Comment by cambalache | Hacker News Reader

Comment by cambalache | Hacker News Reader

Parent Full thread

No communication is possible BUT, I have this toy scenario where a "quasi-communication" may be possible in FTL speed.

"2 generals command 2 armies hundreds of km apart. They want to attack a common enemy, there are 2 options, A) And all-front attack. B) From the flanks. The generals want the plans to remain uncertain until the last second before the attack. So from an intermediate point, they send two "entangled coins" , one to each general, the coins will arrive at both sites at the exact start of the battle.Both will show the same face when "measured". The generals have agreed previously that if they turn out "heads" they will both attack from the flank, in the other case, they will do a front-attack.

Of course you dont need a quantum system for this, you could have agreed on other stuff (like it if it is raining that day at certain place or sending a framed coin by regular mail) but I think the quantum solution is the more elegant, assuming no 3rd party snooping.

There are game theoretical proofs that show that you can use entanglement to enable better-than-classical performance on games that reward cooperation but disallow actual communication.

https://en.m.wikipedia.org/wiki/Quantum_pseudo-telepathy

Sorry but this example doesn't make sense, how it is faster than light?

I think key is that both generals have already agreed on a predetermined "meaning" of each coin flip outcome, rather than having to communicate it on a different, slower medium after the coin has been flipped.

So really it just front-loads that portion (I.e. removes it from consideration as part of the proposed solution) instead of allowing it to slow down solving the overall problem. It requires precomputation/agreement beforehand, rather than during the time allotted to the problem.

The speed of that precomputation would still be unchanged, and still be the slowest portion.

Yes of course that is why it is not real communication, but in a sense is better than a classical solution because nobody actually knows the final decision until the last second. Of course you could get philosophical and say that an enemy can intercept the quantum coin and measure it, thwarting the plan, so in that sense is the same thing that sending just a pic, but then you will be pulled by the same kind of discussion the physicists has spent the last 100 years doing. Check for example the correspondence between Einstein and Born https://www.amazon.com/Born-Einstein-Letters-1916-1955-Frien...

It is not faster than light. I think what he wanted to show was a situation, in which entanglement and quantum mechanics, are superior to classical physics. i.e someone using quantum mechanics would have an advantage over someone who don't.

However, as he said, in this situation the two generals could have agreed on something else, like if it was raining or not.

In fact, something they could have done is to flip a coin, and split with a copy of the result, which they only look at when they launch the attack. It would have the same effect.

With a random variable (the coin flip), that is hidden until revealed, we achieve the same results as quantum mechanics.

Scientist call them "hidden values" and Einstein hopped we could explain the "spooky action at a distance" with such hidden variables. But we can't, Bell proposed an experiment with entangle state which measurement could not be explained by such hidden variables, and Aspect did the experiment and obtained the predicted results.

So there exist situation where we can use entanglement to achieve better results, for example in "non-local games", where players sharing entangled state can win with probability 1, when "classical player" with probability < 1

“ So there exist situation where we can use entanglement to achieve better results, for example in "non-local games", where players sharing entangled state can win with probability 1, when "classical player" with probability < 1”

Do you have an example for this? This is really interesting

It's called "pseudo-telepathy." The wikipedia page has an example of a game:

https://en.m.wikipedia.org/wiki/Quantum_pseudo-telepathy

> In fact, something they could have done is to flip a coin, and split with a copy of the result, which they only look at when they launch the attack. It would have the same effect.

Yes but the difference is that in this case, the plan already exists from the moment the coin is flipped until the result is revealed to the generals. In the QM case no one (not even God) knows what the result will be until it is actually measured,but once that happens the result is known "instantly" at both places (assuming both coins arrive at the same time), so in that sense is a FTL communication (of course it is not real communication)

Ah yes, you're right, I hadn't seen it from that angle.

The "communication" of the final decision is FTL. If you choose a non-QM method, like checking it if rained in some place or using the pic of a flipped coin, that final decision can only be "communicated" at a sub-luminar speed.

Reply on news.ycombinator.com