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Comment by sliken | Hacker News Reader

A staple of sci-fi has been a pair of boxes with quantum entangled particles inside allowing FTL communications once the boxes travel (sub-FTL) to their destination.

By my understanding that's impossible, although I always wondered if you could coordinate FTL, by detecting when a particle was no longer entangled.

Can someone with the background comment?

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Caveat: I only have a B.Science degree in Engineering Physics (Colorado, USA) and hobbyist-type research in PhD-level physics.

I've always found it easier to explain quantum entanglement thusly:

Imagine you have 2 cubes that alternate glowing between purple and orange. And they stay in one color for a random amount of time (100s of milliseconds). Holding both in your hands, you would just have randomly color-shifting cubes. But a neat trick of these cubes is that when you close your eyes, hit them together, and quickly put them into separate closed boxes, you know that three things will happen:

1. they will be locked into one color as long as the box stays closed (no more alternating)

2. each cube will be the opposite color from each other (when one is purple, the other is orange, and vice versa)

3. when you open the box and observe the cube, it will stay in the current color for 1 second, and then resume its randomness once again.

So you take one of the boxes containing a cube and ship it to your friend on the Moon (greater than 1 light-second away). When she gets the box, you call her on your phone and ask her to wait. Then you open your box and see that your cube is purple. And then the cube starts blinking again because the entanglement is broken on your end. So you wait a few seconds and say to her "open your box and I bet you the cube is orange". She does so and confirms the cube was glowing orange before resuming the random blinking. Now the entanglement is broken on her end, but you "knew" that by having the purple cube in your possession that hers would be orange.

Using this system, there is no way to exploit your knowledge of what the colors will be once observed (so sorry, no FTL communication here). The particles no longer influence each other after they are entangled at the start. It's not an active process, but rather a very specific type of setup. But it still bothers physicists (most famously Einstein), that you could "know" something without direct proof.

And people in general hope that this un-intuitive phenomenon will open up whole new worlds of sci-fi tech. But no, as the article explained: "Nothing we knew suggested this goal was unachievable. The significance of this news is not that it was unexpected or that it overturns anything previously believed, but simply that it’s a satisfying culmination of years of hard work."

If you had shipped the other cube-box to the opposite side of the galaxy, thousands of years after you were long dead, someone would open it and still find an orange cube.

i fail to see whats mysterious about knowing something without direct proof. You could put a literal orange cube in a box, ship it across the galaxy, and you'd "know" anyone opening the box would be an orange cube. You don't need to be present at the unboxing to know that with certainty. How is this different? Am I missing something?

That this example has very little to do with entanglement, which is not merely "knowing without direct proof". We do that everyday.

The same way, the issue with Shroedigger's cat is not that "the cat is in a single state, but we just don't know whether that's dead or alive unless we open the box" (as many laymen try to explain it).

That's just "I don't know yet" and has nothing to do with the issue Shroedigger tried to highlight or quantum mechanics (not to mention that this we encounter everyday and we can replicate perfectly with say, a box and a person that throws a dice and based on that releases or not a gas in the box with a cat -- that's not what the paradox is about).

For entanglement, see this:

https://www.sciencealert.com/watch-this-is-how-quantum-entan...

If I understand right, this starts to touch on Bell's Theorem.

The description as is means that the situation is identical if there's "spooky action at a distance" or if there's none and you just don't know what's in each box until you open one (and therefore can guarantee what's in the other). There are predictions from this however that when tested just don't work. There is something else going on. I would go further but I'm sure my explanations would contain huge inaccuracies that probably would cause more problems with understanding what's happening so will link you to a couple of things:

https://en.wikipedia.org/wiki/Bell%27s_theorem

http://www.askamathematician.com/2010/06/q-how-it-is-that-be...

I'm sure there are others here who are well versed in this who can chip in.

Allow me to give a more (but not perfectly) accurate analogy.

I give you a magic piece of paper with a two binary digits (00/01/10/11) on it. I also keep such a paper for myself. If we both start by looking at our first digits then those will mysteriosly match, but if we then look at our second digits they will have just a 50% chance of matching. However, if we both start by looking at the last digits, then those will match, and our first digits will have a 50% chance of matching. If you and I start by looking at different digits, then the digits we read are just standard random bits.

The cool thing about this is that our magic paper will also work instantly even if we are lightyears apart. However, if you decide to look at the last digit and you see a 1, then that tells you nothing about me or which digits (if any) I have looked at, so I can't send you a message with this paper.

It sounds like an interesting, if very expensive, way to get around primes for key exchange, in the world where prime factorization is much more efficient. Need a random number, look a bun of shared digits in a predefined order. You now have a shared random number that's perfectly secure, as long as you shipped out your shared state in advance.

With quantum entanglement, what happens to one particle prior to measurement affects the other even if the effect occurred after they were parted. That's where it gets really spooky.

To belabour the colours and boxes analogy:

We have 2 balls, if one is orange the other is always purple and vice versa.

Now these balls have an extra property. If you shake them vigorously they may or may not change colour randomly.

So, we put them in boxes without looking and take them far away from each other. Now, some time after they are far apart and before you look in the box, you shake one of the boxes. You then look.

In QM, if the ball you observe is orange(purple) then you know the other is purple(orange). But the other ball can't have known ahead of time that a) you were going to shake your one and b) that it changed colour (or didn't). That's why it's spooky.

Sounds like they never were two separate entities to begin with or never parted location. Which is spooky too, in a different way.

Too build on this thought - is it possible that these quantum properties aren't affected by locality? Which is obviously true given the explanation above... I guess I'm asking if there are other intuitions that can be made if locality is explicitly considered unrelated.

When I first started to be interested in this area, it occurred to me that it might be something like a 'stretchy' spacetime i.e. the particles are actually locally connected but appear unconnected if you use a static spacetime as a reference as we tend to. I guess something analogous to the idea that someone moving close to c experiences time different from someone in a slow moving reference frame.

Unfortunately, I have neither the experience nor ability to progress that thought much further so, if someone wants to go off and get a Nobel from it, feel free...

>>Sounds like they never were two separate entities to begin with or never parted location. Which is spooky too, in a different way.

Sounds interesting. I wouldn't be surprised if it turns out that these two particles are really just a single particle in a different dimension that we don't know it exists yet.

Experimentally, when they entangle photons, they just pick two random photons.

If two random photons used in an experiment weren't ever really separate entities to being with, even before the experiment ever started, then one should probably conclude that /all/ photons are not really separate entities.

Otherwise, every quantum experiment we've ever done has just /happened/ to pick two photons that were actually one entity, unlike most other photons.

> Sounds like they never were two separate entities to begin with

“Separate entities” may just be an artifact of intelligences trying to divide sense data about the universe into chunks they are capable of processing.

What is the equivalent to "shaking" in QM? I'm trying to figure out why it matters whether you shake the box or not (i.e. could the shaking simply not have an effect if you're not looking?)

The shaking represents any sort of action that doesn't perform a measurement. It was the first thing I thought of that could affect the ball without opening the box.

It could be that the shaking didn't have an effect. But equally, it could have had an effect. The spooky bit is that the other ball 'knows' whether it did or not in a non-local way.

But isn't it possible that performing "shaking" might not affect any "ball" when it's "inside of a box"?

The "spookiness" is more subtle than that. You could argue that the cubes have decided on their color as soon as you close the box. Why doesn't this argument work for real quantum systems?

In reality, you're not measuring colors of boxes, but spins of particles. You generate two particles with opposite spins along a known axis. If you measure the spins along that same axis, you always get the same result: one is spinning "clockwise", the other "counterclockwise".

On the other hand, if you measure one of the spins along a different axis, you'll randomly get "clockwise" or "counterclockwise" according to the angle between the original spin axis and the measurement axis. Measuring at a right angle to the original axis gives you random results, which is also what you'd expect. But what about the angles in between? Classical statistics says the correlation should vary linearly with the angle, but the actual results have a cosine factor (https://en.wikipedia.org/wiki/File:Bell.svg). This cosine factor can't be explained by any kind of classical statistical randomness. That's the "spooky" part.

But Bell's equality means that either the system is non-local or non-deterministic.

So either it's determined when you close the box, but it's spooky action at a distance. Or it's not determined on box closure (no spooky action) but it has "random-ness".

Is this fair? The real trick is science found a way to create a pair of of particles with predictable relatable spin values (though not known until observed) and the ability isolate those particles from interference.

The emphasis always seems to be on the "spookiness" of the observation, but the real trick is isolation and "entanglement" (pairing of values) in the first place.

Furthermore, nothing particularly real occurs at the time of observation, there's no way to tell that an observation of either of the particles has occurred, thus they aren't "tangled" as much encoded, isolated and separated.

Sounds a little like Quanta's balloon analogy:

https://www.quantamagazine.org/quantum-entanglement-puzzle-s...

[I'm not a physicist] I like your analogy for color entanglement. However, it doesn't show the "weirdness" of superposition (the balls don't have a concrete value until one of the boxes are opened, at which point, this triggers the determination of the open box's color value), nor the "spookiness" of action-at-a-distance (opening one box instantly influences the state of the ball in the other box -- which now must be the opposite color when its box is opened).

Intuitively, someone might think: well, the ball colors were "decided" while the two balls were next to each other (ballA will be orange, ballB will be purple), and the information about the color is attached to the balls, so the balls know what color to be when they are separated and later opened (they have "hidden variables" indicating the assigned color)....however, the reality (provable statistically by Bell's Theorem) is that the balls do not carry this color information, and instead the act of opening the box, randomly sets the color of BallA and instantly affects the color that BallB will have.

So if both people synchronize the time to open the box (that has some time relativity problems), so that BoxA is opened a fraction of a second before BoxB, then BoxA's color will influence BoxB's color (seen a fraction of a second later), but that will have happened faster than the speed of light would allow if BoxA was sharing its color information with BoxB.

> the balls don't have a concrete value until one of the boxes are opened,

> opening one box instantly influences the state of the ball in the other box

How do you know that without opening the box?

Bohr and Heisenberg interpret it that way.

https://en.m.wikipedia.org/wiki/Copenhagen_interpretation

And Schrödinger's cat never told us what really happened inside the box :)

https://en.m.wikipedia.org/wiki/Schrödinger%27s_cat

This analogy is confusing because it is incorrect in some fundamental ways. You aren't just opening a box and seeing one or the other color. The measurement itself determines a role in the outcome that you couldn't predict in advance.

It is more like you get to choose to measure only one of the red, green or blue channels. If you send a message containing which channel and the measurement, the other person can do the same measurement and find the complementary value. Without the channel and the original measurement the other person just sees random behavior.

The spooky part is that I don't choose the color channel to measure until after the entangled balls are separated

> 1. they will be locked into one color

No, this is wrong! As someone else commented, this is just not knowing something. The weirdness of the quantum entanglement comes from the fact that the behavior of each cube is experimentally correctly described by equations based on the cubes continuing to change color randomly.

I probably should have clarified that my analogy was coming from a perspective of super-deterministic[1] compatibilism[2]. In the absence of knowing the truth, I feel that solution is the best model / explanation. It strips away the 'spookiness' and many other problems, though it admittedly has its own issues with testability and proof.

I do not believe in the idea that particles can super-luminally affect each other during the moment of observation. I think that is a failing of our measurement systems and ability to describe what's going on. Just as we often fail to accurately convey the Schrodinger's Cat thought experiment [3].

But for the record, if a different perspective were definitively proven to be true, I would of course switch to it, as any scientist should. But given that there are multiple avenues of investigation open, I'm exploring super-deterministic compatibilism.

[1] https://en.wikipedia.org/wiki/Superdeterminism

[2] https://en.wikipedia.org/wiki/Compatibilism

[3] http://www.smbc-comics.com/?id=2524

Boojums All the Way through http://www.cambridge.org/catalogue/catalogue.asp?isbn=052138...

Damn, all of these explanations are overly complex! The most basic reason it doesn't work is because measuring your particle breaks the entanglement. So you would know that your partner has the opposite spin (or whatever) but you can't bootstrap any sort of communication system with that.

Not overly complex, but overly simple. I wish people talking about physics learn for the last time that analogies are a horrible way to explain a complex concept that at best adds an unnecessary translation layer increasing cognitive load and at worst hand waves away the most critical parts.

They just get too excited : )

>FTL, by detecting when a particle was no longer entangled

The trouble is you can't detect that. You can just make a measurement on your particle and get some result. That's the same whether it's entangled or not.

> A staple of sci-fi has been a pair of boxes with quantum entangled particles inside allowing FTL communications once the boxes travel (sub-FTL) to their destination.

Of course this is complete nonsense. It's useful for plotting stories, but seems to have imprinted on a large fraction of readers a total falsehood. Do you know where the idea started? I think the first time I saw it was in Orson Scott Card's novel "Ender's Game" (1985).

Looks like it may have started in 1966 with a story by Ursula K. Le Guin. She called the device an ansible, which is what it was also called in OSC's books.

https://en.wikipedia.org/wiki/Ansible

From the book The Dispossessed.

It's a great novel, by the way.

According to that Wikipedia page, Le Guin's ansible might not be relying on entanglement.

It "doesn't involve radio waves, or any form of energy. The principle it works on, the constant of simultaneity, is analogous in some ways to gravity ... One point has to be fixed, on a planet of certain mass, but the other end is portable."

About Card's ansible, it says, "Card's description of the ansible's functions in Xenocide involves a fictional subatomic particle, the philote. In the "Enderverse", the two quarks inside a pi meson can be separated by an arbitrary distance while remaining connected by "philotic rays"."

So perhaps Card's "Xenocide" (1991) is where the entanglement => FTL communication myth began?

It's about as likely as reading a byte from an initialized /dev/urandom and measuring its entropy to see if someone removed a hardware RNG.

I'd guess it's impossible without measuring both particles and communication to compare the results.

A few weeks ago there was another announcement about the advancement in arresting the evolution of a quantum system

https://news.ycombinator.com/item?id=14347231

In this way it seems to me communication may be possible

A sort of sending bytes stead bits

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