Freethought & Rationalism Archive

Lykil · 02-28-2002, 01:32 AM

I have a question about quantum mechanics that is giving me some problems: I recently saw a documentary, in which quantum theory was explained in laymans terms. A great part of the doc. concerned itself with the socalled EPR experiment, in which a pair of twin particles are separated, and then measured at exactly the same time when they are a great distance from each other. The tricky part is that quantum mechanics states that before the particles are measured, the outcome of the measurement is not decided, but still, when they are measured, they always end up being a pair, even if they are measured at exactly the same time, leaving no time for them to send messages to each other. The success of the EPR experiment was said to have proved Albert Einstein wrong, since he objected very passionately to the theory. My problem however is this: Aren’t the particles acting exactly as they would do, if the outcome was decided beforehand? So how has the claim - that the outcome is not decided - ever been verified experimentally? IOW how do we know, that the particles in question actually ‘do not know what they are,’ to use the laymans terms again? And doesn’t the EPR exp. disprove this claim, rather than proving some form of instantanious messaging?

Regards
Mandark

Ps. Hey remember, I’m a layman myself, so don’t be too technical

ohwilleke · 02-28-2002, 08:56 AM

There is certainly well established and absolutely confounding proof that quantum particles do not behave in a manner which is independent of whether they are being observed or not.

For example, suppose that you shoot quantum particles down a long tunnel towards a couple of different exits, one on the left and one on the right. A strategically placed device that observes the path of particles through the tunnel can change the ratio of quanta coming out the exits.

Another example. An important real life phenomena, quantum tunnelling, is used in every transistor. It involves electrons passing through a point in the system that the individual electron shouldn't be able to crosss, yet, a certain percentage of them never the less make it over the barrier.

More particularly, it is theoretically (not just practically) impossible to know what state a quanta will be in what it is observed under certain circumantances. Yet, by observing one twinned particle in one place, you can get this theoretically impossible to know knowledge about the other.

The twin effect is particular weird because one of the implicit assumptions of the standard model in almost every other circumstances is that quanta act independently of each other.

Ipecac · 02-28-2002, 04:06 PM

The things I find most fascinating about quantum physics are the potential computing and communications applications. Instantaneous communications would be an incredible boon for space exploration.

Imagine a network card that is made up of billions of entangled particles that are paired up in groups of a million. Your card can communicate instantaneously with 10000 other cards like it and they in turn can communicate with 10000 others instantly. Or if you prefer the centralized model you split the entangled particles into a host and client configuration. The host has one side of all pairs and coordinates commincation between the clients.

Kosmon · 02-28-2002, 04:18 PM

However, it is impossible for data (information) to be transferred faster than the speed of light... so instantaneous communication across space would only be possible to an extent. This is also why it would be impossible to figure out the spin of one particle here and instantaneously be able to deduce the spin of its partner across the universe.

excreationist · 02-28-2002, 05:13 PM

Quote:

Originally posted by Kosmon:
However, it is impossible for data (information) to be transferred faster than the speed of light... so instantaneous communication across space would only be possible to an extent. This is also why it would be impossible to figure out the spin of one particle here and instantaneously be able to deduce the spin of its partner across the universe.

What if there were two entangled particles and one was taken by a human to a place several light-hours away. Then they decided what time the first one would force their particle to have a certain spin (I think that's how it works). Then an hour later, the other human could see which way their entangled particle is spinning...
So that communication would be faster than the speed of light, wouldn't it?

Quote:

There is certainly well established and absolutely confounding proof that quantum particles do not behave in a manner which is independent of whether they are being observed or not.
For example, suppose that you shoot quantum particles down a long tunnel towards a couple of different exits, one on the left and one on the right. A strategically placed device that observes the path of particles through the tunnel can change the ratio of quanta coming out the exits.

Wouldn't observation just deflect the particles (photons being extracted or something)?

Kosmon · 02-28-2002, 05:21 PM

Quote:

Originally posted by excreationist:

What if there were two entangled particles and one was taken by a human to a place several light-hours away. Then they decided what time the first one would force their particle to have a certain spin (I think that's how it works). Then an hour later, the other human could see which way their entangled particle is spinning...
So that communication would be faster than the speed of light, wouldn't it?

Sorry if I misunderstood what you said, but it would appear to me that communication wouldn't be travelling faster than the speed of light, since it would take 'an hour' for the other human to see the spin. This is assuming that they are one light-hour away.

excreationist · 03-01-2002, 04:00 AM

Quote:

Originally posted by Kosmon:
Sorry if I misunderstood what you said, but it would appear to me that communication wouldn't be travelling faster than the speed of light, since it would take 'an hour' for the other human to see the spin. This is assuming that they are one light-hour away.

But they are *several* light hours away. So the communication would be happening faster than the speed of light... (?)

Malaclypse the Younger · 03-01-2002, 06:33 AM

The conclusion of QM is that quanta do "communicate" nonlocally. However, that communication only establishes correlation.

To detect quantum nonlocality, I must first determine the other observer's results and correlate them with my own. I must use a local (speed-of-light) method of communication to determine the other observer's results.

Without the other observer's results, I see only what appears to be random results. Essentially, nonlocal quantum correlation turns one set of random results into another set of random results. However, by the definition of random, those two sets of results are indistinguishable without the corresponding information.

excreationist · 03-01-2002, 05:25 PM

I still don't get it though....

just say you entangled two particles, and you moved them faraway (e.g. several light hours away).... can you force one to spin clockwise (or something) and then change and then does the other one change instantly? I mean can you transmit messages using a single entangled pair? Or does the message transmission only work once per pair? And can you change the state of one of them so that the other one changes?

Or is it just the same as two objects being in boxes, that have a predetermined state, where you see the state when you open the box? (look at it)

And as I said before, the part about when detectors are used - maybe this just affects the particles rather than the particles actually being in an intermediate fuzzy state.

Basically I'm just looking for evidence that this intermediate fuzzy state must exist.

Malaclypse the Younger · 03-01-2002, 10:00 PM

It's hard to explain on a message board, without diagrams.

I recommend:

<a href="http://www.amazon.com/exec/obidos/ASIN/0316328197" target="_blank">Schrodinger's Kittens</a>
<a href="http://www.amazon.com/exec/obidos/ASIN/055326382X" target="_blank">The Dancing Wu Li Masters</a>
<a href="http://www.amazon.com/exec/obidos/ASIN/0553246259" target="_blank">The Cosmic Code</a>
<a href="http://www.amazon.com/exec/obidos/ASIN/0385235690" target="_blank">Quantum Reality</a>

[ March 01, 2002: Message edited by: Malaclypse the Younger ]

02-28-2002, 01:32 AM	#1
Lykil Junior Member Join Date: May 2001 Location: T�rshavn, Faroe Islands (north of GB) Posts: 99	Problems with quantum theory I have a question about quantum mechanics that is giving me some problems: I recently saw a documentary, in which quantum theory was explained in laymans terms. A great part of the doc. concerned itself with the socalled EPR experiment, in which a pair of twin particles are separated, and then measured at exactly the same time when they are a great distance from each other. The tricky part is that quantum mechanics states that before the particles are measured, the outcome of the measurement is not decided, but still, when they are measured, they always end up being a pair, even if they are measured at exactly the same time, leaving no time for them to send messages to each other. The success of the EPR experiment was said to have proved Albert Einstein wrong, since he objected very passionately to the theory. My problem however is this: Aren’t the particles acting exactly as they would do, if the outcome was decided beforehand? So how has the claim - that the outcome is not decided - ever been verified experimentally? IOW how do we know, that the particles in question actually ‘do not know what they are,’ to use the laymans terms again? And doesn’t the EPR exp. disprove this claim, rather than proving some form of instantanious messaging? Regards Mandark Ps. Hey remember, I’m a layman myself, so don’t be too technical

03-01-2002, 10:00 PM	#10
Malaclypse the Younger Regular Member Join Date: Apr 2001 Location: nowhere Posts: 416	It's hard to explain on a message board, without diagrams. I recommend: <a href="http://www.amazon.com/exec/obidos/ASIN/0316328197" target="_blank">Schrodinger's Kittens</a> <a href="http://www.amazon.com/exec/obidos/ASIN/055326382X" target="_blank">The Dancing Wu Li Masters</a> <a href="http://www.amazon.com/exec/obidos/ASIN/0553246259" target="_blank">The Cosmic Code</a> <a href="http://www.amazon.com/exec/obidos/ASIN/0385235690" target="_blank">Quantum Reality</a> [ March 01, 2002: Message edited by: Malaclypse the Younger ]</p>

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02-28-2002, 08:56 AM	#2
ohwilleke Veteran Member Join Date: Jun 2000 Location: Denver, Colorado, USA Posts: 4,834	There is certainly well established and absolutely confounding proof that quantum particles do not behave in a manner which is independent of whether they are being observed or not. For example, suppose that you shoot quantum particles down a long tunnel towards a couple of different exits, one on the left and one on the right. A strategically placed device that observes the path of particles through the tunnel can change the ratio of quanta coming out the exits. Another example. An important real life phenomena, quantum tunnelling, is used in every transistor. It involves electrons passing through a point in the system that the individual electron shouldn't be able to crosss, yet, a certain percentage of them never the less make it over the barrier. More particularly, it is theoretically (not just practically) impossible to know what state a quanta will be in what it is observed under certain circumantances. Yet, by observing one twinned particle in one place, you can get this theoretically impossible to know knowledge about the other. The twin effect is particular weird because one of the implicit assumptions of the standard model in almost every other circumstances is that quanta act independently of each other.

02-28-2002, 04:06 PM	#3
Ipecac Regular Member Join Date: Jan 2002 Posts: 135	The things I find most fascinating about quantum physics are the potential computing and communications applications. Instantaneous communications would be an incredible boon for space exploration. Imagine a network card that is made up of billions of entangled particles that are paired up in groups of a million. Your card can communicate instantaneously with 10000 other cards like it and they in turn can communicate with 10000 others instantly. Or if you prefer the centralized model you split the entangled particles into a host and client configuration. The host has one side of all pairs and coordinates commincation between the clients.

02-28-2002, 04:18 PM	#4
Kosmon Junior Member Join Date: Feb 2002 Posts: 7	However, it is impossible for data (information) to be transferred faster than the speed of light... so instantaneous communication across space would only be possible to an extent. This is also why it would be impossible to figure out the spin of one particle here and instantaneously be able to deduce the spin of its partner across the universe.

03-01-2002, 06:33 AM	#8
Malaclypse the Younger Regular Member Join Date: Apr 2001 Location: nowhere Posts: 416	The conclusion of QM is that quanta do "communicate" nonlocally. However, that communication only establishes correlation. To detect quantum nonlocality, I must first determine the other observer's results and correlate them with my own. I must use a local (speed-of-light) method of communication to determine the other observer's results. Without the other observer's results, I see only what appears to be random results. Essentially, nonlocal quantum correlation turns one set of random results into another set of random results. However, by the definition of random, those two sets of results are indistinguishable without the corresponding information.

03-01-2002, 05:25 PM	#9
excreationist Veteran Member Join Date: Aug 2000 Location: Australia Posts: 4,886	I still don't get it though.... just say you entangled two particles, and you moved them faraway (e.g. several light hours away).... can you force one to spin clockwise (or something) and then change and then does the other one change instantly? I mean can you transmit messages using a single entangled pair? Or does the message transmission only work once per pair? And can you change the state of one of them so that the other one changes? Or is it just the same as two objects being in boxes, that have a predetermined state, where you see the state when you open the box? (look at it) And as I said before, the part about when detectors are used - maybe this just affects the particles rather than the particles actually being in an intermediate fuzzy state. Basically I'm just looking for evidence that this intermediate fuzzy state must exist.

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