# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Atomic-S on 30/10/2015 01:19:43

Title: What is the largest possible rest mass of the photon?
Post by: Atomic-S on 30/10/2015 01:19:43
Certain theorists have theorized that like the neutrino, the photon cannot have a strictly zero rest mass.  If it is not, it clearly is very small. What is the upper limit based on known science upon any possible such mass? The main consideration appears to be that if the photon has a nonzero rest mass, then it travels at slightly less than the speed of "light" and therefore it is possible, for any given photon, to find a reference frame in which it at rest. (Which, of course, is inevitable by definition if it has a rest mass).  However, a particle at rest has a quantum wave function that has an oscillatory component
eiωt in which ω is controlled by its energy, which is nonzero in this case and therefore cannot be zero . But if ω is not zero, then there must exist for electromagnetic waves a minimum possible frequency, and if we knew what that was, we would be able to calculate the rest mass of the photon. As for measuring experimentally the minimum possible frequency, the first thing that comes to mind is that we can do so easily be constructing a DC circuit, whose frequency is zero, thereby proving that the photon cannot have a rest mass. We could also cite permanent magnets, whose field is unvarying.  However, neither of those cases describe an electromagnetic wave propagating in free space, which is what is required for this calculation.  What we need to do is ask what is the lowest frequency radio wave ever found traveling in empty space. Whatever that frequency is, we know that the lowest possible frequency cannot be higher than it, thereby setting an upper bound for the possible mass of the photon. Does anyone know?
Title: Re: What is the largest possible rest mass of the photon?
Post by: evan_au on 30/10/2015 07:44:28
It is very hard to detect radio waves using an antenna which is much smaller than the wavelength. So in practice, it is hard to measure propagating radio waves with a frequency of < 1Hz, where the wavelength is larger than the Earth.

However, the Sun is effectively a low-frequency transmitter, producing 1 cycle of radiation from its solar magnetic field about every 22 years (approximately). The amplitude of this 22-year cycle is reflected in the 11-year sunspot cycle. However, due to the size of the Sun's magnetosphere, only a small fraction of the energy in this oscillating magnetic field would be released as freely-propagating EM waves.
Title: Re: What is the largest possible rest mass of the photon?
Post by: alancalverd on 30/10/2015 09:53:09
The lowest possible frequency is any number greater than zero. However as Evan says, the lowest detectable frequency depends on the length of your antenna and your patience. As the earth's magnetic field drifts and reverses every billion years or so, there's at least one known photon out there with a lower frequency than sunspots, but you'd have a tough job to find it.
Title: Re: What is the largest possible rest mass of the photon?
Post by: PmbPhy on 30/10/2015 15:41:56
The largest possible mass which is consistent with the experimental data that we have is 4x10-51 kg. This is the value stated in Classical Electrodynamics - 3rd Ed. by J. D. Jackson, page 7.
Title: Re: What is the largest possible rest mass of the photon?
Post by: Atomic-S on 31/10/2015 03:53:03
OK, so we know that it would definitely have to be small.
Title: Re: What is the largest possible rest mass of the photon?
Post by: AndroidNeox on 31/10/2015 21:59:51
The rest mass will have to be zero or the energy of every photon would be infinite. Or, special relativity must be fundamentally wrong (which I don't think is plausible).

Note: The speed of light in vacuum isn't an attribute of light. It's a result of the geometry of spacetime. Anything with zero rest mass travels at the speed of light.
Title: Re: What is the largest possible rest mass of the photon?
Post by: PmbPhy on 01/11/2015 13:12:48
The rest mass will have to be zero or the energy of every photon would be infinite. Or, special relativity must be fundamentally wrong (which I don't think is plausible).

Note: The speed of light in vacuum isn't an attribute of light. It's a result of the geometry of spacetime. Anything with zero rest mass travels at the speed of light.
Why would you think that the photon's energy have to be infinite?
Title: Re: What is the largest possible rest mass of the photon?
Post by: chiralSPO on 01/11/2015 17:07:51
The rest mass will have to be zero or the energy of every photon would be infinite. Or, special relativity must be fundamentally wrong (which I don't think is plausible).

Note: The speed of light in vacuum isn't an attribute of light. It's a result of the geometry of spacetime. Anything with zero rest mass travels at the speed of light.
Why would you think that the photon's energy have to be infinite?

If the photons are actually traveling at c would they not have infinite kinetic energy if they had any non-zero rest mass?

However, I think it is possible (though highly unlikely) that they have a finite non-zero, but incredibly small mass, and are moving at nearly c. An arbitrarily small mass moving at arbitrarily close to c could have arbitrarily large or small kinetic energy, depending on how arbitrary we want to be :-)
Title: Re: What is the largest possible rest mass of the photon?
Post by: PmbPhy on 01/11/2015 19:22:25
Quote from: chiralSPO
If the photons are actually traveling at c would they not have infinite kinetic energy?
Not if their proper mass is zero as I hold it to be. You should know this, don't you?
Title: Re: What is the largest possible rest mass of the photon?
Post by: chiralSPO on 01/11/2015 21:26:01
Quote from: chiralSPO
If the photons are actually traveling at c would they not have infinite kinetic energy?
Not if their proper mass is zero as I hold it to be. You should know this, don't you?

Sorry forgot to finish the phrase. Please see my corrected post, which asks if we assume a non-zero rest mass, then must the kinetic energy be infinite at c.
Title: Re: What is the largest possible rest mass of the photon?
Post by: jeffreyH on 02/11/2015 00:43:02
Rest mass assumes a hypothetically 'stationary' object. Relativistic mass accumulates through an increase in velocity. The photon with none of this stationary or rest mass has to gain relativistic mass via some other mechanism and related to frequency. The wave determines the mass. Whereas the momentum determines the wave for tardyons.
Title: Re: What is the largest possible rest mass of the photon?
Post by: puppypower on 02/11/2015 12:25:46
If you look at a photon, it travels at the speed of light, which is the same in all inertial references. Photons also have wavelength and frequency. Wavelength and frequency is different and is not limited to one state like the speed of light. Wavelength and frequency have a wide variety, with this variety also being relative to inertial reference; red and blue shift.  Photons have two distinct aspects, which act as though separate from each other. Changes in wavelength and/or red shift have no impact on the speed of light aspect.

The speed of light is the fixed aspect of energy, common to all photons. The speed of light is analogous to the ground in electrical circuits. We can vary the circuit voltage and current to a wide variety of states, but all these circuit states will be connected to the same ground. The speed of light is the ground state of the universe.

The rest mass of the photon is connected to the variable aspect of the photon, which can be influenced by inertial references; wavelength and frequency. The ground at C is not dependent on the rest mass of the circuit.

This approach to the photon is different but is consistent with the observation that changes in the rest mass, wavelength and frequency of the photon is connect to its inertial aspect, not to changes C. SR only applies to inertial things and not the ground state.
Title: Re: What is the largest possible rest mass of the photon?
Post by: alancalverd on 05/11/2015 09:21:11
However, I think it is possible (though highly unlikely) that they have a finite non-zero, but incredibly small mass, and are moving at nearly c.
This is either ridiculous or profound.

Ridiculous: c is defined as the speed of photons.

Profound: if c was the theoretical speed of a massless particle and real photons have mass, then the observable universe is slowing down.

This makes a lot more sense than string theory, despite the lack of evidence (which has not hindered the expenditure of megabucks on string theory conferences!)
Title: Re: What is the largest possible rest mass of the photon?
Post by: PmbPhy on 05/11/2015 13:49:12
Quote from: alancalverd
Profound: if c was the theoretical speed of a massless particle and real photons have mass, then the observable universe is slowing down.
I don't understand this comment. Please describe your reasoning that led you to this conclusion. Thanks.
Title: Re: What is the largest possible rest mass of the photon?
Post by: alancalverd on 05/11/2015 18:38:37
We measure everything, ultimately, by reference to photons. Now if a photon has mass it cannot travel at c, so the "yardstick" by which we actually measure everything, call it c',  is shorter than we think it is. But our thought is constrained by our measurement of c', which appears to be invariant and equal to c. The onlhy way you can have c' <  c and c' = c = constant is if c is decreasing.
Title: Re: What is the largest possible rest mass of the photon?
Post by: jeffreyH on 06/11/2015 00:30:15
Now this is a philosophical debate I am interested in following.  [8D]
Title: Re: What is the largest possible rest mass of the photon?
Post by: alancalverd on 06/11/2015 08:50:52
Yuk! Philosophy is the antithesis of science.
Title: Re: What is the largest possible rest mass of the photon?
Post by: jeffreyH on 06/11/2015 13:03:32
That sounds like a philosophical postulate to me.  [;D]
Title: Re: What is the largest possible rest mass of the photon?
Post by: evan_au on 06/11/2015 20:43:42
Quote from: alancalverd
Yuk! Philosophy is the antithesis of science.
I heard it said that if you pick almost any topic in Wikipedia, and click on the first* hyperlink in the article, and then on the first hyperlink in that article, etc ... you will eventually end up with "Philosophy".

So I tried it!

Since this thread is about the rest mass of photons, I started with "Photon". It led me down a merry trail: Photon, Elementary particle, Particle physics, Physics, Natural science, Science, Knowledge, Awareness, Consciousness, Quality (philosophy), Philosophy.

So I would say, based on this little experiment, that Philosophy is the basis of Science.

But they have a different focus: Philosophy deals with pure thought-stuff, while Science tries to apply it to something more concrete, and Engineering tries to apply it to solve real problems.

*When I selected the "first" link in the article, I found myself in a little sideline explaining the origins of words, so I decided to click on the first link that was not merely explaining the linguistic origin or pronunciation of the word.

But when I came back to follow this trail to the end, I found:
....Physics, Ancient Greek, Greek language, Modern Greek, Help:IPA for Greek, International Phonetic Alphabet, Alphabet, Letter (alphabet), Grapheme, Phoneme, Phone (phonetics), Phonetics, Linguistics, Science ... Philosophy!
Title: Re: What is the largest possible rest mass of the photon?
Post by: alancalverd on 07/11/2015 00:55:26
Science is the recursive application of the algorithm "observe, hypothesise, test". Philosophy is something else. It usually involves an incomplete observation, a wholly irrelevant hypothesis, and an excuse to do something bizarre or evil. Testing and recursive refinement are beneath the lofty intellect of a philosopher, which is why they tend to flock with those other arrogant purveyors of ignornace, priests and politicians. If you allow philosophy to infect science, you end up with such perversions as Nazi science, Lysenkoism, Maoism, ruin and starvation. Evolution is a scientific observation, creationism is a philosophy. There is no scientific justification for beheading infidels, but a powerful philosophy can sanctify it.

The overweening arrogance of philosophers is apparent from Evan's posting. As is the fatuous nature of philosophy

Quote
Photon, Elementary particle, Particle physics, Physics, Natural science, Science, Knowledge, Awareness, Consciousness, Quality (philosophy), Philosophy.
It all makes sense until you get to "consciousness", a term which is never defined by its users: everything thereafter appears to derive its place in the thread, from a meaningless word. Philosophically, a photon must be a wave or a particle, so the properties of electromagnetic radiation depend on some notion of duality and observer-dependence...what utter rubbish! We scientists use the term photon to denote the cause of a number of linked phenomena: it doesn't "have to be" anything other than itself, and we admit that we don't have a unique mathematical description of all its properties. That is a far more mature, and far less arrogant and athropocentric way of looking at the universe.
Title: Re: What is the largest possible rest mass of the photon?
Post by: evan_au on 07/11/2015 08:55:37
I guess Philosophy is "thinking about thinking", in an attempt to understand "the meaning of meaning".

No wonder Philosophers seem to spend time going around in circles...