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Jetlack
2008-Jul-31, 10:32 AM
I was wondering if anyone could help me with 2 questions:

1) When they say the effects of entanglement are FTL, are they saying they are FTL but still not completely instantaneous? I've been looking at studies on quantum tunelling for instance, and it appears the difficulty in getting a clean "transit" reading is because one has to seperate the classical measurement time. I would presume timing the effects of quantum entanglement are just as difficult.

2) Would there be a difference in our understanding of quantum mechanics if entanglement speeds were clocked to be instantaneous as opposed to just a little bit faster than light?

Jeff Root
2008-Jul-31, 12:22 PM
I have zero expertise in the subject, but I suspect that "quantum entanglement"
effects have no speed of any kind. It has more to do with information theory
than with physics, I think. Nothing about the observed things change. Only
your knowledge changes. One moment you have no information about that
distant particle; the next moment you do; as if information about that distant
particle had been transmitted to you instantaneously -- in other words, FTL.
But nothing was actually transmitted so nothing had any speed.

-- Jeff, in Minneapolis

Colby
2008-Jul-31, 02:38 PM
It's one of the things I find more confusing than helpful in the current lay-person description of things. On the one hand, you'd want complicated things described so that lay-people will get excited about science and understand a bit of how the universe operates, OTOH over-simplifing scientific principles and calling it "weird quantum actions at a distance" (and similar) is misleading. It implies that information can travel FTL. I have the same issue with many other physics descriptions (Schrodinger's cat, time dilation, big bang, string theory, multiple universes) that are attempts to explain a mathematical description of an effect as if it really worked that way. I strive for a conceptual understanding of how things work more than a mathematical one. If the math helps me calculate the area of the universe, then great and math has done its job, but that's not the same as understanding the concepts involved.

(I'm way less of an expert than Jeff, but I do ask many questions and get similar answers to Jeff's on this one.)

John Mendenhall
2008-Jul-31, 02:48 PM
Try looking at it this way: the speed of light, as you measure it in your reference frame, defines the minimum amount of time in which you can perceive information about anything in the rest of the universe (including the screen you are reading and the farthest galaxies).

Ken G
2008-Jul-31, 03:50 PM
I have zero expertise in the subject, but I suspect that "quantum entanglement"
effects have no speed of any kind. It has more to do with information theory
than with physics, I think. Nothing about the observed things change. Only
your knowledge changes. Yes, this is the perspective I take on it as well. Entanglement does put certain limits on the way physics must work (essentially it rules out "local realism", so that means particles don't "carry with them" all the information that could relate to predictions about the particle). But once you accept that's how things are, you can separate what is "real" from what is "information" and never have a problem with FTL propagation of anything.


I have the same issue with many other physics descriptions (Schrodinger's cat, time dilation, big bang, string theory, multiple universes) that are attempts to explain a mathematical description of an effect as if it really worked that way. Right on, I have the same issue with that same list.


Try looking at it this way: the speed of light, as you measure it in your reference frame, defines the minimum amount of time in which you can perceive information about anything in the rest of the universe (including the screen you are reading and the farthest galaxies).But that is just what is violated by entanglement. You can do an experiment on a particle in your lab and gain knowledge, instantly, about particles halfway across the universe. What's more, that information will violate "Bell's inequality", which would have to hold if local realism worked. There's no contradiction there if you relax local realism, as the information is in your head and does not need to 'travel to' that other particle. That's what's bizarre, but there is no propagation involved there so no speed limit.

Colby
2008-Jul-31, 04:54 PM
That's what's bizarre
Maybe I'm missing something, but I don't even find it the least bit bizarre.

If I glue two coins to two pieces of paper such that one paper has a heads-up coin and the other paper has a heads-down coin, then I close my eyes, randomly pick one coin/paper, then take it into another room (far far away if you like). When I look at my coin/paper and see a heads-up, I instantaniously, FTL know the other room contains a heads-down coin. Weird, isn't it? NOT. Substitute spin-up and spin-down particles, and it's no more strange.

cosmocrazy
2008-Jul-31, 05:01 PM
Maybe I'm missing something, but I don't even find it the least bit bizarre.

If I glue two coins to two pieces of paper such that one paper has a heads-up coin and the other paper has a heads-down coin, then I close my eyes, randomly pick one coin/paper, then take it into another room (far far away if you like). When I look at my coin/paper and see a heads-up, I instantaneously, FTL know the other room contains a heads-down coin. Weird, isn't it? NOT. Substitute spin-up and spin-down particles, and it's no more strange.

Not nit picking or anything, cause i like your analogy :) but it takes time for the light to travel from the coin to your eyes and even longer for your brain to process the information. Is it really instantaneous? :confused:

cosmocrazy
2008-Jul-31, 05:06 PM
Also as Ken mentioned, there is no propagation of information from the unobserved coin as you already new which way up it is based on what you observe from the coin you see. The information exchange is from the observed coin.

Colby
2008-Jul-31, 05:47 PM
Not nit picking or anything, cause i like your analogy :) but it takes time for the light to travel from the coin to your eyes and even longer for your brain to process the information. Is it really instantaneous?
I didn't see it in the research papers, but I'm pretty sure it would time for the equipment to measure particle spin, yet they still described it as "spooky". The information is immediately available, even if your mind is too slow to recognize it. J/K :) Take the coin/paper to Alpha Centauri if you want more time to think about it, and the information will still be available FTL.

Ken G
2008-Jul-31, 06:01 PM
Maybe I'm missing something, but I don't even find it the least bit bizarre.What is bizarre is actually very subtle. The scenario you describe would not violate "Bell's inequality", so is consistent with an interpretation that the reality is "encapsulated" in the two coins, as if everything you could possibly know was locally encoded in the coins themselves. That's how we tend to imagine reality. But violations of Bell's inequality cannot be achieved with that picture, and do indeed happen with entanglement. That's what some people interpret as an "influence", but I use a tighter meaning of that word that would require actual signal propagation.

kleindoofy
2008-Jul-31, 06:55 PM
... particles don't "carry with them" all the information that could relate to predictions about the particle ...
That and other things said here sound like bad news for KV550 (= Mozart's symphony no. 40, g minor). ;)

John Mendenhall
2008-Jul-31, 07:32 PM
But that is just what is violated by entanglement. You can do an experiment on a particle in your lab and gain knowledge, instantly, about particles halfway across the universe. What's more, that information will violate "Bell's inequality", which would have to hold if local realism worked. There's no contradiction there if you relax local realism, as the information is in your head and does not need to 'travel to' that other particle. That's what's bizarre, but there is no propagation involved there so no speed limit.



The particle you are observing only got here at the speed of light. You cannot send a message back to the other entangled particle. You are receiving information about the other side of the universe, but it got here at the speed of light, riding on the particle.

I think we are in agreement about this, but looking at it differently.

Regards, John M.

Ken G
2008-Jul-31, 08:59 PM
That is more like it, the earlier remark seemed to be saying things about the speed at which you can know about other things. If there is an event that creates two particles a light year away, a year later you can learn about another particle that is 2 light years away. So in 1 year after it is created, you can learn about an object that is now 2 light years away. That would seem to violate that speed limit you were talking about before, but not the way you've described it above.

Colby
2008-Jul-31, 10:36 PM
[QUOTE=Ken G;1293369]The scenario you describe would not violate "Bell's inequality"[QUOTE]
Thanks Ken. You're the first person to explain the error in my analogy, and I've asked many people on other physics sites. I do find Bell's inequality test results on photons to be bizarre. Intuitively, I'd guess the problem isn't with locality, but then I don't pretend to know enough to debate it, and relying on intuition is notoriously error-prone in any case.

"...our minds are limited by our models." <-- exactly :)

Len Moran
2008-Aug-01, 09:29 AM
....I do find Bell's inequality test results on photons to be bizarre.


This question has come up a few times and there have been some very good and informative discussions on it that you may find interesting, but especially the threads "spooky matter at a distance" and "is quantum entanglement true". Both of these in part, look at what I would term a descriptive method and predictive method, the former invokes a notion of "influence" (and in my opinion bizarre behaviour), the latter does not. On the basis of much of what has said by Ken G and my own reading, I definitely fall into the predictive method camp. As far as I can make out, such a method does not invoke any notion of faster than light influence at all and removes any notion of bizarre behavior. It does not offer a "realist explanation" of course, but I don't subscribe to such notions anyway, it seem far more satisfactory to me, at the quantum level, to think in terms of predictive observations rather than trying to invoke any "real" descriptive elements as existing prior to the observation.

As best as I can understand things from this perspective, predictive observations derived from a joint wave function yield the joint probability of observing a given pair of results at each detector for any orientation of the pair of photons. Those predictions give us the probabilities of observing spin, and they simply state that a spin of (say) up found "here" will correspond to a spin down "there". It's not as if I can select a particular observation in order to influence the other detector, I have to take what I get, and the spin that I do get has it's compliment at the other detector in accordance with the predicted outcome. We could do some book keeping at a later date and compare results, but rather than assigning some macroscopic notion of influence in terms of a "description" of events that suggests a wave function being a real materialistic entity, we instead stick to quantum mechanics as consisting of predictive observations, the book keeping simply confirms the joint probabilities.

RussT
2008-Aug-01, 10:18 AM
Here is that Spookie thread...

Pay close attention to Grey's responses, because IMHO, to just allow KenG's approach to win out, will simply aid in the answer never being found.

http://www.bautforum.com/questions-answers/60731-spooky-matter-distance.html

Jetlack
2008-Aug-01, 12:23 PM
Guys,

Thanks for all the answers. However i realise no signal or information or mass is being propogated. :)

But would it still not be possible to time whether the effects of entanglement were instantaneous as opposed to the just FTL? Okay so maybe we'd need a long distance experiment but is it not feasible?

Jeff Root
2008-Aug-01, 01:35 PM
Exactly what would you time?

-- Jeff, in Minneapolis

Ken G
2008-Aug-01, 02:13 PM
\Pay close attention to Grey's responses, because IMHO, to just allow KenG's approach to win out, will simply aid in the answer never being found.

http://www.bautforum.com/questions-answers/60731-spooky-matter-distance.html

And of course I feel there is no answer to find that is not already found out. But you are certainly correct that in the interest of "equal time', that is an important thread to consider when thinking about spooky "influences", and I am glad you linked to it. No one died and left me the expert on entanglement, I just think a lot of hooey gets circulated on that topic!

Jetlack
2008-Aug-01, 02:39 PM
Exactly what would you time?

-- Jeff, in Minneapolis

The time between the initial measurement of the particle and the effect it has on its entangled twin. Lets say we were able to fire an entangled pair of photons in opposite directions and we allowed them to travel for a lightyear. At measurement point a, someone checks the polarity of particle a, and at measurment point b (slightly delayed) it checks particle b's polarity. Okay we would have to take into account the known "delay" but as long as we get the predicted entangled result it would confirm more or less that entanglemens influence is instantaneous as opposed to just FTL.

I guess what one is timing is the difference in collapse times between the measured particle and its entangled pair.

Is that not possible in theory?

Jeff Root
2008-Aug-01, 03:10 PM
But it has no effect on its entangled twin. All you would be measuring is
the amount of time between two measurements, not between a cause and
an effect.

You would be vastly better off detecting the two photons a meter apart
than a light-year apart.

-- Jeff, in Minneapolis

Jetlack
2008-Aug-01, 04:47 PM
But it has no effect on its entangled twin. All you would be measuring is
the amount of time between two measurements, not between a cause and
an effect.

You would be vastly better off detecting the two photons a meter apart
than a light-year apart.

-- Jeff, in Minneapolis

I know what you mean it would seem an extravagant experiment to confirm entanglement's effects; however it would also confirm that the spooky action at a distance is bonafide instantaneous, not just at some higher speed aboce light. For all we know it takes 1.5 X c, which would be rather paradoxical because then the person at measuremnt point b would have taken his measurement before entanglement could have time to occur.

Obviously im assuming to keep the universe consistent it is instantaneous otherwise that would be weird, but we dont really know unless its tested over very large distances. I believe Anton Zeilinger has proposed an experiment but on a smaller scale using the space station as a relay or something:

"Testing “spooky action-at-a-distance” on the International Space Station"

http://arxivblog.com/?p=460

John Mendenhall
2008-Aug-01, 04:52 PM
That is more like it, the earlier remark seemed to be saying things about the speed at which you can know about other things. If there is an event that creates two particles a light year away, a year later you can learn about another particle that is 2 light years away. So in 1 year after it is created, you can learn about an object that is now 2 light years away. That would seem to violate that speed limit you were talking about before, but not the way you've described it above.

10/4.

Sorry, I have a good friend who is a CB freak. They communicate at the rate of slurred speech.

Regards, John M.

mugaliens
2008-Aug-01, 06:01 PM
I've been looking at studies on quantum tunelling for instance, and it appears the difficulty in getting a clean "transit" reading is because one has to seperate the classical measurement time. I would presume timing the effects of quantum entanglement are just as difficult.

Experiment (more thought than anything):

1. Set up two QT entanglement sites, separated by a distance of several meters. Sensors are linked to recording equipment located equidistant from both sites, and via equidistant transmission medium lengths.

2. At site A, you have both a sender, A1, and a receiver, B2. Same for B (B1 and A2). Thus, 1's send, 2's receive.

3. A1 is QT'd with A2, a few meters away. Same for B2. Thus, you have two QT pairs, for a total of four particles.

Simultaneous perform the experiment. Repeat many times. Compare the results.

I hypothesize that the QT occurs either at the speed of light or instantaneously, not somewhere between.

Jeff Root
2008-Aug-01, 06:53 PM
But it has no effect on its entangled twin. All you would be measuring is
the amount of time between two measurements, not between a cause and
an effect.

You would be vastly better off detecting the two photons a meter apart
than a light-year apart.
I know what you mean it would seem an extravagant experiment to
confirm entanglement's effects;
Evidently you do not know what I mean. I didn't suggest that
there is anything extravagant about such an experiment. I suggested
two things:

1) The "experiment" doesn't measure anything useful.

2) The experiment is physically impossible, because a single photon
cannot be bounced off an orbiting satellites and detected on the ground,
despite what the article claims. A few trillion photons can be sent up to
the satellite; a few thousand will bounce back; maybe one or two of
those will be detected on the ground.

However, any experiment intended to show quantum entanglement
that theoretically works over a distance of 100 or 200 km will work
just as well over a distance of 1 or 2 meters.

But the "experiment" doesn't measure anything useful. All it measures
is the amount of time between your measurements, which can be any
amount of time you want. If you take your measurements fifteen
minutes apart, your result is that it took fifteen minutes. Pointless.



however it would also confirm that the spooky action at a distance
is bonafide instantaneous, not just at some higher speed above light.
For all we know it takes 1.5 X c, which would be rather paradoxical
because then the person at measuremnt point b would have taken his
measurement before entanglement could have time to occur.
Entanglement doesn't take any time. Entanglement is not an event
or a sequence of events. There isn't anything to time.

How much time elapses between the head side of a coin landing "up"
and the tail side landing "down"?

If you think that there is something that can be timed, please tell me
exactly what it is, and how it would be timed.

-- Jeff, in Minneapolis

grant hutchison
2008-Aug-01, 07:42 PM
And once we have found a pair of events we want to call "The Spooky Action at a Distance", we still need to agree the reference frame from which we are to judge "instantaneity" (a word I just learned from Ken G). That would seem to be a way to break Special Relativity.

Grant Hutchison

Ken G
2008-Aug-01, 08:35 PM
One thing more I'd like to stress on all this entanglement business is that so far no experiment has ever achieved a result that did not agree with the prediction of quantum mechanics, in regard to entanglement. People forget that the purpose of entanglement experiments was originally to say "here's a situation where we know by "common sense" that the quantum mechanics must make a wrong prediction, so let's set it up", but lo and behold, the QM prediction is always fine. So I won't pay any attention to any more entanglement experiments until they get a result that is not predicted by quantum mechanics (delayed choice, for example, is predicted by quantum mechanics). Furthermore, none of them get results that don't agree with relativity, if relativity is interpreted quantum mechanically (time measurements, for example, are notoriously incorrectly interpreted in light of the uncertainty principle).

Jetlack
2008-Aug-02, 07:33 AM
One thing more I'd like to stress on all this entanglement business is that so far no experiment has ever achieved a result that did not agree with the prediction of quantum mechanics, in regard to entanglement. People forget that the purpose of entanglement experiments was originally to say "here's a situation where we know by "common sense" that the quantum mechanics must make a wrong prediction, so let's set it up", but lo and behold, the QM prediction is always fine. So I won't pay any attention to any more entanglement experiments until they get a result that is not predicted by quantum mechanics (delayed choice, for example, is predicted by quantum mechanics). Furthermore, none of them get results that don't agree with relativity, if relativity is interpreted quantum mechanically (time measurements, for example, are notoriously incorrectly interpreted in light of the uncertainty principle).

Can you elaborate on the time measurements being notoriously difficult to interpret? Are you talking about the Schrodinger equation? I find the concept of time and quantum mechanics hard to associate, because i dont understand how we can know/predict how time evolves inside a wave function.

Jetlack
2008-Aug-02, 07:53 AM
Jeff Root,

"Evidently you do not know what I mean."

Ouch - tetchy :lol:

It's funny because all of a sudden you are a big expert on this subject; having previously said "I have zero expertise in the subject, but I suspect that "quantum entanglement" effects have no speed of any kind.."

Ya okay I get your point. Anton Zeilinger is wrong and there's no point being interested in this whatsoever. Move right along folks - nothing to see here. :)

Neverfly
2008-Aug-02, 07:59 AM
Jeff Root,

"Evidently you do not know what I mean."

Ouch - tetchy :lol:

It's funny because all of a sudden you are a big expert on this subject; having previously said "I have zero expertise in the subject, but I suspect that "quantum entanglement" effects have no speed of any kind.."

Ya okay I get your point. Anton Zeilinger is wrong and there's no point being interested in this whatsoever. Move right along folks - nothing to see here. :)

A wise person can recognize modesty when he sees it.
Jeff Root's knowledge base is QUITE impressive.

In the meantime, Jetlack, you really need to also recognize what the difference is between action and perception in Quantum Entanglement.

Jetlack
2008-Aug-02, 08:12 AM
A wise person can recognize modesty when he sees it.
Jeff Root's knowledge base is QUITE impressive.

In the meantime, Jetlack, you really need to also recognize what the difference is between action and perception in Quantum Entanglement.

Yes I'm sure he has impressive knowledge but I'm not sure modesty becomes itself by bashing other learned Professors such as Zeilinger. The great thing about modern humanity is that because there are alot of people with impressive knowledge, one can afford to discriminate about where to aquire one's information.

Also as you know I have a low tolerance for rudeness. In the mean time, please do enlighten me with your knowledge on the differences between action and perception in quantum mechanics. Perceptions??? You heretic! :D

Neverfly
2008-Aug-02, 08:17 AM
Yes I'm sure he has impressive knowledge but I'm not sure modesty becomes itself by bashing other learned Professors such as Zeilinger. The great thing about modern humanity is that because there are alot of people with impressive knowledge, one can afford to discriminate about where to aquire one's information.


Percival Lowell was an educated and accomplished man and astronomer.

So is former astronaut Ed Mitchell.

It is not a matter of bashing learned people, but of bashing ideas.
Furthermore, one can choose to acquire information from reliable and scientific sources.... or not.

Jetlack
2008-Aug-02, 08:31 AM
Percival Lowell was an educated and accomplished man and astronomer.

So is former astronaut Ed Mitchell.

It is not a matter of bashing learned people, but of bashing ideas.
Furthermore, one can choose to acquire information from reliable and scientific sources.... or not.

Yes no-one is perfect. And you are quite right about reliable sources. I was on a thread just the other day and this guy was talking about how Hitler had marched through India! Then he said he'd be right back with a cite and never re-appeared again! :D


What about your insights on perception? I was licking my chops at that one :)

Neverfly
2008-Aug-02, 08:41 AM
Yes no-one is perfect. And you are quite right about reliable sources. I was on a thread just the other day and this guy was talking about how Hitler had marched through India! Then he said he'd be right back with a cite and never re-appeared again! :D
Reappeared? I haven't disappeared.
And it was Nazi Troops, not Hitler, personally, that marched- Not through India- but the northern part around where Nepal is.
Finding CITES for it is NOT easy and I would appreciate it if you got up off my back about it and waited patiently like the guy who I was actually talking to is doing.


What about your insights on perception? I was licking my chops at that one :)

If I line up a few million people, across the United States- coast to coast... Then tell them via radio communication to all put their hands up in da air and wave 'em around like just don't care...

Does the wave travel faster than the speed of light?

THIS is the flaw in your perceptions, Jetlack.

Jetlack
2008-Aug-02, 08:48 AM
Neverfly,

"Reappeared? I haven't disappeared.
And it was Nazi Troops, not Hitler, personally, that marched- Not through India- but the northern part around where Nepal is.
Finding CITES for it is NOT easy and I would appreciate it if you got up off my back about it and waited patiently like the guy who I was actually talking to is doing."

Will that be before or after the big rip? :D Okay i am only joshing with you Neverfly.

"If I line up a few million people, across the United States- coast to coast... Then tell them via radio communication to all put their hands up in da air and wave 'em around like just don't care...Does the wave travel faster than the speed of light?"

No it doesn't. I'm not sure there is any perception paradox in that example. :)

Jeff Root
2008-Aug-02, 12:48 PM
Jetlack,

I have zero expertise in the subject of quantum entanglement. I do know
some bits and pieces in other subjects. I assumed that the experiment you
are interested in required a single photon to be emitted, reflected, and
detected again. However, it appears that the experiment conducted by
Zeilinger and others emits a moderate number of photons toward the mirror
in space, and on average less than one photon is reflected back, so that
for the majority of pulses, no returning photon is detected. The point
of the Zeilinger experiment appears to be that usually only one photon at
most is sent from the satellite to the ground in each pulse, not that a
single photon can be sent from the ground, reflected at the satellite, and
detected on the ground.

http://arxiv.org/ftp/arxiv/papers/0803/0803.1871.pdf



Experimental verification of the feasibility of a quantum
channel between Space and Earth

P. Villoresi, T. Jennewein, F. Tamburini, M. Aspelmeyer, C. Bonato,
R. Ursin, C. Pernechele, V.Luceri, G.Bianco, A.Zeilinger, and C. Barbieri

The measured count rate in the peak is 5 counts per second, corresponding
to a probability to detect a photon per emitted laser pulse of 3x10^-4 .
Taking into account the losses due to detection efficiency (-10dB) and the
losses in the detection path (-11dB), the average photon number per pulse,
mu, emitted by the satellite and acquired by our detector is approximately
4x10^-2 , i.e. well within the single photon regime.
It appears that if they count, say, 90 photons, about 60 of those are
background noise and the other 30 are signal.

-- Jeff, in Minneapolis

cosmocrazy
2008-Aug-02, 09:34 PM
Entanglement doesn't take any time. Entanglement is not an event
or a sequence of events. There isn't anything to time.

How much time elapses between the head side of a coin landing "up"
and the tail side landing "down"?

If you think that there is something that can be timed, please tell me
exactly what it is, and how it would be timed.

-- Jeff, in Minneapolis



And once we have found a pair of events we want to call "The Spooky Action at a Distance", we still need to agree the reference frame from which we are to judge "instantaneity" (a word I just learned from Ken G). That would seem to be a way to break Special Relativity.

Grant Hutchison

Just to back up what Neverfly has stated...

Jetlack, read these 2 quotes then think about what you are asking? You have had some good answers from Ken, John & Jeff with some good food for thought.;)

Jetlack
2008-Aug-03, 12:40 PM
Jeff,

Yes i understand Zeilinger's experiments seem more about proving either local or non-local realism is incompatible with quantum theory. I just read another article on Zeilinger's research in Seed magazine.

http://seedmagazine.com/news/2008/06/the_reality_tests_1.php?page=all&p=y

It does seem measuring entanglement "time" is not the issue. I apologise to all for being such a buffoon for ever having thought entanglement could be measured. :whistle: