What is the current understanding of the nature of photons?

[Mephisto]

I'm confused. It's a particle and it's also a wave; I'm actually starting to think that photon is just a distortion of spacetime or something... It's a small electro-magnetic field, where one always induces the other and thus this ever-changing electromagnetic pulse travels through space. Do physicists actually have a well accepted idea of what a photon ACTUALLY is?

I'm also thinking that as photon comes near an electron, the way it gets absorbed would be that somehow the electric field interacts with the electrons electric field and it then pulls/pushes that electron and converts this energy stored in the photon's electric field into the kinetic energy of an electron. Or something like that. I don't know. I haven't exactly figured out emittance of a photon yet.

How right/wrong am I? What is the accepted view?

 

[Shackleford]

I believe the simplest answer is that a photon is a quantum of energy. The wave-particle duality is just that - a mathematical duality that accurately describes the same physical object's properties, motion, and so forth. Don't think of it as being a wave but actually a particle but actually a wave...ad infinitum. I don't believe anyone knows why light behaves both as a particle and a wave.

I'm just getting back into physics, so I could be a rusty on everything. Correct me, someone, if I am mistaken!

 

[malawi_glenn]

a photon is a photon, an electron is an electron, etc

What properties they have (particle, wave, pointlike, electric charge) etc, its the physicists role to assign, and how they interact etc.

 

[f95toli]

As far as I know the most stringent definition is something like "an excitation of a tempo-spatial mode".
Basically, every system "contains" an infinite number of electromagnetic modes and each mode can be occupied by 0 or more excitations that we call photons
This is somewhat analogues to other phenomena in e.g. acoustics. There is an inifinte number of acoustic modes in a room, when you clap your hands some modes are excited but when the the room is silent no mode is occupied (but they are still there).

 

[lightarrow]

I'm confused. It's a particle and it's also a wave; I'm actually starting to think that photon is just a distortion of spacetime or something... It's a small electro-magnetic field, where one always induces the other and thus this ever-changing electromagnetic pulse travels through space. Do physicists actually have a well accepted idea of what a photon ACTUALLY is?

I'm also thinking that as photon comes near an electron, the way it gets absorbed would be that somehow the electric field interacts with the electrons electric field and it then pulls/pushes that electron and converts this energy stored in the photon's electric field into the kinetic energy of an electron. Or something like that. I don't know. I haven't exactly figured out emittance of a photon yet.

How right/wrong am I? What is the accepted view?

I have the Copyright of that question!
https://www.physicsforums.com/showthread.php?t=143975

 

[malawi_glenn]

I have the Copyright of that question!
https://www.physicsforums.com/showthread.php?t=143975

hehe yeah you do ;) Always good to search the forums for old threads before one posts. Certainty when one have questions about quite genreral and fundamental things.

 

[Mephisto]

lightarrow i think its pretty funny that the topic name is exactly the same :D

I went through the topic, but it seems that most of the people are really quite vague in their description of a photon, as if they were afraid to say something definite about what it is. My overall impression then is that they simply don't know. They also like to describe it sometimes using math, but it's in reverse! Math should describe the photon, not the other way around.
Moreover, there is a lot of mentions of a detector, but detector has nothing to do with the nature of a photon in the physical realm... But maybe that's just because of the way you phrased the question, talking about the "revelators".

I was just trying to see what it could mean that a photon is both a wave and a particle, and how can it have 0 mass. The picture I came up with i think makes at least some sense. The best answer I saw so far, (and also the one that seems most physical) was made in your topic by hellfire:
"A photon is an excitation of the free electromagnetic field."
even though I am not exactly sure if i understand it correctly.

 

[malawi_glenn]

Mephisto: A photon is a photon. An electron is an electron. (as far as we know today) Why must the answer be anything else?

Also, how can we find information about what an object is and behaves if we did not use detectors?...

"A photon is an excitation of the free electromagnetic field." Is an abstract mathematical approach to it.

 

[Bright]

If you would ask question: "What my gransma watch is?", you could find one or another answer.

Now, you are looking for a SIMILAR answer to the question "What photon is?"

Unfortunately, there is NO SIMILAR answer. Because in terms of existing physics photon cannot be represented as an entity, consisting of a simpler and easy to understand parts.

In General Physics there is ONE concept of photon.
In Nonrelativistic QM - there is another concept.
In Quantum Fielt Theory - absolutely different from both above.
In String theory - something else.
In Final Theory the photon is just some garbage, when we compare for example photon and electron, which is not garbage.

So, up to you...

 

[Arham]

According to qunatum physics every particle have wave-like properties. Photon is a particle (like electron and ...) and therefore possesses these wave-like properties. But special relativity implies that there is one difference between photon and other ususal particles: Its rest mass is zero.

I hope my english is not so bad!

 

[Bright]

Its rest mass is zero.

I have NO IDEA what is rest mass of photon, because photon never stops.
However, if photon travels, at speed of light of course, it's mass is not zero already.

Its energy = hw
according to equivalence principle,

hw = m*c^2

So, photon equivalent mass should be

m = hwc^-2

 

[Arham]

Yes. Its relativistic mass is not zero. But according to m = m0 / sqrt(1-v^2/c^2), m0 must be 0, since v = c.

 

[malawi_glenn]

Bright : now you are just talking rubbish.

 

[Bright]

But according to m = m0 / sqrt(1-v^2/c^2)

This relationship is good only for v < c

If v = c, it does not have any sence.
Of course you may try introduce any kind of sense to it even if v = c
But, this is just your (and others) misunderstanding... an attempt introduce any sence, when actually there is no sence...

 

[Bright]

Bright : now you are just talking rubbish.

Which part of my posts is rubbish?

P.S. Or you think all of them? )

 

[Arham]

Bright: It's simple. Suppose we have a particle of rest mass m0 which is not equal to zero. This particle can't reach to velocity of light, because to do this we must give it energy infinitley. So the m0 of photon must be zero.

 

[peter0302]

Whether a photon has "rest mass" is irrelevant since the photon is never "at rest."

But as Bright's "rubbish," points out the photon does indeed have energy, and therefore momentum, which gives it many of the properties of a massive particle. The question becomes how you define "mass." If mass is energy that obeys inertia, then the photon obviously doesn't have mass. But if mass is simply localized energy, then the photon does indeed have mass.

Here's a more important question! Does light have gravity?

 

[jtbell]

Whether a photon has "rest mass"

Ahem. Real Physicists (TM) call it invariant mass and calculate it using

(mc^2)^2 = E^2 - (pc)^2

which gives a well-defined value of zero for a real (as opposed to a virtual) photon, for which E = pc.

 

[Mephisto]

Mephisto: A photon is a photon. An electron is an electron. (as far as we know today) Why must the answer be anything else?

Also, how can we find information about what an object is and behaves if we did not use detectors?...

"A photon is an excitation of the free electromagnetic field." Is an abstract mathematical approach to it.

i appreciate you trying to help me with this, but your answers are probably the most cryptic of them all... I'm asking what a photon is, and you are telling me that a photon is a photon. What kind of an answer is that?! What am I supposed to get from that?

I guess maybe I should rephrase my question... I don't have a clear view right now of the difference between normal matter and a photon... I would be ok with just thinking that photon is a packet of energy, but it seems there is more to it!
Why do they say that light is electromagnetic radiation? Light is made up of photons. So surely photons have to have something to do with electro-magnetism.

 

[quantumfireball]

An harmonic oscillator of frequency v,excited to an energy level n is equivalent to n photons.

 

[Riogho]

Q: What is a photon?

A: A ripple in the electromagnetic field

 

[malawi_glenn]

i appreciate you trying to help me with this, but your answers are probably the most cryptic of them all... I'm asking what a photon is, and you are telling me that a photon is a photon. What kind of an answer is that?! What am I supposed to get from that?

I guess maybe I should rephrase my question... I don't have a clear view right now of the difference between normal matter and a photon... I would be ok with just thinking that photon is a packet of energy, but it seems there is more to it!
Why do they say that light is electromagnetic radiation? Light is made up of photons. So surely photons have to have something to do with electro-magnetism.

And as many have pointed out here, you can THINK of a photon as many things, and describe it according to a theory. Like in string theory, the photon is composite of strings that oscillates in a 11dim space etc. ALL our destriptions of the elementary particles is limited. Which descritption you use depends on what you want to do.

You can easy describe it as an excited mode of the electromagnetic field. And the describtion of the electron, you can also describe it as an excitation of a(nother) field. But the very fundamental, is that photons and electrons are not composite systems, and therefore they are point-like objects with quantum properties.

 

[pediejo]

Going back to the wave vs. particle question:
I believe that when one asks this question (what really is a photon?), he or she is looking for an analogy that compares a photon to a macroscopic phenomenon. This is what physicists are doing in a way, but instead of looking for an analogy they are searching for a mathematical model (since quantum, we have realized that we don't have a macroscopic analogy). We can compare a photon to a particle (a mass that exists at a point) if its wavelength is very small. This is because its small wavelength makes it more likely to be found in a smaller region of space (its position is well defined). Particles that we are familiar with (billiard balls for example), have a well defined position (you can catch a baseball!) and we have a way of describing their motions and interactions. In this way we apply our macroscopic vision of balls colliding, with small wavelengthed photons (x-rays, gamma rays). Alternatively, we can compare a photon to a wave (a periodic variance of some quantity in space and/or time) if its wavelength is relatively large. This is because its large wavelength smears out the probability that a photon will be found in a certain area. So with a bunch of photons, their intensity will be periodic. We are familiar with waves in water, we know how they reflect, refract, act constructively and destructively (not like Tsunamis). This model works well for describing and conceptualizing photons as they travel through space, reflect, refract, and diffract. Now, as De Broglie kindly points out, all particles have a wavelength and all waves have an associated particle. It makes sense to view the distinction between the two as a result of our inability to describe the whole of it as a single entity, due to the fact that there are not many macroscopic phenomena which behave like quantum phenomena (although it is interesting that the phenomena we perceive are a result of quantum principles, however difficult they may be to describe in this way.) All in all, nature does not care whether we think of a photon as a wave or a particle or an excitation of the free electromagnetic field, it keeps acting how it always had, while we do our best to conceptualize and predict.

P.S. Isn't it funny how readily we distinguish between ourselves and nature, and even more funny how we think that we think that we think that we think... ad infinitum. When will we understand what this means?

-PJ

 

[pediejo]

Here's a more important question! Does light have gravity?

According to general relativity, energy curves space. I would assume this energy wouldn't exclude photons.

 

[malawi_glenn]

According to general relativity, energy curves space. I would assume this energy wouldn't exclude photons.

I don't know what the original poster meant by "does light have gravity?". But according to general relativity photons "feel" the gravitational force, that's why we have gravity lenses and gravitational red shift. But if a photon itself can deform the space-time, I don't know.

 

[lightarrow]

i appreciate you trying to help me with this, but your answers are probably the most cryptic of them all... I'm asking what a photon is, and you are telling me that a photon is a photon. What kind of an answer is that?! What am I supposed to get from that?

I guess maybe I should rephrase my question... I don't have a clear view right now of the difference between normal matter and a photon... I would be ok with just thinking that photon is a packet of energy, but it seems there is more to it!
Why do they say that light is electromagnetic radiation? Light is made up of photons. So surely photons have to have something to do with electro-magnetism.

At your place, I would proceed gradually and I would begin by asking if a photon can be described as a "corpuscle" moving from source to detector.

 

[Bright]

Whether a photon has "rest mass" is irrelevant since the photon is never "at rest."

A++

But if mass is simply localized energy, then the photon does indeed have mass.

Here's a more important question! Does light have gravity?

But if a photon itself can deform the space-time, I don't know.

According to General Relativity, the source of gravitational fiels is the MASS-ENERGY tensor. Photon has energy. So, a component of that tensor, which correspond to photon energy is nonzero. Thus, photon creates gravitational field.

There is an independent proof. Assume we have proton and antiproton in a box with mirror wallls. These particles are source of gravitation. Now, assume the particles annihilate and we have high energy photons instead of particles. The gravitation from the box is the same. That means photons are sources of gravitation.

 

[Bright]

Q: What is a photon?

A: A ripple in the electromagnetic field

Assume the frequency of electromagnetic field is w and energy of the ripple is E

Now, if E < hw (for instance E = homework / 10 )
then I do not believe that ripple is a photon.

If E = hw then MAYBE that ripple is a photon.

If E = 1.2345 * hw then I have no idea what that ripple is.

 

[ZapperZ]

There is an independent proof. Assume we have proton and antiproton in a box with mirror wallls. These particles are source of gravitation. Now, assume the particles annihilate and we have high energy photons instead of particles. The gravitation from the box is the same. That means photons are sources of gravitation.

Er... this "proof" assumes a priori that there is a "conservation law" for gravitation. Is there such a thing?

Zz.

 

[Bright]

I don't have a clear view right now of the difference between normal matter and a photon...

Normal matter (electrons, protons etc) can stay in REST, v = 0
or can move, v > 0, but v < c

Photon CANNOT stay in the rest. Its velocity can be only v = c (in vacuum)

Why do they say that light is electromagnetic radiation? Light is made up of photons.

In Classical Physics, light is just electromagnetic radiation and nothing more.

In Quantum Physics, that electromagnetic radiation is separated in portions, quanta.
The value of energy of each such a quanta is exactly E = hw

 

[Bright]

Er... this "proof" assumes a priori that there is a "conservation law" for gravitation. Is there such a thing?

Zz.

You are right. There is a weak point in my proof. And I do not know about conservation of the gravity at the moment.

Ok! Below is a better proof. It is well established, that black holes has only mass (and sometimes momentun).

Instead of the box we have black hole. We drop proton and antiproton toward black hole. These proton and antiproton ADD mass to the original mass of black hole. Now, assume that inside black hole two our particles annihilate. This process CANNOT reduce mass of resulting (original black hole plus two our particles) mass of black hole. So, this is a proof that photons have gravitational mass.

P.S. Inertia mass of photon is more evident. Inertia is an ability of an object to RESIST to its acceleration (or deacceleration). Now, if speed of photon is v = c, there is NO WAY to make it faster or slower. That means that inertia mass of photon is INFINITY.

 

[Bright]

... and therefore they are point-like objects

In modern quantum physics, there are two main approaches.
1. Particles are point-like objects
2. Particles are extended objects

As I know, there is no proof until now that one of the approaches is wrong.

P.S. But, there are many unsolved problems with both of them. So, maybe BOTH approaches are wrong.

 

[ZapperZ]

You are right. There is a weak point in my proof. And I do not know about conservation of the gravity at the moment.

Ok! Below is a better proof. It is well established, that black holes has only mass (and sometimes momentun).

Instead of the box we have black hole. We drop proton and antiproton toward black hole. These proton and antiproton ADD mass to the original mass of black hole. Now, assume that inside black hole two our particles annihilate. This process CANNOT reduce mass of resulting (original black hole plus two our particles) mass of black hole. So, this is a proof that photons have gravitational mass.

P.S. Inertia mass of photon is more evident. Inertia is an ability of an object to RESIST to its acceleration (or deacceleration). Now, if speed of photon is v = c, there is NO WAY to make it faster or slower. That means that inertia mass of photon is INFINITY.

I think you're digging a bigger hole with this one. If the inertial mass of a photon is infinite, then how are we able to stop it so easily?

Zz.

 

[Bright]

I think you're digging a bigger hole with this one. If the inertial mass of a photon is infinite, then how are we able to stop it so easily?

Zz.

Actually, we are not able TO STOP photon. Because its inertial mass is infinity.

But, we can DESTROY photon. If a photon would be detected by any device, it would not exist after detection. That means we would destroy both photon itself and its infinite inertial mass.

Bottom line. Photon cannot be stopped until it exists, but it can be destroyed. :)

 

[Bright]

P.S. There are ANNIHILATION operators in quantum field theory that can destroy photon.
But, I never heard about any operators that could STOP photon.

 

[dst]

P.S. There are ANNIHILATION operators in quantum field theory that can destroy photon.
But, I never heard about any operators that could STOP photon.

http://www.hno.harvard.edu/gazette/1999/02.18/light.html

 

[Bright]

http://www.hno.harvard.edu/gazette/1999/02.18/light.html

That example with Bose-Einstein condensate is very interesting. But everything depends on interpretation.

Consider an example, when light is slowed not 20 billion times, as in your citation, but only 1.5 times, as it happen in glass.

Sped of light according to regular, commonly accepted understanding is

v = c / n

where n is refractive index of material.

Now, let us recollect a piece of Feynman explanation of refraction.
In his explanation, in materials the speed of light is still = c.
But, material generates secondary waves, so superposition of initial and secondary waves LOOKS LIKE SLOW wave.

The similar happens in cited recent experiment. The behaviour of light in Bose-Einstein condensate is very complex. There is no stright trajectories for light propagation anymore, as it was in vacuum. Light is absorbed and emitted billion times before it leaves the area of BE condensate. The total effect of such absorbtion-emission again LOOKS LIKE SLOW wave.

 

[lightarrow]

Actually, we are not able TO STOP photon. Because its inertial mass is infinity.

But, we can DESTROY photon. If a photon would be detected by any device, it would not exist after detection. That means we would destroy both photon itself and its infinite inertial mass.

Bottom line. Photon cannot be stopped until it exists, but it can be destroyed. :)

What is "inertial mass" in a photon? How could it be infinite? It's mass is zero, it's energy divided by c^2 is finite, so what are you talking about?

 

[dst]

That example with Bose-Einstein condensate is very interesting. But everything depends on interpretation.

Consider an example, when light is slowed not 20 billion times, as in your citation, but only 1.5 times, as it happen in glass.

Sped of light according to regular, commonly accepted understanding is

v = c / n

where n is refractive index of material.

Now, let us recollect a piece of Feynman explanation of refraction.
In his explanation, in materials the speed of light is still = c.
But, material generates secondary waves, so superposition of initial and secondary waves LOOKS LIKE SLOW wave.

The similar happens in cited recent experiment. The behaviour of light in Bose-Einstein condensate is very complex. There is no stright trajectories for light propagation anymore, as it was in vacuum. Light is absorbed and emitted billion times before it leaves the area of BE condensate. The total effect of such absorbtion-emission again LOOKS LIKE SLOW wave.

That's true, it just gets repeatedly absorbed and retransmitted but that's the closest we'll get to "slowing light". Everyone has already acknowledged that the speed of light is constant no matter what.

I can't see why a massless particle would have infinite inertial mass unless inertial mass was inversely proportional to mass of a particle and that's obviously a retarded concept.

 

[lightarrow]

You are right. There is a weak point in my proof. And I do not know about conservation of the gravity at the moment.

Ok! Below is a better proof. It is well established, that black holes has only mass (and sometimes momentun).

Instead of the box we have black hole. We drop proton and antiproton toward black hole. These proton and antiproton ADD mass to the original mass of black hole. Now, assume that inside black hole two our particles annihilate. This process CANNOT reduce mass of resulting (original black hole plus two our particles) mass of black hole. So, this is a proof that photons have gravitational mass.

You don't need a black hole: when a couple electron/positron (with total momentum = 0) annihilates, it generates a couple of gamma photons of identical energy traveling in opposite directions and the mass of the system of the two photons is exactly = the total energy of the initial couple electron/positron.

But one single photon has mass = 0.

 

[ZapperZ]

Actually, we are not able TO STOP photon. Because its inertial mass is infinity.

But, we can DESTROY photon. If a photon would be detected by any device, it would not exist after detection. That means we would destroy both photon itself and its infinite inertial mass.

Bottom line. Photon cannot be stopped until it exists, but it can be destroyed. :)

You say destroy, I say stop. The end effect is the same, i.e. the momentum is gone or transfered. Would you care to tell me what happen if I transfer the momentum of something with an infinite inertial mass?

Zz.

 

[Bright]

What is "inertial mass" in a photon? How could it be infinite? It's mass is zero, it's energy divided by c^2 is finite, so what are you talking about?

In physics, there are TWO masses.
One mass, M1, is in definition of gravitational potential

Potential ~ M1 / r

This is gravitational mass.
Another mass is in acceleration formula

a = F / M2

This is inertial mass.
Many experiment confirmed that with GREAT precision

M1 = M2

But, all the experiments were performed only for regular bodies, not for fields.
So, may be for fields, like photon

M1 is not equal M2

As for photon, it travels only with speed c, and never slower or faster (restrict themselsf for simplicity only considering vacuum). So, we cannot change absolute value of photon speed. Than can be interpreted as INFINITE photonic INERTIAL mass.

 

[Bright]

I can't see why a massless particle would have infinite inertial mass unless inertial mass was inversely proportional to mass of a particle and that's obviously a retarded concept.

I am not sure that inertial mass was inversely proportional to mass of a particle...
maybe there are TWO different definition of the INERTIAL mass. Could you please give any link to definition of the inertial mass which is inversely proportional to mass of a particle?

 

[Bright]

P.S. I just found in Wikpedia definition of inertial mass.
http://en.wikipedia.org/wiki/Mass
It seems to me that it is NOT inverse proportional to the mass of particle, but just equal to that in the Newtonian law F = ma

 

[Bright]

... the mass of the system of the two photons is exactly = the total energy of the initial couple electron/positron.

But one single photon has mass = 0.

Sorry, I do not understand...
Mass of two photons is NOT equal to zero. OK, well...
Why a single photon has mass = 0?

 

[dst]

I am not sure that inertial mass was inversely proportional to mass of a particle...
maybe there are TWO different definition of the INERTIAL mass. Could you please give any link to definition of the inertial mass which is inversely proportional to mass of a particle?

I'm saying it is difficult to believe that a photon could have an infinite inertial mass unless inertial mass is inversely proportional to rest mass.

 

[Bright]

You say destroy, I say stop. The end effect is the same, i.e. the momentum is gone or transfered. Would you care to tell me what happens if I transfer the momentum of something with an infinite inertial mass?

Zz.

OK! It seems to me that we nedd THREE masses for every particle.
Two of them, well known in hystory of physics, are inertial and gravitational masses.
The third is MOMENTAL (associated with moment) mass

The MOMENTAL mass of photon is not infinite and is equal to its gravitational mass (something like homework / c^2 )

But, inertial mass of photon is still infinity. And there is NO conservation of inertial mass. When photon is destroyed (or stopped, as you prefer), the inertial mass just DISSAPEAR, but not transformed to another body like detector of photon.

 

[Bright]

I'm saying it is difficult to believe that a photon could have an infinite inertial mass unless inertial mass is inversely proportional to rest mass.

OK! If you give such a definition of inertial mass, then it would be easy to believe...
but I do not understand why such a definition is necessary.

Wikpedia says about inertial mass the following:
"Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily. "

Photon resists SO MUSH when we try change its velocity, when we try make it larger or smaller than c. That means its inertial mass is VERY large.

 

[jcsd]

OK! If you give such a definition of inertial mass, then it would be easy to believe...
but I do not understand why such a definition is necessary.

Wikpedia says about inertial mass the following:
"Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily. "

Photon resists SO MUSH when we try change its velocity, when we try make it larger or smaller than c. That means its inertial mass is VERY large.

Whilst a photons speed is fixed at c, it's very easy to change it's velocity (speed is the magnitude of velocity), by changin it's direction.

We're talking relativity here, so an all-encompassing definition of inertial mass is not obvious as F=ma no longer holds true. Howvwre any reasonably sensible defitnion of inertial mass does not diverge for a photon.

 

[Bright]

Whilst a photons speed is fixed at c, it's very easy to change it's velocity (speed is the magnitude of velocity), by changin it's direction.

OK! Inertial mass of photon has TWO components. One tangential to its velocity. This component equals to infinity. And another component is normal to its velosity. This component of the inertial mass is FINITE.