Faster than light and time travel

No, the problem is, that what you read about the state of the entanglement at the receiving end does not tell you anything - unless and until you communicate with the source - which you can only do at luminal or subluminal speeds.
Okay, let me take this hypothetical scenario. Interstellar travelers from Earth are going to Alpha Centauri, and with them they take a quantum-entanglement Internet link to Earth, so they can communicate in a reasonable amount of time (assume they have no FTL technology, that is way too speculative for this era and forum).

For our purposes, let's assume that information is sent from Earth to Alpha Centauri with a binary system, which at Alpha Centauri is "flipped over" so to speak in order to cancel out the flipping which happens with quantum entanglement. So 0s on Earth are converted to 1s in entanglement which are converted to their true form of 0 by the computer. The computer was programmed to do the conversions.

So does this mean that the programmer who wrote the program to "make sense of "the entanglement results, and the fact that the skills to use a computer to "make sense of" the data were taught to the crew, was the necessary subluminal communication component? How does it work, I don't really have a physics background so I don't understand it too well.

DrGreg
Gold Member
All you can do with entangled particles is read what state they are in. As soon as you try to write a new state, the particles are no longer entangled.

If the spacetime continuum is an unviolated principle of nature, then faster than light travel requires backwards time travel. A violation of SR would be a violation of the one or more assumptions about spacetime, as presently defined. If the neutrinos are verified to travel faster (in a local frame) than light speed observed in a vacuum (in a local frame), then given that the scientists at CERN have measured a positive velocity in their experiments, then discovery of FTL travel would be directly incompatible with the existence of a space-time continuum, as presently defined.

If the spacetime continuum is an unviolated principle of nature, then faster than light travel requires backwards time travel.
It requires backwards time travel in some frames of reference but not in all frames of reference.

given that the scientists at CERN have measured a positive velocity in their experiments, then discovery of FTL travel would be directly incompatible with the existence of a space-time continuum, as presently defined.
That the scientists at CERN have measured a positive velocity in their own rest frame wouldn't mean that the velocity is positive in every frame of reference.

It requires backwards time travel in some frames of reference but not in all frames of reference.

That the scientists at CERN have measured a positive velocity in their own rest frame wouldn't mean that the velocity is positive in every frame of reference.
What do you call a velocity that is "positive" when going from point A to point B and "negative" when going from point B to point A? (Hint: It is not velocity as measured by an observer relative to that observer.)

And the focus here anyway is the velocity of neutrinos measured from the laboratory frame in which point A and point B are fixed coordinates. This is what actually matters. The scientists at CERN didn't discover that the neutrinos travelled at 60 m/s now did they?

And how exactly does one travel such that otherwise backward time travel is not observed as backwards? Wouldn't such a traveler be traveling backwards in time too?

What do you call a velocity [...]?
The derivation of the position with respect to the time.

And the focus here anyway is the velocity of neutrinos measured from the laboratory frame in which point A and point B are fixed coordinates.
If you want to have "backwards time travel" you will have to change the frame of reference.

The derivation of the position with respect to the time.
Not that I disagree with that, but you obviously misunderstood what I meant by positive and negative. There are three spatial dimensions and one time dimension. So let's guess what I meant by positive and negative.

That's right, the line crossing point A and point B in the experiment determines which direction is positive and which one is negative.

If you want to have "backwards time travel" you will have to change the frame of reference.
I'm not talking about comparing frames of reference. We are discussing velocity respect to the laboratory frame, not acceleration with respect to an arbitrary observer. This is where we address the measured speed of neutrinos, which is still being debated.

Perhaps I've got this wrong, and the tachyons (or other alleged faster-than-light particles) would simply be aging in reverse (time reversal limited to the "internals" of the particle, if any) without having to arrive at the detector before being fired from the accelerator?

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All you can do with entangled particles is read what state they are in. As soon as you try to write a new state, the particles are no longer entangled.
Yes, that is what I was trying to say. Manipulating the atoms would end the entanglement and make interstellar Internet impossible.

On the upside, if any FTL technology for spacecraft is discovered (I will not go into too much detail, trying to avoid over-speculation) postal services will thrive.

I'm not talking about comparing frames of reference. We are discussing velocity respect to the laboratory frame
Than your statement "faster than light travel requires backwards time travel" is wrong.

DaveC426913
Gold Member
So does this mean that the programmer who wrote the program to "make sense of "the entanglement results, and the fact that the skills to use a computer to "make sense of" the data were taught to the crew, was the necessary subluminal communication component? How does it work, I don't really have a physics background so I don't understand it too well.
You generate a stream of entangled particles. For simplicity's sake we'll say the information in them is 1s and 0s. Here's the first 3 pairs:
p1a+p1b, p2a+p2b, p3a+p3b
You do not yet know which of p1a+p1b is a 1 and which is a zero.

Now, you send one of the pair off to A. Centauri and keep one for yourself.

You observe your p1a and see it is a 1. You now now that A. Centauri just received a 0.
You observe all your particles and see that they are 1,0,1. You now know that A. Centauri has received the sequence 0,1,0.

The particles at A.Centauri did not have a defined sequence at all until you onbserved yourse, at which piont the aprticvles at A. Centauri somehow instantly became 0,1,0. That is the spooky part that happened instnatly across 4 light years.

But what use is the sequence of 010 to you or to A.Centauri? The sequence of 1's and 0's occurred before you could observe them, so it carries no information that you could encode in it.

Than your statement "faster than light travel requires backwards time travel" is wrong.
This is a statement I didn't agree with anyway.

If the spacetime continuum is an unviolated principle of nature, then faster than light travel requires backwards time travel
If A implies B, my response would be, not B, and therefore not A.

http://en.wikipedia.org/wiki/Tachyon

Wikipedia said:
As noted by Gregory Benford, among others, special relativity implies that tachyons, if they existed, could be used to communicate backwards in time[7] (see Tachyonic antitelephone article).
A violation of what special relativity implies for a given situation is more palatable than a violation of causality, in my view.

If A implies B, my response would be, not B, and therefore not A.
That is correct but with limitation to a single frame of reference A doesn't implies B.