Photon question -- is it a particle?

[thetexan]

is a photon a particle?

Tex

 

[rootone]

A photon can be considered to be either a particle or as a fluctuation in an electromagnetic field, field depending on what it it is you want to consider.

 

[Drakkith]

There's no easy, accurate answer to that question. You either get an easy answer or an accurate answer, not both.

 

[sophiecentaur]

is a photon a particle?

Tex

Yes, you can consider a photon as being a particle in some contexts. BUT it is not a particle like a tiny bullet and you cannot make any 'mechanical' type assumptions about how it will behave. You cannot, for instance, tell 'where it is', until it has interacted with something. It can be literally anywhere, which is an odd notion until you come to terms with its abnormal particle nature.

 

[Nugatory]

is a photon a particle?

The word "particle", as used in quantum electrodynamics, has a specific meaning. A photon is a particle according to that definition.

But there's a catch here... The meaning of the word "particle" used in quantum field theories does not match the non-technical English-language meaning of the word, so saying that "a photon is a particle" isn't saying what you might be thinking it means.

 

[jtbell]

The Particle Data Group (the "official" repository of particle data for particle physicists) includes data for the photon in the same table as the W, Z, gluons and Higgs:

http://pdg.lbl.gov/2015/tables/contents_tables.html

(in that table of contents, it's called the "gamma.")

 

[thetexan]

If a single photon is emitted must you be in its "line of fire" to detect it? If I was 30 feet left or right wouldn't I miss detecting it?

 

[Vanadium 50]

Is its wavelength less than about 30 feet?

 

[Nugatory]

If a single photon is emitted must you be in its "line of fire" to detect it? If I was 30 feet left or right wouldn't I miss detecting it?

What is this "line of fire" of which you speak? This thing isn't a little bullet.

 

[thetexan]

Let's say a photon is emitted from a source and we know the direction that's it emoted. Let's say 100 people are evenly distributed around a one mile radius sphere centered on the source.

Clearly all 100 people cannot detect the photon. Only one (if any) will be able to detect the photon due to his position in relation to the direction of the emission.

Can we make some general statements about this such as...

Only the person in the direction of the emission will be able to detect the emission. This assumes the particle acts like a particle or a little bullet so that only someone in its line of fire will detect it. Or...

Everyone will be able to detect it. This assumes it acts like a wave and everyone will be hit by the wave and thus able to detect it. If it acts as a wave its potential energy will be continually reduced by the inverse of the distance and would be nearly non existent after only a short distance ( many miles).

So, if a single photon is emitted who among the 100 is most likely to detect it?

Tex

 

[jtbell]

Let's say a photon is emitted from a source and we know the direction that's it emoted

How do we know the direction that it's emitted?

 

[thetexan]

let's say it is emitted in the direction of point A on the sphere using a little photon emitting gun

 

[Dale]

Only the person in the direction of the emission will be able to detect the emission. This assumes the particle acts like a particle or a little bullet so that only someone in its line of fire will detect it. Or...

Everyone will be able to detect it. This assumes it acts like a wave and everyone will be hit by the wave and thus able to detect it. If it acts as a wave its potential energy will be continually reduced by the inverse of the distance and would be nearly non existent after only a short distance ( many miles).

Neither of these "this assumes" statements are correct. You are asking classical questions about a quantum system.

let's say it is emitted in the direction of point A on the sphere using a little photon emitting gun

It doesn't work like that. Photons are not bullets that can be fired from a little gun.

If you prepare a photon state with a well defined position then the momentum will be undefined. So you could prepare states with a wide variety of wavefunctions, each with different detection characteristics.

If you made the photon state tightly concentrated on one detector, then the probability of being detected by other detectors would be small. If you made the photon state more dispersed then the probability would be higher that other detectors would detect it.

 

[f95toli]

It doesn't work like that. Photons are not bullets that can be fired from a little gun.

That is true in general. However, there are single photon sources that can send one (and only one) photon in a particular direction (or even launch it into a waveguide). Hence, there is nothing intrinsically wrong with that thought experiment.
In fact, on-demand single photon sources are actually sometimes (informally) referred to as "machine gun-type sources" (presumably because they send out as photons when triggered)

Also, if it is in fact a single photon it can only be detected once. However, most sources do NOT send out single photons.

 

[thetexan]

If I have a powerful point or nearly point light source and surround it with a light proof sphere no light will get out. Now if I put a pinhole in the sphere only someone in line with the source and the hole will see the light. No one even slightly to one side will see the light. In fact there will only be spot of light on a distant surface.

Therefore the photon(s) are clearly distinct and directional. Whether they are particles or waves is at dispute. Particles would explain the observation. If waves then one would have to explain how the waves AS WAVES could be so precisely contained to a ray of light. A laser is a good example of this.

Im aware of the slit experiment. I'm not questioning that light and photons act as waves and particles. My simple question is this...is a photon a particle as it travels from source to receiver or wave?

 

[DrStupid]

If I have a powerful point or nearly point light source and surround it with a light proof sphere no light will get out. Now if I put a pinhole in the sphere only someone in line with the source and the hole will see the light. No one even slightly to one side will see the light. In fact there will only be spot of light on a distant surface.

How do you avoid light diffraction?

 

[lightarrow]

If I have a powerful point or nearly point light source and surround it with a light proof sphere no light will get out. Now if I put a pinhole in the sphere only someone in line with the source and the hole will see the light.

Everyone will be in line with the source and a hole: given two points in space, there always is a straight line passing through them.

No one even slightly to one side will see the light. In fact there will only be spot of light on a distant surface.

Cannot understand here.

Therefore the photon(s) are clearly distinct and directional.

They are clearly distinct if you generate them in such a way, but they are not clearly directional. They don't have "nadelsstrahlung" (needle-like radiation) as even Einstein initially believed. Prove: light diffraction.

Whether they are particles or waves is at dispute.

Maybe you intended "corpuscles". In fact they are particles: as Nugatory wrote, "particle" in QM means something which has both corpuscolar and wave behaviour.

Particles would explain the observation. If waves then one would have to explain how the waves AS WAVES could be so precisely contained to a ray of light. A laser is a good example of this.

It's easy to describe this behaviour as waves, instead. You only have to take a monochromatic wave with low enough wavelenght (low with respect to all the other dimensions involved in the experiment = geometrical optics approximation) and which is very collimated (you can obtain this making the radiation pass through distant holes made on parallele screens). Nothing particularly difficult.

--
lightarrow

 

[thetexan]

Well here the point I'm driving at. Let's just take one star...I ca seethe light front he star no matter where I move my head. Therefore light from the star permeates every cubit centimeter of space. Therefore every cubit centimeter of space is filled with "something" from the one star.

What is the something? Jillions of photon particles or a sea of photon waves?

 

[DrStupid]

What is the something? Jillions of photon particles or a sea of photon waves?

A sea of quantum objects which are neither particles nor waves but show properties of both (depending on the conditions).

 

[f95toli]

I

Therefore the photon(s) are clearly distinct and directional. Whether they are particles or waves is at dispute. Particles would explain the observation. If waves then one would have to explain how the waves AS WAVES could be so precisely contained to a ray of light. A laser is a good example of this.

It is much more complicated than that. If the photons are emitted by a true single photon source they are indeed quite "particle like" (with undetermined phase). However, if the source is coherent (say a laser) it is more akin to classical source and it will be -in general- behave a bit more like a classical wave. In the the full theory of light (QED) we refer to different types of fields (and states of light). Note for most states the number of photons in not even fixed so talking about a number of photons is meaningless.

It is important to realize that there is no mystery here. We have an extremely accurate theory for light (quantum electrodynamics, QED) meaning the "nature" of photons is fully understood, but you need to know a fair amount of math to understand the explanation: there is no simple intuitive picture.

 

[Drakkith]

What is the something? Jillions of photon particles or a sea of photon waves?

The energy of an electromagnetic wave is quantized, meaning that when I put a detector, such as my eye or a camera, in the path of the wave, the transfer of energy from the wave to my detector is not continuous, but takes place in discrete amounts. A photon is this discrete transfer of energy from the wave to the detector. In laymen terms it is a little packet or bundle of energy given up to or from the wave during an interaction.

 

[Dale]

Now if I put a pinhole in the sphere only someone in line with the source and the hole will see the light. No one even slightly to one side will see the light.

This is not the way light works even classically. Because of diffraction a small pinhole acts essentially as a point source.

With photons it becomes even weirder because a single photon has no single well defined path and can take multiple paths. This is clearly seen in the two slit experiment and (even more convincing IMO) in diffraction gratings.

 

[Dale]

Therefore every cubit centimeter of space is filled with "something" from the one star.

What is the something? Jillions of photon particles or a sea of photon waves?

Classical EM works fine for this. There is no need to consider quantum effects. It is just a field of essentially spherical incoherent EM radiation.

 

[Alastair McD]

The Texan wrote: "My simple question is this...is a photon a particle as it travels from source to receiver or wave?"

My simple answer is that as it travels it is a wave, but when it reaches the observer the wave function collapses into a photon.

 

[ASK]

When photon moves, it behaves like a particle having kinetic energy .It behaves like a particle having rest mass zero with electromagnetic properties.

 

[sophiecentaur]

What is the something? Jillions of photon particles or a sea of photon waves?

I suggest you read some general stuff about QM because I think you need to immerse yourself in this stuff. At the moment, you seem to want a Classical answer to your questions and there are actually no satisfactory classical answers (which is why 'they' had to invent QM).
There is a list of Free textbooks in the Science and Maths Textbooks section of the Forums Contents Page. That could be worth browsing through - it will only cost you your time and you may find something that suits you.

 

[Dale]

At the moment, you seem to want a Classical answer to your questions and there are actually no satisfactory classical answers (which is why 'they' had to invent QM)

I actually think that the problem is almost the opposite. There is simply no reason to introduce quantum concepts to answer his underlying question. The problem is actually that he has brought quantum terminology into a classical question and seems not to recognize that the quantum terminology requires a quantum theory to make sense.

 

[sophiecentaur]

no reason to introduce quantum concepts to answer his underlying question.

His first question was about photons, though, so you can hardly answer it without pointing out their quantum nature. But clearly, confusion rules. (as with all of us, I guess)

 

[Nugatory]

His first question was about photons, though, so you can hardly answer it without pointing out their quantum nature. But clearly, confusion rules. (as with all of us, I guess)

The first question "Is a photon a particle" is pretty clearly about photons so cannot be answered without reference to QM. However, the many followup questions such as #15 and #18 aren't about photons at all, they're about the propagation of electromagnetic radiation, and DaleSpam is spot on.

Edit: Maybe someday I'll understand how people can go years without feeling a compulsion to introduce phonons into discussions of sound waves, but somehow when it comes to light the first thing they think of are photons.

 

[Dale]

His first question was about photons, though, so you can hardly answer it without pointing out their quantum nature. But clearly, confusion rules. (as with all of us, I guess)

Yes, that is true. It wasn't until quite a bit later that the clarification was provided indicating that the underlying question was classical.

The word "photon" is too often used inappropriately. I wish there were a way to get people to say "light pulse" when they had a classical question and "photon" only when they had a quantum question. Or maybe the problem is more with the word "particle" which can refer either to classical or quantum concepts which are very different in many important ways.

 

[my2cts]

What about electrons? Should we say charge pulse, unless we are using QM? I don't advocate that.
I vote that the photon is a particle. For many years I believed "photon" to be a convenient fiction
but not any longer.

 

[lightarrow]

Edit: Maybe someday I'll understand how people can go years without feeling a compulsion to introduce phonons into discussions of sound waves, but somehow when it comes to light the first thing they think of are photons.

Maybe because light can also propagate in the void and someone still believe that only corpuscles/spaceships/asteroids/planets/etc (and not waves) can propagate in the void. Furthermore, tv programs and popular books talk about "photons" very often (and quite never about phonons) and people easily and quickly makes in his head an image of flying corpuscles like little bullets.

--
lightarrow

 

[Dale]

What about electrons? Should we say charge pulse, unless we are using QM? I don't advocate that.

I would say "charge" or "current". There is rarely a need to talk about electrons in classical EM. I find that a lot of unnecessary confusion for circuits comes from "electrons" which are not needed.

 

[HunterThomson]

Photon is a particle and a wave structure phenomenon at the same time.

video

 

[Alastair McD]

The TED talk does not say "Photon is a particle and a wave structure phenomenon at the same time". It says that light sometimes behaves like a particle and sometimes like a wave. It can't do both at the same time! I am saying that it behave like a particle when the wave function has collapsed, and it behaves like a probability wave of direction until it collapses, by being detected, or hitting an object. You don't know its exact path until it is detected.

 

[f95toli]

The TED talk does not say "Photon is a particle and a wave structure phenomenon at the same time". It says that light sometimes behaves like a particle and sometimes like a wave. It can't do both at the same time! I am saying that it behave like a particle when the wave function has collapsed, and it behaves like a probability wave of direction until it collapses, by being detected, or hitting an object. You don't know its exact path until it is detected.

That is not quite correct either. First of all. there is no wavefunction for the photon, at least not in the usual sense (you can define quantities that are a bit similar to a wavefunction). Secondly, it is possible to create states of light where it behaves very much like a particle (so-called Fock states), but these are unusual and not created by any conventional source of light (including lasers); most of the time classical (wave) EM works fine.

However, in general all we can say is that light behaves as if it sometimes has particle-like or/or wave-like properties; but this does not means that it "is" (whatever that means) either a wave or a particle (or even a mixture of the two). Note also that the "nature" of light is very well defined in the (complete) theory of light (quantum electrodynamics) so there is no unsolved mystery here. The issue in this context is that you need to know a fair bit of math to understand QED: there is no easy way to explain it and you can certainly not use classical concepts to do so if you want to be accurate.

The best popular description of what light really "is" I've come across is Feynman's book on QED.

 

[Alastair McD]

I don't think my description differs from that of Feynman.

 

[my2cts]

there is no wavefunction for the photon

The potential is the wave function. It obeys a massless Klein-Gordon equation.

 

[Nugatory]

I am saying that it behave like a particle when the wave function has collapsed, and it behaves like a probability wave of direction until it collapses, by being detected, or hitting an object. You don't know its exact path until it is detected.

That picture doesn't quite work either. Even if we gloss over f95toli's point about the problem with attaching a wave function to the photon at all, and even if we note that a state that is collapsed in one basis is uncollapsed in another (so we can't tie the particle/wave distinction to a collapsed/uncollapsed distinction because that latter distinction doesn't exist)... We still don't know the exact path. All we know is that a photon was detected at a particular point.

 

[ontheroad]

At our size scale, and in our environment, quantum phenomena do not exist: I mean that bullets and baseballs, ocean waves and sound waves do not behave identically to photons. Therefore we have no innate understanding of quantum phenomena. However, photons do display some of the same phenomena as the above mentioned, and this is useful.

 

[Alastair McD]

Nugatory wrote: "so we can't tie the particle/wave distinction to a collapsed/uncollapsed distinction because that latter distinction doesn't exist."

Before we open the box Schrodiger's cat is neither dead nor alive. After we open the box it is dead or alive. The cat's state has changed from an unknown probability state (wave function) to a physical cat's body just as real as a photon.

 

[Nugatory]

Nugatory wrote: "so we can't tie the particle/wave distinction to a collapsed/uncollapsed distinction because that latter distinction doesn't exist."

Before we open the box Schrodinger's cat is neither dead nor alive. After we open the box it is dead or alive. The cat's state has changed from an unknown probability state (wave function) to a physical cat's body just as real as a photon.

That doesn't affect the point about there not being a collapsed/uncollapsed distinction. Consider the common example of a photon approaching a vertically oriented polarizing filter. The photon has been prepared in the state $(|V\rangle+|H\rangle)/\sqrt{2}$; this is the uncollapsed superposition of the states $|V\rangle$ and $|H\rangle$, vertical and horizontal polarization respectively. When the photon encounters the filter, its state collapses to either $|V\rangle$ and it passes the filter or to $|H\rangle$ and it is absorbed by the filter. It's tempting to say that before the interaction we had an uncollapsed state and after the interaction we had a collapsed state... But we don't. The apparently collapsed and unsuperposed post-interaction state $|V\rangle$ is the exact same state as the uncollapsed and superposed state $(|L\rangle+|R\rangle)/\sqrt{2}$ where $|L\rangle$ and $|R\rangle$ are polarization along the +45 and -45 degree axes. For that matter, the initial and apparently uncollapsed state $(|V\rangle+|H\rangle)/\sqrt{2}$ is also exactly the state $|L\rangle$.
Thus there's no way of distinguishing between collapsed and uncollapsed states; it's just a matter of how we look at them. However, as I said above, Schrodinger's cat is unrelated to this question - neither "live cat" nor "dead cat" are pure states to which the notion of superposition and collapse applies. Instead, they're both mixed states.

You may also be misunderstanding the point of Schrodinger's thought experiment. He was not seriously suggesting that the cat would be neither dead nor alive. He was pointing out a problem with the then current (eighty-plus years ago) understanding of quantum theory, namely that there was nothing in the theory that explained why the cat wouldn't end up in such a superimposed state even though everyone knew perfectly well that it didn't. It took almost a half-century to find the answer to that problem - you may want to google for "quantum decoherence" or give this relatively gentle book a try.