Is violating causality via FTL really a problem? Can someone explain?

[theprestige]

I'm trying to think about this, but right away my brain stalls out:

1. I observe an effect, necessarily arising from a cause.

2. I teleport instantaneously to a location in spacetime where the cause has not yet occurred.

3. ???

4. Violation of causality! Everything in the light cone ceases to exist! The universe begins to unravel!

But what actually happens? Or would happen, hypothetically? Theoretically?

[Planigale]

Originally Posted by theprestige

I'm trying to think about this, but right away my brain stalls out:

1. I observe an effect, necessarily arising from a cause.

2. I teleport instantaneously to a location in spacetime where the cause has not yet occurred.

3. ???

4. Violation of causality! Everything in the light cone ceases to exist! The universe begins to unravel!

But what actually happens? Or would happen, hypothetically? Theoretically?

You would be exterminated by Daleks and space time would return to normal and the Daleks would cease to exist. Daleks are purely a mechanism the universe uses to deal with those who try to violate causality.

[Thermal]

Didn't we essentially do this on the Time Travel thread? There isn't a "location" in spacetime. There is a location in space, and a point in time, but Spacetime is not a calendar where the temporal points actually exist simultaneously.

[theprestige]

Originally Posted by Thermal

Didn't we essentially do this on the Time Travel thread? There isn't a "location" in spacetime. There is a location in space, and a point in time, but Spacetime is not a calendar where the temporal points actually exist simultaneously.

If you point me to the essential explanation of my conundrum in the other thread, I will be much obliged. The temporal paradox is well-trodden ground in speculative fiction and casual conversation. Here I'm interested in the related but distinct question of how exactly causality violation is supposed to work, and what it implies, with FTL travel.

[Horatius]

I think your problem is, you're thinking about causality as it exists in our universe, where FTL is impossible, and asking what it tells us about causality in a universe where FTL is possible.

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

Quote:

In both Einstein's theory of special and general relativity, causality means that an effect cannot occur from a cause that is not in the back (past) light cone of that event. Similarly, a cause cannot have an effect outside its front (future) light cone. These restrictions are consistent with the constraint that mass and energy that act as causal influences cannot travel faster than the speed of light and/or backwards in time.

Were it possible to travel FTL, then the rules of causality would be different. "Nothing outside the light cone" arises because of the fundamental limit to the speed of light. If that's not really a fundamental limit, then things would be different.

[Thermal]

Originally Posted by theprestige

If you point me to the essential explanation of my conundrum in the other thread, I will be much obliged. The temporal paradox is well-trodden ground in speculative fiction and casual conversation. Here I'm interested in the related but distinct question of how exactly causality violation is supposed to work, and what it implies, with FTL travel.

Because your #2 begs the question that that point where the cause hasn't happened yet could exist. It seems to be saying "then I travel back in time", but substitutes "another location" while still literally meaning "backwards in time".

Or do you mean is that by transporting instantaneously, that the light/information/detectability of the cause hasn't reached that other point yet?

[Olmstead]

Originally Posted by Horatius

I think your problem is, you're thinking about causality as it exists in our universe, where FTL is impossible, and asking what it tells us about causality in a universe where FTL is possible.

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



Were it possible to travel FTL, then the rules of causality would be different. "Nothing outside the light cone" arises because of the fundamental limit to the speed of light. If that's not really a fundamental limit, then things would be different.

I don't think that's quite accurate. FTL travel is theoretically possible if we were to discover something weird like wormholes. But that would violate causality, and that would be even weirder. And since causality violations don't make sense, we don't think there are FTL enabling wormholes.

[Ziggurat]

Originally Posted by theprestige

But what actually happens? Or would happen, hypothetically? Theoretically?

We have absolutely no idea. And that's sort of the problem. We have no idea of how to the universe could work with causality breaks. Do you get diverging timelines? An unraveling of the universe? The whole universe resetting with each jump back in time? Not a clue. There are no good theories, because all the theories that we do have which make sense basically assume causality as a prerequisite.

But suppose you developed a theory that could handle time travel. Well, there's no way to test if that theory is any good unless you time travel. And we don't know how to do that. So even developing such a theory would basically be a waste of time (pun intended).

[theprestige]

Originally Posted by Thermal

Because your #2 begs the question that that point where the cause hasn't happened yet could exist. It seems to be saying "then I travel back in time", but substitutes "another location" while still literally meaning "backwards in time".

Or do you mean is that by transporting instantaneously, that the light/information/detectability of the cause hasn't reached that other point yet?

Thanks, but I'm actually asking for the physical theory that describes the mechanism of causality violation, and its consequences, from FTL travel. The obvious similarity to time travel in the lay view is noted and understood, but tangential to my question.

[theprestige]

Originally Posted by Ziggurat

We have absolutely no idea. And that's sort of the problem. We have no idea of how to the universe could work with causality breaks.

Okay, but how exactly does causality break? Is there a toy scenario with a spherical cow that you can use to explain how it comes about?

[Olmstead]

Originally Posted by theprestige

Okay, but how exactly does causality break? Is there a toy scenario with a spherical cow that you can use to explain how it comes about?

I'm not sure which part you have a problem with?

At 8 a.m., I send an FTL message to a spaceship that is moving away from me at close to light speed. The pilot receives my message at 5 a.m. based on his relativistic clock. He sends me an FTL reply not to send him the message. I receive that message at 2 a.m. based on my clock, i.e. before having send the original message. I don't send that message.

The above doesn't make sense, so FTL travel isn't possible.

[theprestige]

Originally Posted by Olmstead

I'm not sure which part you have a problem with?

At 8 a.m., I send an FTL message to a spaceship that is moving away from me at close to light speed. The pilot receives my message at 5 a.m. based on his relativistic clock. He sends me an FTL reply not to send him the message. I receive that message at 2 a.m. based on my clock, i.e. before having send the original message. I don't send that message.

The above doesn't make sense, so FTL travel isn't possible.

Sorry, you changed clocks halfway through the scenario.

What if we use your clock throughout?

1:00 AM - You prepare a message for transmission at 1:30 AM.

1:05 AM - You receive a message telling you not to transmit the message you had prepared.

1:06 AM - You bin the message and get a cup of coffee instead.

[Olmstead]

Originally Posted by theprestige

Sorry, you changed clocks halfway through the scenario.

What if we use your clock throughout?

1:00 AM - You prepare a message for transmission at 1:30 AM.

1:05 AM - You receive a message telling you not to transmit the message you had prepared.

1:06 AM - You bin the message and get a cup of coffee instead.

Short answer: The universe doesn't work like that.

Everyone has their own relativistic clock, and when you travel really fast, things get really weird, and this has been proven through experimentation.

When the pilot left earth, both our clocks showed exactly the same time. But now that he's in Alpha Centauri travelling at near light speed, an FTL message send by him will always arrive three hours in the past based on MY clock, while an FTL message send by me will always arrive three hours in the past based on HIS clock (three hours is an arbitrary number because I hate to do math; depending on the speeds involved the difference could be more or less).

[Robin]

Originally Posted by theprestige

Sorry, you changed clocks halfway through the scenario.

What if we use your clock throughout?

1:00 AM - You prepare a message for transmission at 1:30 AM.

1:05 AM - You receive a message telling you not to transmit the message you had prepared.

1:06 AM - You bin the message and get a cup of coffee instead.

So how did the guy on the spacecraft know you sent a message if you didn't send it?

Look.at it this way. The spacecraft is set to repeat the message back to you.

Your apparatus is set to transmit the binary opposite of what you send.

You receive a "1" at 1:05 am and your apparatus sends a 0. The apparatus on the spacecraft returns a "0" which is the message you received at 1:05 am.

Not possible because you sent "0" based on your receiving a "1".

(On the other hand, I don't think that Olmstead's or my scenario can happen in Relativity, I would have to work that out).

[theprestige]

Originally Posted by Robin

So how did the guy on the spacecraft know you sent a message if you didn't send it.

THAT IS MY QUESTION.

Where exactly does the causality violation come into play? Is there a diagram that shows it?

Some sort of equation or description that demonstrates how messages from the other guy's future necessarily end up in my past?

[Olmstead]

Originally Posted by theprestige

THAT IS MY QUESTION.

Where exactly does the causality violation come into play? Is there a diagram that shows it?

Some sort of equation or description that demonstrates how messages from the other guy's future necessarily end up in my past?

There's this:

http://www.physicsmatt.com/blog/2016...es-time-travel

But might be confusing if you've never seen a space time diagram.

[theprestige]

Originally Posted by Olmstead

There's this:

http://www.physicsmatt.com/blog/2016...es-time-travel

But might be confusing if you've never seen a space time diagram.

Thanks! I'll study that.

[The Man]

Originally Posted by theprestige

Okay, but how exactly does causality break? Is there a toy scenario with a spherical cow that you can use to explain how it comes about?

In your scenario you go back in time to stop something before it happens. In that case causality remains intact since you had to stop it before it happened.

The time travel paradox then comes how do you know or remember it happening if it never happened. The general SIFi type answer are that you may not remember it anymore having stopped it. Becoming part of that modified timeline or you do remember and are now in a new district timeline. Bottom line, as Ziggurat noted, we don't know and if you come up with a scenario that does actually break causality in that scenario then it breaks in that scenario however the scenario requires it to break as it is all just speculation.

[Olmstead]

Originally Posted by Robin

(On the other hand, I don't think that Olmstead's or my scenario can happen in Relativity, I would have to work that out).

I mean, it can't happen because FTL communication can't happen, and the numbers are probably off, but I think that's basically how relativity works.

When the pilot starts travelling away from earth, his "clock" will run slower from my point of view due to time dilation. But because speed is relative, it's just as valid for the pilot to say that he is standing still and the earth is travelling away from him, which means that my "clock" will run slower from his point of view. And this "point of view" isn't just perception, but an actual physical thing that happens. If FTL messages were possible, the message would always arrive in the past for the other person.

This ignores acceleration, but all that would do is change the numbers and isn't really relevant.

Of course, I'm just a sci-fi fan who read some science books, so if there are holes in my understanding, I'd be happy to learn more.

[Robin]

The point is that each frame is supposed to represent the same situation.

If a message is received before it is sent in one frame and this represents a physical situation then a transform of all the co-ordinates to the rest frame of another observer must also represent a physical situation. If there was FTL communication then you would have cases where a situation that could be described in one frame with all world line moving forward in time could be transformed to the rest frame of another observer with some world lines moving back in time.

[Robin]

Carol is travelling 0.2 c with respect to Alice.

Alice sends a message to Carol at ten times the speed of light.

Carol receives the message and sends it back at ten times the speed of light.

Alice will receive Carol's reply before she sends the message.

Alice has the instruction - send the same message you receive.

Carol has the instruction - send a message different from the one you receive.

That's the causality violation - it cannot be that Alice and Carol complete their instructions correctly.

[Robin]

The key being that if Alice can send a message to Carol at 10 times the speed of light, Carol can receive it at a time when the time that Alice sent it is still in Carol's future.

So if Carol can send the reply at 10 times the speed of light too then Alice will receive the reply before she sent the message.

So there is the causality violation described above.

[Cat Not Included]

The really important thing to keep in mind being "that's not how reality actually works, none of this stuff is actually possible".

[Robin]

The question of what would happen is, I think, a non-question since it can't happen.

[Puppycow]

Originally Posted by Thermal

Didn't we essentially do this on the Time Travel thread? There isn't a "location" in spacetime. There is a location in space, and a point in time, but Spacetime is not a calendar where the temporal points actually exist simultaneously.

I wonder if that part is even true.

I think maybe the concept of "now" is a completely subjective and local phenomenon according to GR. Even the concept of simultaneity may be wrong. Like, two events that may appear to happen simultaneously to one observer could happen at different times to another observer.

I saw an interesting talk recently about this year's Nobel Prize in Physics.

The same person wrote an article about it.

It seems that General Relativity and Quantum Mechanics don't fit together very well.

[smartcooky]

Originally Posted by Horatius

I think your problem is, you're thinking about causality as it exists in our universe, where FTL is impossible, and asking what it tells us about causality in a universe where FTL is possible.

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


Were it possible to travel FTL, then the rules of causality would be different. "Nothing outside the light cone" arises because of the fundamental limit to the speed of light. If that's not really a fundamental limit, then things would be different.

OK so let say not FTL (because FTL is not even theoretically possible in our universe), so instead, lets go with something that is theoretically possible - a wormhole/Einstein-Rosen bridge.

So, to restate theprestige's question

Quote:

1. I observe an effect, necessarily arising from a cause.

2. I pass through an Einstein-Rosen bridge to a location where the cause has not yet occurred.

3. ???

4. Violation of causality! Everything in the light cone ceases to exist! The universe begins to unravel!

But what actually happens? Or would happen, hypothetically? Theoretically?

[Ziggurat]

Originally Posted by theprestige

Okay, but how exactly does causality break? Is there a toy scenario with a spherical cow that you can use to explain how it comes about?

Sure. It’s cliched, but the whole time travel to kill your grandfather so you were never born thing.

Or are you asking how FTL leads to breaking causality?

[theprestige]

Originally Posted by Ziggurat

Or are you asking how FTL leads to breaking causality?

Yes, and also what breaking causality would actually look like in practice.

[The Great Zaganza]

Luckily, the energy expended by a space ship to slow down from FTL would create a shockwave that would obliterate nearby planets. So there wouldn't be a place left on which you could violate causality

[Thermal]

Originally Posted by Puppycow

I wonder if that part is even true.

I think maybe the concept of "now" is a completely subjective and local phenomenon according to GR. Even the concept of simultaneity may be wrong. Like, two events that may appear to happen simultaneously to one observer could happen at different times to another observer.

I saw an interesting talk recently about this year's Nobel Prize in Physics.

The same person wrote an article about it.

It seems that General Relativity and Quantum Mechanics don't fit together very well.

Agreed, the whole topic of time can get pretty heady. What I'm not understanding about theprestige's question is how time travel seems to get slid in there somewhere. As I am reading his posit, from the vantage point of an omnicient observer:

1. A thing happens at 1:00.. Using the magic instantaneous phone, theprestige calls his buddies on Alpha Centauri to tell them the thing happened. The light has not reached AC to let them see this event has taken place.

2. AC receives the call at 1:01, and using their magic phone, responds at 1:02 to say not to make the thing happen.

3. TP receives the call at 1:02 saying not to let the thing happen, but it is too late. The thing happened already.

4. AC sees the time delayed 'echo' of the event some minutes later.


Even though the event had not 'happened' yet on AC (the information hadn't been received yet), time continued moving on it's arrow, and one event followed the next. That the visual information hadn't reached AC yet while the magic phone was working doesn't mean the event hadn't occurred yet. I'm not seeing where anything traveled backwards in time to where it's cause had not yet occurred.

[The Man]

Originally Posted by Puppycow

I wonder if that part is even true.

I think maybe the concept of "now" is a completely subjective and local phenomenon according to GR. Even the concept of simultaneity may be wrong. Like, two events that may appear to happen simultaneously to one observer could happen at different times to another observer.

[Polite snip]

If I recall correctly that is only if the two events are separated spatially. Simultaneous evets that happen at the same place do so for all observers.

[The Man]

Originally Posted by Thermal

Agreed, the whole topic of time can get pretty heady. What I'm not understanding about theprestige's question is how time travel seems to get slid in there somewhere. As I am reading his posit, from the vantage point of an omnicient observer:

1. A thing happens at 1:00.. Using the magic instantaneous phone, theprestige calls his buddies on Alpha Centauri to tell them the thing happened. The light has not reached AC to let them see this event has taken place.

2. AC receives the call at 1:01, and using their magic phone, responds at 1:02 to say not to make the thing happen.

3. TP receives the call at 1:02 saying not to let the thing happen, but it is too late. The thing happened already.

4. AC sees the time delayed 'echo' of the event some minutes later.


Even though the event had not 'happened' yet on AC (the information hadn't been received yet), time continued moving on it's arrow, and one event followed the next. That the visual information hadn't reached AC yet while the magic phone was working doesn't mean the event hadn't occurred yet. I'm not seeing where anything traveled backwards in time to where it's cause had not yet occurred.

Which was my point before, the whole scenario is going back in time and stopping/or changing the cause before it occurs. So causality remains intact, what's left is just your typical time travel paradox.

[Crossbow]

Originally Posted by theprestige

Yes, and also what breaking causality would actually look like in practice.

If I may chime in here, I do not know if this is the type of answer that you are looking for, but since it has not yet been discussed, then I will attempt to do so now.

So anyway, I am sure that you already know that it takes energy in order to accelerate any object with a non-zero rest mass.

Additionally, any object with a non-zero rest mass that is moving, has a certian amount of momentum.

However, as the said object approaches the speed of light, then the momentum of said object increases by the factor gamma.

gamma = (1 - (v/c)²)^-1/2

And as the momentum of said object increases, then the more energy that it takes to further accelerate said object.

Accordingly, sooner or later one will reach the point where there is not enough energy in the entire universe to accelerate said object to the speed of light, let alone exceed the speed of light.

Therefore, since travelling faster than the speed of light is necessary in order to violate causality, then it is impossible to violate causality since doing so would take more energy than is available in the entire universe.

Or put another way, in order for said object to violate causality, then all of the mass from an infinite number of universes would need to be converted to energy so that said object could exceed the speed of light. But, providing all of this energy would destroy all of the mass in all of these universes (including the one universe that we live in).

I hope this helps.

[Olmstead]

Originally Posted by Thermal

Agreed, the whole topic of time can get pretty heady. What I'm not understanding about theprestige's question is how time travel seems to get slid in there somewhere. As I am reading his posit, from the vantage point of an omnicient observer:

1. A thing happens at 1:00.. Using the magic instantaneous phone, theprestige calls his buddies on Alpha Centauri to tell them the thing happened. The light has not reached AC to let them see this event has taken place.

2. AC receives the call at 1:01, and using their magic phone, responds at 1:02 to say not to make the thing happen.

3. TP receives the call at 1:02 saying not to let the thing happen, but it is too late. The thing happened already.

4. AC sees the time delayed 'echo' of the event some minutes later.


Even though the event had not 'happened' yet on AC (the information hadn't been received yet), time continued moving on it's arrow, and one event followed the next. That the visual information hadn't reached AC yet while the magic phone was working doesn't mean the event hadn't occurred yet. I'm not seeing where anything traveled backwards in time to where it's cause had not yet occurred.

If TP and AC are travelling at different velocities, TP would receive the call from AC before 1:01.

[Ziggurat]

Originally Posted by theprestige

Yes, and also what breaking causality would actually look like in practice.

We've got absolutely no idea what it would look like in practice, because as far as we can tell it's not possible.

As for how FTL breaks causality, basically going faster than light is the same thing as going backwards in time. Let's say you can go twice the speed of light in Earth's reference frame. In Earth's reference frame, you're still going forward in time.

But there's another reference frame where you were going backwards in time. In other words, there's no real difference between going faster than c and going backwards in time (you're moving to the side too, but that's fixable by having a second segment to the trip).

Much of the weirdness of relativity comes from what happens when you change reference frames. Normally, this constrains you from going faster than light. Let's say you're on earth, and you send out a rocket at 0.8c. Then that rocket sends out a mini rocket ahead of it, going 0.8c relative to it. The mini rocket isn't going 1.6c relative to you, but something closer to but still less than c. Well, those weird reference change things get even weirder when going faster than c.

Another way to look at it is that for sub-light speed travel, whether you're moving to the left or the right depends on reference frame, but moving forward in time is invariant. But if you go faster than c to the right, then you're always going to the right, but whether you're going forward or backward in time is reference frame dependent.

[Thermal]

Originally Posted by Olmstead

If TP and AC are travelling at different velocities, TP would receive the call from AC before 1:01.

Which is the calendar problem again. As soon as you draw a timeline, or calendar, or any other spacetime grid to represent the past and present existing at the 'same time', you have screwed the proverbial pooch, logistically.

Thre was a representation of what you are describing on the Time Travel thread (or following liknks or something) where the timeline had a plane moving forward in the direction of time. All well and good. But then to account for the discrepancies in velocities, the timeline was angled. Seems well and good...kind of. The problem is, if that angled timeline continued its trajectory, it would flip till time stood still, then stared going backwards. IIRC, there was some discussion about limits approaching zero to mitigate this logical absurdity.

But all of it hinges on the calendar/linear representation. A calendar is the Supermodel of time. It works like gangbusters to predict and explain, just like QM/GR and all. It can predict when I will have a neap tide on my beach years into the future. It has no competition. It is the be-all and the end-all of time models.

Yet if you place a pawn on today on this calendar, you can't slide it to last week and declare "Therefore, Time Travel! It is proven via the Supermodel of time which has no competition!" The model, indisputably powerhouse that it may be, does not reflect reality.

[Finster]

Pretty good explanation with space time diagrams - Pretty detailed but you don't need a PhD in physics to understand - http://www.physicsguy.com/ftl/html/FTL_part1.html

[pzkpfw]

(deleted: image hosting issues.)

[Olmstead]

Originally Posted by Thermal

Which is the calendar problem again. As soon as you draw a timeline, or calendar, or any other spacetime grid to represent the past and present existing at the 'same time', you have screwed the proverbial pooch, logistically.

Thre was a representation of what you are describing on the Time Travel thread (or following liknks or something) where the timeline had a plane moving forward in the direction of time. All well and good. But then to account for the discrepancies in velocities, the timeline was angled. Seems well and good...kind of. The problem is, if that angled timeline continued its trajectory, it would flip till time stood still, then stared going backwards. IIRC, there was some discussion about limits approaching zero to mitigate this logical absurdity.

But all of it hinges on the calendar/linear representation. A calendar is the Supermodel of time. It works like gangbusters to predict and explain, just like QM/GR and all. It can predict when I will have a neap tide on my beach years into the future. It has no competition. It is the be-all and the end-all of time models.

Yet if you place a pawn on today on this calendar, you can't slide it to last week and declare "Therefore, Time Travel! It is proven via the Supermodel of time which has no competition!" The model, indisputably powerhouse that it may be, does not reflect reality.

Attempts at simple representation will always have their issues. There really isn't a debate here. Physicists in relevant fields agree that FTL messages, especially magical instantenous phones, would lead to time paradoxes if they were possible. A layman like myself isn't going to be able to poke holes into something they have been studying for decades. At that point, all I can do is to accept it and to try to wrap my head around the concepts as much as possible.

[William Parcher]

Would you be able to win the lottery with this?

After a lottery drawing you send the winning numbers to a distant spaceship at FTL. Then they send them back to you at FTL. The winning numbers arrive before the drawing and you play them for the win.

Is that one of the time travel paradoxes?