# Simultaneously Measuring Entangled Particles

In summary, when measuring the spins of entangled particles simultaneously with respect to the reference frame of the source, they will always be opposite spins as predicted by the theory. However, it is uncertain if the particles can be measured instantly, and it is believed that they will remain opposite even during the uncertain, non-measured state. The distance between the particles does not affect the "instant" action of the measurement, leading to the concept of "spooky" action at a distance. The Copenhagen interpretation suggests that the wave function collapses as soon as one particle is measured, while the Many Worlds interpretation suggests that both possibilities exist in different worlds. There is no single experiment that cannot be explained without the wave function collapse, and there is no known explanation for

Let's say you have entangled particles emitted from a source. What happens if you measure the spin of both entangled particles simultaneously with respect to the reference frame of the source?

Would the spins measured still be opposite (anti-correlated)? Would you even get a definite measurement? Thanks.

They will be opposite spins as the theory goes...

How ever - I am not sure we really can measure them instantly at the same time, even computers probably are not always going to ping dead on time (even if were talking unimaginably small timing differences)

Me personally the more exciting question is how are they connected over these distances. Is it a new energy that can travel faster than light, are they connected in a higher dimension outside of our time dimension etc.

sirchick said:
They will be opposite spins as the theory goes...

Could you elaborate? Since the measurements are being taken at the same time with respect to the reference frame of the source, aren't the spins of both the particles still indefinite at the instant before the measurements? As a result, isn't there a probability that the particles will have the same measured spin? I think I'm misunderstanding something.

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The short answer is we would get opposite spins if measured both of them instantly from what we believe. I don't believe we ever could measure them instantly how ever.

If a zero spin particle decays to two entangled particles - their spin total when combined has to equal the zero spin because that's where they came from.

So they always have to be opposite (and I am assuming here even during the uncertain non measured state - both of them are still opposite). But as we can never know that without measuring which collapses the uncertainty we can only assume logically they stay opposite to keep every thing balanced to zero spin. But weather they are up or down spin is completely random.

As far as we know - when you measure one or both, the spins are set "instantly" no time has past when this occurs - so if our computers really could measure instantly (which they probably cannot even if they difference is so small it appears instant to us mere mortals...) they would still be opposite spins.

Also the distance between the particles does not change this "instant" action. They could span the universe and appear to change instantly...we have tested such things on quite large distances on Earth with no delay.

This is why its "spooky".

It does not matter at which time you measure the spins.
In addition, "at the same time" cannot be exact, the measurement itself requires some finite time.

Is it a new energy that can travel faster than light, are they connected in a higher dimension outside of our time dimension etc.
No. It depends on your favourite interpretation of QM, but they all have some way to explain this.

How does the Copenhagen interpretation explain it?

As soon as you measure (at least) one particle, the wave function of both collapses into a state corresponding to your measurement.

Many worlds gives you two worlds - if you measure "horizontal"/"vertical", for example, one world ("A horizontal, B vertical") and one ("A vertical, B horizontal").

Sure. Now what does "as soon as" mean, in a relativistic context? In which rest frame are they simultaneous. If I measure particle A, how does the Copenhagen interpretation decide when to affect particle B?

It does not matter in which frame you look at the system. One frame might say "the measurement of A came first and collapsed the system", an another frame could say "the measurement of B came first and collapsed the system".

As I never saw some equation handling those collapses, I do not like the Copenhagen interpretation, but it is somehow well-defined what happens.

mfb said:
No. It depends on your favourite interpretation of QM, but they all have some way to explain this.

What do you mean no - its still entirely possible they communicate with some kind of energy unknown to us, or are connected in higher dimensions and so the wave collapses much faster than light.

Can't rule those out.

Right, or the collapse fairies.

Ok, thanks, mfb.

Bill_K said:
Right, or the collapse fairies.

Ok, thanks, mfb.

Mock if you must but there's no real reason to suggest what I'm saying is any more wrong/right than what you believe. We know zero about how it happens instantly so its any ones guess.

Well, Bill_K is right: You can invent anything, but as long as there is no experimental test for this, you should be careful with words with a fixed meaning. Why would you call that "energy" or "connection in higher dimensions", if you have absolutely no hint that it is in any way related to energy or spacetime geometry?

Okay good point, but is there really any good ideas that can be experimented regarding how it happens ?

Experiments have been performed to measure them as close in time as possible. It makes no difference to the outcome how you order the observations (according to both theory and within experimental limits). A before B, B before A, or same time.

If there would be an experiment showing collapse at all (independent of the mechanism), collapse interpretations would be a theory, not interpretations.
In other words: There is no single experiment which cannot be explained without collapses of wave functions. To avoid triple negation: All experiments can be explained without collapses.

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So we have no real explanation how their spins become set instantly regardless of distance between them upon a measurement of either, or both??

I've also heard a suggestion that the "first" measurement may send a signal backwards in time to when the entangled particles were first produced, ensuring that the spins are always opposite.

This seems to solve the issue about the ordering of the measurements, and removes a bit of the "spookiness" involved with the particles being spatially separated.
I'm not sure what other issues this might create though. And I can't think of an experiment which could test for this.

Interesting though ...

Joncon said:
I've also heard a suggestion that the "first" measurement may send a signal backwards in time to when the entangled particles were first produced, ensuring that the spins are always opposite.

This seems to solve the issue about the ordering of the measurements, and removes a bit of the "spookiness" involved with the particles being spatially separated.
I'm not sure what other issues this might create though. And I can't think of an experiment which could test for this.

Interesting though ...

Time Symmetric Quantum Mechanics. Beautiful interpretation. Search it through the Forum. In some threads you will find links to very interesting papers. It preserves locality but ruins causality.
I really like not only the preservation of locality but also leaving causality aside because it makes you think as time being much more similar to space than what one usually does (in this interpretation the arrow of time problem sort of does not occur in microscopic world and arises in the macroscopic human world -by the way my thought about it is that it is a human illusion which arises as a consequence of the fact that we, human beings, are consumers of ordered energy and producers of entropy, and, because of this, we order our life from situation of low entropy -past- to situations of high entropy -future--)

Ps: Remember that all that I said is just an interpretation which probably will never be tested! (Nevertheless I have faith that someone much more intelligent than me will do it!)

Thanks the_pulp, I'll take a look at those

the_pulp said:
Time Symmetric Quantum Mechanics. Beautiful interpretation. Search it through the Forum. In some threads you will find links to very interesting papers. It preserves locality but ruins causality.
I really like not only the preservation of locality but also leaving causality aside because it makes you think as time being much more similar to space than what one usually does (in this interpretation the arrow of time problem sort of does not occur in microscopic world and arises in the macroscopic human world -by the way my thought about it is that it is a human illusion which arises as a consequence of the fact that we, human beings, are consumers of ordered energy and producers of entropy, and, because of this, we order our life from situation of low entropy -past- to situations of high entropy -future--)

Wow. That's deep stuff.

mfb said:
It does not matter in which frame you look at the system. One frame might say "the measurement of A came first and collapsed the system", an another frame could say "the measurement of B came first and collapsed the system".

So causality can be violated?

Since you cannot know both the position and momentum of each particle with precision knowing whether or not you preformed the measurement instantaneously is impossible.

the_pulp said:
Time Symmetric Quantum Mechanics. Beautiful interpretation. Search it through the Forum. In some threads you will find links to very interesting papers. It preserves locality but ruins causality.
I really like not only the preservation of locality but also leaving causality aside because it makes you think as time being much more similar to space than what one usually does (in this interpretation the arrow of time problem sort of does not occur in microscopic world and arises in the macroscopic human world -by the way my thought about it is that it is a human illusion which arises as a consequence of the fact that we, human beings, are consumers of ordered energy and producers of entropy, and, because of this, we order our life from situation of low entropy -past- to situations of high entropy -future--)

Ps: Remember that all that I said is just an interpretation which probably will never be tested! (Nevertheless I have faith that someone much more intelligent than me will do it!)

If you could construct a closed timelike curve where entropy always increases and Novikov's Self Consistency Principle is satisfied I don't see any problem with this.

Originally Posted by the_pulp
Time Symmetric Quantum Mechanics. Beautiful interpretation. Search it through the Forum. In some threads you will find links to very interesting papers. It preserves locality but ruins causality.
I really like not only the preservation of locality but also leaving causality aside because it makes you think as time being much more similar to space than what one usually does (in this interpretation the arrow of time problem sort of does not occur in microscopic world and arises in the macroscopic human world -by the way my thought about it is that it is a human illusion which arises as a consequence of the fact that we, human beings, are consumers of ordered energy and producers of entropy, and, because of this, we order our life from situation of low entropy -past- to situations of high entropy -future--)

Ps: Remember that all that I said is just an interpretation which probably will never be tested! (Nevertheless I have faith that someone much more intelligent than me will do it!)

If you could construct a closed timelike curve where entropy always increases and Novikov's Self Consistency Principle is satisfied I don't see any problem with this.

Sorry, I can't follow you completely because I am not too strong in General Relativity (just the basics from second volume of Landau Lifgarbagez), so:

1) How can we make a closed timelike curve where entropy always increases? Isn't entropy a function of the state. If I follow a closed timelike curve I should arrive, after a 2pi loop, to the same state and, as a consequence, to the same entropy. I can't see it.
2) When you say that there is no problem with this, what's this? what could have had a problem? Which was that problem?

So causality can be violated?
No. In every frame, there is a description which does not involve any propagation backwards in time. And all frames give the same measurements.

There is no movement(and hence no causality) in spacetime, according to GR. Causality must be emergent/secondary in quantum mechanics as well, experiments like that of Aspect prove this(unless one believes in instant communications between particles which is absurd for the reason that particles have no known abilities to communicate). Claims that particles are able to communicate ftl are crackpottery and the basis for such claims is someone's philosophical beliefs.

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There is movement in spacetime in GR, and a local time direction everywhere.

Causality must be emergent/secondary in quantum mechanics as well, experiments like that of Aspect prove this(unless one believes in instant communications between particles which is absurd for the reason that particles have no known abilities to communicate).
Well, some interpretations of QM are time-reversible or at least do not have a unique "direction of time". In this case, you could view the future as cause for the present, this is just a bit unconventional in terms of entropy.
You do not need the concept of causality in an explicit way to calculate anything, do you mean this with "emergent/secondary"?

mfb said:
There is movement in spacetime in GR, and a local time direction everywhere.

You confuse clocks ticking with an implied preferred frame of reference, which does not exist. Without a preferred FOR time does not move and objects do not change. Observed time in personal experience probably has to do with how we experience it, not how it really is. The experienced arrow of time shares the same fate.

Well, some interpretations of QM are time-reversible or at least do not have a unique "direction of time". In this case, you could view the future as cause for the present, this is just a bit unconventional in terms of entropy.
You do not need the concept of causality in an explicit way to calculate anything, do you mean this with "emergent/secondary"?

Yes, that's what i meant, causality comes out of the theory at the classical limit.

You confuse clocks ticking with an implied preferred frame of reference, which does not exist. Without a preferred FOR time does not move and objects do not change.
Why? Locally, clocks tick in every frame.
If you consider objects as 4-dimensional part of spacetime, then they do not change their 4-dimensional shape... well this is obvious, you just reduced time to a coordinate. This is just playing with words.

Joncon said:
I've also heard a suggestion that the "first" measurement may send a backwards in time to when the entangled particles were first produced, ensuring that the spins are always opposite.

This seems to solve the issue about the ordering of the measurements, and removes a bit of the "spookiness" involved with the particles being spatially separated.
I'm not sure what other issues this might create though. And I can't think of an experiment which could for this.

Interesting though ...

retrocausality from the
Two State Vector Formalism.

the_pulp said:
Time Symmetric . . it through the Forum. In some threads you will to very interesting papers. It preserves locality but ruins causality.
I really like not only the preservation of locality but also causality aside because it makes you think as time being much more similar to space than what one usually does (in this the arrow of time problem sort of does not occur in microscopic world and arises in the macroscopic human world -by the way my thought about it is that it is a human illusion which arises as a consequence of the fact that we, human beings, are consumers of ordered energy and producers of entropy, and, because of this, we order our life from situation of low entropy -past- to situations of high entropy -future--)

Ps: Remember that all that I said is just an which probably will never be tested! (Nevertheless I have that someone much more intelligent than me will do it!)

Physical Review A 79, 052110
Multiple-time states and multiple-time measurements in quantum mechanics
Yakir Aharonov, Sandu Popescu, Jeff Tollaksen, and Lev Vaidman

... Finally we discuss the implications of our approach to quantum mechanics for the problem of the flow of time...

Maui said:
There is no movement(and hence no causality) in spacetime, according to GR.

the block universe, recently i opened a thread about, but it was deleted.

JPBenowitz said:
If you could construct a closed timelike curve where entropy always increases and Novikov's Self Consistency Principle is satisfied I don't see any problem with this.

from CTCs

Perfect State Distinguishability and Computational Speedups with Postselected Closed Timelike Curves
Foundations of Physics March 2012, Volume 42, Issue 3, pp 341-361

...an entangled state efectively creates a noiseless quantum channel into the past...

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and there are Timlike Entanglement , i.e. entanglements in time.

Physical Review A 85, 012306 (2012)
...entangled between timelike separated regions of spacetime...
...non-separability across time...

----
and an experiment proposed:

Physical Review Letters 109, 033602 (2012)
...We propose a realistic circuit QED experiment to test the extraction of past-future vacuum entanglement to a pair of superconducting qubits...
...We show that this experiment can be realized with current technology and discuss its utility as a possible implementation of a quantum memory...

i have the complete papers if somebody wish it.

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mfb said:
arXiv has them, too:

Extraction of timelike entanglement from the vacuum (your link there seems broken)
Extracting past- vacuum correlations using circuit QED

yes, i know, but... mod`s do not like to much, that forums users put arxiv papers.
you know...

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They are just different links to the same thing - the publications were accepted (and therefore passed peer review), so they shouldn't be completely crap.

Rules said:
References that appear only on http://www.arxiv.org/ (which is not peer-reviewed) are subject to review by the Mentors. We recognize that in some fields this is the accepted means of professional communication, but in other fields we prefer to wait until formal publication elsewhere.
I did not highlight "only", this is already done in the rules.