Effect Preceding Cause: A Quantum Physics Thought Experiment on Usenet

[Cthugha]

I don't think this is a problem for the path integral description. I'd say it's the exact opposite of a problem!

Oh, I never intended to say, that it is a problem for the path integral formalism. Putting such a detector in just makes the whole experiment uninteresting by destroying any interference pattern. Path integrals are fine as a model although they are nasty and unpractical when it actually comes to calculate stuff.

Also, in the case of the first image I posted, where it looks as though an interference pattern is formed after both exit ports of the recombining beamsplitters (I really wish I hadn't posted that image, but rather, only the second image in post #41), when we block the long-path beam, I'm now picturing the interference pattern being diminished before we block the beam rather than completely erased, until 5ns after we block the path, and then it is completely erased. After all, only the subject photons would stop interfering, while the photons taking the long path (of their 2-path journey) before we block the long path would keep interfering. That is, we make the short path "subject" photons distinguishable at the detector while the older photons taking the long path (or their 2-path journey) still interfere because they are still indistinguishable...?

No matter which image I look at (though I much prefer the second image), I still see effect preceding cause, unless photons are interfering with other photons and not themselves, which counters everything I've read in books on the subject...sigh!

Ok, letme at first comment on the first image.
Where exactly do you think, that different photons interfere? You can't really say, that photons taking one path keep interfering, while those taking the other path don't. The key to interference is, that you can't distinguish photons taking the short path from photons taking the long path. So in fact the photon taking the long path and the photon taking the short path are the same single photon, which produces the interference pattern. Although this seems counterintuitive, one must keep in mind, that we have a coherent state. The key property of a coherent state is, that the uncertainty of the moment, when one certain photon is emitted is roughly the order of the coherence time. So it is this one single photon, which interferes with itself. It takes the long path and is emitted early or it takes the short path and is emitted later. As long as those paths (or better realizations or histories) are indistinguishable, there will be interference. Just to stress it, you do not need one older photon traveling the long path and one newer photon traveling the short path, but just one photon, which could have taken both paths and could have been emitted at both times with equal probability.

The second image does not change anything in principle.

 

[Robert Noel]

Where exactly do you think, that different photons interfere? You can't really say, that photons taking one path keep interfering, while those taking the other path don't.

I'm not saying that at all. I'm saying that the older photons taking both paths interfere while the newer ones forced to take one path don't.

The key property of a coherent state is, that the uncertainty of the moment, when one certain photon is emitted is roughly the order of the coherence time. So it is this one single photon, which interferes with itself. It takes the long path and is emitted early or it takes the short path and is emitted later. As long as those paths (or better realizations or histories) are indistinguishable, there will be interference. Just to stress it, you do not need one older photon traveling the long path and one newer photon traveling the short path, but just one photon, which could have taken both paths and could have been emitted at both times with equal probability.

This is the part I just cannot get my mind to accept. Each atom emits its own photon, wether only one atom is doing it or billions. This idea that one photon could have come from two atoms, or that an atom could have emitted a single photon at two different times just doesn't sound realistic. I can accept that a photon can "take both paths" (given my strange relativity-inspired view on how a photon might "percieve" its knowable universe), but the idea that we can consider a single photon as having come from two sources, or a single source at two different times, just doesn't fly with me. That said, you do have me thinking...is it any stranger that a photon can be emitted at two different times than that it can be absorbed at two different times? Hmmm...food for thought...I think you've finally convinced me that there's something seriously wrong with how I think...again, thanks!

Edit: I think the main reason for my flawed thinking is in reading about how "an electron drops from a higher orbit to a lower orbit and emits a photon in the process", which suggests a single determined cause (and effect)...obviously it must not be that simple.

 

[Cthugha]

I'm not saying that at all. I'm saying that the older photons taking both paths interfere while the newer ones forced to take one path don't.

Ah, ok. I misunderstood. Sorry.

Edit: I think the main reason for my flawed thinking is in reading about how "an electron drops from a higher orbit to a lower orbit and emits a photon in the process", which suggests a single determined cause (and effect)...obviously it must not be that simple.

Yes, exactly. One important thing is that the light source matters. One might picture a simple single electronic transition as a single determined cause for photon emission, but the important thing is, that just having a lot of such transitions does not produce coherent light. Coherent light is different. If you have a look at how a laser works, you will notice, that the main source of photon emission is stimulated emission. So you have some atom/molecule/quantum dot or whatever your active medium is in an excited state. Another photon, which was emitted by some other atom/molecule/quantum dot and is resonant with the electronic transition comes along and triggers stimulated emission. The atom/molecule/quantum dot is back in its ground state again and you do now have two photons with equal phase, wavelength and direction. Can you tell, which one was emitted in the process of stimulated emission and which was the stimulating photon?

 

[Robert Noel]

Heh heh, which unveils yet another mystery: How a photon can stimulate the emission of another photon with the same properties (interact with another atom) without having its own properties changed.

But one mystery at a time is quite enough...I'll be spending a lot of time trying to understand all you've revealed to me in this thread.

I can't thank you enough Cthugha. Your input, and your patience, is much appreciated...as is everyone else's!

 

[dkv]

Yes how can a photon emit another photon?

 

[Nick666]

So are there any experiments where the effect is preceding its cause ?

 

[Cthugha]

Yes how can a photon emit another photon?

It doesn't. It just stimulates another atom to emit another photon.

 

[dkv]

How can the emitted photon have the same properties?

 

[Phrak]

How can the emitted photon have the same properties?

http://en.wikipedia.org/wiki/Stimulated_emission" if you want to read about it

 

[dkv]

The incident photon interacts with excited atom which means it must loose some of its original properties to do so... I mean how can some thing interact without interacting at all!