FTL communication

[cantab]

Indeed, in what reference frame should the transmission be instantaneous? Earth's? Mars's? The Cosmic Microwave Background? It's not clear why nature should pick any one over all other possibilities.

[78stonewobble]

I still have a hard time imagining, not the physics...

But why anyone controlling a mars rover, would use slower than light communication to it, from earth, if we can have a FTL ship in orbit around mars.

I mean, either upgrade everyone to FTL or just remote control it from that ship.

That way... No paradox shenanigans.

...

Or offcourse... The "the wheel turned left" message from the future, will offcourse be the reason for the rover operator to turn the wheel left.

[N_las]

But why anyone controlling a mars rover, would use slower than light communication to it, from earth, if we can have a FTL ship in orbit around mars.

I mean, either upgrade everyone to FTL or just remote control it from that ship.

That way... No paradox shenanigans.

The command-station-explosion-paradox only contains FTL signals, nothing about slower than light communication in it.

Or offcourse... The "the wheel turned left" message from the future, will offcourse be the reason for the rover operator to turn the wheel left.

Not if we program the command station to explode if it gets the "turned left" message from the future. Then the "the wheel turned left" message from the future, will offcourse be the reason for the rover operator to NOT turn the wheel left.

[78stonewobble]

The command-station-explosion-paradox only contains FTL signals, nothing about slower than light communication in it.

Not if we program the command station to explode if it gets the "turned left" message from the future. Then the "the wheel turned left" message from the future, will offcourse be the reason for the rover operator to NOT turn the wheel left.

There is no paradox then.

If the signal is sent to mars at ie. 2x times the speed of light, it will still take y amount of time to get there and any return signal at 2x times the speed of light will also take y amount of time to get home. The observer cannot send a return signal, before the original signal has been sent and arrived at mars (which takes y amount of time) and the observers return signal will take it's own time to get back (2 times y amount of time).

Sure, it'll happen faster than we can "see" it, with normal equipment, but there is no paradox.

[N_las]

There is no paradox then.

If the signal is sent to mars at ie. 2x times the speed of light, it will still take y amount of time to get there and any return signal at 2x times the speed of light will also take y amount of time to get home. The observer cannot send a return signal, before the original signal has been sent and arrived at mars (which takes y amount of time) and the observers return signal will take it's own time to get back (2 times y amount of time).

Sure, it'll happen faster than we can "see" it, with normal equipment, but there is no paradox.

In your mind the universes works just like Gallileo and Newton describe it, so you don't see the paradox. You seem to have a fixed clock in the back of your universe that tickes for all observers at the same rate. Let me spell it out again:

If you would send the command signal from earth to mars at the speed of light, you have to wait y amount of time for it to arrive. For a 3rd observer that speeds past mars, this amount of time could be compressed arbitrarily small. If he would go at the speed of light, the amount of time would compress to zero. Thats paradox free.

But if you send the signal at 2x the speed of light, this 3rd observer could observe the rover-reaction before the command decision on Earth. This situation is possible as soon as the signal speed is more than 1c. And this third observer can now send you the "future prediction" back. As long as it is faster than 1c, it is possible to reach you before your command decision.

This is even possible if you use 1.0000001 times the speed of light signals. The only thing that has to change is that the 3rd observer has to be faster.

ANY FTL communication will allow for time travel situations and paradoxes. Thats simply because in special relativity, what happens in the time interval t1 and the distance x1 can look like a different time interval t2 at distance x2 for someone with different velocity than yours. FTL travel and time travel are the same thing from different perspectives. There isn't one without the other.

It is like length and height. Any object that has a height will look like it has a length from a different observer (here a 90° rotated observer)

Edited September 24, 2014 by N_las

[cantab]

ANY FTL communication will allow for...paradoxes.

This is where I disagree. It's easy to contrive paradoxes based on FTL communication or time travel (which are two sides of the same coin). But just because our mind can contrive them doesn't mean they can be in the Universe.

It's my belief that there aren't any paradoxes in the Universe, and indeed that's axiomatic: the Universe, considering its full extent in spacetime, is consistent. Crucial in understanding why the Universe could contain time travel without having paradoxes is that the contrived situations implicitly assume human free will in setting them. Free will is an extremely difficult idea that would warrant its own thread to debate, but by abandoning it it becomes much easier to just say "paradoxes don't occur". Nobody will ever successfully set up a self-destroying signal or a grandfather paradox, even if a naive view suggests they could, because they do not happen in a consistent Universe.

[N_las]

I have met some people who argue that FTL and time travel is possible, and therefore causality is only valid locally. And many people argue that causality has to be global, and therefore FTL and time travel is impossible.

Your mix idea is very unsatisfying. You not only have to show that the universe as a whole is consistent, you also have to show a magic mechanism that prevents certain events for seemingly no local reason, just to allow for some FTL.

[cantab]

Well, this is just the http://en.wikipedia.org/wiki/Novikov_self-consistency_principle by the way.

When you speak of a "magic mechanism", I suspect you're imagining a situation where events would be about to cause a paradox and suddenly something randomly goes kaput to stop it. I don't think that's very likely; rather, there wouldn't even be "near-misses", or they'd be extremely rare.

Supporting this sort of idea is the study done of the following time paradox, discussed in more detail on the wiki article:

Take a wormhole that has its two ends at different times.

Send a ball in through the "future" end so it will come out the "past" end and knock its previous self away from the wormhole, meaning it never goes through so never knocks itself off course, meaning....etc etc etc.

It seems a problem, but there's a self-consistent solution. The ball exits the wormhole not on the expected course, but a slightly different one, giving its previous self a glancing blow to put it on that slightly different course.

Detailed study found that those self-consistent trajectories are "easy", there are loads of possibilities, while finding an initial trajectory that does not have a self-consistent solution defeated the students and may well be impossible.

Now that's just one situation for one type of time machine, but the flavour of the argument is instructive.

Perhaps it's not entirely satisfying, but I don't find the alternative argument that "some time travel situations are paradoxes therefore all time travel situations are impossible" satisfying either.

[Mr Shifty]

Free will is an extremely difficult idea that would warrant its own thread to debate, but by abandoning it it becomes much easier to just say "paradoxes don't occur". Nobody will ever successfully set up a self-destroying signal or a grandfather paradox, even if a naive view suggests they could, because they do not happen in a consistent Universe.

Yes, and the way the universe enforces consistency is to set luminal speed as an upper limit. QED

Note that free will is probably illusory. There've been studies on reaction times that show decisions being acted upon before the conscious mind registers them. (Look up Benjamin Libet.) It suggests your brain is informing your mind of 'free' decisions after the fact, constructing an illusion whereby you 'chose' the decision.

EDIT: In Charles Stross's novel Singularity Sky, humanity has discovered FTL travel, but there is a cosmic censor called the Eschaton that acts decisively from the far future to prevent any causality violations that might preclude its own existence. ("Thou shalt not violate causality within my historic light cone. Or else.")

Edited September 25, 2014 by Mr Shifty

[WestAir]

That's mildly subjective. If I chose to go to the store to buy icecream, I guarantee my mind had more to do with it than my reflexes. If we're talking about gun fights or shoot-outs, you're probably right, but we're not.

[cantab]

Yes, and the way the universe enforces consistency is to set luminal speed as an upper limit. QED

That remains plausible, indeed probable, of course. But general relativity does seem full of loopholes to get round it, and even though many of them require silly stuff like negative mass there's no general theoretical reason as yet to say they'll all be like that.

However, what I'm arguing isn't so much that FTL communications are possible, but rather that time travel paradoxes do not make FTL communications impossible. There may still be something totally different standing in the way of FTL.

[N_las]

EDIT: In Charles Stross's novel Singularity Sky, humanity has discovered FTL travel, but there is a cosmic censor called the Eschaton that acts decisively from the far future to prevent any causality violations that might preclude its own existence. ("Thou shalt not violate causality within my historic light cone. Or else.")

I read that, it was nice. You have to pay attention to what you are planing, or a giant meteor falls on your head to stop you from it. And if there is no meteor, than you can be quite sure that your time-travel will fail to have an effect, thus conserving causality.

[ZetaX]

Why is everyone arguing like time travel needs to be restrained to a single instance of the universe¿ Just have a multitude of universes and there will be no causality problems. You just leave this one and enter another, possibly at a different point in time. This also resolves all the problems coming from information being transmitted.

You could even have a similiar maximum speed like the speed of light that cannot be broken, but which is only accounted correctly if you factor in the movement "orthogonal to the universe". The way those 1 second per second time machiens in "Primer" work might be an example of such a thing.

That's mildly subjective. If I chose to go to the store to buy icecream, I guarantee my mind had more to do with it than my reflexes.

Yes, but that's beside the point. Your mind was just a clockwork initiating the necessary actions. No free will or similiar things needed here.

[N_las]

Well, this is just the http://en.wikipedia.org/wiki/Novikov_self-consistency_principle by the way.

When you speak of a "magic mechanism", I suspect you're imagining a situation where events would be about to cause a paradox and suddenly something randomly goes kaput to stop it. I don't think that's very likely; rather, there wouldn't even be "near-misses", or they'd be extremely rare.

Supporting this sort of idea is the study done of the following time paradox, discussed in more detail on the wiki article:

Take a wormhole that has its two ends at different times.

Send a ball in through the "future" end so it will come out the "past" end and knock its previous self away from the wormhole, meaning it never goes through so never knocks itself off course, meaning....etc etc etc.

It seems a problem, but there's a self-consistent solution. The ball exits the wormhole not on the expected course, but a slightly different one, giving its previous self a glancing blow to put it on that slightly different course.

Detailed study found that those self-consistent trajectories are "easy", there are loads of possibilities, while finding an initial trajectory that does not have a self-consistent solution defeated the students and may well be impossible.

Now that's just one situation for one type of time machine, but the flavour of the argument is instructive.

Perhaps it's not entirely satisfying, but I don't find the alternative argument that "some time travel situations are paradoxes therefore all time travel situations are impossible" satisfying either.

You could use that to build a perfect differential equation solver. I assume that testing a proposed solution to a differential equation is always easy, while finding this solution without its proposal is nearly impossible.

1: We build a machine that can communicate through time.

2: We receive a solution from the future (we can do that even before we define the differential equation).

3: We define the differential equation.

4: We test the solution.

5a: If the solution is a valid solution of the differential equation, we send it back in time (to point 2 in this list).

5b: If the solution is invalid, we mulitply it with any other function and send it back.

If there was a magic anti-grandfather-paradox mechanism build into our universe, the probablity of 5b will be zero, beause it will create a paradox (we received a different solution as was send). So only 5a is possible. So the solution will always be valid.

Or do you think a magic anti-paradox lightning bolt will strike my machine? It will give me a solution to any differential equation, even before I input the equation into the machine.

Edited September 25, 2014 by N_las

[ZetaX]

This won't work: how do you send a function¿ You can send an expression, yes, but not a function (there are way more than finite length expressions). Just use any differential equation that does not have a solution expressible on your symbols (such things exists), then your solver is no better than:

- Recieve a single bit.

- Flip it.

- Send the new bit back in time.

Obviously causing a paradox, being forbidden, so your solver never even gets to exist.

But you can find a similiar thing in Harry Potter and the Methods of Rationality. There it is used to factor a large integer. And the cycle has a nonexpected fixed point.

Edited September 25, 2014 by ZetaX

[N_las]

This won't work: how do you send a function¿ You can send an expression, yes, but not a function (there are way more than finite length expressions). Just use any differential equation that does not have a solution expressible on your symbols (such things exists), then your solver is no better than:

- Recieve a single bit.

- Flip it.

- Send the new bit back in time.

Obviously causing a paradox, being forbidden, so your solver never even gets to exist.

Yeah, then it will only work for finite symbolic solutions. It is still a pretty neat machine... that showcases how unsatisfying the Novikov self-consistency principle seems to me. I really doubt that this machine can work, and the most probable point of failure seems to be #1.

Edited September 25, 2014 by N_las

[ZetaX]

I also find the principle unnecessarily complex. But you can salvage it in this situation: assume you are planning the scheme you mentioned. But when waiting for the solution from the future, you instead get something else that still makes you or someone else send back exactly that. This might for example be one of:

a) A very convincing message that very bad things might happen to you if you break this cycle (possible coming together with a temperoral agent that makes it clear that he will kill you and send [him and] the message himself, if necessary).

Nothing at all because for whatever reason you cannot control, the time machine fails, and this knowledge by itself is worthless to fix this.

c) A solution you falsely believe to be correct and send back. The reason for that one may be numerical or else, and you may find the error later, but you would still send it in this case.

[N_las]

I also find the principle unnecessarily complex. But you can salvage it in this situation: assume you are planning the scheme you mentioned. But when waiting for the solution from the future, you instead get something else that still makes you or someone else send back exactly that. This might for example be one of:

a) A very convincing message that very bad things might happen to you if you break this cycle (possible coming together with a temperoral agent that makes it clear that he will kill you and send [him and] the message himself, if necessary).

Nothing at all because for whatever reason you cannot control, the time machine fails, and this knowledge by itself is worthless to fix this.

c) A solution you falsely believe to be correct and send back. The reason for that one may be numerical or else, and you may find the error later, but you would still send it in this case.

Thats all assuming that the universe tries to be mean. Its basically the magic lightning bolt that hits the machine before it's turned on. If this self-consistency principle is true, then it should be definetly possible to use it for computation. I think I will write the code for an time-travel FEM-Solver this weekend. The time travel isn't there yet, but writing the software would be funny regardeless.

Whats funny is, that some lines of code will be never executed, but they are still crucial for the programm to work.

[ZetaX]

In case you haven't seen this already: http://blogs.msdn.com/b/oldnewthing/archive/2014/06/27/10537746.aspx . It has similiar features to what you describe.

[Beowolf]

That's mildly subjective. If I chose to go to the store to buy icecream, I guarantee my mind had more to do with it than my reflexes. If we're talking about gun fights or shoot-outs, you're probably right, but we're not.

Sure, and I'd make the same guarantee about my own actions. "Reflexes" is the wrong term, though.

When they actually built brain scanners good enough to measure what's going on and designed controlled experiments, it turned out we're wrong. I'm uncomfortable with it too. But the results have been independently confirmed multiple ways. When they actually measure both the moment you start acting on a decision, and the moment you consciously make the decision, it comes out backwards. We make the decision, THEN our conscious mind (appears to) come up with a reason to justify it. We're only talking about a fraction of a second. Googling "decision making brain scan" without the quotes gets you a page full of citations.

It doesn't really say we don't have free will, though. Just that our conscious thoughts have less to do with our immediate actions than we believed. And nobody yet knows how (if at all) this applies to long-term decisions like whether to go to college or to get married. Maybe it's like our subconscious "animal" mind is piloting the plane, but our consciousness is the one scheduling where the flight goes and the air traffic controller managing the route. I'm sure we'll know more in another decade or two.

[ZetaX]

Even without further science, the result should be rather clear: the brain is essentially a very complicated but deterministic clockwork. Under the very same conditions it will do the same. Don't let clueless people tell you that quantum physics makes a difference, because the size of a neuron is way to big to give this any kind of realistic probability (and since when is randomness comparable to "free will" anyway¿).

[rtxoff]

Claiming such things could be a very dangerous path. For an example you could just as well say: "let all the criminals out of their jails because they are not guilty, they had no choice to do different"

[N_las]

Claiming such things could be a very dangerous path. For an example you could just as well say: "let all the criminals out of their jails because they are not guilty, they had no choice to do different"

"I don't bring my broken car to the repair man, because it doesn't broke down on purpose."

"I don't put this dangerous acid in a special secure container, because it doesn't disolve things on purpose."

"I don't stop this stone from rolling down the mountain, because it doesn't cause avalanches on purpose."

You see how stupid that sounds?

Deterrence (prevention), rehabilitation, incapacitation and societal protection, restoration, education and denunciation, and even retribution... they are all valid reasons for punishment, even if the offender has no free will.

But apart from that argument, I do belive in free will. I just think that it doesn't matter. The universe and our society would be indistinguishable either way.

Edited September 25, 2014 by N_las

[ZetaX]

Claiming such things could be a very dangerous path. For an example you could just as well say: "let all the criminals out of their jails because they are not guilty, they had no choice to do different"

No. A lack of free will does not imply such a thing and this argument is quite old, quite bad, and has been discussed a lot already. What I wrote is mostly factual (there have been some studies that test this determinism), and the part that is not is mostly just assuming that there is no magical transcendental effect that deviates brain physics from the physics in the rest of the universe.

At the worst, I could just get away with "my lack of free will dictates me to still put them in jail"/"still lets me act as if I have one". You can easily find better ones (mostly based on incentives and/or consequences and such) ones, though.

Edit: Also, ninja'd by N_las.

[rtxoff]

No. A lack of free will does not imply such a thing and this argument is quite old, quite bad, and has been discussed a lot already.

No it doesn't ??? What exactly does it then? Please explain what's so old about the argument and what came out at these discussions.