Why do we need a photon to mediate the electromagnetic force?

[lightarrow]

Why the photon?

Ya know, there are people to this day that still resist QM. I can only imagine the nuttyness that people must have thought when it was first being introduced as “reality.” But what changed? The idea of “calling something” an electron, a photon? The particle picture of light came before the wave picture. QM is what made physicists say, whoa, this light particle stuff is serious business, we better call it something.

And the photon will always be. Just like Newtonian physics will always be, but not like how the Thompson Model turned out. As time progress’s, we learn more about, “what we call a photon” and our picture of it may change, but that thing called a photon will still remain. Hey, someday the photon may be a vibrating string and we will then say, that string with that mode corresponds to……….a photon.

In type II superconductors, some call the penetration of flux lines, fluxons and treat them as particles. There is a quanta of flux. While I admit that calling these flux tubes particles is a stretch of the definition, it doesn't change the fact that this quanta of flux exists. Whether it's a particle can be debated.
In general,
Doubting what people call "fluxons" means you doubt that there is a quanta of flux.

Doubting what people call "photons" means you doubt that there is a quanta of EM energy.

If QM is just some lucky model that does so well in predicting our Universe, but its explanation of reality is totally wrong, then what a colossal misfortune.

I’m not a betting man, but I’m gunna have to go with…….QM has a lot of explanations that exist in reality. The negative seems to implausible.

With all the electronics surrounding me right now as I type on this incredible device called a computer, I cannot accept this all as luck or coincidence. If I did, then I would be a physicist that believes in absolutely nothing. My only belief being, coincidence dominates.

But at some point, we have to use inductive reasoning.

Ok, I agree on everything that you said. But:
Let's assume we don't know that light can be thought of as electromagnetic waves, but we know how its made of energy packets (photons) and how to relate different colours to different energies of these packets. We make experiments with coherent light (light which packets have a well defined energy, measured for example through photoelectric effect, ecc.) and we discover how, making light go trough slits on a screen, we find regular fringes on a distant screen. We find rules that relates the fringes spacings to the distance between the slits, the distance between the two screens and the energy of the packets (light's colour).

Then, someone come in and ask: but, why all this happens?
And all people could say: "physics it's not about <<why things happens>>, but how it works. We have our rules, they are physics principles, things happens in this way, and that's all".

Then, one day, someone else comes, and say: wait a moment! But, if light is a wave, then everything it's explained much better in this way...

So, maybe QM (and QED) is the best we can know about light, or maybe one day...

(Why E = hv?).

 

[reilly]

As has happened before on this topic, there is here much ado about nothing. Common sense, and sophisticated physics knowledge cannot answer whether or not there is an objective reality, let alone a real photon. The point is that the idea of a photon turns out to be a powerful and practical concept--you pays your money and takes your chances.

Philosophers and their kin want more than descriptions -- well, go to it, beat us dirt-farmer physicists at our own game. Do something that brings new data or experiments to the table, something that conventional physics cannot duplicate. Some folks for years have been criticizing QM as lacking this that or the other thing -- talk is cheap, and the anti-QM
stuff has lead to no new physics over that last 50 years or so.

Regards,
Reilly Atkinson

PS How could you prove the existence of "objective reality"

 

[Fra]

Having little to do with the photon, but in defence of all the philosophers out there... :)

Philosophers and their kin want more than descriptions

Physics and philosophy has a common history and from my limited history knowledge some of the QM founders was still quite philosophical, which does not contradict with simultaneous formal rigor. They didn't have all the answers but they did acknowledge several deep questions, which is important. If you skip the question and jump right into manipulating formalisms with rigor, IMO the real point is missed.

Not all philosophers are into old style realism. That's perhaps one branch of philosophers, but beeing a philosopher doesn't mean your necessarily some conservative realist thinking that beneath there is a hidden universal structure that would maintain the old ideals.

Sometimes it's claimed that you need no philosophy in physics, because experiment will tell us right from wrong and "why it's right" is metaphysics. You just learn howto apply the theory, and produce the predictions.

But I think that is an oversimplification, which even hides the most important step. First of all the confidence in any real experiment is always finite. So the confidence in any piece of evidence is also finite. Right or wrong, needs to be replaced with more or less confidence in something.

Second comparing a prediction with experiment and conclude that they either disagree or not seems very trivial.

The non-trivial part of this interaction is howto update your predictions in response to conflicting information. The no-philosophy arguments seems to trivialize the only truly non-trivial step.

Yes, I want more than a description. I want a description of howto evolve my description when I'm wrong. The cases when I am right is trivial and doesn't teach me much. Obviously the "minimum intelligence" evolution is to just scramble everything up and you come up with a new random model you can test in a experiment with rigor. But we all know we can do far better than that.

PS How could you prove the existence of "objective reality"

I consider myself philosophical (though I don't study philosophy as such), but I do not think there is any sensible objective reality in the classical sense. And it is not a problem.

One of my current issues with physics is that it seems a bit amgious. That doesn't means it's wrong. It just means that it's hard to see the coherent line of reasoning. This has bugged me since high school. Some people, who are differently minded seems minimally disturbed by this. But this philosophical think makes progress. I've made a lot of progress for myself, and I attribute a lot of it to the fact that I never allowed myself to ignore the important questions because they were fuzzy.

Perhaps the experimentalists may consider the theorists as a black box, to which they report results, and expect out in the other end an update predictive model. And they design new tests to test the updated model. Even a group coming up with random theories should be able to make progress, but it would be far less efficient than if the black box had more intelligence. Thus there seems to be something that determines the efficienct of this black box? what? can we learn more about this?

I am interested to know exactly how the black box works, in as much as I like to know how a particle "works".

I have a feeling that sometimes the underlying tone of reasoning is that it doesn't matter how this box works, the important parts is that whatever comes out of it, is confirmed or not confirmed by experiment.

But what happens in the case of rejection? the theory was wrong, then what? shouldn't the nature of the rejection infere the best correction?

I want to see the essence of reasoning, built into our models. This is currently missing. The scientific method and the models (output of the scientific method) should implement a feedback, until the scientific method is unified with the models.

/Fredrik

 

[Fra]

talk is cheap

Indeed, also in the litteral sense. If you don't get payed to do research, perhaps the cheap way is the only way So let's hope we've invested our taxmoney on the right horse. For a fair comparasion, perhaps the results should be weighted with the investments made in each approach?

/Fredrik

 

[LorentzR]

And the photon will always be. Just like Newtonian physics will always be
.

I can’t agree with this logic since Newtonian physics is a working paradigm which successfully makes predictions about observations on macroscopic events, given an initial set of conditions. Newtonian physics is permanently integrated into our culture.

On the other hand the photon is a literal convenience associated with experiments whose outcome lacks full deductive explanation. Until such a time as a deductive explanation becomes available any description should be regarded as provisional and treated with the utmost caution.

We already have had to discard the aether as the mediator of light, the failure of the idea of the photon to give non-contradictory properties to the phenomenon of light suggests it too might have limited life expectancy.

But at some point, we have to use inductive reasoning.

Unthought-of possibilities will catch out those who abandon deductive logic.

 

[LorentzR]

Apparently, you do not read what I wrote. In addition, Schwarzschild and Minkowski space-time are 4-dim with the signature {+,-,-,-} which is the consequence that the underlined algebraic structure is defined by 4-dim quadratic normal division algebra of the Hamilton quaternions.

I was simply pointing out the that the metric signature was not (+, +, +, +) required by Riemannian geometry. Sorry if I missed your reference.

I already together with Zz gave you the answer: If you intend to do physics, study it.

Regards, Dany.

P.S. And try to justify your pseudonym.

If a concept is not sustained by deductive reasoning, no matter how long you study it the fundamental weaknesses will remain.

Relativity and quantum mechanics are well validated, the photon is not, and there is nothing currently in human knowledge that fully supports the possibility it has any form of physical reality.

The photon's true status is metaphysical, even though it seems to be accepted by popular culture as intrinsically real.

 

[lightarrow]

As has happened before on this topic, there is here much ado about nothing. Common sense, and sophisticated physics knowledge cannot answer whether or not there is an objective reality, let alone a real photon. The point is that the idea of a photon turns out to be a powerful and practical concept--you pays your money and takes your chances.

I have no doubt about it. Even the concept of heat as a material substance was useful (probably less than the concept of photon), but now we don't use it any longer. If, instead, one day it will result a sligth non-zero rest mass for the photon, I would have much less doubts on that cocept.

Philosophers and their kin want more than descriptions -- well, go to it, beat us dirt-farmer physicists at our own game. Do something that brings new data or experiments to the table, something that conventional physics cannot duplicate. Some folks for years have been criticizing QM as lacking this that or the other thing -- talk is cheap, and the anti-QM
stuff has lead to no new physics over that last 50 years or so.

Regards,
Reilly Atkinson

PS How could you prove the existence of "objective reality"

Measuring it.

This is what makes the difference between physics and philosophy.
A low-energy photon, considered as a "flying particle" is more a philosophycal concept than a physical one.

 

[bruce2g]

...
PS How could you prove the existence of "objective reality"

...
Measuring it.

This is what makes the difference between physics and philosophy.
A low-energy photon, considered as a "flying particle" is more a philosophycal concept than a physical one.

I have thought about this also, and I've come to ask the following question: "does the photon exist before it is detected?" There's a lot of thought behind this question -- can you examine other events to see if a photon has been created, or can there be any evidence of its existence before it interacts with a detector? I think the electromagnetic field strength indicates the probability that a photon might be detected, but I also think that a photon's "existence" is a just another quantum property that cannot be measured until it is detected -- i.e., when its wave function collapses. So personally, I don't think it "exists" before it is detected.

(Incidentally, I apologize for jumping in so late in the discussion, but I've wanted to ask this question for a long time, and I like the way lightarrow thinks (I too am impressed by the anti-bunching evidence)).

So, if you guys aren't sick of this question already, I'd like to divide the original question into two: does a photon exist before its wave function collapses (i.e., before it's detected), and does it exist when it is detected?

 

[vanesch]

So, if you guys aren't sick of this question already, I'd like to divide the original question into two: does a photon exist before its wave function collapses (i.e., before it's detected), and does it exist when it is detected?

... And does the detector exist when you look at it and when you don't ?

You see, "measure it" will ultimately always come down to a kind of subjective perception, at which point you will have to make in any case a *hypothesis* of the ontological existence of something. As such, you can argue endlessly over the (non) existence of theoretical concepts such as photons, electrons, voltmeters and sisters. In the end, it is always a matter of hypothesis. But, as Reilly pointed out so correctly, that's not the point. The point is NOT whether the "photon ultimately exists". The point is that the photon is a very practical theoretical concept which helps us "visualise" and explain a lot of observations. In the same way perhaps, as your sister is.

 

[Fra]

In despite of my defense of the "philosophy", I roughly agree with both reily and vanesch, but I disagree a little tiny bit on one focus point.

But, as Reilly pointed out so correctly, that's not the point. The point is NOT whether the "photon ultimately exists".

I agree completely.

The point is that the photon is a very practical theoretical concept which helps us "visualise" and explain a lot of observations.

I sense that the ambition to get a practical concept, is too modest. I mean, relative to what? Relative to the worst case, anything is more practical.

I want to the extent possible the _most practical_ concept, and the missing measure is the one quantifying "practical". There sure is a point where one would think that ANY randomly chose concept IS the most practical one, because getting stuck in mindloops trying to decide what the next step is, isn't practical either. But I think there should be a balance between the two extremes, yielding some "optimal strategy" to progress. So that our ambition should not just be to make progress, because making progress is almost unavoidable. I think the ambition should be to make the most efficient progress allowed by the limitations at hand.

So, some kind of minimum philosophy of the scientific method is IMO not completely out of place.

This also implies a fundamental level of humbleness as to save us from thinking that we "proove things" when we in fact are just guessing. I think the difference between a scientific guess and a random guess is that the scientific guess is the supposedly the BEST guess, but that's not to mistake it for the "truth", or to tink that the supposedly best guess, in fact IS the best guess.

So we are as it seems just learning by guessing... by then, so why don't we construct our theories in the honest way that is designed for it's scientific evolutionary purpose? That's the future physics I want to see on the table.

/Fredrik

 

[vanesch]

So, some kind of minimum philosophy of the scientific method is IMO not completely out of place.

Those who know me a bit would also know that I don't shun a bit of philosophy! After all, the old name for physics was natural philosophy, which I think was a completely appropriate name.

However, people asking for whether "photons really exist" and say that "scientists claim that they proved that photons exist" are wrong, seem to forget their philosophy themselves, because otherwise they would remember that philosophically, one cannot ultimately prove the existence of anything, and that every form of ontological claim is always based upon some hypothesis.

So one should always take with a grain of salt, and recognize that when a scientist says that "thing A exists", then he really means that *within the currently available paradigm, if we make the corresponding ontological hypotheses that go with that, then we have strong indications that thing A exists*.

In other words, when talking about photons, we place ourselves already in the paradigm of quantum field theory (which is at the origin of the concept of photon), and we make the ontological hypothesis that goes with quantum field theory (that means, that we postulate that its basic entities have some ontology to themselves). Of course, the question of ontology only makes sense as long as the observational predictions of the paradigm are in agreement with what we perceive. So a scientist inquiring whether "photons really exist" tries simply to make as many observations as she can, tries to use the scientific method to discard competing theories/paradigms, and when, amongst a reasonable set of paradigms she can think of, only the paradigm containing the theoretical concept of "photon" survives the experimental confrontation, then that scientist says that "photons seem to exist", simply because no non-photon containing paradigm one could think easily of, remains in the competition. That doesn't exclude of course that such a non-photon containing paradigm will be found one day, and it is still based upon the ontological hypothesis of the one and single known surviving paradigm, but nevertheless, it is not a bad argument, because it says:
"of all paradigms we could reasonably think of as of today, only one seems to hold up against all experimental data we have today, so if we take that paradigm, and we make the ontological hypotheses that go with it, then we arrive at the conclusion that the existence of the photon is part of that paradigm".

That doesn't mean that tomorrow, things cannot be different. But as of today, it seems the best/most practical thing to do. That's all that such a claim of "the photon exists" really means.

This also implies a fundamental level of humbleness as to save us from thinking that we "proove things" when we in fact are just guessing. I think the difference between a scientific guess and a random guess is that the scientific guess is the supposedly the BEST guess, but that's not to mistake it for the "truth", or to tink that the supposedly best guess, in fact IS the best guess.

Yes, but that should be clear from the start.

 

[LorentzR]

I have thought about this also, and I've come to ask the following question: "does the photon exist before it is detected?" There's a lot of thought behind this question -- can you examine other events to see if a photon has been created, or can there be any evidence of its existence before it interacts with a detector? I think the electromagnetic field strength indicates the probability that a photon might be detected, but I also think that a photon's "existence" is a just another quantum property that cannot be measured until it is detected -- i.e., when its wave function collapses. So personally, I don't think it "exists" before it is detected.

(Incidentally, I apologize for jumping in so late in the discussion, but I've wanted to ask this question for a long time, and I like the way lightarrow thinks (I too am impressed by the anti-bunching evidence)).

So, if you guys aren't sick of this question already, I'd like to divide the original question into two: does a photon exist before its wave function collapses (i.e., before it's detected), and does it exist when it is detected?

The honest answer on both counts is we don't know.

However you may be interested in this attachment which purports to be reductionistic hypothesis. It explains the development of the light wave and the impulsive nature of quantum interactions without involving a photon

 

[vanesch]

So, if you guys aren't sick of this question already, I'd like to divide the original question into two: does a photon exist before its wave function collapses (i.e., before it's detected), and does it exist when it is detected?

Look at my previous post:
"to exist" is something you decide for yourself, as you decide upon your favorite color. The only requirement is that for something to be decided by you to exist, it has to be a part of a paradigm that is not in contradiction with observation, and clearly, up to now, photons are not in contradiction to observation. So you're FREE to decide whether you'd like them to "exist" or not. But it is highly practical to do so.

If you find this strange, I can just as well decide to say that nothing of what we call the world exists. I'm a solipsist in that case. But it is not a very practical world view.

 

[Fra]

Those who know me a bit would also know that I don't shun a bit of philosophy! After all, the old name for physics was natural philosophy, which I think was a completely appropriate name.

I am new on here so I really have't gotten to know anyone on here. My comments wasn't meant as "personal" in any case. For what I know, my comments may come out as embarrasingly obvious and redundant. If so, I'll attribute that to my ignorance of this environment But I'm learning.

Yes, but that should be clear from the start.

Yes it should. I guess I pointed what should be obvious out because in my experience not everyone in general appears (to me) to think this is clear, rather something that is kept in denial My point was that an explicit acknowledgment of the actual limitations may in fact be constructive.

/Fredrik

 

[Asiadeep]

hahahahah! you guys absolutely crack me up man
opps *nuclear explosion*

 

[lightarrow]

hahahahah! you guys absolutely crack me up man
opps *nuclear explosion*

That is to say what?

 

[bruce2g]

Look at my previous post:
"to exist" is something you decide for yourself, as you decide upon your favorite color. The only requirement is that for something to be decided by you to exist, it has to be a part of a paradigm that is not in contradiction with observation, and clearly, up to now, photons are not in contradiction to observation. So you're FREE to decide whether you'd like them to "exist" or not. But it is highly practical to do so.

If you find this strange, I can just as well decide to say that nothing of what we call the world exists. I'm a solipsist in that case. But it is not a very practical world view.

Well, I'm willing to just posit that measurements exist, and that the things doing the measuring (meters, photographic plates, counters, etc.) exist. I think that most people are willing to do this.

I really think that it's easier to understand quantum physics if you say that the photon does not exist until it is measured. So you don't have to say 'it goes through both slits' because it doesn't actually exist until it falls out of the wave and interacts with a detector.

The other reason I would say that it doesn't exist until it is detected is this: you cannot detect its creation. As near as I can tell, there's just no way to say 'a photon was created at this point at this time' (or, even that 'a single photon was created within a certain dxdydz within a certain dt.') You might be able to say that an approximate number of photons was created (by the sun, for example), but that just tells you that there's a probability that photons will be detected.

I've looked for experiments where people have detected the creation of a single photon (with certainty, and without actually detecting the photon itself), and I haven't been able to find one. Of course, if someone knows of one, then I guess I'd have to say that a photon can exist before it is measured. But if no such experiment exists, then I would like to propose that it's because the photon does not exist before it is detected.

So, since there is really no evidence that any individual photons exist before they are detected, and since there actually is evidence that they don't exist before detection (interference) , I think it's a pretty safe bet to say they don't exist until they are detected.

It seems to me that this point of view makes some of the quantum weirdness easier to swallow, so unless someone can come up with a big hole in my assumptions I think I'll stick with this one for the time being.

 

[ZapperZ]

The other reason I would say that it doesn't exist until it is detected is this: you cannot detect its creation. As near as I can tell, there's just no way to say 'a photon was created at this point at this time' (or, even that 'a single photon was created within a certain dxdydz within a certain dt.') You might be able to say that an approximate number of photons was created (by the sun, for example), but that just tells you that there's a probability that photons will be detected.

I've looked for experiments where people have detected the creation of a single photon (with certainty, and without actually detecting the photon itself), and I haven't been able to find one. Of course, if someone knows of one, then I guess I'd have to say that a photon can exist before it is measured. But if no such experiment exists, then I would like to propose that it's because the photon does not exist before it is detected.

So, since there is really no evidence that any individual photons exist before they are detected, and since there actually is evidence that they don't exist before detection (interference) , I think it's a pretty safe bet to say they don't exist until they are detected.

It seems to me that this point of view makes some of the quantum weirdness easier to swallow, so unless someone can come up with a big hole in my assumptions I think I'll stick with this one for the time being.

Replace everything you just typed here with "electron", or "proton", etc. Do you also think that an electron "doesn't exist" (whatever that means) until it is detected?

Furthermore, if what you said about single photon is true, then single-photon sources are collosal lies. Devices that produce single-photon on demand should be shot down to pieces.

Zz.

 

[bruce2g]

Replace everything you just typed here with "electron", or "proton", etc. Do you also think that an electron "doesn't exist" (whatever that means) until it is detected?

Furthermore, if what you said about single photon is true, then single-photon sources are collosal lies. Devices that produce single-photon on demand should be shot down to pieces.

Zz.

Thanks for your questions and the reference to the single-photon sources. Well, the electrons and protons have mass. Photons are different -- they have no mass, so without mass the case for them existing is a bit weaker. (They do not have time either, hmm, this is suspicious). Also, it looks like they don't have any size, but no one knows for sure. So, since they appear to lack some existential qualities that the other particles have, I'll just stick with the the photons for now.

Thanks for pointing out the single photon sources. I wasn't aware that they had gotten so good at reducing the probability of doubled photons. In one form of single photon sourcing, a pulse excites an atom or molecule and bumps an electron up to a higher energy state. When the electron comes back down to base level, a photon is emitted. Note, however, that you don't know exactly when the photon is emitted - the time to return to ground state is a random variable. The probability that the electron drops down to its base state gets pretty close to 1.0 pretty fast, but it never actually reaches 1, so you never really know for sure that the photon was created until you detect it.

I still maintain that the word "exists" is the wrong word to use before the wave function collapses. In fact, most people call it a 'wave packet' instead of a 'photon' during this phase of its evolution. You could try and say that 'it exists as a wave packet' which gives a probability distribution that it'll be detected at certain points in time and space; but note that you won't get a probability of 1 for any finite region.

My logic is like this: a 'photon' is a particle. However, before the wave function collapses, light is a wave, so the photon particles do not exist yet.

 

[vanesch]

Thanks for your questions and the reference to the single-photon sources. Well, the electrons and protons have mass.

The question is: why is "mass" such an important criterium to decide whether something exists or not ? Does the EM field exist, and in that case, what is its mass ? Does a classical gravitational force exist, and if so, what's its mass ? Do the natural numbers exist, and if so, what's their mass ?

You see, an arbitrary theoretical concept (mass in this case) can never be a criterium for ontological existence! After all, "mass" is simply a parameter that enters a dispersion relation, as in:

E^2 = p^2 c^2 + m^2 c^4

(a relationship between energy and 3-momentum).

For some things, we have to use m different from 0, for other things we can use m = 0, and maybe for even other things, who knows, the dispersion relation might even not exist in this form (but that would be a serious problem for relativity then).

Photons are different -- they have no mass, so without mass the case for them existing is a bit weaker. (They do not have time either, hmm, this is suspicious). Also, it looks like they don't have any size, but no one knows for sure. So, since they appear to lack some existential qualities that the other particles have, I'll just stick with the the photons for now.

That's reasoning by analogy: because many things have property X, things that seem to have not property X "don't exist" then... You could apply that also, say, to electric charge. Do neutrons exist ? Or to baryon number. Do electrons exist ? Or to, I don't know, price. Things that don't have a price don't exist, say. Or things I CAN'T SEE don't exist. In that case, ONLY photons exist!

You see, what I'm trying to point out is that your choice of criteria of having a certain physical property to decide whether something ontologically exist, is rather arbitrary!

I still maintain that the word "exists" is the wrong word to use before the wave function collapses.

Do the particles in your body have a wavefunction ? Do *you* have a wavefunction ? When does it collapse ? When do YOU exist ?

 

[Anonym]

Do the natural numbers exist, and if so, what's their mass ?

Come on! You urgently need the summer vacations.

Regards, Dany.

 

[Fra]

It seems to me that this point of view makes some of the quantum weirdness easier to swallow, so unless someone can come up with a big hole in my assumptions I think I'll stick with this one for the time being.

If you think quantum mechanics is weird, the human brain weirdness must bug you badly?

If person A observes person B, he can see it's actions, but not his thoughts. Are B's thoughts real, relative to A? Are A's thoughts real relative to A? For A to predict B, he might try to estimate B's thoughts, assuming he has an idea what a given though with yield for kind of action, are to conclude what the action will be. He may find that the model is decent, and gives A an competitive advantage. But what's real and what's not. Does it matter? It's obviously "real" in the sense that we are discussing it - ie considering the possibility is seemingly useful at least.

It seems to me that at the instant you ask, is it real? You instantly admitt that at least it's _possibly_ real. Which means the possibility should be real, relative to the questioner, right?

Or if you instead not ask is it real, you ask is it "physical". Then I think that anything physical needs qualifying evidence and therefore there must be some duality between physical and information theoretic views. For example one way toy that a possibility must have a physical memory representation (the state of a physical system is indeed a physical memory device).

So I like think, that answer to the is it real and is it physical is that it doesn't matter. The problem withing using the word physical is that you might tend to associate to newtionan mechanistic models, this creates a confusion that isn't needed.

I think it's almost a contradiction that we as humans insist in thinking in mechanistic terms, when we are operated by such an amazing brain. Reflecting over my own thinking has given me new insights even into physics. The intuitive picture for a human is that we consume information, but we don't know how. We are in the middle of the information theoretic machinery, and have an intuitive understanding. But to give a physical, chemical understanding of exactly how the brain works is extremely complex. Yet for the one beeing inside it, it all seems so obvious? Obvious enough to hardly require proof? :) How come?

/Fredrik

 

[Careful]

The question is: why is "mass" such an important criterium to decide whether something exists or not ? Does the EM field exist, and in that case, what is its mass ? Does a classical gravitational force exist, and if so, what's its mass ? Do the natural numbers exist, and if so, what's their mass ?

Because mass couples to (and is the origin of) everything (except for the natural numbers, they live in the Platonic world ) : GR simply is a universal (in the sense of unavoidable) theory.

You see, an arbitrary theoretical concept (mass in this case) can never be a criterium for ontological existence! After all, "mass" is simply a parameter that enters a dispersion relation, as in:

E^2 = p^2 c^2 + m^2 c^4

(a relationship between energy and 3-momentum).

Mass was so arbitrary that everything around us reveals its existence Without kidding, if you want a *closed* theory for the universe, then something like a mass field is again unavoidable. I don't say you need a mass parameter (which is very un Mach like) - mass can be an averaged quantity of a dynamical field (a bit a la Brans Dicke - but better). The mass parameter certainly is a flat space approximation, I agree with that.

For some things, we have to use m different from 0, for other things we can use m = 0, and maybe for even other things, who knows, the dispersion relation might even not exist :in this form (but that would be a serious problem for relativity then).

No, it wouldn't be any problem for gravity (for special relativity yes, but for GR no).

 

[lightarrow]

The question is: why is "mass" such an important criterium to decide whether something exists or not ? Does the EM field exist, ... ?

You are right, but I think we should consider this:
we have some apparatus that we call "source of EM field", another one that we call "detector" and void in between; then we make some experiments and we see that, sometimes, the detector's response in time is a signal that increases and then decreases. We interpret this saying that an EM wave packet arrived to the detector. Independently of how can this last statement be questionable (of course it is), could we have the same effect with a single photon?

 

[lightarrow]

The other reason I would say that it doesn't exist until it is detected is this: you cannot detect its creation. As near as I can tell, there's just no way to say 'a photon was created at this point at this time' (or, even that 'a single photon was created within a certain dxdydz within a certain dt.') You might be able to say that an approximate number of photons was created (by the sun, for example), but that just tells you that there's a probability that photons will be detected.

I've looked for experiments where people have detected the creation of a single photon (with certainty, and without actually detecting the photon itself), and I haven't been able to find one. Of course, if someone knows of one, then I guess I'd have to say that a photon can exist before it is measured. But if no such experiment exists, then I would like to propose that it's because the photon does not exist before it is detected.

Probably it's possible to detect an atom's recoil after the emission.

 

[Mentz114]

If the photon is given the same onotological status as the massive particles, then it must be given a wave function which can be interpreted as describing the evolution of probablity amplitude whose square gives the probablity of finding the photon. There are many papers in the journals and arXiv postulating a wave function for the photon ( refs supplied if required).

They range from a 3D bispinor formulation that looks like the Dirac equation to the unmodified Maxwell equations. They all have an interesting issue of interpretation because the 'probablity amplitude' for the photon is just the actual physical intensity of the light, which exists in 3D co-ordinate space, while QM probablity amplitude is not physical and lives in a space that can have infinite dimensionality.

This is hardly seamless. So what is the wave function of the photon ?

[later ]
I can some way to answering my own question ( but this is second quantisation, not a single photon wave equation).

This is how the EM field is quantised in Gerry and Knight, 'Introductory Quantum Optics' (2005)

Start with a single mode field which satisfies Maxwells equations and boundary conditions -

E_x(z,t) = (2\omega^2/(V\epsilon_0))^{\frac{1}{2}}q(t)sin(kz)

B_y(z,t) = \frac{\mu_0\epsilon_0}{k}(2\omega^2/(V\epsilon_0))^{\frac{1}{2}}\dot{q}(t)cos(kz)

where q(t) is a time dependent factor having dimension length.

Which gives Hamiltonian -

H = \int dV[ \epsilon_0E_x^2(z,t) + \frac{1}{\mu_0}B_y^2(z,t)]

This is the Hamiltonian of a HO and is quantised in the usual way by defining creation and annihilation
operators satisfying

[a,a^\dagger ] = 1

The eigenstates of the creation operator presumably form a complete basis.

 

[vanesch]

Because mass couples to (and is the origin of) everything (except for the natural numbers, they live in the Platonic world )

You got my pun. It's another way of looking upon ontological questions.

: GR simply is a universal (in the sense of unavoidable) theory.

Of course, given that we don't have a satisfactory theory which gives us GR and quantum theory as limiting cases, we should leave GR out of the picture for the moment when discussing a potential ontology of "photons" which are theoretical concepts of QFT (which doesn't contain GR).

 

[vanesch]

If person A observes person B, he can see it's actions, but not his thoughts. Are B's thoughts real, relative to A? Are A's thoughts real relative to A? For A to predict B, he might try to estimate B's thoughts, assuming he has an idea what a given though with yield for kind of action, are to conclude what the action will be. He may find that the model is decent, and gives A an competitive advantage. But what's real and what's not. Does it matter? It's obviously "real" in the sense that we are discussing it - ie considering the possibility is seemingly useful at least.

That's exactly the point I was trying to make: "ontology" (saying that concept X is "real") is a convenient way of thinking, if concept X is a helpful thing in helping us to organize our conception of how the world (and ultimately our sensations) behaves.

So in as much as "photons" are helpful concepts in explaining lab experiments, they are "real" (possibly in a similar way as natural numbers are, when dealing with accountants ).

So I like think, that answer to the is it real and is it physical is that it doesn't matter. The problem withing using the word physical is that you might tend to associate to newtionan mechanistic models, this creates a confusion that isn't needed.

Exactly

 

[Careful]

Of course, given that we don't have a satisfactory theory which gives us GR and quantum theory as limiting cases, we should leave GR out of the picture for the moment when discussing a potential ontology of "photons" which are theoretical concepts of QFT (which doesn't contain GR).

It might very well be that photons are derived concepts in a field theory of inertia : actually what I argued is that for consistency of *any* theory, it should be embedded into a theory of inertia.

 

[Careful]

That's exactly the point I was trying to make: "ontology" (saying that concept X is "real") is a convenient way of thinking, if concept X is a helpful thing in helping us to organize our conception of how the world (and ultimately our sensations) behaves.

So in as much as "photons" are helpful concepts in explaining lab experiments, they are "real" (possibly in a similar way as natural numbers are, when dealing with accountants ).

Euh, for an accountant, the physical configuration on the sheet of paper (if he is old fashioned), which can be interpreted as a natural number, is real (as is hopefully the reason why this configuration has been written down ).

Exactly

Euh who says that ``Newtonian´´ mechanistic models cause confusion ? What do you think the (classical) string - gauge correspondence is about?
How do you classify all (succesful) attempts to approximate Maxwell's theory by a spinning fluid theory?
Even within quantum theory itself, one has local ``mechanistic'' models to produce fermions from bosons (as nonlocal stringy configurations) - these are highly useful and have been developped mainly in the 80ties.

But I agree that field theory is a better way of thinking.