# The Naked Scientists Forum

### Author Topic: Why is the photon regarded as a massless particle?  (Read 22915 times)

#### hubble_bubble

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##### Why is the photon regarded as a massless particle?
« on: 04/09/2012 06:36:23 »
If we consider time dilation and length contraction using special relativity does the photon therefore have infinite length contraction and time dilation? If so is that why we see it as a massless particle?
« Last Edit: 11/09/2012 21:42:11 by chris »

#### Emc2

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##### Re: mass of a photon
« Reply #1 on: 04/09/2012 10:17:36 »
having no mass, per se, is the main reason a photon can travel at the speed of light, and thereby establish that rule.

if a photon had mass, it could not achieve the speed of light....

I believe....lol

#### imatfaal

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##### Re: mass of a photon
« Reply #2 on: 04/09/2012 11:59:06 »
If we consider time dilation and length contraction using special relativity does the photon therefore have infinite length contraction and time dilation? If so is that why we see it as a massless particle?

Special relativity relies on the fact that physical laws remain the same for all inertial frames.  One of the products of this axiom is that no inertial frame of reference can travel at light speed.  In order to make the assumptions you are making you need to posit an inertial frame travelling at light speed - once you do that you can no longer use the equations of SR.  We just do not know what happens to objects, observers, clocks, rulers etc within an inertial frame that travels at the speed of light - and if our physics is correct, we can never know because it cannot happen

#### evan_au

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##### Re: mass of a photon
« Reply #3 on: 04/09/2012 12:17:51 »
A slight clarification about the difference between Mass and Rest Mass:
• The photon, traveling at the speed of light does have mass, and is deflected by a gravitational well like the Sun - in 1919 this was one demonstration of Special Relativity http://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun
• This Mass is derived from the photon's energy by the famous E=mc2
• In turn, the photon's energy is derived from it's frequency v by E=hv http://en.wikipedia.org/wiki/Photon#Physical_properties
• This is an example of the prediction of relativity that objects get infinitely more massive as they approach the speed of light. This effect is demonstrated daily in particle accelerators.
• Photons have Zero Rest Mass. This can be roughly described mathematically as: (Finite Mass)/Infinity = 0. If it were possible to slow photons down below the speed of light (in whatever medium they were traveling), they would have no energy, and would cease to be observable.

#### hubble_bubble

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##### Re: mass of a photon
« Reply #4 on: 04/09/2012 16:06:00 »
Light has been slowed and observed.

http://en.wikipedia.org/wiki/Slow_light [nofollow]

The question is why light travels at c in the first place. I can see the point particle principle but if mass increases as we reach light speed then why is the photon not massive? Maybe it is but because of it's other properties this mass can not be detected.

#### imatfaal

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##### Re: mass of a photon
« Reply #5 on: 04/09/2012 16:49:18 »
Slow light is through a medium - not in vacuo; completely different.

[you can think of slow light in terms of the easy heuristic-  time taken is time of absorption and re-emission plus actual travelling time at speed of light.  this is not really correct - but is a very useful model.  the actual reasons are hideously complicated and really tend towards light not being viewed as travelling through as substance as photons.]  what slow light isnt is photons going less than c!

see above - SR cannot cannot be used to predict properties of a particle travelling at speed of light.  inertial frames dealing with massive objects do not smoothly change into new frames dealing with massless objects; there is a disconnect, and laws for one frame do not apply to the other.
« Last Edit: 04/09/2012 17:05:40 by imatfaal »

#### hubble_bubble

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##### Re: mass of a photon
« Reply #6 on: 04/09/2012 20:21:32 »
There is the problem of infinity implicit in any equations connected with c. How can we resolve infinite energy/mass/time dilation? Something is obviously missing. Like the quanta at microscopic scale there must be an equivalent concept for the macro scale. If we look at e=mc2 with e at infinity then m must still be finite as it is always less than e. Infinity multiplied by a value makes no sense.

#### lightarrow

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##### Re: mass of a photon
« Reply #7 on: 04/09/2012 20:42:13 »
The photon, traveling at the speed of light does have mass,
Sorry, it's wrong. There is only one mass, the one you call "rest mass" but that name is not quite good because it's impossible to find a photon at rest, so with your definition it would be impossible to define a mass for a photon (or a gluon), even a zero mass.
If you really want to give it a name, call it "invariant mass", it reminds you that it doesn't depend on the frame of reference (and this is the reason physicists use this as definition).
« Last Edit: 04/09/2012 20:50:56 by lightarrow »

#### lightarrow

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##### Re: mass of a photon
« Reply #8 on: 04/09/2012 20:49:53 »
Light has been slowed and observed.

http://en.wikipedia.org/wiki/Slow_light

The question is why light travels at c in the first place.
c is just a number, because it depends on the units of measure; it is 1 or it is 0.000001 or any number, in other units. The question is another: why light speed is finite?
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I can see the point particle principle
What is this principle?
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but if mass increases as we reach light speed then why is the photon not massive?
On the contrary! As another has written, if it had mass, it couldn't reach light speed.

#### hubble_bubble

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##### Re: mass of a photon
« Reply #9 on: 04/09/2012 21:40:44 »
Light has been slowed and observed.

http://en.wikipedia.org/wiki/Slow_light [nofollow]

The question is why light travels at c in the first place.
c is just a number, because it depends on the units of measure; it is 1 or it is 0.000001 or any number, in other units. The question is another: why light speed is finite?
Quote
I can see the point particle principle
What is this principle?
Quote
but if mass increases as we reach light speed then why is the photon not massive?
On the contrary! As another has written, if it had mass, it couldn't reach light speed.

Yes we can define c as 1 for convenience in equations. Point particles are idealized particles used in equations. There is a dichotomy inherent in what we consider mass. The fermions which make up matter and a boson like the photon behave in distinct ways at velocity. The Fermions can never act like a photon as instead of its mass decreasing with increasing velocity it increases so will never reach a zero point mass. It is going in the wrong direction. The photon is immediately in the correct state when emitted.

#### JP

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##### Re: mass of a photon
« Reply #10 on: 04/09/2012 21:48:57 »
Yes we can define c as 1 for convenience in equations. Point particles are idealized particles used in equations. There is a dichotomy inherent in what we consider mass. The fermions which make up matter and a boson like the photon behave in distinct ways at velocity. The Fermions can never act like a photon as instead of its mass decreasing with increasing velocity it increases so will never reach a zero point mass. It is going in the wrong direction. The photon is immediately in the correct state when emitted.

Yes, all this is true because it matches our observations of the universe.  So what's the problem?  Why should we be troubled that special relativity, which is designed to cover the reference frames only of massive particles, doesn't include reference frames for massless particles?   It still covers all the cases we can measure, since we have mass and our detectors have mass, and produces very accurate results.

#### hubble_bubble

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##### Re: mass of a photon
« Reply #11 on: 05/09/2012 00:50:00 »
Yes, all this is true because it matches our observations of the universe.  So what's the problem?  Why should we be troubled that special relativity, which is designed to cover the reference frames only of massive particles, doesn't include reference frames for massless particles?   It still covers all the cases we can measure, since we have mass and our detectors have mass, and produces very accurate results.

What should trouble us as you say is the connection between the speed of light c, gravity and acceleration. Time dilation slows things down at speeds approaching light speed but also inside a gravitational field, which can be considered an accelerating frame. Under extreme conditions inside the Shwarzschild radius of a black hole the constancy of light's forward motion is breached and escape velocity exceeds the speed of light. If we had a mathematical model of massless particles then we would be on our way to a unified field theory incorporating gravity. We would also finally have an understanding of the mechanics of electromagnetism and gravity.

#### JP

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##### Re: mass of a photon
« Reply #12 on: 05/09/2012 02:27:48 »
But our models do account for massless particles.  We know exactly how they'll act when viewed by any detector we can build!  Special relativity works exceedingly well for predicing how massless particles behave.  The only catch is that we don't know how to describe what a massless detector would see... but building a massless detector doesn't even make sense (since all its particles would fly away from each other at the speed of light!)

Would a theory that includes the reference frame of a photon include any testable predictions?  If not, then while it might seem more complete, it's more a matter of metaphysics than physics.

#### hubble_bubble

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##### Re: mass of a photon
« Reply #13 on: 05/09/2012 04:17:06 »
I don't think that we would detect anything in the reference frame of a photon as theoretically the time dilation would be infinite. This would effectively stop time in that frame. Looking at it this way if we ever could reach light speed we would experience nothing until we impacted some other object. Even then the impact would preclude any interaction with the universe as our matter would vaporize.

The important point is the effect of gravity on light. Gravity affects mass, which suggests that photons must have mass. If not then there is some component missing from the laws of physics. This is fundamental. Photons are also said to have neutral charge, however, this also may be wrong. If the spacetime frame of a photon has infinite dilation then it follows that we would not be able to detect any mass, charge or other reactive properties. Whereas state properties such as luminosity and wavelength are still detectable. This implies a separation between state and reaction. This should also hold for discrete particles of matter. This can be tested under general relativity for ordinary matter and applied to light.
« Last Edit: 05/09/2012 04:18:54 by hubble_bubble »

#### lightarrow

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##### Re: mass of a photon
« Reply #14 on: 05/09/2012 08:47:54 »
There is a dichotomy inherent in what we consider mass. The fermions which make up matter and a boson like the photon behave in distinct ways at velocity. The Fermions can never act like a photon as instead of its mass decreasing with increasing velocity
Mass decreasing with velocity? Even if you talked of "relativistic mass" (which is not "mass") I can't understand how it could decrease with increasing velocity.
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so will never reach a zero point mass.
What is a "zero point mass"?
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It is going in the wrong direction.
What does it mean?
Quote
The photon is immediately in the correct state when emitted.
What does it mean?

#### Emc2

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##### Re: mass of a photon
« Reply #15 on: 05/09/2012 08:48:45 »
having no mass, per se, is the main reason a photon can travel at the speed of light, and thereby establish that rule.

if a photon had mass, it could not achieve the speed of light....

I believe....lol

http://en.wikipedia.org/wiki/Photon

Physical properties

The photon is massless,[Note 2] has no electric charge,[12] and is stable.

HA !! I was right, no mass......

#### lightarrow

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##### Re: mass of a photon
« Reply #16 on: 05/09/2012 08:52:53 »
The only catch is that we don't know how to describe what a massless detector would see... but building a massless detector doesn't even make sense (since all its particles would fly away from each other at the speed of light!)
Infact a massless detector would be, e.g., another photon. Since Even if photon-photon scattering hasn't been observed yet (as far as I know), because of its exceedingly low cross section, it shouldn't be anything particularly bizarre.
« Last Edit: 05/09/2012 13:22:45 by lightarrow »

#### hubble_bubble

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##### Re: mass of a photon
« Reply #17 on: 05/09/2012 10:57:40 »
There is a dichotomy inherent in what we consider mass. The fermions which make up matter and a boson like the photon behave in distinct ways at velocity. The Fermions can never act like a photon as instead of its mass decreasing with increasing velocity
Mass decreasing with velocity? Even if you talked of "relativistic mass" (which is not "mass") I can't understand how it could decrease with increasing velocity.
Quote
so will never reach a zero point mass.
What is a "zero point mass"?
Quote
It is going in the wrong direction.
What does it mean?
Quote
The photon is immediately in the correct state when emitted.
What does it mean?

What I was saying is that mass INCREASES with velocity so it will never be in a massless state. Going in the wrong direction as in increase of mass with velocity instead of a decrease. The mass is getting less like a photon with velocity. The photon is massless when emitted and doesn't lose mass over time.

#### evan_au

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##### Re: mass of a photon
« Reply #18 on: 05/09/2012 13:56:41 »
Preface: I see that current practice prefers to talk in terms of a particle's momentum, rather than it having a relativistic mass which increases with its speed (http://en.wikipedia.org/wiki/Mass_in_special_relativity#Relativistic_mass). However, I am a bit old-school, so apologies in advance...

Let's ignore the photon for a while, and take a simpler example of the difference between Relativistic Mass and Rest Mass: the electron neutrino.
• Mass and the Energy are equivalent, due to E=mc2
• Particle physicists quote the Mass of a Particle by its Energy, in electron Volts (eV) or Mega-electron Volts (MeV).
• A neutrino has a Rest-Mass, thought to be around 0.04eV to 2.2eV (but it's very hard to measure: http://en.wikipedia.org/wiki/Neutrino#Mass)
• This means that if you could ever capture a neutrino, stop it moving, and turn it entirely into energy, you would get somewhere around 1 eV  (give or take a decimal place)
• However, neutrinos emitted from a nuclear fission reactor have energies of up to 10MeV. (http://en.wikipedia.org/wiki/Neutrino#Artificial)
• This means that if a neutrino from a nuclear reactor interacts with a hydrogen nucleus (eg in http://en.wikipedia.org/wiki/KamLAND), a neutrino from a nuclear reactor will produce particles with an energy millions of times larger than the energy of a stationary neutrino
• This increase in energy (equivalent to an increase in momentum, and an increased relativistic mass) is because they are traveling at very close to the speed of light; a rough calculation suggests that a 4MeV neutrino will be traveling somewhere around 99.999999999997% of the speed of light
• So their momentum and energy is much higher than you would expect from their velocity (effectively c) and their rest mass (1eV).
• No matter how much energy you give the classical neutrino (ie how much you increase its momentum), it cannot reach the speed of light, as this would imply infinite momentum (or infinite relativistic mass, in older terminology).

So, for a 4MeV neutrino:
• It has a small rest mass (= a small energy) of somewhere around 1 eV
• It has a non-zero and finite relativistic mass (which may be millions of times larger)
• It can never reach the speed of light in a vacuum
• Time is slowed down significantly, but it has not stopped, so we can still see phenomena like neutrino oscillations (http://en.wikipedia.org/wiki/Neutrino#Flavor_oscillations)
• The neutrino will have severe length contraction - but not to zero

Heading back to the original question: if we contrast this with a violet photon
• It has a small relativistic mass of around 0.5 eV (= a small energy), similar in magnitude to a stationary neutrino
• The photon always travels at exactly the speed of light in a vacuum.
• Time is stopped, so the photon does not change (unless it interacts with something)
• Length contraction will also apply to photons, but the wave nature of light as a transverse electromagnetic wave means that the size will be non-zero at right-angles to the direction of travel.
• If you could ever slow down a photon in a vacuum, and turn it entirely into energy, you would detect 0eV, ie the photon's rest-mass would be zero.

PS: If you find an error in my rough calculations, please feel free to point it out!
« Last Edit: 05/09/2012 14:11:59 by evan_au »

#### JP

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##### Re: mass of a photon
« Reply #19 on: 05/09/2012 14:03:44 »
The only catch is that we don't know how to describe what a massless detector would see... but building a massless detector doesn't even make sense (since all its particles would fly away from each other at the speed of light!)
Infact a massless detector would be, e.g., another photon. Since Even if photon-photon scattering hasn't been observed yet (as far as I know), because of its exceedingly low cross section, it shouldn't be anything particularly bizarre.

There's pair production, but that has to happen in the presence of matter, and there are also people looking at the idea of vacuum nonlinearity (which would be photon-photon interactions) at high energy densities.  These effects still don't require a theory of the photon's reference frame, though, since they can be adequately described in terms of any experiment we know how to do, in terms of special relativity's reference frames.

#### JP

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##### Re: mass of a photon
« Reply #20 on: 05/09/2012 14:19:03 »
I don't think that we would detect anything in the reference frame of a photon as theoretically the time dilation would be infinite. This would effectively stop time in that frame. Looking at it this way if we ever could reach light speed we would experience nothing until we impacted some other object. Even then the impact would preclude any interaction with the universe as our matter would vaporize.

This is the point that's tripping you up, hubble_bubble.  Check what Imatfaal said above.  There's no such thing as a photon reference frame.  The claims that time stops for photons or that if we traveled at a photon's speed, we'd see a static universe are science fiction, not science.  The problem is that special relativity, which is used for this claim, is designed specifically to only work in reference frames in which you, as the observer, see light (in vacuum) always moving at the speed of light.  If you were to somehow ride along on a photon at its speed, it couldn't be moving at the speed of light with respect from you, so all of special relativity would break down.

The problem is that if you simply plug in the speed of light for the observer's velocity, you get a lot of infinities in the equations of special relativity (which is a sign that they've broken down).  A lot of time these infinities get misinterpreted as the idea that time stops for a photon.

A photon reference frame might be interesting if we could only explain certain phenomena by introducing it, but it doesn't appear to be useful to try to add photon reference frames into special relativity.  There is no problem (at least that I know of) that we need them for.  And there is now way (again, that I know of) to measure them if they do exist.

The important point is the effect of gravity on light. Gravity affects mass, which suggests that photons must have mass. If not then there is some component missing from the laws of physics. This is fundamental.
On this point, gravity does affect mass, but it does not only affect mass.  If you look at what general relativity says, gravity alters trajectories through space-time.  Even massless particles have trajectories.  In fact, general relativity predicts very precisely how the gravity massive objects should affect massless photons, and this has been confirmed by observation.

As for creating gravity, anything that has energy or momentum can create gravity.  Mass has a lot of energy (E=mc2), so mass creates gravity.  But photons, which are massless, have energy and momentum, so they can also create gravity.
« Last Edit: 05/09/2012 14:26:42 by JP »

#### yor_on

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##### Re: mass of a photon
« Reply #21 on: 05/09/2012 15:44:03 »
Evan, you're not the only one thinking of it in terms of 'relativistic mass', myself I think of in terms of 'invariant mass or as they say restmass, but depending on terminology relativistic mass also works, as I understands it. Although, if we are going to discuss the definition of 'invariant mass' then it is that it has to be the same from all frames measured as I understands it, no matter if on a event horizon or in 'flat space' without gravitation.

How do we know what that mass is?
'Energy' right :)

And what the he* is 'energy'?

#### stu

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##### Re: mass of a photon
« Reply #22 on: 06/09/2012 07:18:14 »
If E=mc^2

then

y not the mass stays same and the energy amount changes dependant on the speed of which the photon moves.

I know this is not the way you guys see either stationary or moving at 'c' it but if you had a method to slow the photon down incrementilly then surely the energy it has
would be inverse to the speed at which it travells

i know very little on this subject but is good to read your conversations

#### Emc2

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##### Re: mass of a photon
« Reply #23 on: 06/09/2012 07:58:24 »
yes stu.

E-MC2

energy of object is equal to the mass of said object x the speed of light squared.

8.98755179×10^16 m^2/s^2             I believe is speed of light squared.

so this is the rule as it applies to photons.

photons do not have mass, but they do have momentum. The proper, general equation to use is E2 = m2c4 + p2c2 So in the case of a photon, m=0 so E = pc or p = E/c. On the other hand, for a particle with mass m at rest (i.e., p = 0), you get back the famous E = mc2.

This equation often enters theoretical work in X-ray and Gamma-ray astrophysics, for example in Compton scattering where photons are treated as particles colliding with electrons.

« Last Edit: 06/09/2012 11:33:25 by Emc2 »

#### Emc2

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##### Re: mass of a photon
« Reply #24 on: 06/09/2012 11:37:04 »
A slight clarification about the difference between Mass and Rest Mass:
• The photon, traveling at the speed of light does have mass,and is deflected by a gravitational well like the Sun

photons do not have any "mass'
mass
A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.

a Photon does not affect other particles gravitationally, cause it has zero mass....

No, photons do not have mass, but they do have momentum. The proper, general equation to use is E2 = m2c4 + p2c2 So in the case of a photon, m=0 so E = pc or p = E/c. On the other hand, for a particle with mass m at rest (i.e., p = 0), you get back the famous E = mc2.

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

http://www.weburbia.com/physics/photon_mass.html

http://home.fnal.gov/~pompos/light/light_page31.html

The photon is massless,[Note 2] has no electric charge,[12] and is stable
http://en.wikipedia.org/wiki/Photon

« Last Edit: 06/09/2012 11:43:55 by Emc2 »

#### The Naked Scientists Forum

##### Re: mass of a photon
« Reply #24 on: 06/09/2012 11:37:04 »