Register to reply

How big is a photon

by alvaros
Tags: photon
Share this thread:
Schrodinger's Dog
#217
Jul29-07, 02:36 PM
Schrodinger's Dog's Avatar
P: 1,136
I don't think it's going to suffice, it's not so much smashing them apart but how you detect and measure such tiny portions of energy amongst the mass of energetic particles created in a collision; besides establishing the mass or ever more precise upper limits of a photon is not as important as establishing the existence or non existence of say the Higgs Boson, so since detection methods aren't precise enough scientists probably have better things to do with it. I'm not an expert not even close, but I don't think we are anywhere near being refined enough with the machinery atm. I'd ask Zz...
rewebster
#218
Jul29-07, 02:51 PM
P: 880
I wasn't too serious when I suggested it---just the idea of 'bigger and better' came up.

I wonder how many betting 'pools' have come up as to how many new particles will be 'discovered'/'created' with the first year there.


well, that's good for you though that's one less worry/thing to think about if it is 'at rest' now.
LorentzR
#219
Jul30-07, 03:04 AM
P: 34
Quote Quote by ZapperZ View Post



But one can say that to every single quantum particles.

Zz.
No! I don’t agree with this statement. The idea of the photon was at least partly introduced as a solution to the old problem of what causes “action” at distance. But the question presupposes the notion of locality. This in the context of modern physics would be the assumption that the “particles” composing matter can be assigned, for any given time, a unique position in space. However, correlation experiment results showing the violation of Bell’s inequality suggest that our world has a non local flavour; casting doubts on the idea that particles of matter can be assigned exact locality relative to our inertial reference frames. This in turn puts into doubt the need for carrier particles to mediate action at a distance. If the locations of “particles” are not fixed exactly relative to our inertial reference frames then neither are the gulfs that separate them and direct interaction between pairs of particles becomes possible when the gulf between them has zero magnitude.

The “particles”/ “fields” of matter must remain in our philosophy it is only the photon (the suggested mediator of force) that is surplus to the explanation of electromagnetism.
LorentzR
#220
Jul30-07, 03:07 AM
P: 34
Quote Quote by ZapperZ View Post
At the same token, photons and other quantum particles have definite position upon measurement, or else, what exactly are those photomultipliers and CCD detectors measuring? So what exactly do you mean by "cannot have exact locality" here?

Zz.
No one has ever observed the position of a photon or any other quantum particle. What you observe is a macroscopic response in a particular experimental set up. The timing and position of the response may be measured with exquisite accuracy but the observable response exists at the end of a sequential causal chain that is necessary to amplify a quantum event to an observable event.

You are observing the location of a macroscopic event that is initiated by a quantum event. There is no clear logical path that links what is observable to where and when the quantum event took place. To give a photon or other quantum particle a definite position on this evidence is unsustainable. All we can say is that somewhere in the four dimensional manifold an event has occurred that has affected our detection device in a particular way. There is no justification based on experimental evidence for assigning a definite position to a quantum object.
ZapperZ
#221
Jul30-07, 05:19 AM
Emeritus
Sci Advisor
PF Gold
ZapperZ's Avatar
P: 29,242
Quote Quote by LorentzR View Post
No one has ever observed the position of a photon or any other quantum particle. What you observe is a macroscopic response in a particular experimental set up. The timing and position of the response may be measured with exquisite accuracy but the observable response exists at the end of a sequential causal chain that is necessary to amplify a quantum event to an observable event.

You are observing the location of a macroscopic event that is initiated by a quantum event. There is no clear logical path that links what is observable to where and when the quantum event took place. To give a photon or other quantum particle a definite position on this evidence is unsustainable. All we can say is that somewhere in the four dimensional manifold an event has occurred that has affected our detection device in a particular way. There is no justification based on experimental evidence for assigning a definite position to a quantum object.
Then you need to write a rebuttal to all the experimental high energy physics papers, and even to condensed matter papers using anything that measures electron and neutron spectroscopy such as what I've shown in my avatar. Look under ANY papers in any physics journal that actually detects these particles and tell them that they're not really detecting that 'quantum event'. Remember, in angle-resolved photoemission experiment, the location where the electron hits the detector just doesn't signify that this electron was there at that time, but also carries information about its energy and "in-plane" momentum while it was in the solid!

Did I see electrons? Of course not! But did I detect that at the instant the detector made its detection, that quantum particle was there? Sure I did! Is this macroscopic? Of course it is! All our measurements are not only macroscopic, they are also classical! That is why I can talk about "position" and "momentum" and "energy", because these are all classical concepts. It is also the CAUSE of the weirdness at the quantum level, because we are asking for properties that are not that well-defined at the quantum world.

Note that in the HUP, there's nothing here that says that you cannot measure, as accurately as technologically possible, the position and momentum of just ONE measurement. The accuracy of these measurements do not depend on the HUP, but depends on the accuracy of the instrumentation and technique. The HUP says nothing about a single measurement. The HUP, however, tells you (i) the accuracy that you can predict the NEXT measurement, given the accuracy that you have imposed on one of the non-commutating observable, and (ii) the spread in the value of that observable after repeated identical measurement.

Zz.
ZapperZ
#222
Jul30-07, 05:22 AM
Emeritus
Sci Advisor
PF Gold
ZapperZ's Avatar
P: 29,242
Quote Quote by LorentzR View Post
No! I don’t agree with this statement. The idea of the photon was at least partly introduced as a solution to the old problem of what causes “action” at distance. But the question presupposes the notion of locality. This in the context of modern physics would be the assumption that the “particles” composing matter can be assigned, for any given time, a unique position in space. However, correlation experiment results showing the violation of Bell’s inequality suggest that our world has a non local flavour; casting doubts on the idea that particles of matter can be assigned exact locality relative to our inertial reference frames. This in turn puts into doubt the need for carrier particles to mediate action at a distance. If the locations of “particles” are not fixed exactly relative to our inertial reference frames then neither are the gulfs that separate them and direct interaction between pairs of particles becomes possible when the gulf between them has zero magnitude.

The “particles”/ “fields” of matter must remain in our philosophy it is only the photon (the suggested mediator of force) that is surplus to the explanation of electromagnetism.
Huh?

What's with the Bell inequality all of the sudden?

Here's what you can do. Write down the wave function for a free particle. Now find <x>. Tell me what is "local" here.

Zz.
LorentzR
#223
Aug14-07, 06:52 AM
P: 34
Quote Quote by ZapperZ View Post
Then you need to write a rebuttal to all the experimental high energy physics papers, and even to condensed matter papers using anything that measures electron and neutron spectroscopy such as what I've shown in my avatar. Look under ANY papers in any physics journal that actually detects these particles and tell them that they're not really detecting that 'quantum event'. Remember, in angle-resolved photoemission experiment, the location where the electron hits the detector just doesn't signify that this electron was there at that time, but also carries information about its energy and "in-plane" momentum while it was in the solid!



Zz.
Sorry for this late response I only rarely get chance to look at your forum

Far be it from me to issue a rebuttal; a lead balloon comes to mind. Within the context of the ARPES experimental setup and other scattering experiments the electron and the “photon” seem to behave as if the were particles. This has been known for many years and the advance in the energy resolution and angular resolution (momentum) does not alter this perspective. But because in this particular experimental set up we can explain the observed “classical” outcome by regarding the electron and “photon” as particles does not mean that is what they actually are. We can not pick and choose our experiments to fit our favourite theories we also have to take into account other experimental circumstances where the idea of a particle fails to explain the observed outcomes.

I cannot agree that particles are actually detected, an observer detects a classical event a change in the observable state of the detector and we only infer it is a response to a quantum event. I have no doubt that the observed change in the state of the detector is consequential on a quantum event occurring. Whether or not the quantum event has a specific location relative to our reference frame and the location of the detector I would suggest is unknowable.
LorentzR
#224
Aug14-07, 06:54 AM
P: 34
Quote Quote by ZapperZ View Post

Did I see electrons? Of course not! But did I detect that at the instant the detector made its detection, that quantum particle was there? Sure I did!

Zz.
This is your inference not what you actually detected or what’s likely to have happened at quantum level?
LorentzR
#225
Aug14-07, 07:12 AM
P: 34
Quote Quote by ZapperZ View Post
Is this macroscopic? Of course it is! All our measurements are not only macroscopic, they are also classical! That is why I can talk about "position" and "momentum" and "energy", because these are all classical concepts. It is also the CAUSE of the weirdness at the quantum level, because we are asking for properties that are not that well-defined at the quantum world.



Zz.
Then perhaps you will agree that the measured values for "position" and "momentum" and "energy" are properties of the changes in the detector(s) and do not necessarily represent the state of a quantum entity immediately before the detection?

Quote Quote by ZapperZ View Post
Note that in the HUP, there's nothing here that says that you cannot measure, as accurately as technologically possible, the position and momentum of just ONE measurement. The accuracy of these measurements do not depend on the HUP, but depends on the accuracy of the instrumentation and technique.

Zz.
This depends on the group behaviour of the quantum systems forming the measuring apparatus and their interaction within themselves and with the quantum system to be measured as well as the group behaviour of the quantum systems forming the brain and creating the mind of the observer ( Locality for instance may be a property of the observer and not the quantum object). The resulting measurements may be extremely accurate at macroscopic level(classical outcome) but this does not necessarily mean it is providing precise information, say, about the position of something that may be inherently fuzzy.

We must have a better understanding of the group behaviour of quantum systems that form the experimental set up since it is their collective behaviour that allows us to obtain accuracy of measurement relative to our inertial reference frames. Our observations are limited to the macroscopic outcome of experimental situations. We may be capable of fixing the position of these outcomes with extreme accuracy, but this accuracy is created by the group behaviour of the quantum systems forming the detector and the “clocks” and “rulers” needed to fix the response’s location. The place and time of a quantum interaction initiating a detection event cannot be specified from the information we have available to us.
LorentzR
#226
Aug14-07, 07:14 AM
P: 34
Quote Quote by ZapperZ View Post
The HUP says nothing about a single measurement. The HUP, however, tells you (i) the accuracy that you can predict the NEXT measurement, given the accuracy that you have imposed on one of the non-commutating observable, and (ii) the spread in the value of that observable after repeated identical measurement.

Zz.
This reinforces the belief that the quantum objects do not possess specific locations; their interactions do not possess specific locations and their particle like behaviour is an illusion created by the integrated behaviour of all the quantum systems participating in the experimental set up. It also reinforces the idea that the wave-function controls of how the distribution of the intensity of a “particle’s” likelihood to interact is projected onto our reference frames.
LorentzR
#227
Aug14-07, 07:21 AM
P: 34
[

Quote Quote by ZapperZ View Post
Huh?

What's with the Bell inequality all of the sudden?
Zz.
My principle reason for doubting the existence of the photon was that its raison deter was to mediate the electromagnetism; to bridge the gulf between charged quantum entities. But if quantum entities don’t have uniquely defined positions in the world then the gulfs separating them are also undefined; opening up the possibility of direct interaction between the charged systems. I therefore brought the evidence of the violation of Bell’s inequality to reinforce the idea that charged quantum systems do not have unique positions in space, since here was further evidence of a non local flavour to our world.



As I wrote earlier you can’t pick and choose your experimental results to reinforce your favourite theory. Our underlying understanding of how things happen must explain all known experimental results if it is to be valid.

Locality is fundamental to classical physics, but the conventional definition seems incapable of explaining in a consistent way all the experimental results obtained by physics. At the heart of the problem is the Born rule that allows one classical description of a quantum entity; where the entity is spatially distributed; to be transformed into possible classical outcomes where the entity becomes point like.

Explaining how this happens in a relativistically friendly manner is one of the great problems of natural philosophy.

Quote Quote by ZapperZ View Post
Here's what you can do. Write down the wave function for a free particle. Now find <x>. Tell me what is "local" here.

Zz.
Further evidence that you cannot pin point a quantum entity!
Measures of location are facilitated by the collective behaviour of quantum systems and their group “position” in the manifold.

I stand by my assertion that the weight of this evidence suggests that charged systems interact directly, so there is no requirement in nature for man’s photon. So the answer to Alveros’s question seems to be; something that does not exist does not have a size!!!!!
ZapperZ
#228
Aug14-07, 07:24 AM
Emeritus
Sci Advisor
PF Gold
ZapperZ's Avatar
P: 29,242
Quote Quote by LorentzR View Post
Then perhaps you will agree that the measured values for "position" and "momentum" and "energy" are properties of the changes in the detector(s) and do not necessarily represent the state of a quantum entity immediately before the detection?
Come again?

Other than the fact that this issue is getting pretty old and I've lost interest in it already, I have no idea what you just said here. If the detector is more of a property of itself, then you have to seriously explain why it just didn't make a detection trigger all by itself. Why does it need a photon (or anything else) to cause a measurement?

You are also forgetting that everything that you detect (including all of your sense) is an interaction of something with something. As an experimentalist, I have to be keenly aware of what I'm measuring and how I'm measuring it. Still, this doesn't address the "size of a photon", because you still have to "detect" the size. While I can point to you a position operator in QM, can you define for me a "size" operator?

This depends on the group behaviour of the quantum systems forming the measuring apparatus and their interaction within themselves and with the quantum system to be measured as well as the group behaviour of the quantum systems forming the brain and creating the mind of the observer ( Locality for instance may be a property of the observer and not the quantum object). The resulting measurements may be extremely accurate at macroscopic level(classical outcome) but this does not necessarily mean it is providing precise information, say, about the position of something that may be inherently fuzzy.

We must have a better understanding of the group behaviour of quantum systems that form the experimental set up since it is their collective behaviour that allows us to obtain accuracy of measurement relative to our inertial reference frames. Our observations are limited to the macroscopic outcome of experimental situations. We may be capable of fixing the position of these outcomes with extreme accuracy, but this accuracy is created by the group behaviour of the quantum systems forming the detector and the “clocks” and “rulers” needed to fix the response’s location. The place and time of a quantum interaction initiating a detection event cannot be specified from the information we have available to us.
I have no clue on what you just said here. You somehow made it sound as if this is something we know nothing about. How do you think they detect those exotic particles in high-energy collisions at CERN and Fermilab? Do you think they are not keenly aware of how exactly the detection works and how they interact with what they're trying to detect? I mean, give these hard-working (and very intelligent) people SOME credit!

Zz.
ZapperZ
#229
Aug14-07, 07:26 AM
Emeritus
Sci Advisor
PF Gold
ZapperZ's Avatar
P: 29,242
Quote Quote by LorentzR View Post
I stand by my assertion that the weight of this evidence suggests that charged systems interact directly, so there is no requirement in nature for man’s photon.
Then I want you to submit a paper to PRL to explain the results of the anti-bunching experiments without invoking the photon picture. Till you do that, I strongly suggest you cease making such statements on here that clearly violates the PF Guidelines. This is the only warning that you will get on this matter.

Zz.
reilly
#230
Aug14-07, 12:05 PM
Sci Advisor
P: 1,082
Quote Quote by LorentzR View Post
[



My principle reason for doubting the existence of the photon was that its raison deter was to mediate the electromagnetism; to bridge the gulf between charged quantum entities. But if quantum entities don’t have uniquely defined positions in the world then the gulfs separating them are also undefined; opening up the possibility of direct interaction between the charged systems. I therefore brought the evidence of the violation of Bell’s inequality to reinforce the idea that charged quantum systems do not have unique positions in space, since here was further evidence of a non local flavour to our world.



As I wrote earlier you can’t pick and choose your experimental results to reinforce your favourite theory. Our underlying understanding of how things happen must explain all known experimental results if it is to be valid.

Locality is fundamental to classical physics, but the conventional definition seems incapable of explaining in a consistent way all the experimental results obtained by physics. At the heart of the problem is the Born rule that allows one classical description of a quantum entity; where the entity is spatially distributed; to be transformed into possible classical outcomes where the entity becomes point like.

Explaining how this happens in a relativistically friendly manner is one of the great problems of natural philosophy.



Further evidence that you cannot pin point a quantum entity!
Measures of location are facilitated by the collective behaviour of quantum systems and their group “position” in the manifold.

I stand by my assertion that the weight of this evidence suggests that charged systems interact directly, so there is no requirement in nature for man’s photon. So the answer to Alveros’s question seems to be; something that does not exist does not have a size!!!!!

Wow. A few things: photons were invented by Planck and Einstein to explain black body radiation and the photoelectric effect. The invention of QED - Heisenberg, Pauli and Dirac, and Fermi -- brought Gauge invariance into play. so, in the Coulomb gauge, with divA=0, the particle-particle Coulomb interaction is a direct interaction; in the Lorentz Gauge, for practical purposes, the Coulomb interaction is mediated by photons -- Dirac gives an elegant discussion of the Coulomb gauge in his Quantum theory text.

Gauge Invariance is a big deal, an important deal; photons are tricky, no doubt, but no one has come close to presenting a better idea.Best to check it out.

Re positions: read about the basics of probability theory, then you will see the fallacy in your arguments about positions and interactions and the like.Note also that if positions and "gulfs' are uncertain then the probabilty of direct interactions will be very low. Why?

Regards,
Reilly Atkinson
lightarrow
#231
Aug14-07, 02:13 PM
P: 1,521
Quote Quote by ZapperZ View Post
Come again?

Other than the fact that this issue is getting pretty old and I've lost interest in it already, I have no idea what you just said here. If the detector is more of a property of itself, then you have to seriously explain why it just didn't make a detection trigger all by itself. Why does it need a photon (or anything else) to cause a measurement?
The particle-like behaviour is manifest in the interaction, not in flight. All we know is that light behaves as wave from source to detector and as particle at the detector. Period.
ZapperZ
#232
Aug14-07, 02:34 PM
Emeritus
Sci Advisor
PF Gold
ZapperZ's Avatar
P: 29,242
Quote Quote by lightarrow View Post
The particle-like behaviour is manifest in the interaction, not in flight. All we know is that light behaves as wave from source to detector and as particle at the detector. Period.
We do? How do you know? Because it can "interfere" with itself? Even single photons? How do you explain the which-way experiments and the anti-bunching experiments, all of which have to occur "during flight" and then detected?

This is getting silly. We are now turning this thread into a "photon" thread. I guess this is probably because the issue of the "size of a photon" is dead and buried but people just can't leave that photon well-enough alone? Again, talk is cheap. Please show a citation where an alternative explanation and description has been done for those two experiments that I cited, plus the multi-photon photoemission.

Zz.
LorentzR
#233
Aug14-07, 06:17 PM
P: 34
Quote Quote by ZapperZ View Post
Come again?

Other than the fact that this issue is getting pretty old and I've lost interest in it already, I have no idea what you just said here. If the detector is more of a property of itself, then you have to seriously explain why it just didn't make a detection trigger all by itself. Why does it need a photon (or anything else) to cause a measurement?

Zz.
I mean that your interaction as the observer is with the detector and not the particle. Locality (of the detection) is a property of the group behaviour of the quantum systems forming the detector and the accuracy of the detector depends on their group behaviour.. Of course the detector needs to interact with something but the position of the actual interaction is not definable, only the response of the detector can have known position. Quantum mechanics only predicts the probable observable and measurable outcomes of experiments





Quote Quote by ZapperZ View Post
You are also forgetting that everything that you detect (including all of your sense) is an interaction of something with something.
Zz.
Sorry I thought that was exactly what we were talking about; that is the locality of the quantum interaction the initiates a response at an observable position at the detector.

Quote Quote by ZapperZ View Post
As an experimentalist, I have to be keenly aware of what I'm measuring and how I'm measuring it.
Zz.
I’m sure you are an excellent experimentalist; even so, nature will only permit you to observe the response of the detector to a quantum event and not the event itself.



Quote Quote by ZapperZ View Post
Still, this doesn't address the "size of a photon", because you still have to "detect" the size. While I can point to you a position operator in QM, can you define for me a "size" operator?
Zz.

If my reasoning is correct such an operator is unnecessary.


Quote Quote by ZapperZ View Post
I have no clue on what you just said here. You somehow made it sound as if this is something we know nothing about. How do you think they detect those exotic particles in high-energy collisions at CERN and Fermilab? Do you think they are not keenly aware of how exactly the detection works and how they interact with what they're trying to detect? I mean, give these hard-working (and very intelligent) people SOME credit!

Zz.
I do, I think you should conduct a survey and ask them how exactly the detection works and how they interact with what they detect. I suspect you might be in for a surprise.
LorentzR
#234
Aug14-07, 06:19 PM
P: 34
Quote Quote by ZapperZ View Post
Then I want you to submit a paper to PRL to explain the results of the anti-bunching experiments without invoking the photon picture.
Zz.
Sounds like reasonable challenge.

Quote Quote by ZapperZ View Post
Till you do that, I strongly suggest you cease making such statements on here that clearly violates the PF Guidelines. This is the only warning that you will get on this matter.

Zz.
Sorry! just got carried away


Register to reply

Related Discussions
Speed of a photon relative to another photon Special & General Relativity 26
Photon+deuteron-&gt;p+n , E(photon)=? Advanced Physics Homework 4
Atomic photon versus cavity photon Quantum Physics 4
Photon-Photon Repulsion-Attraction Quantum Physics 12