# Complete predictability contradicts relativity I think

#### bogdan

If we can find a formula to determine the state of a system in a future moment then we have the informations about that system before light could travel to us...got it ?
So...isn't this in contradiction with relativity ?
Or something like time travel ? (in the future)

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#### plus

No, this is related to inference.

If you can infer things from the information available to you, then that information is already with you, it does not have to travel faster than light.

#### Locutus

Yes.

Imagine a 2 men left the planet earth, 1 with a cat and the other with a dog. They were each headed in opposite directions until one man reached Venus and the other reached Mars. When the man's spaceship arrives on Venus, the colonists on Venus (use your imagination) don't know what he is carrying with him, but they know it is either a cat or a dog. As soon as they see the man was carrying a dog with him, they instantly know that the man who travelled to Mars brought along a cat.

Therefore, information about the spaceship on Mars was discovered virtually instantaneously. This also would seemingly violate relativity, but common sense tells us this is not true. I think this idea may also have something to do with quantum nonlocality, but I'm not the right person to ask.

#### Hydr0matic

Therefore, information about the spaceship on Mars was discovered virtually instantaneously. This also would seemingly violate relativity, but common sense tells us this is not true.
Indeed this is not true. Information about either of the animals will yield information about the other, so all info is obtained at Venus and none has to travel from Mars. No quantum weirdness here.

#### Locutus

Okay then can someone explain what is quantum nonlocality? I originally thought it would have something to do with this because the experiment that proved it involved measuring the spin after splitting up pions (i think).

#### ultrafind

Forgive my naivity, but isn't this analogous to the EPR paradox and yes quantum nonlocality? What do we have to say? ...

#### damgo

Quantum nonlocality refers to the fact that measurements done on an object in one location can instantaneously affect another object far away, in seeming violation of relativity.

eg, we can imagine setting up two pet carriers A,B in a quantum state that is 50% |dog in A, cat in B> and 50% |cat in A, dog in B>. They travel apart to Mars/Venus, and then A opens his carrier an instant before B. If A is measured to be a dog, then B will always find a cat, even though relativity forbids the sending of signals between the two.

Einstein (cf EPR Paradox) took this to mean that in each case there is something determining whether the dog will be found in A or B, since if the measurement really affected it locality would be violated. But a rather clever idea (cf Bell inequality) and experiment (cf Aspect) proved that this was not the case; the measurement at A does affect B, apparently instantaneously.

However, it turns out you can't actually send any information this way -- you always have to "compare notes" afterwards to notice the effect.

#### bogdan

Just an idea...don't take it serious...

#### Hydr0matic

But a rather clever idea (cf Bell inequality) and experiment (cf Aspect) proved that this was not the case; the measurement at A does affect B, apparently instantaneously.
There are loopholes in this experiment .. see:
http://users.aber.ac.uk/cat/Html/vigier.htm [Broken]

I agree with the author of this site and believe that such a ridiculous concept as nonlocality should be investigated more thoroughly..

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#### Locutus

Originally posted by damgo
Quantum nonlocality refers to the fact that measurements done on an object in one location can instantaneously affect another object far away, in seeming violation of relativity.

eg, we can imagine setting up two pet carriers A,B in a quantum state that is 50% |dog in A, cat in B> and 50% |cat in A, dog in B>. They travel apart to Mars/Venus, and then A opens his carrier an instant before B. If A is measured to be a dog, then B will always find a cat, even though relativity forbids the sending of signals between the two.

Einstein (cf EPR Paradox) took this to mean that in each case there is something determining whether the dog will be found in A or B, since if the measurement really affected it locality would be violated. But a rather clever idea (cf Bell inequality) and experiment (cf Aspect) proved that this was not the case; the measurement at A does affect B, apparently instantaneously.

However, it turns out you can't actually send any information this way -- you always have to "compare notes" afterwards to notice the effect.
How does this example differ from the one I originally posted, which hydr0matic said was not an example of quantum nonlocality?

#### Hydr0matic

How does this example differ from the one I originally posted, which hydr0matic said was not an example of quantum nonlocality?
There are no quantum states in the macroscopic world, so when you say the animal is either a cat or a dog, this usually means just that, not that the animal is in a 50/50-cat/dog quantum state.

Your example had two flaws - first, you left out the quantum state part... second, you said that the information about the animal on Mars travelled to Venus instantaneously, but as I said earlier the info about both animals is obtained at Venus. It's the collapse of the quantum state that supposedly travels instantaneously (Venus -> Mars).

#### Hurkyl

Staff Emeritus
Gold Member
There are loopholes in this experiment .. see:
http://users.aber.ac.uk/cat/Html/vigier.htm [Broken]

I agree with the author of this site and believe that such a ridiculous concept as nonlocality should be investigated more thoroughly..
You should be wary of any scientific paper whose abstract involves a conspiracy theory.

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#### Hydr0matic

You should be wary of any scientific paper whose abstract involves a conspiracy theory.
Did you read the paper though ? I've read almost everything on her site and I believe she's right in questioning the experimental results of the Bell tests. A concept such as nonlocality should not be accepted without overwhelming empirical evidence.

#### maximus

Originally posted by bogdan
If we can find a formula to determine the state of a system in a future moment then we have the informations about that system before light could travel to us...got it ?
So...isn't this in contradiction with relativity ?
Or something like time travel ? (in the future)
well, we could go one with IFs, but the truth is that at this point, that is impossible so there really isn't any reason to ask it. and also the uncertanty prinsiple tells us that we cannon know the complete state of a system. that was called determainism...

#### maximus

Re: Re: Complete predictability contradicts relativity...I think...

Originally posted by maximus
well, we could go one with IFs, but the truth is that at this point, that is impossible so there really isn't any reason to ask it. and also the uncertanty prinsiple tells us that we cannon know the complete state of a system. that was called determainism...

i'm sorry. i was completely wrong. we must always question our knowledge.

#### Hurkyl

Staff Emeritus
Gold Member
Did you read the paper though ? I've read almost everything on her site and I believe she's right in questioning the experimental results of the Bell tests. A concept such as nonlocality should not be accepted without overwhelming empirical evidence.
I have read the paper several times. I don't know enough about the experiment and the terminology involved to follow the technical details of the paper.

However, I can follow her train of thought. While she's right to question things, she appears to approach it from a crackpot's perspective and not a scientific perspective. I would have taken her much more seriously if she wasn't accusing experimentalists of fakery without proof, and making the classic fallacy of taking the (alledged) lack of evidence for nonlocality as proof for her alternative pet theory (for which she does not give one shred of evidence in that paper, though she talks about it a lot).

#### heusdens

Originally posted by damgo
Quantum nonlocality refers to the fact that measurements done on an object in one location can instantaneously affect another object far away, in seeming violation of relativity.

eg, we can imagine setting up two pet carriers A,B in a quantum state that is 50% |dog in A, cat in B> and 50% |cat in A, dog in B>. They travel apart to Mars/Venus, and then A opens his carrier an instant before B. If A is measured to be a dog, then B will always find a cat, even though relativity forbids the sending of signals between the two.

Einstein (cf EPR Paradox) took this to mean that in each case there is something determining whether the dog will be found in A or B, since if the measurement really affected it locality would be violated. But a rather clever idea (cf Bell inequality) and experiment (cf Aspect) proved that this was not the case; the measurement at A does affect B, apparently instantaneously.

However, it turns out you can't actually send any information this way -- you always have to "compare notes" afterwards to notice the effect.
This quantum locality thing seems to me nothing but a weird interpretation of things, and has much to do with interpretation.

In the experiment there is a particle split in two parts. One has spin up, the other has spin down. But which particle has spin up or spin down, we do not know in advance, but can only be measured.

QM interprets that each particle is in a 50-50% state of spin up/spin down. But that is just an interpretation. A more comprehensible interpretation is that the state is simply unknown, untill detected.
And as soon as we observe one particle to have spin down, we already known that the other (wether it is observed or nor) has spin up.

If we just conform to the more comprehensible idea of the state of the particle to be "unknown", but correlated (one is spin up, the other down, but which is which we only know after observing at least one). There is no information travelling to be assumed here.
That makes things far too complicated as has to.

The experiment is not different from a macro scale experiment.
We take two cards, one is Ace the other King. Put them face down on the table. Some other person enters the room who knows about the cards but did not see them, and as soon as he takes one card and observes it, he know what the other card is. Suppose the other card is not on the same table, but on a table on the moon, this makes no difference. Did information travel from earth to the moon in a fraction of a split second, even faster then the speed of light? Of course not.

#### ahrkron

Staff Emeritus
Gold Member
Originally posted by heusdens
QM interprets that each particle is in a 50-50% state of spin up/spin down. But that is just an interpretation. A more comprehensible interpretation is that the state is simply unknown, untill detected.
And as soon as we observe one particle to have spin down, we already known that the other (wether it is observed or nor) has spin up.
It is not that simple; think about the following:

In a double slit experiment, photons will hit the screen behind the slits here and there. After a while, a pattern emerges that shows an interference pattern.

Now, things start to be weird when you realize that, according to QM, this should happen (and does happen) even if you send one photon at a time.

This is probably already enough to see what the "collapse" implies. Here is why:

Consider one of these photons. If the wavefunction was just a description of our ignorance, as you claim, then the photon would go through one slit, and be unaffected by the presence of the other slit (the hit it produces on the screen shoes that the photon is far smaller than the slits, and also smaller than the distance between them). As a result, the pattern you get with two slits should be the same as the sum of two single-slit experiments.

But that is not the case. When two slits are open, you get a very different hit pattern (an interference pattern). So, somehow, the pattern formed in the screen has information about both slits.

This means that the wavefunction (which allows you to describe the formation of the interference pattern) has more to it than just our incomplete knowledge.

Also, say you put a photon detector at one of the slits (that allows the photon to continue the journey to the screen). The moment you do this, the intreference pattern is lost.

i.e., the experiment has been transformed from one in which the wavefunction can interfere with itself, to one in which at some point it gets collapsed into "Passed though slit one".

So, even in this simple experiment, the moment you detect a photon to have passed (or not passed) through slit1, say, the wavefunction collapses, changing instantaneously what happens at the other slit, and the overall result of the experiment (interference pattern vs. sum of single slit experiments).

#### Hydr0matic

Also, say you put a photon detector at one of the slits (that allows the photon to continue the journey to the screen)
This can't be done, can it ? You can't detect a photon without disturbing it.

How does this photon/wavefunction model explain the superpositioning aspect of the interference experiment ? Because, as I understand it, there is no superpositioning taking place with the photons, right ?

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