a "world line" made of discontinuous segments. My suspicion is that it doesn't make a difference, but I don't know how to prove it one way or another.

*As I understand it, it only takes finite energy to travel faster than c, but since there's a singularity in the energy-velocity curve at c, you can't get there from here

 AndrewGPaul wrote:
Specifically, if two bodies are moving towards each other at 0.9c (relative to a third party observer) then each body will see the other approaching at 0.9944c, not 1.8c.

http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel2.html#c3

The spacetime diagrams in the links posted above are fairly straightforward for "continuous" FTL in "normal" spacetime*, but offhand I'm not sure about teleportation - that would give you a "world line" made of discontinuous segments. My suspicion is that it doesn't make a difference, but I don't know how to prove it one way or another.

*As I understand it, it only takes finite energy to travel faster than c, but since there's a singularity in the energy-velocity curve at c, you can't get there from here

From E=ymc^2, where y is the Lorentz factor, an object with mass travelling at the speed of light has infinite energy as when an object travels at the speed of light the Lorentz factor is one divided by zero. If you are travelling faster than light, then you end up with a non-real value of energy.

 A Town Called Malus wrote:

 Orlanth wrote:

This is how I see it also. The problems of 'paradox' exist when multiple objects travelling faster than c interact with each other. I am not even sure that these are in fact paradoxes as the one raw FTL experiment we have achieved that of accelerating particles at CERN to collide at combined velocities higher than c has odd by measurable effects.

Special Relativity prevents any combined velocities from exceeding the speed of light. Even if you had 2 particles moving directly towards each other and both travelling at 99.9999999% of the the speed of light relative to the rest frame of the laboratory, neither particle would observe the other moving faster than the speed of light.

Sometimes it can appear that an object is travelling faster than light, but when you actually work out the physics of the problem the velocities always end up being below c.

Ok, thanks for the explanation.