The Future Predestined? Near Lightspeed Travel & Quantum Teleportation

[blackwizard]

Lets say we mastered near lightspeed travel and instantaneous communication using quantum teleportation. Then we send the spaceship off to someplace 1 lightyear away. It turns around and for a brief moment travels twords Earth at near lightspeed. During this moment the spaceship asks Earth how things are goin and Earth responds. The spaceship reduces it velocity to 0 relative to Earth now the spaceship has information about Earth from 1 year in the future, because of the change in the alignment of its inertial frame, and has the ability to tell Earth all about it.

Can any1 see a flaw in that logic? Otherwise it seems that everythin is predestined.

 

[Gelsamel Epsilon]

The first and foremost flaw is that you your conclusion begs the question. If and only if we ever master near lightspeed travel and quantum teleportation (these things may very well be impossible). I'm sure there may be other flaws but I'm too lazy to think about it (and I probably won't find them). But that is the biggest problem with your situation.

 

[DaveC426913]

The first and foremost flaw is that you your conclusion begs the question. If and only if we ever master near lightspeed travel and quantum teleportation (these things may very well be impossible). I'm sure there may be other flaws but I'm too lazy to think about it (and I probably won't find them). But that is the biggest problem with your situation.

Fair enough, but this is only a difficulty in scale and possibility, not a flaw in logic.

With what we know today, is there any fdlaw in the logic, and thus any reason it cannot be done in theory?

 

[JesseM]

Fair enough, but this is only a difficulty in scale and possibility, not a flaw in logic.

With what we know today, is there any fdlaw in the logic, and thus any reason it cannot be done in theory?

Quantum teleportation does not allow for FTL communication, even in theory--in fact I'm pretty sure it's been proven that quantum effects can never be used this way. So your thought-experiment is contradicting known laws of physics, although you're right that relativity says that if FTL communication were possible somehow, and it worked the same way in all inertial frames, then it would also be possible to send information into your own past.

 

[Tzemach]

In 1935; Einstein, Rosen and Podolsky (EPR) presented a famous paper its title was “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” They considered what Einstein called the "spooky action-at a- distance communication” that seemed to be part of Quantum Mechanics, and concluded that the theory must be incomplete if not outright wrong. Bell's Theorem also seems to indicate the probability of faster than light (instantaneous) communication, I think it is too early to write off any of these concepts.

The only problem with our spaceship is that if it traveled at very close to light speed for a distance of say fifty light years the communication that it would receive from Earth would be the 1956 Olympics from Melbourne, Australia.

 

[JesseM]

In 1935; Einstein, Rosen and Podolsky (EPR) presented a famous paper its title was “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” They considered what Einstein called the "spooky action-at a- distance communication” that seemed to be part of Quantum Mechanics, and concluded that the theory must be incomplete if not outright wrong. Bell's Theorem also seems to indicate the probability of faster than light (instantaneous) communication, I think it is too early to write off any of these concepts.

Bell's theorem and the EPR results indicate that if there are hidden variables there must be a sort of FTL coordination (or backwards-in-time coordination, perhaps) between entangled particles, but most interpretations of QM do not feature hidden variables. If you use something like the many-worlds interpretation of QM, for example, than the results of the EPR experiment can be explained without the need for anything to travel faster than light. In any case, even if the particles themselves are somehow exchanging hidden signals faster than light, I'm pretty sure it's been shown that experimenters can never actually use these effects to send their own signals faster than light, in much the same way that even if you believe in hidden variables and the idea that particles actually do have well-defined positions and momentums at all times, the uncertainty principle shows that we can never measure them simultaneously to arbitrary accuracy unless the theory of quantum mechanics is totally wrong. For example, see quant-ph/9801014 for a possible proof that the no-cloning theorem forbids superluminal signalling, although this is disputed in quant-ph/980303 and I don't know enough about the subject to evaluate the arguments.

The only problem with our spaceship is that if it traveled at very close to light speed for a distance of say fifty light years the communication that it would receive from Earth would be the 1956 Olympics from Melbourne, Australia.

But if you could send a signal that was FTL in the Earth's frame, in the ship's frame the ship might receive the signal at an earlier time than the signal was sent from the earth. And if the ship immediately sent a reply that was FTL in the ship's frame, in the Earth's frame this reply might arrive at Earth at an earlier time than the ship sent it, with it being possible that the Earth would receive the reply before it sent out the original signal. Of course this is assuming that FTL would work the same way in every reference frame, if there were a preferred frame for FTL signals (a frame in which no FTL signal can ever arrive before it is sent) then you could have FTL without violating causality, at the expense of destroying Lorentz-symmetry.

 

[DaveC426913]

Quantum teleportation does not allow for FTL communication, even in theory--in fact I'm pretty sure it's been proven that quantum effects can never be used this way.

*slaps forehead* I knew that.

 

[selfAdjoint]

Lets say we mastered near lightspeed travel and instantaneous communication using quantum teleportation. Then we send the spaceship off to someplace 1 lightyear away. It turns around and for a brief moment travels twords Earth at near lightspeed. During this moment the spaceship asks Earth how things are goin and Earth responds. The spaceship reduces it velocity to 0 relative to Earth now the spaceship has information about Earth from 1 year in the future, because of the change in the alignment of its inertial frame, and has the ability to tell Earth all about it.

Can any1 see a flaw in that logic? Otherwise it seems that everythin is predestined.

What you have done is rediscover the theorem in SR that if you have FTL motion or communication - in your case instantaneous - then some observers will see you moving or communicating into your own past. It's a perfectly correct theorem, and when I say "see" I mean you really can get news from the future this way.

That's SR. It has not yet been proven in QM or anywhere else that nature truly forbids FTL. In my opinion the QM conclusions are too broad for the narrow experimental result. Consider the delayed choice quantum eraser, on which there is a long thread in the Quantum Mechanics forum. Does it or doesn't it show the electron "knowing" what the future choice in the experiment will be?

 

[blackwizard]

I know SR forbids ftl travel for an object with mass. Iv only just come to grips with SR so I am really just speculating but i can't see how one would deduce that information also can't b sent ftl or even instantaneous.

My understanding of QM is limited to the concepts and the basic 1s at that. From what iv read I am quantum computation, information is stored in the state of the qubits.

This site talks about Quantum Teleportation:

http://researchweb.watson.ibm.com/quantuminfo/teleportation/

So if like this the state of an atom can be teleported 2 the atoms (qubits) of the spaceship computer. couldn't that be used as instananeous communication. I am not 100% on the instantaneous bit.

I think the practicality of near light speed travel is irrelevant, its possible

 

[JesseM]

This site talks about Quantum Teleportation:

http://researchweb.watson.ibm.com/quantuminfo/teleportation/

So if like this the state of an atom can be teleported 2 the atoms (qubits) of the spaceship computer. couldn't that be used as instananeous communication. I am not 100% on the instantaneous bit.

I think the practicality of near light speed travel is irrelevant, its possible

No, because quantum teleportation always requires a "classical channel" where information about the system being teleported is transported by normal classical means like a light signal--without this information the people at the other end won't know what measurements to perform to create the teleported quantum state. In the diagram on the page you linked to, the classical channel is represented by the white line titled "send data". They also refer to it indirectly in their description of the process (I put the part about the classical channel in bold):

By contrast, in quantum teleportation, two objects B and C are first brought into contact and then separated. Object B is taken to the sending station, while object C is taken to the receiving station. At the sending station object B is scanned together with the original object A which one wishes to teleport, yielding some information and totally disrupting the state of A and B. The scanned information is sent to the receiving station, where it is used to select one of several treatments to be applied to object C, thereby putting C into an exact replica of the former state of A.

And the more detailed wikipedia article on quantum teleportation says:

The teleportation scheme combines two "impossible" procedures. If we remove the shared entangled state from Alice and Bob, the scheme becomes classical teleportation, which is impossible as mentioned before. On the other hand, if the classical channel is removed, then it becomes an attempt to achieve superluminal communication, again impossible (see no communication theorem).

And the article on the "no communication theorem" linked to there is relevant to the subject of instantaneous communication too...at the end it says:

The no communication theorem thus says shared entanglement alone can not be used to transmit quantum information. Compare this with the no teleportation theorem, which states a classical information channel can not transmit quantum information. (By "transmit" we mean transmission with full fidelity.) However, quantum teleportation schemes utilize both resources to achieve what is impossible for either alone.

 

[blackwizard]

I remember the "by applying to C a treatment depending on the scanned-out information" bit now!

Does that mean their talk about a Quantum Internet is a load of BS? Considerin the isp guys would hav 2 phone you up and tell you how 2 treat each qubit.

(This has become more about QM than SR now!)