icemith, from what I gather, each photon will follow the interference distribution. That is the surprising result – the wave is not made up of a lot of component photons, each photon has wave aspects and particle aspects.

“Also there’s the fact that photons diffract when they pass through a small slit.”

I have a problem with that statement. It seems to be illogical. I understand that ‘diffraction’ is the process whereby a wave, or a wavefront is split into many parts, as it passes through a narrow slit or close to an edge. We see that effect in a rainbow display, or a prism where light is split into various colors depending upon its wavelength.

I take it that means broken into photons, but I guess that could mean many different sized bits, depending on the actual ‘roughness’ of the edge (edges) involved. Maybe even single photons. However our correspondent is implying that the diffraction (subsequently ?) also has an effect on photons, and they diffract, ie, also break down to componant parts.

And is a photon considered a ‘particle”, or are *they*
made up of particles, or vicky-virky?

Sorry, but I need a Q.M. 101 course.

Ivan.

Let’s assume the following:

There is a detector that determines the “type” of photon – particle or wave – as the photon passes the detector. This will be called “Detector A”. It is not “adjustable” and will report faithfully what type of photon passes.

There is a type of detector that, by manipulation, will “influence” the type of photon you detect – whether it is “read as” a particle or a wave. This will be called “Detector B”

The detector that cannot be manipulated reads a photon that has only passed through free space.

The detector that can be manipulated is reading (and influencing) the photon that has run through the fibre and is delayed by 30 ms.

Now – here is what I fail to understand about this experiment:

The photon that is detected by A is detected “first”, the photon detected by B is detected 30ms later – BUT B can influence the detected type of the photon, presumably at A.

If we know that A has detected a particle, and then (one presumes a computer does this) we then “influence” B to detect a wave, does our detection of the particle at A then change?

To me, it seems to say that A influences the detection at B, but I may just be missing the entire point. If B’s positioning is determined ahead of time, the photons are emitted and detection at A must then agree with what is seen at B. If B is set to detect a wave, then all photons at A should be seen to be waves.

However, if you detect the photon at A and then change B so that it it is opposite to what you just saw at A………

I guess I just don’t understand the experiment well enough.

JC

As for detecting whether the photon is a particle or wave, it’s not really a simple case of “Okay, now it’s a wave… now it’s a particle.” The meaning of wave-particle duality is that some experiments show photons exhibiting properties we expect from waves (such as diffraction and interference), while others show them exhibiting properties we’d expect from particles (such as being quantized into single photons).

When you get into a full QM picture, our old definitions of “particle” and “wave” just don’t cut it. Everything we’ve tested on this scale appears to propagate as a probability, but then resolve into a single point when “measured.”

So, with that in mind, here’s basically how the double-slit detector works. A photon is fired out of some source, and its propagation wave can pass through one of two slits. The wave then propagates out of both slits and hits a detector screen. If the photon were acting like a particle, we would expect the resulting pattern on the screen to be the sum of the diffraction patterns from each individual slit. If it were acting like a wave, we’d expect a much stranger interference pattern.

So then we run this experiment. With many photons being shot out, we see the interference pattern form, so they act like a wave here. If we shoot out the photons one at a time, then surprisingly we also see this pattern in the probability distribution of where they hit. The result is the surprising fact that a single photon seems to propagate as a wave which gives its probability of resolving at a given point if measured.

With this in mind, I don’t why a photon would ever register as a “particle” in a detector like that. Yes, a light distribution matching that predicted for a particle would imply this, but all the evidence we’ve seen tells us that that won’t happen.

Aside: That being said, it is possible to get the “particle” distribution to appear if we alter the experiment slightly. What we have to do is force the photon to resolve its wavefunction when it’s in one of the slits. This is tricky to do with a photon, but if we switch to electrons and shine light on them, we can do this (and yes, electrons normally will show wave-like interference if we don’t do this).

>Now, even after you take that into consideration, their physics is still wrong. Simply put, the problem is that photons always travel as waves, and then resolve as single particles when they’re measured. Detecting the first particle would force the second to resolve somewhere, but after that it spreads out into a wave again before it hits the detector.

This is an interesting suggestion. Someone should email it to the researchers.

>With that in mind, my hypothesis for this experiment is that they will simply detect both photons as waves. They may then go on to incorrectly interpret this as confirmation of their own hypothesis.

They will get the first detected however it hits (50% split? I don’t know what is typical). The second will register depending upon what they look for. This will leave them perplexed, but conflict with their hypothesis.

And I don’t think avataru was unaware of interference, I think he/she was picking on your wording.

Simply put, the problem is that photons always travel as waves, and then resolve as single particles when they’re measured.

Except when they’re measured as waves (interference).

“The other photon will be sent toward … a movable detector, he said.

Adjusting the position of the detector … determines whether it is detected as a particle or a wave. ”

So – the first one detects what it is, the second one is influenced to be a P or W, thereby (in theory tested) influencing the first one.

JC

Many scientists claim that it’s entropy that gives time its direction, but this is only partly true. The problem with this is that entropy isn’t a physical law; it’s a statistical one, and simply describes a trend. Entropy may give time its direction, but we still need something to give entropy its direction. The collapse of a wavefunction is what accomplishes this.

It means you’re late!!

Jack Hagerty: regarding “Forbidden Planet” – Just got my 50th Anniversary DVD today. I’ve seen the movie many times, of course – I’ll have to keep my ears open for the tiny wrenches.

jbs

Because photons exhibit wave interference just as sound waves or ripples on a pond. Search Double-slit experiment on Wikipedia.

Because photons exhibit wave interference just as sound waves or ripples on a pond. http://en.wikipedia.org/wiki/Double-slit_experiment

Because photons exhibit wave interference just as sound waves or ripples on a pond. http://en.wikipedia.org/wiki/Double-slit_experiment

Maybe they need one of Chief Quinn’s “Quantum Mechanics” (they guys with the really tiny wrenches…).

For the reference impaired: That’s from “Forbidden Planet”, the 50th anniversary restoration DVD of which was released this week. They did a great job, you should check it out.

- Jack

[i]Now, even after you take that into consideration, their physics is still wrong. Simply put, the problem is that photons always travel as waves, and then resolve as single particles when they’re measured. Detecting the first particle would force the second to resolve somewhere, but after that it spreads out into a wave again before it hits the detector.[/i]

Well, how do scientists know photons travel as waves when every time they take a measurement they see a particle?

ouch, my head hurts….

April fools,,,just a bit early, eh???

Oh, wait, didn’t someone just say that,,,,

Ack,,,Deja, vuja,,

I think Bill the Cat just bit me. But then, I may have already said that?

Heck, I’m going back to bed,,,

GAry 7

Now, even after you take that into consideration, their physics is still wrong. Simply put, the problem is that photons always travel as waves, and then resolve as single particles when they’re measured. Detecting the first particle would force the second to resolve somewhere, but after that it spreads out into a wave again before it hits the detector.

With that in mind, my hypothesis for this experiment is that they will simply detect both photons as waves. They may then go on to incorrectly interpret this as confirmation of their own hypothesis.

This just goes to show: Even quantum physicists don’t always understand quantum physics.

Aside: Now, if we were to take two particles and have one move at a relativistic speed, and the other at a non-relativistic speed, we might be able to see something interesting, but that’s another story.

skeptigirl said:
>The words, ’subatomic equivalent’ are quite buried in that description.

I had the same complaint. There’s something fishy going on when people are leaping from “I can possible get a reading that corresponds to a decision I make 50 milliseconds later” to sending messages back in time. Yes, the principle becomes established, but there’s still a huge leap to go to practical application. And the mechanism relies on miles of fiberoptic cable to delay one photon. Seems a challenge to use that effectively.

We may get some interesting and perplexing info from the experiment, but it’s a far cry from time travel or time communication.

Or is he still working on the receiver?

Answer:The pope

I hope the experiment works. It would validate the concept, “,,,and then, everything happened all at once, but it took us forever to see it,,,”

Gary 7

Wrong!
The rooster came first!