There’s an article I read yesterday that is haunting me. No matter how many times I read it, I just don’t understand it. If there are any readers of this blog that are a) really good with physics and b) really good at explaining complex physics principles to people with absolutely no knowledge of physics, I’d really love to hear from you.
Here’s the story:
If his experiment with splitting photons actually works, says University of Washington physicist John Cramer, the next step will be to test for quantum “retrocausality.”
That’s science talk for saying he hopes to find evidence of a photon going backward in time.
“It doesn’t seem like it should work, but on the other hand, I can’t see what would prevent it from working,” Cramer said. “If it does work, you could receive the signal 50 microseconds before you send it.”
“There’s a whole zoo of quantum paradoxes out there,” Cramer said. “That’s part of the reason Einstein hated quantum mechanics.”
One of the paradoxes of interest to Cramer is known as “entanglement.” It’s also known as the Einstein-Podolsky-Rosen paradox, named for the three scientists who described its apparent absurdity as an argument against quantum theory.
Basically, the idea is that interacting, or entangled, subatomic particles such as two photons — the fundamental units of light — can affect each other no matter how far apart in time or space.
“If you do a measurement on one, it has an immediate effect on the other even if they are separated by light years across the universe,” Cramer said. If one of the entangled photon’s trajectory tilts up, the other one, no matter how distant, will tilt down to compensate.
This is where going backward in time comes in. If the entanglement happens (and the experimental evidence, at this point, says it does), Cramer contends it implies retrocausality. Instead of cause and effect, the effect comes before the cause. The simplest, least paradoxical explanation for that, he says, is that some kind of signal or communication occurs between the two photons in reverse time.
We’re going to shoot an ultraviolet laser into a (special type of) crystal, and out will come two lower-energy photons that are entangled,” Cramer said.
For the first phase of the experiment, to be started early next year , they will look for evidence of signaling between the entangled photons. Finding that would, by itself, represent a stunning achievement. Ultimately, the UW scientists hope to test for retrocausality — evidence of a signal sent between photons backward in time.
In that final phase, one of the entangled photons will be sent through a slit screen to a detector that will register it as either a particle or a wave — because, again, the photon can be either. The other photon will be sent toward two 10-kilometer (6.2-mile) spools of fiber optic cables before emerging to hit a movable detector, he said.
Adjusting the position of the detector that captures the second photon (the one sent through the cables) determines whether it is detected as a particle or a wave.
The trip through the optical cables also will delay the second photon relative to the first one by 50 microseconds, Cramer said.
Here’s where it gets weird.
Because these two photons are entangled, the act of detecting the second as either a wave or a particle should simultaneously force the other photon to also change into either a wave or a particle. But that would have to happen to the first photon before it hits its detector — which it will hit 50 microseconds before the second photon is detected.
I just don’t get this.
2 responses so far ↓
1 jon // Dec 1, 2006 at 10:53 am
I think Doc Brown just slipped and hit his head on the sink after slipping off the toilet - the Flux Capacitor is born!
2 Travis // Dec 9, 2006 at 4:02 am
Okay, the article is vague about what the first detector for the first proton will do, so I do not have inofrmation about that part. But I will attempt to explain his idea a little bit better:
There is an actual feature called quantum entanglement, and even if the photon pairs are separated by miles, if you do something to one photon, it is reflected in the other pohoton simultaneously. You don’t have to wait for the time it takes light to travel or anything, once you adjust something, it automatically changes the other photon. They usually refer to the “spin” of a photon, meaning if you create an “up” spin in one photon, your measurement of the other photon will immediately register it having a “down” spin. (spin means intrinsic angular momentum; spin is easier to say)
For his measurement, he is proposing that he can measure a change in one proton. The change in one proton will affect the other, but he is manipulating them in a way that the second photon will not be detected to have a certain disposition until after the first photon has reached its target. If they can demonstrate that (relatively) the first photon registers as what they want it to before the second photon can send the order, then they make the second photon one specific thing 50 microseconds later, then they have managed to order the second photon 50 microseconds late, but still affect the first photon. Ergo, they have had a cause later in time affect the past state of something. Retrocausality.
By definition, if one photon is a wave, for example, the other MUST be a wave as well. The retrocausality comes into effect because the first photon will have to register as a wave at the first detector, and will arrive there first. But if they can adjust the second detector to influence the outcome, and they adjust it so the second photon is detected as a particle, then the first photon will have “no choice” but to also register as a particle.
Since the instruction to be a particle is “decided” by the second detector, but they send the second photon farther so the first photon hits the first detector first, then they will conclude that they transmitted an “order” back in time to tell the first photon to be a particle.
The potential flaws I see that aren’t really covered by the article are this:
How do they avoid determining causality with the first detector? Won’t it register it as a particle or wave before the second photon arrives at the second detector? The article does not get into that particular detail.
Also, according to a certain physicist friend of mine, we might not have the correct language to describe quantum effects yet. Photons can behave as both particles or waves depending on how you measure them, so when you ask, “Is it a wave?” the answer is “yes.” As soon as you measure it differently and ask, “Is it a particle,? the answer is “yes.” So if it can be both a wave and a particle and exhibit the same qualities of both at the same time, how can we tell that both photons aren’t measured as a wave, but still retain the qualities that are relevant to particles the whole time?
I’m not convinced this experiment is entirely sound, but the nature of quantum mechanics makes it an interesting- if mind-bending- thing to examine.
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