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Offline flr

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a single photon: particle or wave?
« on: 22/09/2013 05:35:56 »

Is a single photon alone a wave or a particle?



 

Offline alancalverd

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Re: a single photon: particle or wave?
« Reply #1 on: 22/09/2013 06:47:08 »
Yes.

Or more precisely, no.

The photoelectric effect is best modelled as a particle interaction, and single photon events can be detected.

The double-slit self-interference property of electromagnetic radiation is best modelled as a wave phenomenon and persists down to single photon events.

A photon is a photon. We describe it as a wave or a particle depending on the circumstances. X-ray crystallography, particularly of biological molecules,  is challenging because the useful aspect is in the  diffraction of photons, accurately modelled as waves, but individual photon particle events can damage the structure we are trying to study.     
 

Offline Bill S

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Re: a single photon: particle or wave?
« Reply #2 on: 22/09/2013 13:16:04 »
Would it be right to say that a photon is neither a wave nor a particle; rather, it is something of which we have no analog in the macro world?  It seems that the nearest we can come to describing this "something" is to say it has properties of a particle and a wave, depending on how we "look at" it.
 

Offline flr

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Re: a single photon: particle or wave?
« Reply #3 on: 22/09/2013 17:04:12 »
Quote
The double-slit self-interference property of electromagnetic radiation is best modeled as a wave phenomenon and persists down to single photon events

As I understand it, the interference and other wave-like properties are observed when an ensemble of very many photons are sent. If a single photon is sent then the interference pattern is gone and instead of getting that energy of 1 single photon spread out over the entire screen (as in an interference pattern) we only get 1 single spot in 1 very localized area of the screen as if the 1 photon alone is a particle.

The interference patterns reappears when very many photos are sent over the screen even if they are not all at once but instead one at the time.
However this pattern is not generated by a single photon but by very many of them (sent one at the time).

Therefore 1 single photon must be particle.
Yet, somewhere down the path from source to screen (probably at double slit) something modified the path of photon in a sense that it will hit the screen in unexpected places for a particle (and this is better seen when we sum up billions of photons sent one at the time), yet a single photon hit the screen in a localized area as if it is a particle.   
 
 

Offline alancalverd

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Re: a single photon: particle or wave?
« Reply #4 on: 22/09/2013 19:03:04 »
But if the photons arrive one at a time, no photon knows where its predecessors went. So how does it know where to go? If it merely passed through one slit or the other, you would get an image of two fuzzy slits, each with something like a gaussian distribution. But you don't! You get a distribution with multiple peaks, however many photons were in transit at any time, so each photon must have "known" about both slits.
 

Offline flr

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Re: a single photon: particle or wave?
« Reply #5 on: 22/09/2013 20:01:42 »
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But if the photons arrive one at a time, no photon knows where its predecessors went.
They have to be independent to each other. If there will be spaces on screen where photons will preferentially go then summing up very many independent photons will reveal those domains where photons preferentially go. There is no need to know the history of all other photons.
The same like flipping a coin: if the coin is biased then flipping it many times will reveal the bias if each coin flipping is random and independent to any other coin flipping.

Now back to the initial question:
A single photon will not generate interference pattern on the screen, instead all its energy will be find in a very localized point on screen as is the photon was a particle.

Also, a single measurement (of 1 single photon) will tell me nothing about how photon behaved at the double slit based on where it hit the screen. Only when I sum up many photon and I analyze their distribution on screen I can reach a conclusion that the superimposed image of billions of photons looks like an interference pattern.




 

 

Offline evan_au

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Re: a single photon: particle or wave?
« Reply #6 on: 22/09/2013 23:02:23 »
Dr Karl suggests this simplification: A photon travels as if it is a wave, but is detected as if it is a particle.

That resolves a couple of the dilemmas above.
 

Offline alancalverd

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Re: a single photon: particle or wave?
« Reply #7 on: 23/09/2013 00:22:48 »
Only when I sum up many photon and I analyze their distribution on screen I can reach a conclusion that the superimposed image of billions of photons looks like an interference pattern.

Even a few hundred will suffice. But the underlying question is why, if each event is independent and the result of a single particle travelling through one slit or another, the distribution has more than two peaks? Well, independence is a "given", so the answer must be that it is not the result of a single particle travelling through one slit or the other.

Like the man says, a photon travels like a wave (Maxwell's equations describe it perfectly) and interacts like a particle. The operative word is "like".   
 

Offline flr

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Re: a single photon: particle or wave?
« Reply #8 on: 23/09/2013 01:31:12 »
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But the underlying question is why, if each event is independent and the result of a single particle travelling through one slit or another, the distribution has more than two peaks?
If the events are independent to each other, we should recover whatever 'inherent bias' there may be.
Like in a Monte Carlo integration if you want (the analogy may not be perfect).
Or if one throws randomly very many times 1 dice (with 6 faces), one will notice that only 1 in 6 trials generated face with number (say) 2. That does not mean that each event knew about others such that they sum up to 1/6 for face 2. It means that a random generator recover the probability associated with an event after many trials.

For photon hitting the screen after going though double slit , the 'inherent bias' consists in the complex interference patterns with domains where photons are less likely to end up and domains where photons are more likely to end up.
Why the 'inherent bias' (revealed after measuring an ensemble of photons) in similar to an interference patterns rather than 2 spots? I guess that is a clear signature of a wave-like character manifested at the double slit. However, at any point before and after slit we can identify the photon as a particle because if we put a screen to detect it we only get 1 single localized spot on the screen.
« Last Edit: 23/09/2013 01:36:54 by flr »
 

Offline Pmb

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Re: a single photon: particle or wave?
« Reply #9 on: 24/09/2013 03:37:05 »

Is a single photon alone a wave or a particle?
This question doesn't have a simple yes or no answer. The wave nature of the photon tells us where it will be while the particle nature tells us that when we do find it the photon will be located at a point. So yes or no is insufficient to answer your question.
 

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Re: a single photon: particle or wave?
« Reply #9 on: 24/09/2013 03:37:05 »

 

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