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Author Topic: What is a photon? But read the PDF first thanks.  (Read 3090 times)

Offline yor_on

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What is a photon? But read the PDF first thanks.
« on: 02/02/2010 14:55:14 »
If you read this short paper you will find the statement.

"Such an attosecond pulse was recently described in Nature (2004 427:817) and was 150as which means that it was 45nm in length. But how can this be? How can a photon be shorter than its wavelength? Basically, it can not."

Now, when questioning this I got the answer "quantum particles are equally well represented in the momentum space (aka. wavenumber space, aka wavelength space) as in physical space. Here you have a particle well-localized in physical space but very poorly localized in wavelength or momentum."

Which I then presume to be a statement of HUP, reading the article, do you agree?

1. So my question is, do you agree to this statement, but please, read the article first thanks.

===

But if we then assume that it is HUP making its statement here, does this mean that we have a clear definition for what HUP is? As we actually then have proof of this light becoming shorter than its wavelength. Can you see my question?

---Quote---

The other thing is the Heisenberg Uncertainty Principle. I have been having another look at the EPR experiment because I`d like, eventually, to get to Bell`s theorem, and it seems that it is a good idea to understand EPR first! There seem to be several versions/takes on the EPR experiment, but this question is more fundamental. Does the HUP mean that we cannot know both x and p_x to better than the standard uncertainty because the act of measuring means that we disturb the system, or does it mean that the actual of existance of x and p_x are ruled by the HUP - in other words, it wouldn`t matter even if we could measure without disturbing the system, the information we seek is inherently affected by the HUP anyway, ie it is not there in the first place?  If the latter is the case, why do so many books talk about the act of measuring disturbing the system (thus giving rise to the HUP) when in fact it is irrelevant to the existence of the HUP?  So perhaps with the act of measuring we get a double effect, a combination of the HUP per se plus the `jolt` suffered by the system as a result of taking a measurement?

---End of quote--

2. Now, this is an old question, but it seems to me that if the photon indeed act this way due to HUP the question should be answered? It's not the measurements failing, it really is an indeterministic world. So, do you agree?

My last question :)

Assuming that we have no limit to the size of a photon except for their possible energy.

3. At what level of energy will they transit Planck scale, and, can they transit it?
Or is there a boarder set by times arrow between photons and virtual photons?
Similar to 'c' in a vacuum f.ex




===
PS: Oh yes, another thing, do you consider this paper as made by an 'crank?'?
Some called the site 'Crackpot central = Journal of theoretics.' and made all kind of derogatory statements towards both the article and the site?
==




 

Offline graham.d

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What is a photon? But read the PDF first thanks.
« Reply #1 on: 26/08/2011 12:30:28 »
yor_on, I don't know the journal from which you got this but the article seems wrong, or provocatively contraversial, in many areas. On the particular statement regarding the pulse length being shorter than the wavelength, they did not explain how they measured this pulse length. You do not even need to invoke Heisenberg here; Fourier analysis would be sufficient to show a short "pulse" would have a very wide spectrum indeed. The pulse being a change in an em field represented by a photon.

I think what they may be doing is timing a photon impact of some sort; without explanation it is hard to say. If this is the case then it is true that it is in the nature of wave-particles to resolve themselve to be a particle in this case, and that this particle can transfer its energy in a period shorter than the wavelength. What can't be determined is exactly where in the wavepacket this will occur (Heisenberg). With a good laser the coherence length is very long (the frequency band is very narrow). When the intensity is reduced so that photons are emitted infrequently, there is no way to tell the position of the photon. The resolution is the photoelectric effect of the detector when the energy is transferred, not by a gradual build up of em energy being dispersed as heat (for example) but as though from a particle impact.

It seems to me that the article is making a statement about wave-particle duality but I cannot see what they are saying that is any more hard to understand than the concept of wave-particle duality itself.
 

Offline rosy

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What is a photon? But read the PDF first thanks.
« Reply #2 on: 26/08/2011 13:22:40 »
Graham: note that yor_on posted this in February 2010.. the thread came up the recent posts list as a result of a spam post (since deleted).
 

Offline graham.d

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What is a photon? But read the PDF first thanks.
« Reply #3 on: 26/08/2011 13:37:06 »
Oh yes, Rosy, I see the date now. D'oh! Anyway, I didn't see this post the first time around. I expect yor_on is still interested though.
 

Offline lightarrow

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What is a photon? But read the PDF first thanks.
« Reply #4 on: 26/08/2011 19:58:23 »
That is a perfect example of what is a physical photon compared with the QED definition of photon, as we have also recently discussed in the last 4 posts of this thread:
http://www.thenakedscientists.com/forum/index.php?topic=40635.0
 

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What is a photon? But read the PDF first thanks.
« Reply #4 on: 26/08/2011 19:58:23 »

 

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