[qq12as]

Does someone have a good explanation/intuition for why you cannot exploit quantum entanglement to send information faster than light?

If me observing the particle in Australia alters the probability distribution of your particle in USA, can't I only observe the particle when I want to communicate 1 and never observe it when I want to communicate 0?

Edit: thanks a lot for the answers! I guess it boils down to the fact that the Australian guy cannot condition his decision on the (unknown) spin of his particle -- if he could (eg: had access to the local hidden information) then he would be able to update the USA's probability distribution instantaneously and use it to communicate

[317070]

I have a bag with two balls, one red, one blue. I randomly send one of the balls to Australia, one of the balls to the USA. The moment you look at your ball in Australia, you also know the color of the ball in the USA. How would you use that fact to send information?

The crux with quantum mechanics is that you can show (very easily, understandable by the layman, look up the Bell inequality) that in the quantum case the ball only takes on a color the moment you look at it, and so instantaneously is setting the color in the USA as well.

However, the basic setup still applies. You cannot send information by merely observing something.

[TrinaryWorksToo]

If the ball only takes a color when it's observed (loosely), couldn't that timing be exploited to convey information?

[GuB-42]

No, in fact it doesn't matter who opened the box first.

Imagine we both open our boxes at around the same time, and communicate each other results at the speed of light.

- From my point of view, I opened the box first, got red and caused you to get blue. Your message confirms my hypothesis.

- From your point of view, you opened the box first, got blue and caused me to get red. My message confirms your hypothesis.

We can repeat this experiment thousands of times, every time, we both get a 50/50 chance of red vs blue, and every time, the hypothesis is confirmed.

In order to know who really was first, the only way is to wait for the other person result. For example: when you open the box, start a timer, stop it when you receive my message. It the time is less than the times it takes for light to travel between us, I was first. But in order to have this information, you have to wait, so it is not faster-than-light anymore.

[mwaitjmp]

By viewing the color of the ball you’re not notifying the other ball to change its state(in a way that can be measured). You will only know that once the ball is observed it will have a certain color.

The only way I can foresee this being used to transmit data faster than light is that if you both agree to perform an action depending on what color ball they see. If you both view the ball at some agreed point in the distant future, you will instantly know what action the other person will do.

[Groxx]

By looking, you're forcing it to take a color if it didn't have one. You can't tell if it was randomly chosen by your action, or if it was chosen by the other side's action before yours, without communicating by some other means.

[onlyrealcuzzo]

I've always wondered - what constitutes an "observation"? How does the particle know it has been observed?

[HackOfAllTrades]

In Quantum Mechanics things travel as waves and interact as particles. Between interactions, everything is described by the Schrodinger Wave Function, which evolves to include every possible path. But at the point of an interaction the wave function must collapse to satisfy conservation rules.

Consider an electron fired at a dual slit with a phosphor screen. While traveling from the electron gun, thru the slits, to the screen, the electron is described by a Wave Function. It has no fixed position or momentum. The Schrodinger wave passes through both slits and interferes with itself on the other side. The wave function evolves into a series of lines.

But when the electron interacts with the screen it always appears as a single point. It must do so by the laws of conservation. At the interaction it must have a specific location and momentum in order for there to be conservation of charge, momentum, energy, etc.

This interaction is enough to 'collapse the wave function'. No 'observation' is required.

How does this happen? There is no localized mechanism that can possibly make this work. The conservation laws are not local restrictions. They are universal.

Please note that this is my own explanation of now QM works, and does not necessarily reflect the official position of any school of thought. It does, however, reflect the actual use of Quantum Mechanics, in that systems evolve via the Schrodinger Equation and interactions must obey conservation laws. And No, it cannot explain how entanglement works.

[kgwgk]

> In Quantum Mechanics things travel as waves and interact as particles. Between interactions, everything is described by the Schrodinger Wave Function, which evolves to include every possible path. But at the point of an interaction the wave function must collapse to satisfy conservation rules.

That would make some sense if by interaction you mean "interaction with the macroscopic environment". When small-enough quantum systems (like two particles) interact there is no collapse and the evolution is unitary.

> This interaction is enough to 'collapse the wave function'. No 'observation' is required.

How do you distinguish the interactions that 'collapse the wave function' from those who do not?

[HackOfAllTrades]

The idea that Measurement = "interaction with the macroscopic environment" is part of the Copenhagen interpretation, not a requirement of QM itself.

Aside: (Personally, I see this more as Bohr's way of dodging questions he had no answer to, and not a viable way to think about Quantum Mechanics. A better answer would have been "I don't know. Let's figure it out." But that was impossible for political reasons. Bohr was being attacked by Einstein for 's sake. He can be forgiven for adopting Ali's "rope-a-dope" tactics if he felt that Einstein was trying to destroy his entire field in its infancy. But I find "there is no quantum world" simply unacceptable.)

Now to answer your question as best I can, an interaction must collapse the wave function when it is required to fulfill a conservation rule. For example, if an electron is captured by a nucleus it becomes bound and emits a photon. This is an interaction that must conserve momentum, angular momentum, energy, and charge. Because of that, the electron can no longer be represented by a non-localized wave function. The universe must concentrate those properties down to a point in order to "do the accounting" necessary for the conservation rules.

No, I don't know how it does that. But then, NONE of the available interpretations answer that question. This indicates to me we are thinking about it wrong.

What I like about Stuckey's paper is that it adds another factor: besides conservation rules the universe seems to require that "measurements" obey the Relativity Principle (No Preferred Frame of Reference). I have yet to figure out how to incorporate that.

[kgwgk]

Is the idea of “collapsing the wave function” a requirement of QM itself? In that context, a “measurement” would be to be what you call “an interaction that must collapse the wave function”.

And your answer is simply wrong. An excited atom can emit a photon, for example, and the system will still be described by a “non-localized wave function”. It won’t even be well defined if the spontaneous emission has happened or not yet.

The evolution of a quantum system according to Schrödinger’s equation doesn’t violate conservation rules. And, in case it’s not clear, the quantum system described by the wave function in the example above is the atom-photon(-or-not) pair.

You’re definitely thinking about it wrong.

[warent]

My understanding is that any kind of measurement will do, it doesn't matter how you get it. You could use photons to do the measurement but there are other ways which all have the same result.

It's a mystery how the particle "knows" (In other words, nobody knows when the wave function collapses) but one popular interpretation is that the particle exists in all states, i.e. in a pure description of reality. When any quantum system interacts with it, then it becomes entangled with the result of that measurement, branching it into a new universe (edit for clarification: a new world where it was as if it was never a wave, and it was always a particle). That's my understanding of the many-worlds theory.

That entanglement propagates across nearby particles, so it doesn't have anything to do with eyes or consciousness. If the air molecules around your body interact with the particle then that entanglement propagates through your body and places you in the new world.

[tabtab]

Re: When any quantum system interacts with it, then it becomes entangled with the result of that measurement, branching it into a new universe. That's my understanding of the many-worlds theory.

This is a case of a simple theory that indeed models the mystery well. However, it seems "wasteful" in that it would branch into gazillion trees of reality. In Occam's Razor, does "simplicity" include quantity of "stuff" needed? Because sometimes the brute force algorithm/model is the "simplest" if we ignore quantity of stuff and time, such as bubble-sort. Bubble-sort is one of the simplest sorting algorithms known, but is inefficient from a time and resource standpoint.

If there are "free" dimensions to spare out there, then the "wasteful" multi-verse model may not really be wasteful. We humans are used to thinking in terms of economic trade-offs, and a model that uses up large quantities of space/time rubs our instincts wrong.

If true, the theory means that in some universe somewhere I'm a billionaire who married a supermodel.

[warent]

I think you're mixing up metaphysics and human intuition with what the math describes. The current math says there may be essentially infinite worlds created in infinite time, where yes there is least one in which you are a billionaire married to a supermodel. The only constraint is in the properties of nature (e.g. a world will never be created in which an electron has 0 spin).

However, I agree with you that it seems implausible because it implies absurd situations like, there is a world in which someone lives a life of celebrity because every time they roll some dice it always lands on 6, and every time they flip a coin it lands on heads, etc.

[tsimionescu]

Even worse, many-worlds doesn't really solve the problem anyway - it still doesn't explain WHY you only observe one result, when the Schrodinger equation predicts several. That is, why can't you see the other worlds?

[slavik81]

Don't you have the same problem in classical mechanics? Let's say you're standing at the edge of a pond, and you see waves rippling across the surface. The deviation in height of the surface of the water is described by h = cos(r + t) where r is the distance from the centre of the pond and t is the current time.

Why can you see the solution of the equation for the entire surface of the pond at once, but only for a single instant of time at any given moment?

[warent]

That doesn't seem like a question that can be answered mathematically, does it? That's like asking, why do electrons have a spin of 1/2? Why is the speed of light 299,792,458 m/s? These are just properties of the universe.

[tabtab]

Re: doesn't explain WHY you only observe one result

If the alternative universes are in different dimensional planes, it's pretty obvious why we couldn't observe them.

[RobertoG]

>>"However, it seems "wasteful" in that it would branch into gazillion trees of reality."

In a way, it could be interpreted as very efficient. Only the branches where some "measurement" is done are "calculated". I suppose the others are garbage collected at the end of time, or something like that.

And maybe it's not a tree, but a graph of universes. In the same way that a universe split in two, two universe could also fuse into one when they share the previous state. Somehow it feels like this have to be connected to reversible vs. non-reversible computation.

Ah.. it's a good feeling being a fearless dilettante.

[tabtab]

Re: Only the branches where some "measurement" is done are "calculated". I suppose the others are garbage collected at the end of time, or something like that.

But that's adding complexity back into it. You are increasing complexity of the theory/model by adding a complex cleaner/trimmer in order to reduce the quantity of resources consumed.

[tim333]

If the universe is a mathematical object there being an infinity of universes isn't any more wasteful than there being an infinity of integers for example. From Occam's point of view it's simpler if all integers exist rather than there being a cap if that were even logically possible. So yeah go supermodel!

[tabtab]

Re: If the universe is a mathematical object...

Math is a modeling technique, not a "thing". To me it doesn't make sense to say the universe "is" math. Maybe it's a machine "running" math notation (programming code), but that's not the same as it "being" math.

(Is "God" the server admin?)

[Waterfall]

The universe isn't mathematical, it is explained by math, a country is not Chinese because I wrote a tour guide in Chinese about it. Infinite universes isn't really applicable here, you're thinking of a growing block universe. A simpler point is a block universe. https://en.m.wikipedia.org/wiki/Eternalism_(philosophy_of_ti...

[thaumasiotes]

> one popular interpretation is that the particle exists in all states, i.e. in a pure description of reality. When any quantum system interacts with it, then it becomes entangled with the result of that measurement, branching it into a new universe (edit for clarification: a new world where it was as if it was never a wave, and it was always a particle).

The two-slit experiment contradicts this. You get different results depending on when you perform the observation(s).

So the new world is a world where the particle was originally a wave, and became a particle when it was observed. Not a world where the particle was always a particle.

[Waterfall]

Ever heard of pilot wave theory? https://en.m.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theo... It's an easy visualization that can be shown with speakers and liquid

[flubert]

Here's one interesting silicone oil pilot-wave video:

https://www.youtube.com/watch?v=nmC0ygr08tE

[tabtab]

But to fit experimental observations, the "wave" would have to be faster than light.

[jsmith45]

Yeah. On the other hand it does give decent intuition about how certain interactions would result, especially if talking about massive particles traveling much slower than light.

I remember reading an article here a while back that involved a macroscopic re-creation of the double slit experiment results, but where mere observation remained possible, because light did not sufficiently influence the substrate. In that experiment the particles were droplets traveling on top of a set of waves, working in the pilot wave fashion.

Any attempt to use anything of similar scale to the particles to observe which slit the drop went through would break the interference pattern, but mere light did not, allowing one to visually see how a pilot wave style interpretation could work, if it were not for that whole (photons travel at the speed of light, so these would need to be faster than light propagating pilot waves) thing.

Indeed it looks like flubert linked a video from an earlier study of the same basic mechanics, prior to the more recent one that included the double slit experiment replication.

[thaumasiotes]

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

You can't observe something without sending information. In order to make an observation, you must interact with whatever is being observed, so that information about the interaction can come back to you.

In the bag example above, we can observe the Australian ball and know the color of the American ball, and we cannot use this interaction in Australia to send information to America. But we cannot avoid sending information to the Australian ball when we observe it.

>> You cannot send information by merely observing something.

This is, at the least, very poorly phrased. As explained above, not only can you send information by observing something, it's impossible not to do so. The question here is where the information goes.

[brlewis]

I assume, for the sake of my own sanity, that "observation" means the particle becomes a cause for some kind of effect, e.g. colliding with something in a way that changes the something's state. Quantum mechanics experts, please don't tell me it's weirder than that.

[centimeter]

In many formulations, e.g. multiverse, the apparatus doing the measuring (doesn't have to be a human or anything) becomes entangled with the thing being measured. This is still not super well understood.

[burkaman]

Particles don't know anything, but you have to interact with it in order to observe it. You have to bounce a photon off of it or something like that in order to get any information out of it.

[flubert]

"A new interpretation of quantum mechanics suggests that reality does not depend on the person measuring it"

https://phys.org/news/2020-10-quantum-mechanics-reality-pers...

https://www.mdpi.com/2073-8994/12/9/1533

[jdashg]

Phew!

[cgh]

You might find this helpful: https://en.wikipedia.org/wiki/Quantum_decoherence

[tabtab]

Re: "How does the particle know it has been observed?"

It's as if God's code looks something like:

    if (event.thisParticle.isBeingObserved()) {
       thisParticle.assignAttributes();
    }

[pjmorris]

"extreme late binding of all things?"

[tabtab]

Lack of compile-time type-checking gives us dodgy politicians and lawyers.

[smusamashah]

How does a ball know that it has been observed and in effect changing the color of the other paired ball?

[317070]

When you observe it, you collapse the wave-function of which color the ball has into a particular value (red or blue). Before you observed it, the ball was in a superposition of the two colors. And this collapse instantaneously also collapses the wave-function to the American ball.

Now, that is obviously not true for macroscopic objects like balls. Those are not in a superposition of colors until they are observed, but it is true for quantum objects like electrons.

[AnthonyMouse]

> Now, that is obviously not true for macroscopic objects like balls. Those are not in a superposition of colors until they are observed, but it is true for quantum objects like electrons.

But then what is it that can I do with two entangled electrons that I can't do with two literal billiard balls known to be different colors than one another?

[kgwgk]

You can prepare a pair of photons in a state such that when you measure the polarization of both of them along the same axis, for whatever direction you want to choose, you get the same result. But they are entangled, each photon considered separately is not in a well-defined state.

You can also prepare two photons in the same state, so the have the same polarization for some direction chosen at that time. But the measurements along other axis won't be perfectly correlated (if they are correlated at all).

The red/blue color example is too simple to be interesting.

[Koshkin]

You cannot paint a billiard ball with a superposition of colors.

[AnthonyMouse]

But what is the implication of that? What do those words cause to be different in practice?

[aeternum]

The ball example is insufficient and misleading. It is unfortunately too simple as you need 3 inputs and 2 output to demonstrate the effect (aka Mermin device).

The way to think about it is a box with 3 buttons. There is no such thing as 'observation', the only way you can interact is to push one of the 3 buttons and as a result the box will output either a red or green light.

You must push a button to get the light, but the button may mutate the internal state of the box. Using this model, there's nothing special about human or conscious observation. Every interaction via a particle or otherwise is simply pushing a button.

The crazy thing is.. no matter how clever an algorithm you write to drive the lights from the buttons, you cannot match the observed probabilities. (100% if the same button is pushed, 25% if different buttons are pushed).

[kgwgk]

> Using this model, there's nothing special about human or conscious observation. Every interaction via a particle or otherwise is simply pushing a button.

But there is something kind-of-special about the box with the buttons and the lights.

Not every interaction is simply pushing a button that lights one lamp or another. Keeping the analogy, the result of an interaction between two particles may be a combination of the "red on", "green on" states. You need to keep adding particles to have a box with buttons and lamps that works as expected.

[tim333]

Parallel realities. In one red goes to the US in another blue. In another vice versa. They decohere to one with someone who's seen red one who's seen blue.

[EliRivers]

That's the "spooky action at a distance" (in the case of the entangled particles, obviously; with the two balls it's not so spooky). Spooky.

[nemothekid]

Maybe this is a basic question - but what I don’t understand is why this is called “spooky” action.

My intuition is you have two particles, and you don’t know what concrete states they are in, but you know all possible states (that may be represented as some sort of system of equations).

By observing a single particle you unlock a variable in that system of equations and can therefore solve the whole thing. To me it would be more straightforward to say the concrete state of the particle is simply unknown until it is observed. The concept of superposition seems like an overly complex description for this phenomenon.

I understand my view is wrong, but I don’t understand how I’m wrong

[tabtab]

Here's a layman's explanation of why "hidden local variables" theory doesn't match experiments:

https://www.wired.com/2014/01/bells-theorem/

In other words, modeling particle pairs as having matching static hidden "meta data" in them doesn't work. They do act as if there is instantaneous communication between the particles, but in a limited way that prevents us from using them for instant communication. Quantum mechanics is a weird tease, having magical properties that always serve up loopholes when we try to leverage the magic for real-world benefits. The quantum universe seems built by insurance lawyers who are masters at screwing consumers with fine-print when they go to make a claim.

[EliRivers]

I observe my particle here, and in doing so its state is decided.

The state of the entangled particle over there, a light year away (for example) is also decided. Instantly. Faster than the speed of light. Nothing travelled from here to there. No particle, no photon, nothing. How does over there "know" that I did something over here?

Sure feels kind of spooky.

To me it would be more straightforward to say the concrete state of the particle is simply unknown until it is observed.

It's not just unknown. It's undecided. It has no concrete state. It's not that it IS a one or a zero and you just don't know it. It's not yet been decided whether it's a one or a zero, but as soon as the decision is made for one of the entangled particles, the decision is also made for the other one, a light year away. Instantly. Spooky.

[rrss]

What you are describing is a hidden variable theory - i.e. there is some concrete state of the particles, but it is hidden.

John Bell demonstrated that in order for a hidden variable theory to make predictions in agreement with quantum mechanics, it must have nonlocal interactions, which means any workable hidden variable theory must also be pretty spooky.

https://en.wikipedia.org/wiki/Bell's_theorem

[brianush1]

Obligatory "not a quantum physicist," but the only way to observe something is to throw something at it (e.g. a photon) and see what bounces back. The problem is that when you throw something at it, you're interacting with and affecting the ball.

[monktastic1]

This is not quite correct. There are ways to gain information about a quantum state without interacting with it: https://en.m.wikipedia.org/wiki/Interaction-free_measurement

[mrkstu]

Do (non)measurements taken this way 'collapse the wave function' anyway? Or can you only get information that is still open to change during the actual measurement?

[monktastic1]

"Collapse" is just one way to explain how measurements happen. These are "actual" measurements, because they give you actual information.

[Koshkin]

Yeah, this is another popular non-explanation. (It does not explain why the ensuing randomness is subject to strict and perfectly deterministic laws).

[centimeter]

> the quantum case the ball only takes on a color the moment you look at it, and so instantaneously is setting the color in the USA as well

We don't actually know that this is an accurate description of what is happening, although it is consistent with what is happening.

Very likely, the underlying physical process still operates below the speed of light. "Instantaneous" isn't something that makes physical sense in this context.

[gmadsen]

There have been experiments confirming entanglement. It does not "tell" the particle at the speed of light. The mutual wave function collapses as a whole unit.

it is very easy to measure something entangled at the same time (or at least within a margin that is faster than light travel) and confirm you always get the correct results. If the wave function didn't collapse together, you would get results that break quantum laws, such as measuring two entangled particles with both up-spin.

[centimeter]

You misunderstand the objection. The fact that you didn't expand on your definition of simultaneity suggests that you're missing a lot of background here. There are many ways that physics could appear to violate causality with collapse without actually sending information faster than light (e.g. how most of the multiverse theories work).

[lisper]

This is a Google Tech Talk I gave about this about 8 years ago:

https://www.youtube.com/watch?v=dEaecUuEqfc

It is based on this (unpublished) paper:

http://www.flownet.com/ron/QM.pdf

The key insight is that measurement and entanglement are actually the same physical phenomenon. A measurement is nothing more than a very large network of mutual entanglements.

[EliRivers]

When the person at the other end looks at their particle, and sees it either 1 or 0 and has no other information, how do they know if you've looked at your particle or not?

[fjfaase]

And even if they would know, due to the distance and general relativity, there is no concept of one of the two persons looking first. It could be that from Alice's point of view Bob looked later, while from Bob's point of view Alice looked later. So, in which direction did the information go?

[Gunax]

This is my favourite explaination. I have had this bookmarked for over 10 years and keep coming back to it:

http://www.felderbooks.com/papers/bell.html

[phkahler]

>> Does someone have a good explanation/intuition for why you cannot exploit quantum entanglement to send information faster than light?

Because physicists can not tell the difference between a particle whose "wave function collapsed" and one that didn't.

The faster than light communication would be equivalent to modulating the collapsedness of a stream of particles by measuring-or-not their entangled counterparts. Since the is no discernable difference between particles pre and post collapse, no information can be transmitted.

[mppm]

Initially, your particle in Australia and its entangled twin in the USA exist in a superposition of 0 and 1. When you "measure" the state of your particle, you force it to assume a definite state, and entanglement forces the other particle to assume e.g. the opposite definite state ("spooky action at a distance"). This allows you to synchronize information across large distances, but you cannot send anything, because you cannot chose the outcome of the quantum measurement.

[willis936]

If you can guarantee a random distribution of the color of the balls, it seems like a decent way to handle secure key echange.

[mppm]

Yes, that is in fact one of the practical ways to do quantum key distribution.

[teekert]

What if we manage to get our whole world into a superposition and only collapse to the world where the particles did contain the message we wanted? Sorry... I've been reading a lot of Greg Egan lately ;)

[GoblinSlayer]

Entanglement works like synchronized clocks. You can't send information, but you can coordinate actions to a similar effect. Much like you can coordinate actions of separate groups using synchronized clocks with no communication except for passing time.

[tabtab]

That's the "hidden local variables" theory. Experiments seem to suggest it's wrong. How one pair is observed appears to affect (change) the other instantaneously. But, not in a way we can use to our advantage to get instant communication.

[GoblinSlayer]

Exact mechanism is different from synchronized clocks, it's an illustration how you can have correlation without communication and how you can use it in practice, an this correlation works with both entanglement and synchronized clocks.

[tabtab]

It seems some aspects can be explained by (modelled as) synchronization, but not all. "Spooky action at a distance" resembling faster-than-light communication still exists.

The two top explanations are either some info can travel faster than light (from our perspective), or the universe forks into copies when needed.

[8note]

you have to send classical information to get useful stuff out of the entanglement like quantum teleportation.

The simplest one is that you need to be able to tell the partner when you've made your changes and that you're ready for them to measure