# Quantum Entanglement: Hidden Information vs. Time

• B

## Main Question or Discussion Point

So let me preface this by saving, I am a computer programmer, not a physicist, and the main reason I became interested in quantum mechanics has a lot to do with the recent developments in quantum computing and quantum entanglement in general, although I do have a lot of curiosity about the inner workings of the universe from both a mechanical and philosophical perspective. So therefore, I may be making incorrect assumptions about quantum mechanics that would lead me to erroneous conclusions, so please bear with me.

My understanding of entanglement is that causality is not violated because while taking a measurement of an entangled particle, while you do gain information about the particle it is entangled with (due to conservation of angular momentum), you cannot affect the outcome of the measurement directly without also influencing the entangled particles in such a way that would collapse the quantum system. In other words, if the spins of two entangled particles were measured in different locations, while you would learn information about both particles at the same time (regardless of locality), because there is no way to potentially know more about the spin of the particle before measuring it than the probability it will be in a given state (and no way to influence the probability of a given measurement that wouldn't also affect/collapse the entangled state), there is therefore no way to send information faster than the speed of light via entanglement.

This is my understanding, and please feel free to correct me if it is in any way flawed. But if all this is correct, there is a somewhat interesting conclusion you can derive from this information.

So say Alice is in outer space, in some sort of futuristic spaceship, approximately one light year from earth. Before leaving earth, Alice and Bob created 1,000,000 pairs of entangled photons (for good measure), and Bob kept half the entangled photons on earth, while Alice took her half aboard her spaceship. She also took an atomic clock with her, and they both agreed to measure the states of their entangled photons exactly one hour (relativistically speaking) apart, once Alice had reached the distance of one light year from earth. Through painstaking physical isolation, the quantum states of all entangled photons had been preserved in such a way, so that with their highly accurate measuring equipment, the statistical probability that any one given photon would measure up or down, was within almost complete statistical certainty, 50%. Well, let's say with a 0.1% margin of error. This statistical margin of error, had been rigorously proven, and all variables had been accounted for, to the point that if even 50.2% of the measured particles were definitively measured with the same spin, something interesting must have occurred.

Now let's say Bob had a little trick up his sleeve. Let's say that Bob's measuring equipment allowed him to take measurements with Planck scale accuracy, and through exhaustive testing, he had discovered a method to stage the timing of his measurements, so there was a slight bias toward measuring an up or down spin that had no bearing on the quantum system until the exact moment of measurement. A sort of hidden resonance that correlated the results of his measurements with time. So when Alice goes to take her measurements, and notices a bias in her data toward measuring a down spin of 0.2%, she knows Bob must have staged his measurements to have a bias toward measuring an up spin, and Alice and Bob have successfully communicated information faster than the speed of light.

This scenario of course hinges on the assumption that the state of the quantum system is influenced by time (that the entangled photons are in a real sense oscillating between two states in time), rather than staying in a completely static state until measurement. If such were the case there would be nothing Bob could do time wise that would influence the results. So the obvious question here is, do we know if entangled photons experience time? If not, I'm not sure what the implications are exactly, but it seems at least to be an interesting observation. But what if they did experience time, in such a way that time could affect the results of the measurement?

Well, in this case, we have a conundrum. Unless the state of the quantum system has no correlation with the timing of observation whatsoever (true randomness), then it is at least theoretically possible to exchange information faster than light. Even the slightest, most minute correlation would be sufficient to violate causality if measurements were precise enough. The problem with this observation, is that nothing can be proven to be random. If you observe a phenomenon for any given finite amount of time, and observe no discernible pattern whatsoever, there will always be a non-zero possibility of a pattern eventually emerging. No amount of observation is sufficient to rule out this possibility. Therefore, you must come to the conclusion that causality cannot be preserved without a source of true randomness, but that it is also impossible to prove that true randomness exists.

I'm really not sure what to make of this, does anyone here have thoughts? Keep in mind my lack of background in physics, so fancy mathematical symbols and terminology will probably be lost on me. ^^;

Related Quantum Physics News on Phys.org
mfb
Mentor
Let's say that Bob's measuring equipment allowed him to take measurements with Planck scale accuracy
That doesn't mean anything for spin measurements.
he had discovered a method to stage the timing of his measurements, so there was a slight bias toward measuring an up or down spin that had no bearing on the quantum system until the exact moment of measurement.
You cannot have such a bias.

It doesn't matter when you make measurements. The result will always be the same.
(that the entangled photons are in a real sense oscillating between two states in time)
They don't.
rather than staying in a completely static state until measurement
You *can* influence their state, but everything that will shift the fraction of photons measured to be spin up vs. down has to start with a measurement of the spin.
So the obvious question here is, do we know if entangled photons experience time?
Massless particles don't "experience" anything.

You *can* influence their state, but everything that will shift the fraction of photons measured to be spin up vs. down has to start with a measurement of the spin.
So I think this is where my confusion about the situation might be coming from. If we are saying that the passage of time has no effect on the result of the collapse of a quantum entangled system, why is this so called spooky action at a distance so spooky? Aren't we more or less just saying, we are creating a state where the spin of two photons is correlated in such a way where there is uncertainty about what the spins will be until we measure at least one of them. That is to say, no matter how we try to look for a correlation between time and what we measure, we won't find one, because the passage of time and what we measure just have nothing to do with each other. So in this sense it's not really "spooky" at all when a quantum state collapses across a distance, because there is no more or less uncertainty about this situation than there is about any quantum system until you measure it. Quantum states are always resolved by measurement/observation, and the only thing different about this situation is that the state we left the system in after entangling it can't fully resolve itself until measurement, right? Or that is to say, nothing about the state of the system between entanglement and measurement is allowed to influence causality without collapsing the entangled state.

mfb
Mentor
It is spooky because the particles cannot communicate with each other but they are 100% correlated no matter which basis you choose. If you could just measure "up/down" (actually "horizontal" and "vertical" for photons) then you could reproduce this with classical particles. But you can also measure them at a 45 degree angle and still get perfect correlation. If they would be horizontal/vertical (and you just don't know which one) this would not be possible.

PeroK
Homework Helper
Gold Member
My understanding of entanglement is that causality is not violated because while taking a measurement of an entangled particle, while you do gain information about the particle it is entangled with (due to conservation of angular momentum), you cannot affect the outcome of the measurement directly without also influencing the entangled particles in such a way that would collapse the quantum system. In other words, if the spins of two entangled particles were measured in different locations, while you would learn information about both particles at the same time (regardless of locality), because there is no way to potentially know more about the spin of the particle before measuring it than the probability it will be in a given state (and no way to influence the probability of a given measurement that wouldn't also affect/collapse the entangled state), there is therefore no way to send information faster than the speed of light via entanglement.

This is my understanding, and please feel free to correct me if it is in any way flawed. But if all this is correct, there is a somewhat interesting conclusion you can derive from this information.
That it is in a nutshell! Or, perhaps even more concisely: you cannot send information by observation

It is spooky because the particles cannot communicate with each other but they are 100% correlated no matter which basis you choose. If you could just measure "up/down" (actually "horizontal" and "vertical" for photons) then you could reproduce this with classical particles. But you can also measure them at a 45 degree angle and still get perfect correlation. If they would be horizontal/vertical (and you just don't know which one) this would not be possible.
Well, why would they need to communicate with each other? They just share a state. That state was set at the moment of entanglement, and the only thing that changed upon measurement was that we learned what the state was. Nothing particularly spooky about that, just a shared probability between two elements. The takeaway being I guess, that information is not created until causality demands it.

Edit: Oh, also, other slightly off topic question, what is the difference between polarization and spin when it comes to light?

mfb
Mentor
If they would be independent you wouldn't get the observed correlations.
Sure, if you just apply the mechanism of quantum mechanics you get the right result. That's how quantum mechanics was made. But it is surprising that this mechanism works. There is nothing like that in classical mechanics.
The takeaway being I guess, that information is not created until causality demands it.
But then it is created in two places that cannot communicate with each other in a consistent way.
Edit: Oh, also, other slightly off topic question, what is the difference between polarization and spin when it comes to light?
Polarization is the "orientation" of the spin. Don't take that too literal. Linearly polarized light is a superposition of "clockwise spin" and "counterclockwise spin", and the phase of this superposition determines the polarization axis.

If they would be independent you wouldn't get the observed correlations.
Sure, if you just apply the mechanism of quantum mechanics you get the right result. That's how quantum mechanics was made. But it is surprising that this mechanism works. There is nothing like that in classical mechanics.
To me it seems that this situation is really just an artifact of how measurement/observation works with quantum mechanics. Say you entangled a pair of photons, measured them both locally, and determined that photon A had an up spin and photon B had a down spin. Based on what you said about travel through space-time not having any affect on the entangled state of the two photons (assuming no outside forces break the entanglement), it would therefore logically follow, that even if you had sent the photons to separate locations one light year away from each other, and measured them both independently, photon A would still have an up spin, and photon B would still have a down spin. The only issues here is that this is not provable, because there is no way to create a situation where you would know the spin of one of the photons without measuring it. And if there was, it would logically follow, that the measured spin of the photon would agree with that information, which would theoretically be knowable if you could gain a complete understanding of the circumstances under which the entanglement took place. The analogy I would use, would be sending two sealed envelopes to two separate locations with the word up in one envelope and the word down in the other. If you at any point opened one of the envelopes, you would know the value of the other, but I wouldn't say this violates locality in any way.

mfb
Mentor
it would therefore logically follow, that even if you had sent the photons to separate locations one light year away from each other, and measured them both independently, photon A would still have an up spin, and photon B would still have a down spin.
That is the part classical particles could do as well. But quantum mechanics can do more. See above: It works no matter how you orient your polarizers. And that is something you cannot do with classical particles. The particles cannot have a polarization before measurement. Not even one you cannot measure. You either need superluminal communication in some way or the result cannot be unique (you get the many worlds interpretation or something similar).

Okay, let's take a little step back and see if I can state my objections to this situation a little more eloquently. We can say time has nothing to do with the measurement of an entangled particle's spin. I'm still not entirely convinced this is the case, but let's just go with it. So the question becomes, what does? Based on my understanding, we can measure an entangled photon in such a way, that the probability is basically 50/50 we will get an up or down spin, and then when we measure the spin of the other entangled photon in a similar manner, we must get the opposite spin of the first photon we measured to preserve conservation of angular momentum.

But, and again, correct me if I'm wrong here, we must say that the measurement of the first spin was a truly random event, because if we could in any way influence the outcome of the measurement that preserved the entangled state, we could in fact transfer information faster than the speed of light. So here is my problem with this. We already know it's impossible to prove that anything is random. We can theorize that is might be, but even that is a problem. Why? Well, what is randomness exactly? Again, speaking as a computer programmer, my understanding of randomness, is that randomness is information. If you have a perfect compression algorithm, that took all information in the universe and distilled it down to the basic information from which it was derived, what you would get would essentially be a mathematical seed from which the universe was derived. In other words, the only true information that the universe contained.

So if we are saying the measurement of the first photon's spin is truly random, we are essentially saying that by measuring it, we are introducing new information into the universe. Except you can't do that. That would violate the theory of conservation of information from thermodynamics. So you really can't have it both ways. You are either violating the conservation of information by creating randomness, or you are doing something that is in some way (however difficult) is predictable, meaning that you can transfer information faster than the speed of light and break causality. So which is it?

mfb
Mentor
we are introducing new information into the universe. Except you can't do that.
You can - in some interpretations of quantum mechanics.
Thermodynamics doesn't help because it doesn't know about quantum mechanical processes, but even in thermodynamics entropy can increase.

It is fundamentally unpredictable what you will measure. It doesn't have to be random. But there are just three ways to get this: Some superluminal interaction in a way you cannot use to transfer information, a universe where all measurements happen but in disconnected spaces (many worlds), or universes with retrocausality (events in the future influence the past). Different interpretations of quantum mechanics choose different options here.

You can - in some interpretations of quantum mechanics.
Thermodynamics doesn't help because it doesn't know about quantum mechanical processes, but even in thermodynamics entropy can increase.
Hmm... well that is a little convenient isn't it? We can just throw out the theory of conservation of information when we talk about quantum mechanics because... But even if you do that, you are not really addressing the fundamental problem. Even if you add this extra layer of complication by assuming quantum mechanics generates it's outcomes from information that is not correlated with anything else in the universe, how do you know that information is random? Again, you may suspect it, but you could never prove it. Well, at least not without infinite time.

It is fundamentally unpredictable what you will measure. It doesn't have to be random. But there are just three ways to get this: Some superluminal interaction in a way you cannot use to transfer information, a universe where all measurements happen but in disconnected spaces (many worlds), or universes with retrocausality (events in the future influence the past). Different interpretations of quantum mechanics choose different options here.
Basically, what I am saying is that an event in the future could influence the past, so that sounds like retrocausality to me. In any case, there are two points I really wanted to make here.

1. You cannot prove randomness exists.
2. In a case where you successfully predicted the result of the measurement of a quantum entangled particle, without influencing it in a way that broke the entanglement (which again, if you accept point #1 as being true, cannot be proven to be impossible). you could send information into the past.

So it still seems to me, that we do live in a universe where information could, and perhaps does, travel back in time. Maybe you could even go as far as to say, out future is actively informing and influencing our present.

OK, it seems like a lot of people are missing the fundamental idea here when referring to retrocausality, superluminal knucklewraps, whatever. The seperation of the photons when measured is SPACELIKE, hence one cannot provide any time ordering to the measurements. For spacelike seperated events there will always be a Lorentz transform that changes the time ordering of the measurements. Or equivalently, there will always be a lorentz transform to a frame in which the measurements happen at exactly the same time. That pretty much rules out any causal connection between the measurements, by the very definition of "same time."

The lab frame clock is completely irrelevant when it comes to specifying which photon is measured first, second or at the same time, (except in the lab frame) because for each of those possibilities, you can find a lorentz frame in which it is true. There is no way to time order spacelike seperated events. Now, let me pose a question. Given that relativity tells you that which measurement happens first is frame dependent, can you think of a more elegant solution than quantum mechanics provides? Relativity requires that spacelike seperated events cannot be causally related and quantum mechanics gives you just enough freedom for that to be the case.

This sort of experiment has been done very elegantly with moving beam splitters (Stefanov, Zbinden, Suarez and Gisin, Phys Rev Lett 88 12
25 March 2002 and https://arxiv.org/abs/quant-ph/0210015). Oddly enough, the motivation for the experiment was to test an attempt at making bohmian mechanics a "relativistic theory."

1. You cannot prove randomness exists.
2. In a case where you successfully predicted the result of the measurement of a quantum entangled particle, without influencing it in a way that broke the entanglement (which again, if you accept point #1 as being true, cannot be proven to be impossible). you could send information into the past.

So it still seems to me, that we do live in a universe where information could, and perhaps does, travel back in time. Maybe you could even go as far as to say, out future is actively informing and influencing our present.
1- The correct term to use is determinism not randomness, all quantum interpretation preserves randomness but they disagree about determinism, for example you can have perfectly random deterministic model, or a non-deterministic random model, but even in the non-deterministic interpretations you can't rule out super-determinism.

2- No you can't send information to the past, because you can't send information through entanglement in first place. In other words you can't send information by observation.

mfb
Mentor
Hmm... well that is a little convenient isn't it? We can just throw out the theory of conservation of information when we talk about quantum mechanics because...
That's why we have so many different interpretations of quantum mechanics. Different people prefer different unintuitive results. But every interpretation has some unintuitive element, you cannot avoid that. "Spooky action at a distance" is one option.
how do you know that information is random? Again, you may suspect it, but you could never prove it. Well, at least not without infinite time.
It does not matter if it is random or not.
"Random" and "fundamentally unpredictable" are different things.
2. In a case where you successfully predicted the result of the measurement of a quantum entangled particle, without influencing it in a way that broke the entanglement (which again, if you accept point #1 as being true, cannot be proven to be impossible). you could send information into the past.
If you could that. But you cannot. And that you can prove.

Boing3000
Gold Member
They just share a state.
OK, that's actually what entangled mean. There is one state.

That state was set at the moment of entanglement, and the only thing that changed upon measurement was that we learned what the state was.
But the mistake is to think you know that state at entanglement. Whatever the value is, the distant observer are only able to test it along some arbitrary angle. And then there is a randomness in the result...

Maybe you'll get convinced that information don't go anywhere (it is not local) by this little simulator

DarMM
Gold Member
Let's try a different approach. I'll do this in steps.

The correlations in this case are stronger than Quantum Mechanic's, but simpler to explain and discuss.

Imagine you have two photons and either can be measured in one of two directions: $X, Z$ with two possible outcomes $0, 1$, representing horizontal and vertical polarisation in that direction. If you don't know photons well enough to know polarisation, just ignore all that and just imagine the photons can be measured in two ways, each with two outcomes.

Now imagine the following correlation existed between them.
1. If they both measure $X$, the results are the same. $\frac{1}{2}$ chance of either $0, 1$
2. If they one measures $X$ and the other measures $Z$, the results are the same. $\frac{1}{2}$ chance of either $0, 1$
3. If they both measure $Z$, the results will be different. $\frac{1}{2}$ chance of measuring either of the two cases, those being: First person gets $0$ and Second gets $1$ and vice versa.
Try to obtain a list of possible values for $X, Z$ for each photon that obeys the above rules.

morrobay
Gold Member
It is fundamentally unpredictable what you will measure. It doesn't have to be random. But there are just three ways to get this: Some superluminal interaction in a way you cannot use to transfer information, a universe where all measurements happen but in disconnected spaces (many worlds), or universes with retrocausality (events in the future influence the past). Different interpretations of quantum mechanics choose different options here.
So two entangled photons are in superposition HV and with 1/2 probability of measuring horizontal or vertical on first photon A that is measured at any angle θ.
When photon B is measured at same angle θ it is always correlated with photon A , either both vertical or both horizontal.
Well there is a fourth way to account for these correlations : They were created correlated when entangled. But this deterministic local realistic model was dis proved by Bells inequality violations ( upper bound of 2 ) for model

The analogy I would use, would be sending two sealed envelopes to two separate locations with the word up in one envelope and the word down in the other. If you at any point opened one of the envelopes, you would know the value of the other, but I wouldn't say this violates locality in any way.
You might initially think that was true but it turns out that in quantum mechanics that is not the case, and this has been proved by numerous experiments. Look up Bell's Inequality or Bell's Theorem.

http://math.ucr.edu/home/baez/physics/Quantum/bells_inequality.html

Recently, we have been able to close loopholes in Bell's Inequality.

https://physicstoday.scitation.org/doi/10.1063/PT.3.3039?journalCode=pto&

https://astronomynow.com/2018/08/21/closing-a-loophole-in-bells-theorem-with-light-from-ancient-quasars

1- The correct term to use is determinism not randomness, all quantum interpretation preserves randomness but they disagree about determinism, for example you can have perfectly random deterministic model, or a non-deterministic random model, but even in the non-deterministic interpretations you can't rule out super-determinism.
Superdeterminism is just a synonym for god. It's religion cloaked in technobabble as it has no physical content. Along those same lines, no one can rule out creationism either, but creationism also is not science.