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n2doc

(47,953 posts)
Thu Jan 30, 2014, 05:14 PM Jan 2014

The Quantum Mechanics of Fate

BY GEORGE MUSSER

"The objective world simply is, it does not happen,” wrote mathematician and physicist Hermann Weyl in 1949. From his point of view, the universe is laid out in time as surely as it is laid out in space. Time does not pass, and the past and future are as real as the present. If your common sense rebels against this idea, it is probably for a single reason: the arrow of causality. Events in the past cause events in the present which cause events in the future. If time really is like space, then shouldn’t events from the future influence the present and past, too?

They actually might. Physicists as renowned as John Wheeler, Richard Feynman, Dennis Sciama, and Yakir Aharonov have speculated that causality is a two-headed arrow and the future might influence the past. Today, the leading advocate of this position is Huw Price, a University of Cambridge philosopher who specializes in the physics of time. “The answer to the question, ‘Could the world be such that we do have a limited amount of control over the past,’ ” Price says, “is yes.” What’s more, Price and others argue that the evidence for such control has been staring at us for more than half a century.

That evidence, they say, is something called entanglement, a signature feature of quantum mechanics. The word “entanglement” has the same connotations as a romantic entanglement: a special, and potentially troublesome, relationship. Entangled particles start off in close proximity when they are produced in the laboratory. Then, when they are separated, they behave like a pair of magic dice. You can “roll” one in Las Vegas (or make a measurement on it), your friend can roll the other in Atlantic City, N.J., and each die will land on a random side. But whatever those two sides are, they will have a consistent relationship to each other: They could be identical, for example, or always differ by one. If you ever saw this happen, you might assume the dice were loaded or fixed before they were rolled. But no crooked dice could behave this way. After all, the Atlantic City die changes its behavior depending on what is going on with the Las Vegas die and vice versa, even if you roll them at the same moment.

The standard interpretation of entanglement is that there is some kind of instant communication happening between the two particles. Any communication between them would have to travel the intervening distance instantaneously—that is, infinitely fast. That is plainly faster than light, a speed of communication prohibited by the theory of relativity. According to Einstein, nothing at all should be able to do that, leading him to think that some new physics must be operating, beyond the scope of quantum mechanics itself.

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http://nautil.us/issue/9/time/the-quantum-mechanics-of-fate

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The Quantum Mechanics of Fate (Original Post) n2doc Jan 2014 OP
Essentially, they're trying to replace non-locality with retrocausation mindwalker_i Jan 2014 #1
If we're going to be able to send info back in time FiveGoodMen Jan 2014 #2
No, it wouldn't mindwalker_i Jan 2014 #3

mindwalker_i

(4,407 posts)
1. Essentially, they're trying to replace non-locality with retrocausation
Thu Jan 30, 2014, 06:24 PM
Jan 2014

It does very much seem like, when one particle of an entangled pair is measured, it retroactively makes the whole sequence look like the particles had their values the entire time. A recent experiment created two pairs of entangled photons. One from each pair were measured for polarization, and the other two would travel through the aparatus (in other words, there was a delay), then a measurement COULD be done on those two photons that would entangle them, swapping the entanglement such that the two photons that had already been measured were entangled. The result was that, when the entanglement swapping was done, the polarizations of the two measured photons correlated.

This demonstrates that entanglement doesn't just involve instantaneous communication, or at least that's my take on it. There was some sort of explanation denying that conclusion that had to do with looking at it from the photons' point of view, and I didn't understand it.

There have been a lot of explanations as to why one can't use entanglement for communication and a bunch of proofs. I haven't come anywhere near the proofs! However, the explanations say that, when one particle is measured, it take on a value at random. The other particle also takes on a value that is correlated to the (random) value of the first particle. As such, one can't distinguish between measuring a particle first (and getting a random value) or measuring it after the first (and getting the compliment of a random value).

However, the double-slit experiment essentially measures the superposition state of photons (or particles). My hope is that, by measuring entangled photons not going througha double slit, it will affect the photons that do and destroy (or at least damage in a measurable way) the interference pattern. There is already reason to believe this may work since Birgit Dopfer essentially did that in 1998. However, she filtered out just the entangled pairs with a "coincidence detector" so there was another connection between the photons other than their entanglement. I believe that under the right conditions, meaning good detectors (high efficiency), a high enough percentage of photons in the beams being entangled (like 20+%), and some luck in forms that I have not et determined, it may be possible to demonstrate communication with entanglement.

If that happens, sending information backwards in time can be demonstrated, and I don't think it's an exaggeration to state that all hell will break loose.

mindwalker_i

(4,407 posts)
3. No, it wouldn't
Thu Jan 30, 2014, 07:11 PM
Jan 2014

The information can only be sent back to when the two particles are created (and the rest of the apapratus is built and working), and one can't go back further than that. In fact, to make the information go backwards, one of each entangled pair, or alternatively, one of the beams, has to be delayed. From what I've heard, one beam can go through about 5km of fiber optic before the entanglement is broken on average, which amount to 50us. Going through free space could allow for a longer delay but has a whole new set of problems.

One way that might allow us to get past 50us is to build two of these things, and stagger their usage by half a period. Then, a message could be sent through one, move forward half a perion, be sent through the other, move forward half a period, etc. The amount of time that a message could jump would be limited by the accuracy of the transmission, which is essentially an engineering problem.

However, you would still be limited by when the two "jumps" had been built.

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