What's wrong with this FTL experiment?

[San K]

Back to Alice and Bob. Alice is trying to transmit solar flare information faster than light to Bob, say 8 minutes earlier.



Alice is sitting close to the sun, watching for solar flares.

Bob is sitting on earth.

The deal is:

Alice

if there is solar flare before noon, alice will look at the entangled photon A and collapse the wave function.

if there is no solar flare before noon, alice will not look at the entangled photon A...and let the entanglement continue...and the wavefunction would stay "intact"

Bob

Bob will look at entangled photon B at noon plus 1 millisecond.

if the photon B is already in a state that indicates the wave function has collapsed then a solar flare is on its way

if the photon B was intact but Bob collapsed it then no solar flare is coming


or in summary:

can bob (or alice) tell when looking at the photon...if the wavefunction *had* collapsed or they collapsed when they took a look?

 

[Vanadium 50]

What measurement does Bob do?

 

[The_Duck]

can bob (or alice) tell when looking at the photon...if the wavefunction *had* collapsed or they collapsed when they took a look?

No, they can't.

 

[JesseM]

"collapsing the wavefunction" is just something that happens in our mathematical model, if Alice collapses the wavefunction in the model that means our predictions about the probabilities Bob gets various outcomes changes depending on what Alice got, but individually Bob's result still looks randomly, it's only when both their results are compared that you see a correlation. For example, each might get result + randomly on half the trials and - on the other half, but when you compare you see that on every trial where Alice got +, Bob got - and vice versa. So this means that prior to Alice's measurement we'd predict a 50% chance Bob gets + and a 50% chance he gets -, but if Alice performs a measurement and gets -, this collapses the wavefunction so we now predict a 100% chance Bob gets +. This is of no help to Bob, though, because he doesn't know that Alice got -, and it's still true that his results look random with him getting + half the time and - half the time.

 

[San K]

What measurement does Bob do?

Bob looks at Photon B at time noon plus 1 second,

while Alice looks (or does not look) at photon A at noon.

Now when Bob looks at photon at noon plus 1 second, is there any way for Bob to guess/determine whether Alice has already collapsed the wave function at noon?

or simply put...

it there any way to tell if the wave function has collapsed between a photon pair *without* comparing both the photons?

 

[Ryan_m_b]

it there any way to tell if the wave function has collapsed between a photon pair *without* comparing both the photons?

In a word: No

As an analogy think of it this way (note this is an analogy designed to illustrate the concept, it may not be accurate in terms of the effect, I'm not a physicist but a physicist once told me this to get my head around the same question):

Charlie and Dave both have a box, in each box is a spinning coin. To open a box a button must be pressed that opens the lid 1 second later. In that 1 second the coin falls over and is either heads or tails, not only does the coin fall over but the coin in the other box falls over (showing the opposite face). If Charlie presses his button he has no way of knowing if the coin has fallen because Dave has previously pressed his button or if he caused it to fall by pressing his own button. The only way is to ask Dave which removes the FTL aspect I'm afraid

 

[Vanadium 50]

it there any way to tell if the wave function has collapsed between a photon pair *without* comparing both the photons?

No. Hence my question: what does Bob measure.

Here's an example: Bob measures the polarization as up or down. Here's his outcome for the first 30 photons - UDDDUDUUU DDUDUDDDU UDDUDUDUD.

What does that tell us about Alice?

 

[San K]

In a word: No

As an analogy think of it this way (note this is an analogy designed to illustrate the concept, it may not be accurate in terms of the effect, I'm not a physicist but a physicist once told me this to get my head around the same question):

Charlie and Dave both have a box, in each box is a spinning coin. To open a box a button must be pressed that opens the lid 1 second later. In that 1 second the coin falls over and is either heads or tails, not only does the coin fall over but the coin in the other box falls over (showing the opposite face). If Charlie presses his button he has no way of knowing if the coin has fallen because Dave has previously pressed his button or if he caused it to fall by pressing his own button. The only way is to ask Dave which removes the FTL aspect I'm afraid

thanks ryan...good analogy...

now if Alice and Bob made a pact...that if at Noon + 1 second, if Bob gets - (i.e. if Alice got +), Alice would send a blue ball, else a red ball.

So now Bob knows... whether he will get a blue ball or red ball...however this is not useful information ...but maybe its partially useful?

 

[Ryan_m_b]

thanks ryan...good analogy...

now if Alice and Bob made a pact...that if at Noon + 1 second, if Bob gets - (i.e. if Alice got +), Alice would send a blue ball, else a red ball.

So now Bob knows... whether he will get a blue ball or red ball...however this is not useful information ...but maybe its partially useful?

What do you mean by blue ball and red ball exactly? The problem here is that no information is transmitted FTL

 

[San K]

No. Hence my question: what does Bob measure.

Here's an example: Bob measures the polarization as up or down. Here's his outcome for the first 30 photons - UDDDUDUUU DDUDUDDDU UDDUDUDUD.

What does that tell us about Alice?

for alice would it not be: DUUUD...

i.e. exactly opposite with 100% certainty?

 

[San K]

What do you mean by blue ball and red ball exactly? The problem here is that no information is transmitted FTL

ryan

lets make a new Alice and Bob experiment.

At noon, if Alice gets + (i.e. if Bob get - at noon plus 1) then...lunch would be Apples

At noon, if Alice gets - (i.e. if Bob get + at noon plus 1) then...lunch would be Berries

But then this information is not useful, its simply toss of a coin, however is this information not "partially" useful?the random results have been transmitted FTL (as always happens during entanglement, all the time) it might not be useful information but its some information...

I.e. both Alice and Bob know what will be for lunch at the same time (even though sitting on different planets) even if it was a toss, both know what other got for lunch at noon, at noon.
so in a sense...

i.e. information does get transmitted, but its not "useful/causal" information?

i.e. the law of conservation of momentum is transmitted FTL but its not "material/causal/useful" information?

 

[Ryan_m_b]

ryan

lets make a new Alice and Bob experiment.

At noon, if Alice gets + (i.e. if Bob get - at noon plus 1) then...lunch would be Apples

At noon, if Alice gets - (i.e. if Bob get + at noon plus 1) then...lunch would be Berries

But then this information is not useful, its simply toss of a coin, however is this information not "partially" useful?

so in a sense...

i.e. information does get transmitted, but its not "useful/casual" information

the random results have been transmitted FTL (as always happens during entanglement, all the time) it might not be useful information but its some information...

I.e. both Alice and Bob know what will be for lunch at the same time (even though sitting on different planets) even if it was a toss, both know what other got for lunch at noon, at noon.

No information has actually been transmitted faster than light but the results of a physical phenomenon that happens FTL have helped determine a pre-arranged reaction. It's like if we have a woman who always wears one green and one red sock but alternates which foot she wears each on randomly. One day we see that her left sock is green, we know that her right sock is red even though no information has been transmitted.

Alice and Bob know that the other is probably having X for lunch because both agreed to do so based on the results of their entanglement experiment. But this is nothing special, Alice and Bob could have both pulled envelopes out of a hat that they are under orders to open at Noon. The letters either say "Berries" or "Apples" and they know that these are the two options, do you see? There is no FTL communication going on, merely a phenomenon which helps them decide what to do based on pre-arranged rules.

Good question though,

 

[jtbell]

for alice would it not be: DUUUD...

i.e. exactly opposite with 100% certainty?

Yes. However, Alice cannot control whether she gets a U or a D with any particular photon. She simply gets a random sequence of U's and D's. Because she has no control over the sequence, she cannot use it to send a message to Bob.

 

[San K]

the law of conservation of momentum is transmitted FTL but its not "material/causal/useful" information?

 

[maverick_starstrider]

the law of conservation of momentum is transmitted FTL but its not "material/causal/useful" information?

I don't see where conservation of momentum has come in anywhere. I don't know what you mean by that. However, in the situation you're describing absolutely nothing quantum or FTL is occurring. It's an entirely normal situation. A mother is making her kid lunch and they (both the mother and the kid) know that there is only a tuna fish sandwich and a turkey sandwich (and they both know that only one sandwich is given at lunch and that the mother doesn't restock, etc.). The boy gets his lunch box and goes to school and at lunch he opens it and find a turkey sandwich. From that he infers that there is still a tuna sandwich at home and that's what he'll be having tomorrow.

There absolutely nothing quantum or, even, interesting about this situation. No "information" is being transmitted FTL or otherwise. This has nothing to do with the essence of entanglement. Person B is doing an experiment elsewhere to determine the value of some variable X. In this case person B is the kid, the "experiment" is when he opens his lunchbox to see what he has and the variable X is whether the sandwich is tuna or turkey. The results of the experiment is determined by person A at an earlier time (what the mother decided to put in). This really has nothing to do with quantum mechanics.

The kid could open his lunchbox at lunch and see his turkey sandwich and curse to himself because he really hates turkey (nevermind why he didn't tell his mother earlier). But knowing this at noon there's no way he can change the result from earlier that day (i.e. violate causality).

 

[JesseM]

I don't see where conservation of momentum has come in anywhere. I don't know what you mean by that. However, in the situation you're describing absolutely nothing quantum or FTL is occurring. It's an entirely normal situation. A mother is making her kid lunch and they (both the mother and the kid) know that there is only a tuna fish sandwich and a turkey sandwich (and they both know that only one sandwich is given at lunch and that the mother doesn't restock, etc.). The boy gets his lunch box and goes to school and at lunch he opens it and find a turkey sandwich. From that he infers that there is still a tuna sandwich at home and that's what he'll be having tomorrow.

But Bell's theorem proves this sort of explanation of entanglement--where both particles just have identical preexisting values for any of the measurable properties that we find to be perfectly correlated whenever we measure them--cannot actually agree with the statistical predictions of QM. See my [post=3237782]lotto card analogy[/post] to get an idea of why.

 

[San K]

I don't see where conservation of momentum has come in anywhere. I don't know what you mean by that. However, in the situation you're describing absolutely nothing quantum or FTL is occurring. It's an entirely normal situation. A mother is making her kid lunch and they (both the mother and the kid) know that there is only a tuna fish sandwich and a turkey sandwich (and they both know that only one sandwich is given at lunch and that the mother doesn't restock, etc.). The boy gets his lunch box and goes to school and at lunch he opens it and find a turkey sandwich. From that he infers that there is still a tuna sandwich at home and that's what he'll be having tomorrow.

There absolutely nothing quantum or, even, interesting about this situation. No "information" is being transmitted FTL or otherwise. This has nothing to do with the essence of entanglement. Person B is doing an experiment elsewhere to determine the value of some variable X. In this case person B is the kid, the "experiment" is when he opens his lunchbox to see what he has and the variable X is whether the sandwich is tuna or turkey. The results of the experiment is determined by person A at an earlier time (what the mother decided to put in). This really has nothing to do with quantum mechanics.

The kid could open his lunchbox at lunch and see his turkey sandwich and curse to himself because he really hates turkey (nevermind why he didn't tell his mother earlier). But knowing this at noon there's no way he can change the result from earlier that day (i.e. violate causality).

i am not disputing the concept you wrote above (as well as earlier). agreed with that.

however what i am starting a discussion on is:

is entanglement local or non-local.

bell's theorem would say, its non-local

so some property/facet of entanglement (such as conservation of momentum) is transmitted FTL, however that property cannot be used to transfer information.

please see this post...

https://www.physicsforums.com/showthread.php?p=3254573#post3254573

 

[JesseM]

bell's theorem would say, its non-local

so some property/facet of entanglement (such as conservation of momentum) is transmitted FTL, however that property cannot be used to transfer information.

Bell's theorem doesn't necessarily say it's non-local, it just says it's inconsistent with a certain set of assumptions about the laws of physics which physicists label as "local realism". You can mess with the "realism" part if you don't want to mess with the "local" part, for example the many-worlds interpretation drops the assumption that each experiment has a single unique outcome, and thus its advocates say it can preserve locality without violating Bell's theorem (see [post=1647627]this post[/post] for some references).

 

[San K]

Bell's theorem doesn't necessarily say it's non-local, it just says it's inconsistent with a certain set of assumptions about the laws of physics which physicists label as "local realism". You can mess with the "realism" part if you don't want to mess with the "local" part, for example the many-worlds interpretation drops the assumption that each experiment has a single unique outcome, and thus its advocates say it can preserve locality without violating Bell's theorem (see [post=1647627]this post[/post] for some references).

the challenge, in my mind, with MWI is that, it leads to creation of infinite worlds...at each event... worlds with various scenarios ("separated by multiples of plank's constant) are created...

also can you pls take a look at the below post? (beware that its not well articulated)

https://www.physicsforums.com/showthread.php?t=491576

 

[spikenigma]

Is there a difference between an electron changing spin in a high gravitational field and one changing spin in flatter spacetime?