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Comment by gokhan | Hacker News Reader

A comment of the article by Joey N correctly notes that:

Right now, on the top of Mount Everest, time is passing just a little bit faster than it is in Death Valley. That's because speed at which time passes depends on the strength of gravity. Einstein himself discovered this dependence as part of his theory of relativity, and it is a very real effect.

"Just to be clear, the clock in Death Valley appears to run slowly from the point of view of the eagle-eyed observer standing on top of Everest.

To a person in Death Valley, the clock appears to tick at precisely the rate it would appear to tick if both person and clock were on top of Everest."

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But how about a person in the center of the Atlantic listening to a metronome broadcast from Death Valley and one from Mt. Everest, tick-tick-ticktick-ticktick-tick-a-tick-tick-a-tick :-)

He's really bored.

And death valleys ticks slower. He's at sea level, which is higher than Death Valley.

I always wondered what happens to quantum entanglement when time dilation is involved. For example if one of the particles falls into a black hole, can we get info out?

As I understand it, you can't actually use this to transmit information. If you entangle two particles, and then send one away, and attempt to measure the appropriate property (I think it's spin), hurray: you know that the other particle's spin is the opposite.

But that's not communication. It's like slicing a coin down the middle without looking at it, hiding each half in a box, and giving one of the boxes to someone going to the moon. A week later, you both open your box, and you know exactly which side of the coin the other person has - but that's not communication.

(Someone smarter than me, please correct whatever mistakes I've made, and explain this in a better way.)

Wouldn't changing the spin be the way to communicate? Is spin not a changeable property?

Nope, the spin isn't changeable and you cannot communicate via entaglement. If the spin was changeable however we'd have a long distance bit (0 or 1).

Wikipedia says that spin can be changed:

http://en.wikipedia.org/wiki/Spin_(physics)

"Although the direction of its spin can be changed, an elementary particle cannot be made to spin faster or slower."

Spin can be changed, but then you disentangle the two particles. So still no transmission of information.

Isn't disentanglement a form of transmission of information? I.e. if suddenly my particle disentangles, I can assume it's been manipulated? (which is a binary state of tangled/untangles (0,1))

And how can you tell if the particles are no longer entangled without observing both? It's not like there's some outward, observable property of the particle that says "I am entangled". It's simply a statistical correlation between the states of the two particles.

Interesting. Excuse my ignorance(which is certainly present), just trying to learn.

Can you expand on the last part about statistical correlation?

The correlation is that you know from your reading what the other person will detect. For example, if you detect a spin up particle, they will detect spin down. But you don't know when they have, or if they have, done the detection.

So, the statistical correlation between states of particles is literally the fundamental thing that defines quantum entanglement.

Suppose you and I both get a coin and we flip them. You then turn over your hand and reveal heads. If the two coins are "entangled", there is now a greater than 50% probability that my coin will be tails, which would not be the case if the coins weren't entangled.

Quantum entanglement works essentially the same way. "Flipping the coin" is equivalent to observing some state of the particle, like spin. In both cases, entanglement is evidenced by the correlation between the states, nothing more.

Now, imagine we had entangled coins and we were in separate rooms. We flip our coins and you observe yours, noting a heads. In my room, I can't know what you did or didn't observe. So when I observe my coin and see tails, I don't realize it's a consequence of your observation. From each of our individual perspectives the coin flip looks perfectly random. This is why information transfer isn't possible through quantum entanglement.

I now see the relation to the slit experiment.

Very interesting, thanks a lot to everyone who responded!

You would know when you have two particles entangled at the moment of entanglement. You could know you have a half of a pair of entangled particles when the other half is sent away. But you could never know when the pair has become disentangled.

That's the common explanation, but its apparently wrong. There is not a fixed state of each 'quarter' in each 'box'. They are truly not determined until one is resolved; then the other is resolved simultaneously. Still, not communication.

> There is not a fixed state of each 'quarter' in each 'box'. They are truly not determined until one is resolved;

Isn't that unknowable? And aren't both situations (fixed vs undetermined) effectively the same from an observer's point of view?

No. What you are calling 'fixed' is a local hidden variable - an internal state that has yet to be measured but is already determined. This state of affairs is ruled out by Bell's Theorem - https://en.wikipedia.org/wiki/Bell%27s_theorem

That's only true if you take the Copenhagen interpretation. There are other interpretations (like 'consistent histories' for example). See: http://en.wikipedia.org/wiki/Copenhagen_interpretation#Alter...

Other interpretations of quantum mechanics do not generally violate Bell's Theorem, since it appears to be confirmed by experiment. There are other interpretations which choose to abandon locality rather than hidden variables, such as the De Broglie-Bohm pilot wave interpretation. But consistent histories doesn't even do that. It's actually pretty similar to the Copenhagen interpretation.

I didn't say anything about violating Bell's theorem. Yes pilot wave theory is a better alternative that doesn't require spooky action at a distance but consistent histories doesn't require wavefunction collapse (which is a huge source of spookiness as well).

> then the other is resolved simultaneously.

Relativity of course makes a hash of that.

Still it's as close as you'll get with a classical analogy, and if you only have access to a fixed basis is entirely correct. Expanding beyond the classical explanation is only truly necessary when you start measuring in different bases.

Or by many worlds: the quarter is both ways in the box. When you each open your boxes, you yourself are pulled into the doubling. Now there are two pairs of you-and-them. In one pair, you hold heads and they hold tails. In the other pair, you hold tails and they hold heads.

Thinking about this makes it clear how communication is impossible. You will always get both answers - for different values of "you".

Isn't it possible for the astronaut to also know whether the other person has opened their box? If yes, then this is where the analogy breaks down. However I'm probably remembering incorrectly. I just thought there was some property like "It's possible to detect whether the other person has made an observation yet."

I'm probably mistaken, because if this were possible, then you could use it as a 1-bit communication. "If you detect that I've opened my box, then blow up the moon. Otherwise, do nothing."

Time to re-study quantum mechanics... It's strange how easy it is to forget almost everything just by not thinking about it for two years. MIT OCW had some pretty great online courses on the subject.

> Isn't it possible for the astronaut to also know whether the other person has opened their box? If yes, then this is where the analogy breaks down.

The answer is no, and so the analogy doesn't break down. You can't observe the other person except by speed-of-light information transfer.

I asked a similar question on physics stack exchange a few years ago, but I haven't seen a definitive answer. http://physics.stackexchange.com/questions/8708/can-observat...

If you ran two two-slit experiments on entangled particle pairs, shouldn't you be able to detect if your counterpart was observing theirs?

> Isn't it possible for the astronaut to also know whether the other person has opened their box?

How would you know without observing the particle yourself? In which case, how do you know if it collapsed because of your observation or the other person's?

Not an expert by any means, but from what I'm told you can't use it to transmit information, so that's how I think of it.

http://en.wikipedia.org/wiki/No-communication_theorem

Has a nice analogy, that being two radio listeners do know what each other are hearing which is cool, but can't communicate anything between them.

I would have to think about the effect of creating a one time pad.

I've heard of that analogy before Brian Greene uses two boxes with a pair of gloves one box with the left glove and the other box the right glove.

It's kind of like when you go to a doctor and have tests done for a disease if the results are bad, positive, they call you and if the results are negative no call.

At some point a lab tech checks the sample and if it's negative for the disease they know at that moment but you know by the lack of information at some point in time; i.e. no phone call or letter.

The negative results are more like the quantum communication due to the lack of interaction than the positive results. Mine may not be not the best analogy.

Quantum entanglement cannot communicate information, regardless of any other factors. See http://en.wikipedia.org/wiki/No-communication_theorem

Probably nothing more than usual. The local operations you can do to entangled particles are all commutative w.r.t. the operations applied by the other side. So whether they happened at the same time or before or after or slower or faster than the other side doesn't affect the local outcome

(That should also make it clear why you can't use entanglement to communicate instantaneously: the other side must get the same results whether they measure before or after you've sent your message. So if entanglement allowed communication instead of just esoteric types of coordination, you could use it to send messages backwards in time.)

Also may depend on what kind of watch you wear, like quarts watches are sensitive to magnetic field.

Sadly no. That would be a convenient and nifty way to vary frequency for ham radio applications if it were the case. Even just a couple dozen PPM in a magnetic field would be sufficient for transmit receive switching but it just doesn't work. Also if that were the case mounting a crystal nearby a power transformer or fan would generate massive noise in the output signal unless magnetically shielded, which also doesn't happen.

I would imagine magnetizing a mechanical watch would really mess things up, so at a "just under really messed up" threshold it would probably have the claimed effect.

You may be thinking of YIG garnet oscillators which are very dependent on magnetic field but they haven't been common tech in maybe 30 years. Even used on ebay they cost more than modern new microwave VCOs which is annoying. I have a box of old YIG osc in my basement which I actually used for ham radio experiments before modern solid state VCOs took over.

Yeah, they're not sensitive to magnetic fields. But they do drift with temperature. Which leads to fancy semi-improv solutions, like placing the quartz in a small case, together with a heating element and a temperature sensor, to keep that sucker incubated at constant temp. A lot of HAMs used to do this back in the day - or maybe they still do, I dunno.

Also, placing a small variable capacitor in series with the quartz is an easy way to tune it, if it's too fast. I actually have a wall clock that looks nifty but is too fast, and I'm gonna tune it by trial and error, inserting small caps in series with the crystal.

The heating setup is still used today. In fact, you can get integrated components called OCXOs (Oven Controlled Crystal Oscillator) that do the whole job in one small package with precision over time on the order of 1ppb. Cheaper, and less precise, are TCXOs (Temperature Compensated Crystal Oscillators) where a crystal is paired with a circuit that has an opposite dependency on temperature so that the errors tend to cancel each other out.

> Yeah, they're not sensitive to magnetic fields. But they do drift with temperature

Also, gravity: https://www.youtube.com/watch?v=zILwgQhjC_Q

Degauss? You can speed up some clocks with degauss. So isn' it sensitive to magnetic field?

Relativistic time effects are changes in the rate of time itself, not changes in the mechanism of a clock. If there was a magnetic field effect, that's a different, additional thing.

Yeah clock, not time.

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