Motoring2 mins ago
Quantum Entanglement
Ok, here is one for the boffins - Two particles that are 'entangled' are then seperated - if you look at one and detect its spin, the other will automatically be the other spin. This happens faster than the speed of light - so infomation about one particle inflences the result of the another faster than light. How can this be?
Answers
Best Answer
No best answer has yet been selected by jezter. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.
Ooh someone's just come across the EPR paradox!
Well first off special relativity does not say that nothing can travel faster than light. It was that observation that gave rise to relativity, not the other way around so it wouldn't be the end of the world if something were to travel faster than light.
Certain quantum effects can travel faster than light see Bell's inequality (http://en.wikipedia.org/wiki/Bell%27s_theore
m)
Actually although these particles are a long distance apart because they are entangled in this way they are part of the same quantum system there is no signal travelling between them they are part of the same system when you are manipulating one, you are manipulating the other despite the fact that it might be light years distant.
Sorry if this is not in accordance with the "common sense" you have acquired over all these years, but when you start looking at quantum mechanics you have to leave all that "common sense" at home and accept that your intuition is essentially worthless and start again
Well first off special relativity does not say that nothing can travel faster than light. It was that observation that gave rise to relativity, not the other way around so it wouldn't be the end of the world if something were to travel faster than light.
Certain quantum effects can travel faster than light see Bell's inequality (http://en.wikipedia.org/wiki/Bell%27s_theore
m)
Actually although these particles are a long distance apart because they are entangled in this way they are part of the same quantum system there is no signal travelling between them they are part of the same system when you are manipulating one, you are manipulating the other despite the fact that it might be light years distant.
Sorry if this is not in accordance with the "common sense" you have acquired over all these years, but when you start looking at quantum mechanics you have to leave all that "common sense" at home and accept that your intuition is essentially worthless and start again
Except... jake, you are certainly aware there's a cadre of physicists that adhere to the theory that nothing can travel faster than light and use Bell's Inequality theorem to it...
Einstein, Podolsky and Rosen ("EPR") said (basically) " We don't like the idea that the momentum of a particle, if it's position was known, would be completely unknowable--random." and, leaving room for incomplete knowledge wrote their 1935 paper entitled " Can Quantum-Mechanical Description of Physical Reality be Considered Complete?". This remained unchallenged until 1964 when John S. Bell proposed, in his theorem, that very simply, some say elegantly proved that if momentum and position were absolute values (that is, they exist whether they were measured or not) then his so-called inequality would be satisfied.
Einstein countered: "...an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it".
A significant portion of EPR has Einstein and friends imagining a scenario that would let one measure, say, both the position and momentum (as an example) of a particle with absolute certainty, a big no-no in quantum mechanics. They proposed an example: A pion at rest.
Contd.
Einstein, Podolsky and Rosen ("EPR") said (basically) " We don't like the idea that the momentum of a particle, if it's position was known, would be completely unknowable--random." and, leaving room for incomplete knowledge wrote their 1935 paper entitled " Can Quantum-Mechanical Description of Physical Reality be Considered Complete?". This remained unchallenged until 1964 when John S. Bell proposed, in his theorem, that very simply, some say elegantly proved that if momentum and position were absolute values (that is, they exist whether they were measured or not) then his so-called inequality would be satisfied.
Einstein countered: "...an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it".
A significant portion of EPR has Einstein and friends imagining a scenario that would let one measure, say, both the position and momentum (as an example) of a particle with absolute certainty, a big no-no in quantum mechanics. They proposed an example: A pion at rest.
Contd.
Contd.
When the pion decays (a common occurrence in the subatomic world) it no longer is a pion. It splits into two photons that shoot away from each other in opposite directions.
Photons have spin, but these two photons came from a pion with no spin. So, since you know the spin of one photon, you can find out the spin of the other photon because their spins have to add up to no spin at all. Because the photons came from a single pion, it is said that they are entangled.
(Lost you yet... I heard that yawn!)
Well, hold on... Possibility 1 - One of the photons flies to the right. You first measure it's spin along the x-axis with absolute certainty (quite possible). But, alas, quantum mechanics won't let you measure the y-axis spin, since you already know the x-axis spin. So you go to the second photon that flew to the left. You already know its x-axis spin without even measuring it: it is the exact opposite of the other photon. The paradox is this: Can you measure the y-axis spin of the second photon with absolute certainty even though you already know it's x-axis spin without measuring it?
"How would the second photon "know" you measured the first photon? But quantum mechanics says you can't measure the y-axis spin with absolute certainty. It doesn't matter if the two photons were separated by an inch or 10 miles, the very instant you measure the first photon's x-axis spin, the y-axis spin of the second photon is impossible to measure. Relativity says that the "knowledge" of the measurement of the first photon can only travel the speed of light. But quantum mechanics requires the "knowledge" of the measurement to be instantaneous, because they have been entangled. Einstein called it "spooky action at a distance".
Contd.
When the pion decays (a common occurrence in the subatomic world) it no longer is a pion. It splits into two photons that shoot away from each other in opposite directions.
Photons have spin, but these two photons came from a pion with no spin. So, since you know the spin of one photon, you can find out the spin of the other photon because their spins have to add up to no spin at all. Because the photons came from a single pion, it is said that they are entangled.
(Lost you yet... I heard that yawn!)
Well, hold on... Possibility 1 - One of the photons flies to the right. You first measure it's spin along the x-axis with absolute certainty (quite possible). But, alas, quantum mechanics won't let you measure the y-axis spin, since you already know the x-axis spin. So you go to the second photon that flew to the left. You already know its x-axis spin without even measuring it: it is the exact opposite of the other photon. The paradox is this: Can you measure the y-axis spin of the second photon with absolute certainty even though you already know it's x-axis spin without measuring it?
"How would the second photon "know" you measured the first photon? But quantum mechanics says you can't measure the y-axis spin with absolute certainty. It doesn't matter if the two photons were separated by an inch or 10 miles, the very instant you measure the first photon's x-axis spin, the y-axis spin of the second photon is impossible to measure. Relativity says that the "knowledge" of the measurement of the first photon can only travel the speed of light. But quantum mechanics requires the "knowledge" of the measurement to be instantaneous, because they have been entangled. Einstein called it "spooky action at a distance".
Contd.
Contd.
But... some scientists said that there were "hidden variables" that exist in the photons that allow them to behave this way. Hidden variables are variables that we have yet to discover. They would be aspects of each of the photons that are the same, since they were entangled, but that did not depend on the other photon. But the pesky Bell persisted with his inequality equation - Number(A, not B) + Number(B, not C) >= Number(A, not C)...
Almost there...
Problem is Bell's Theorem contains inherent assumptions. T
They are:
Logic is valid.
There is a reality separate from its observation.
No information can travel faster than light.
But, no one knows which assumption may be wrong.
In 1930, Kurt Gödel proved that any theory proposed for the foundation of mathematics will be either insufficient for mathematics, incomplete, or inconsistent thereby making logic incomplete... a radical idea.
Or - There may be no reality separate from its observation. This is where physics melds with philosophy and religion... and no one I know wants to go there.
The popular press likes to claim that quantum physics allows for faster than light communication of information. So far, physicists have not come to this conclusion. Dr. Ken Caviness, chair of the Physics Department at Southern Adventist University in Tennessee, says this:
"I don't know of anyone in the field who seriously proposes instantaneous communication. On the contrary it seems that despite quantum entanglement information cannot be extracted from the system without some (at most) light-speed exchange of information."
(With thanks to Cool Physics Phenomena)...
But... some scientists said that there were "hidden variables" that exist in the photons that allow them to behave this way. Hidden variables are variables that we have yet to discover. They would be aspects of each of the photons that are the same, since they were entangled, but that did not depend on the other photon. But the pesky Bell persisted with his inequality equation - Number(A, not B) + Number(B, not C) >= Number(A, not C)...
Almost there...
Problem is Bell's Theorem contains inherent assumptions. T
They are:
Logic is valid.
There is a reality separate from its observation.
No information can travel faster than light.
But, no one knows which assumption may be wrong.
In 1930, Kurt Gödel proved that any theory proposed for the foundation of mathematics will be either insufficient for mathematics, incomplete, or inconsistent thereby making logic incomplete... a radical idea.
Or - There may be no reality separate from its observation. This is where physics melds with philosophy and religion... and no one I know wants to go there.
The popular press likes to claim that quantum physics allows for faster than light communication of information. So far, physicists have not come to this conclusion. Dr. Ken Caviness, chair of the Physics Department at Southern Adventist University in Tennessee, says this:
"I don't know of anyone in the field who seriously proposes instantaneous communication. On the contrary it seems that despite quantum entanglement information cannot be extracted from the system without some (at most) light-speed exchange of information."
(With thanks to Cool Physics Phenomena)...
Slight problem with all that Clanad
The EPR experiment was actually done 30 years ago in the University of Paris by Alain Aspect.
Einstein lost "spooky action at a distance happens"
As I said though this does not necessarilly mean that there is actual communication if you try to use the phenomina to transmit data yourself you will interact with the medium and break that entanglement (nice description of that here)
http://www.physicspos...?articleId=213&page=3
However again I wouldn't get too upset if it turned out it was possible in some other way. In the same way that Netwonian physics breaks down at extremes and Einstein steps in we may find that Einstein needs modification at extremes too - Dark Energy may well require a reassessment of how Gravity works in Einstein's scheme of things.
Thing is there are still a number of scientists walking about with a rather strong attachment to 19th Century deterministic science that sit's nicely in the mind and doesn't come up with challenging ideas. The last followers of Einstein's God that doesn't play dice and think that there must be something that we don't understand and if we did then the universe would just become reassuring marbles bouncing off each other again.
I tend to think that this is a futile attempt to make the Universe fit the Human mind where the reverse is too difficult
I tend the th
The EPR experiment was actually done 30 years ago in the University of Paris by Alain Aspect.
Einstein lost "spooky action at a distance happens"
As I said though this does not necessarilly mean that there is actual communication if you try to use the phenomina to transmit data yourself you will interact with the medium and break that entanglement (nice description of that here)
http://www.physicspos...?articleId=213&page=3
However again I wouldn't get too upset if it turned out it was possible in some other way. In the same way that Netwonian physics breaks down at extremes and Einstein steps in we may find that Einstein needs modification at extremes too - Dark Energy may well require a reassessment of how Gravity works in Einstein's scheme of things.
Thing is there are still a number of scientists walking about with a rather strong attachment to 19th Century deterministic science that sit's nicely in the mind and doesn't come up with challenging ideas. The last followers of Einstein's God that doesn't play dice and think that there must be something that we don't understand and if we did then the universe would just become reassuring marbles bouncing off each other again.
I tend to think that this is a futile attempt to make the Universe fit the Human mind where the reverse is too difficult
I tend the th
It's like the famous 2 slit experiment, each photon goes through both holes at once until you put a detector there that is! We assume that for the particles to behave as observed there must be some communication between them but that is only a human assumption, essentially we don't know. As the top physicists say, if you "get it" then you haven't really understood!
mmm...all very interesting. Thanks Clanad and JTP for you in depth answers! Ok so we say that the two particles maybe communicating infomation but we couldnt use that to send infomation, so the faster than light issue isnt compromised. Thats fine. Now can I just check on something here, and i may come across as a bit niave, but how do we know that the spin isnt already decided before they seperate (I mean we cant look can we?) so when we determine one spin, the other has to be what it always was - the other spin.
Maybe Geezer you was watching the same program as me on TV the other night about the two slit experiment. I am still thinking about how using one slit you get just once photon pass through but when you have two slits then 3 photons pass through. Maybe I'll have to replay the video to try and make sense of it. Unless Jake can tell us why?
Latest Posts
Crosswords3 mins ago
Body & Soul3 mins ago
Well At Least Sir Kier Is Topping One...
News8 mins ago
ChatterBank9 mins ago
Quizzes & Puzzles12 mins ago
