Physicists Theorize New Method for FTL Travel

[jambaugh]

Another important aspect of this work was the fact that many young people feel that physics is 'boring', 'difficult' and 'irrelevant', and thus an unattractive career option. We believe that the exploration of novel ideas in interstellar propulsion, using advanced physics, encourages new minds to enter our subject.

Don't write checks you can't cash. Any FTL craft is a time machine in an appropriate frame OR you are breaking Lorentz covariance. Invoking entropic time doesn't change this unless you are asserting that the entropy of the entire region across which the flight occurs is being affected (i.e. the attempt causes a BIG Bang.) Two "jumps" (with an intermediate boost) puts you into your own past (and the boost can be replaced with a communication to a second craft.) It doesn't matter if your craft is locally STL due to some funny warp bubble effect.

The Nikolic entropic time paper cited is IMNSHO being misinterpreted. It doesn't provide an "out" for FTL causality paradoxes. It rather seems to impose more restrictions on causal ordering which would imply an impossibility of globally FTL paths being causal even if they are locally STL. (I think the Nikolic paper may relate to the 2nd horizon in certain black-hole/worm-hole solutions where you hit an infinite temperature though you avoid the singularity.)

The basic idea of FTL travel is that you jump in a ship at point A and arrive at point B faster than light could travel by a direct path. Specifically you get ahead of the sphere of light emitted by the television coverage of your departure.

Saying one travels there via some modification of local space-time around the craft doesn't matter. If the arrival event at B is outside the future light cone of the departure event at point A then you can boost the whole mechanism of travel (assuming local Lorentz covariance) so that the arrival event is prior to the departure event and then reverse the boost and reverse the trip so as to arrive at A prior to the original departure.

If this is avoided it must be due to an additional restriction, one which breaks Lorentz covariance and thus you are working with a new theory of physics. Give the rules and empirical predictions so it can be tested. But don't claim FTL is consistent with current tested physics AND avoids causality paradoxes. (Maybe current tested physics is wrong. Maybe causality paradoxes are resolvable some other way...Maybe the universe ceases to exist when someone tries this...or maybe FTL travel is just plain impossible) but the paradox must be resolved in any talk of FTL travel. You can't wish it away by finding a unique mechanism of propulsion.

As to the "Warp Drive" paper by Obousy and Cleaver note that the picture of Alcubierre metric is via the Kaluza-Klein model equivalent to putting a gauge field (say electromagnetic for analogy) around the ship. There will necessarily be a charge distribution effecting this field and affected by it. It seems to me equivalent of the old cartoon lifting oneself by one's own suspenders approach (i.e. ignoring a reaction force). Put a positive charge on a boom in front of the ship, a negative charge on a boom behind the ship, and a negative charge on the ship...ignore the force of the booms and the ship is "pulled forward" but account for the reaction force necessary to maintain the configuration and the ship just sits there.

This might be avoided by allowing the ship to surf a wave but it will also dissipate this wave and cannot accelerate past the wave which will have a fixed speed <= c w.r.t. any outside observer. This is no different than the idea of pushing a light-sail with lasers or microwave transmitters stationed at home. (Now that is an interesting bit of physics!)

The paper is a lot of "gee wizzery" when you look at it with half an eye but it is just plain bad physics no matter how esoteric a theory it invokes.

As to physics being "difficult" it is very hard to be rigorous and careful of bias. As to being boring... well anything is boring unless you develop a passion for it. You can't tempt them in with candy which they'll later find is made of wax. You'll turn them off in the long term. What is worse you'll possibly turn them against science convincing them it has no substance.

 

[Demystifier]

So we have a time loop. How a spaceship appeared there in a first place? It came from the outside. If you draw his world line, can it end with a circle, like in a character "P" ? No, because in the point where his 'entry" worldline is 'attached' to the circle the law of conservation of energy is violatated: 2 objects become one.

The world line has the shape of "O", not "P". The local energy-momentum conservation is not violated; you can interpret it as circulating.

So, for the sake of consistency and Novikov's principle after a full circle spaceship *must* appear at the same time, but in some other place. Time flows normally inside a spaceship and entropy always increases. So even there are timelike loops, matter can not make a full loop, it always spirals... Oly matter which originally was inside the loop can form a loop.

The consistency can be achieved even with true matter loops. The second law of thermodynamics cannot be globally valid and initial conditions cannot be chosen at will, but there is nothing inconsistent with it.

 

[Demystifier]

The Nikolic entropic time paper cited is IMNSHO being misinterpreted. It doesn't provide an "out" for FTL causality paradoxes. It rather seems to impose more restrictions on causal ordering which would imply an impossibility of globally FTL paths being causal even if they are locally STL.

I certainly agree with the last sentence above. But it also does something more. It explains why exactly self-consistent solutions (required by the Novikov's principle) look as paradoxes to us. They look so because we are used to think on time as something fundamentally different from space. Instead, if you think of time merely as a fourth spatial coordinate, and if you think of Universe as a static 4-dimensional object, then no paradox with self-consistent solutions remains. Further, the paper argues that the ONLY reason why we are used to think on time as something fundamentally different from space is the fact that there is a thermodynamic time arrow, while there is no thermodynamic space arrow. Since the thermodynamic time arrow is not fundamental at all (but only corresponds to a property of the specific solution in which we live), it means that there is no fundamental reason to think on time as something different from space (except for the irrelevant opposite sign in the metric signature), implying that there are no true paradoxes with self-consistent solutions.

 

[Dmitry67]

The world line has the shape of "O", not "P". The local energy-momentum conservation is not violated; you can interpret it as circulating.

No
If it is "O" then spacehip had never entered this area from outside, it always existed there.

 

[Demystifier]

No
If it is "O" then spacehip had never entered this area from outside, it always existed there.

Exactly. But such a solution is self-consistent, which is the only requirement.
Of course, your spiralling solutions are also self-consistent. Moreover, your spiralling solutions may be more interesting from the physical point of view. Still, O-solutions are possible as well.

 

[Mr. Paradox]

Exactly. But such a solution is self-consistent, which is the only requirement.
Of course, your spiralling solutions are also self-consistent. Moreover, your spiralling solutions may be more interesting from the physical point of view. Still, O-solutions are possible as well.

How exactly would it be possible?

 

[spock0149]

... Any FTL craft is a time machine in an appropriate frame OR you are breaking Lorentz covariance...

The basic idea of FTL travel is that you jump in a ship at point A and arrive at point B faster than light could travel by a direct path. Specifically you get ahead of the sphere of light emitted by the television coverage of your departure.

Saying one travels there via some modification of local space-time around the craft doesn't matter. If the arrival event at B is outside the future light cone of the departure event at point A then you can boost the whole mechanism of travel (assuming local Lorentz covariance) so that the arrival event is prior to the departure event and then reverse the boost and reverse the trip so as to arrive at A prior to the original departure.

Does anybody know if wormholes suffer from similar problems?

 

[Demystifier]

How exactly would it be possible?

You mean in practice, by acts of an advanced civilization? I am not saying that it is possible in that sense. All I'm saying is that such solutions of the equations of motion exist, so nature may (or may not) realize such solutions in reality. If you want, I can write such solutions explicitly. But if you are familiar with Fourier expansions of periodic functions, you can also do it by yourself.

 

[Dmitry67]

I add Dmitry67 conjecture :)
I state: There are no macroscopic objects without a history

An object without a history is an object, which exists exclusively inside a time loop. So it had been never created and its entropy does not always increase.

Remember my exaple with spiraling inside the time loop? I arrive inside a time loop and see a future copy of my ship. We fly close to each other, and someone arrives to my ship from a future ship. It is a guy called John Doe. he says "Hi, it is really cool here! I will never die, I am immortal! But don't forget to let me go after a full cycle, otherwise we create a causality paradox!"

I drink beer with him, and then I see my own ship - now it is a "past" copy of my ship. I let John Doe jump into that ship, saying farewell to him, then I hit a pedal to the metal and leave the time loop.

So who was that John Doe? He had no mother, father... How could he appear there in a first place? While it might be consistent with the Novikov's principle it sounds impossible.

So while world line of individual particles can be short cut, world lines of macroscopic objects never intersect and never form loops.

 

[Vals509]

i read this article somewhere too but its application will be unlikely as to travel FTL for 1 meter you need to have a size or mass (can't remember which one) the size of jupiter. obviously not feasible

 

[Demystifier]

While it might be consistent with the Novikov's principle it sounds impossible.

If something sounds impossible, it doesn't mean that it IS impossible.
It sounds impossible to you because you are used to think that all events have their cause in the past. But this is only an illusion, caused by the fact that we live in a very UNTYPICAL universe in which entropy increases with time. In reality, there are no causes and consequences. There are only solutions of the equations of motion.

 

[Dmitry67]

It sounds impossible to you because you are used to think that all events have their cause in the past. But this is only an illusion, caused by the fact that we live in a very UNTYPICAL universe in which entropy increases with time. In reality, there are no causes and consequences. There are only solutions of the equations of motion.

I agree with you regarding the nature of time.
However, I still don't believe in that John Doe.

The solution where during my spiraling I meet John Doe is mathematically possible, but the probability of it is not higher then the probability that a human being condense from atoms somehwere due to very lucky coincidence.

 

[Demystifier]

I agree with you regarding the nature of time.
However, I still don't believe in that John Doe.

The solution where during my spiraling I meet John Doe is mathematically possible, but the probability of it is not higher then the probability that a human being condense from atoms somehwere due to very lucky coincidence.

But humans DO exist due to a lucky coincidence (irrespective of the time-travel issue), don't they? Unless God created them, in which case She can also create them in a loop.
So, in a loop, either there will be no humans at all (neither John Doe nor yourself), which indeed is the most probable option, or there will be many "copies" of them.

 

[Dmitry67]

But humans DO exist due to a lucky coincidence (irrespective of the time-travel issue), don't they?

Correct, but there was 13 billions years to filter out 'useful' coincidences
Time loops (around Kerr singularities) are very short.

P.S.
I've just realized, what if the Big Bang is actually an END of the Universe - we live in the epoque when the entropy decreases? :)

 

[Demystifier]

Correct, but there was 13 billions years to filter out 'useful' coincidences
Time loops (around Kerr singularities) are very short.

Correct! So probably the loop will not contain any humans at all.

 

[Dmitry67]

Correct! So probably the loop will not contain any humans at all.

Except the ones coming there from outside
You CAN enter the time loop, even it is a loop.

 

[Demystifier]

You CAN enter the time loop, even it is a loop.

How?
Can you describe the topology of such a universe?

 

[Dmitry67]

How?
Can you describe the topology of such a universe?

Sure. Check the image attached. yes, I know, my drawings are really bad.
But I did it in Paint using a mouse...

So, you see a naked ring singularity. When you approach it due to frame dragging the 'hourglass' rotate finally forming a closed timelike loop (red). Still, you can enter it approaching to it from outside (green line)

In other words, the timelike loop region can be absolutely "naked", there is no horizon preventing from going inside or outside from it.

 

[Demystifier]

Hm!
If I understood you correctly, the trajectory of a traveller is described by both the green and the red curve. However, there is something inconsistent with it. The problematic point is the point at which the two lines (green and red) meet. At that point, the traveller has TWO different 4-velocities, which means that this "solution" is not self-consistent. How do you comment on this?

 

[Dmitry67]

No, red line just shows 1 example of a timelike loop. Timelike loop region contains infinitely many different timelike loop trajectories, including intersecting trajectories.

So red like is just an example of one of them.

Green line shows that he entered that area, then it depends on his decision what to do (so it did not show what happened next to him).

 

[Demystifier]

No, red line just shows 1 example of a timelike loop. Timelike loop region contains infinitely many different timelike loop trajectories, including intersecting trajectories.

So red like is just an example of one of them.

Green line shows that he entered that area, then it depends on his decision what to do (so it did not show what happened next to him).

Show me the whole green line (or at least a much larger part of it) because otherwise I cannot see that there exists any consistent green line at all! I want to see that the green line can make a loop (or something like that) in a consistent way.

 

[Dmitry67]

Ok, so the spaceship finishes a loop, sees his own 'old' copy and commits suicide hitting the singularity.

 

[Demystifier]

Fine, that's consistent!

 

[Mr. Paradox]

Wait so if you are in a time loop than how can you leave the loop if time repeats itself?

 

[Dmitry67]

Check the diagram
After you complete a loop, you end at the same time, but slighlty different position in space. Hence you don't need to repeat all you did before, you can do something different

 

[Demystifier]

Check the diagram
After you complete a loop, you end at the same time, but slighlty different position in space. Hence you don't need to repeat all you did before, you can do something different

This can also be rephrased as follows. You do two different things at the same time, but you also do them at two different positions in space, which makes it consistent.

 

[Dmitry67]

I've realized (and now I remember, I read somewhere about it before) that if there is a closed time-like loop (like the one I drew) then it is possible to go back in time just orbiting around the naked singularity (that is why physicists are so afraid of naked kerr rings?) - at least to the moment of the formation of the naked singularity.

In other words, if there is a closed time-like loop, the causality paradoxes (even we agree with the Demistifier that these paradoxes are not real) "leak" outside of that loop to the whole Universe.