Slashdot Mirror
Quantum Experiment Shows Effect Before Cause
steveb3210 writes "Physicists have demonstrated that making a decision about whether or not to entangle two photons can be made after you've already measured the states of the photons."
Here's the article's description of the experiment: 'Two independent sources (labeled I and II) produce pairs of photons such that their polarization states are entangled. One photon from I goes to Alice, while one photon from II is sent to Bob. The second photon from each source goes to Victor. Alice and Bob independently perform polarization measurements; no communication passes between them during the experiment—they set the orientation of their polarization filters without knowing what the other is doing. At some time after Alice and Bob perform their measurements, Victor makes a choice (the "delayed choice" in the name). He either allows his two photons from I and II to travel on without doing anything, or he combines them so that their polarization states are entangled. A final measurement determines the polarization state of those two photons. ... Ma et al. found to a high degree of confidence that when Victor selected entanglement, Alice and Bob found correlated photon polarizations. This didn't happen when Victor left the photons alone.'
Nevermind -- why bother telling you if you already know :-(
*Looks at physics degree.*
*Tosses it in the trash.*
Victor should decide not to entangle the photons whenever Alice and Bob's polarizations are correlated. That'll rip physics a new one...
The summary doesn't say what the time delay is between when Alice and bob measure their polarization and when victor makes his choice.
FTFA:
Due to the 104-meter fiber-optic cable, Victor's measurements occurred at least 14 billionths of a second after those of Alice and Bob
Victor is Bipolarized making him erratic and unpredictable. Might want to try adding lithium atoms into the mix and see if the results stabilize.
The argument goes like this:
`I refuse to prove that I exist,' says God, `for proof denies faith, and without faith I am nothing.'
`But,' says Man, `The Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED.'
`Oh dear,' says God, `I hadn't thought of that,' and promptly disappears in a puff of logic.
`Oh, that was easy,' says Man, and for an encore goes on to prove that black is white and gets himself killed on the next zebra crossing.
I really which quantum people would stop acting like they know what they are talking about.
This is just a really shitty description/way of looking at a series of events and is more or less wrong in the same way that saying your traveling back in time by looking at old stars in the sky from far off distances.
The only thing out of order here is the observers note taking and logic. Due various other quantum flux it may appear to happen in a certain order even though it didn't and its just a matter of appearance due to propagation effects.
Its a bad observation and bad description of that observation, not a causality violation.
What this article is saying, is that victor's decision to entangle his photons has a direct effect on the results that alice and bob get from their double blind measurements.
So, either there is retrograde communication on time's axis, or....
The decision that victor makes is predetermined, by the act of measurement undergone by alice and bob. (Meaning victor doesn't really have as much free will as he thinks he does.)
Proposed followup experiment:
Alice and bob examine their photons, tell each other, but not victor. Victor decides to entangle or not entangle. Examine new correlation.
This will test "does a correlation between alice and bob indicate that victor will entangle?".
If it does, you have a reasonably strong test case for many worlds.
First of all, quantum effects like this don't allow the passage of information (no quantum entanglement effect does, it would violate relativity). Alice and Bob don't know if their photons are entangled simply by examining them. As a rule, quantum effects are worthless for transmitting information of any kind: both parties know what the other's state is if they know the photon's were entangled, but that is insufficient to transmit any kind of information (it is very useful for encrypting information, but not transmitting it), so you cannot build a useful transistor system using this.
Secondly, the Ars article rightly points out that concluding that effect proceeded cause should be rejected without much much better evidence. I can't explain the results, but throwing out causality so rapidly would be foolish.
One thought I had was that the detectors might actually be in a quantum state (basically, entangled with the photon they observe) after making their observation, which isn't collapsed into an entangled (or not) state with the other photon until Victor makes his decision. In other words, these results might not show up if you increase the timescale, because the quantum state of the detectors after they sense the photons (which, if it lasts long enough, can be affected by Victor after they detect the photon polarization without violating causality) might collapse before he decides to entangle the photons or not. I am, of course, not a quantum physicist, so that might not be possible.
"None can love freedom heartily, but good men; the rest love not freedom, but license." --John Milton
They actually decided to one-up the CERN-OPERA people.
They fired the cable guy before they even hired him!
Free unix account: freeshell.org
I am an OSGS (Optical Sciences Graduate Student) and you don't need Quantum Mechanics to explain the experiment above, all you need is classical wave optics.
Linear polarization is electric field in a specified direction, lets say you have the electric field oscillating in the x direction and in the y direction for the first slit and the second slit respectively. Those directions are orthogonal to one another, so cannot interfere (the inner product is zero). But, if you have some component from both slits in some direction (for your example you will be getting out sqrt[2]/2 of the x component in the 45 degree direction and sqrt[2]/2 of the y component in the 45 degree direction when you insert the 45 degree polarizer, which is basically equivalent to the no polarizer case except you have reduced the amplitude). Then you have slit interference in the classical sense as illustrated here : http://astro1.panet.utoledo.edu/~lsa/_color/14_interference.htm, you will have to scroll down to see the two slit interference. Note that we see a sinusoidal pattern because our eyes view the time averaged irradiance (intensity) of the wave pattern, the the wave pattern itself.
What is different about the quantum case is that you can send, say electrons, through the slits *indivdually*, one at a time and they somehow interfere, that is what is intuitively strange.
So, are they working on something that makes light travel a long distance and/or go slower before making that "decision", thus achieving a substantial delay that could actually be used for "time travelling information"?
Under the simple interpretation, nothing "goes back in time." It's essentially two Schrodinger's cats (A & B) being in a superimposed state for several nano-seconds. Then V adds a constraint, and eventually the A, B, and V information bubbles interact and collapse into an observed state that the scientists record.
The meta-computer that runs our universe probably printed a log message like 'ATOMIC MERGE-OP unexpected long delay on eval: d=7m, t=23ns.' If scientists persist in this sort of research, the person running this universe will probably just ^C the app.