# The Naked Scientists Forum

### Author Topic: Why is the photon regarded as a massless particle?  (Read 22895 times)

#### imatfaal

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##### Re: mass of a photon
« Reply #25 on: 06/09/2012 17:35:14 »
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

I am not sure about your assertion that a photon cannot have a gravitational influence.  General Relativity is much more complicated that just saying that mass is required for gravity.  Photon have energy - energy and energy flux are parts of the calculations to determine curvature.  Now I cannot believe we could possibly measure that gravitational influence at present - but I am not sure that it is zero.

However - one fly in ointment, a photon is a exemplary quantum mechanical object, our theory of gravity is GR which is not quantum.

#### hubble_bubble

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##### Re: mass of a photon
« Reply #26 on: 07/09/2012 02:53:22 »
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 [nofollow]

http://www.weburbia.com/physics/photon_mass.html [nofollow]

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

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

I am not familiar with this equation. For E2 = m2c4 + p2c2 what is p? I am also assuming m^2c^4 + p^2c^2.

#### hubble_bubble

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##### Re: mass of a photon
« Reply #27 on: 07/09/2012 03:08:23 »
Surely this equation could be rewritten E = mc^2 + pc. Unless these are not exponential functions.

#### Emc2

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##### Re: mass of a photon
« Reply #28 on: 07/09/2012 05:56:59 »
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

I am not familiar with this equation. For E2 = m2c4 + p2c2 what is p? I am also assuming m^2c^4 + p^2c^2.

they say it better then I do, hope this helps hubble.

Does the photon have mass?  After all, it has energy and energy is equivalent to mass.

Photons are traditionally said to be massless.  This is a figure of speech that physicists use to describe something about how a photon's particle-like properties are described by the language of special relativity.

The logic can be constructed in many ways, and the following is one such.  Take an isolated system (called a "particle") and accelerate it to some velocity v (a vector).  Newton defined the "momentum" p of this particle (also a vector), such that p behaves in a simple way when the particle is accelerated, or when it's involved in a collision.  For this simple behaviour to hold, it turns out that p must be proportional to v.  The proportionality constant is called the particle's "mass" m, so that p = mv.

In special relativity, it turns out that we are still able to define a particle's momentum p such that it behaves in well-defined ways that are an extension of the newtonian case.  Although p and v still point in the same direction, it turns out that they are no longer proportional; the best we can do is relate them via the particle's "relativistic mass" mrel.  Thus
p = mrelv .

When the particle is at rest, its relativistic mass has a minimum value called the "rest mass" mrest.  The rest mass is always the same for the same type of particle.  For example, all protons, electrons, and neutrons have the same rest mass; it's something that can be looked up in a table.  As the particle is accelerated to ever higher speeds, its relativistic mass increases without limit.

It also turns out that in special relativity, we are able to define the concept of "energy" E, such that E has simple and well-defined properties just like those it has in newtonian mechanics.  When a particle has been accelerated so that it has some momentum p (the length of the vector p) and relativistic mass mrel, then its energy E turns out to be given by
E = mrelc2 ,   and also    E2 = p2c2 + m2restc4 .           (1)

There are two interesting cases of this last equation:

If the particle is at rest, then p = 0, and E = mrestc2.
If we set the rest mass equal to zero (regardless of whether or not that's a reasonable thing to do), then E = pc.

In classical electromagnetic theory, light turns out to have energy E and momentum p, and these happen to be related by E = pc.  Quantum mechanics introduces the idea that light can be viewed as a collection of "particles": photons.  Even though these photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of equation (1) by just considering them to have no rest mass.  That way, equation (1) gives the correct expression for light, E = pc, and no harm has been done.  Equation (1) is now able to be applied to particles of matter and "particles" of light.  It can now be used as a fully general equation, and that makes it very useful.
Is there any experimental evidence that the photon has zero rest mass?

Alternative theories of the photon include a term that behaves like a mass, and this gives rise to the very advanced idea of a "massive photon".  If the rest mass of the photon were non-zero, the theory of quantum electrodynamics would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalisable; also, charge conservation would no longer be absolutely guaranteed, as it is if photons have zero rest mass.  But regardless of what any theory might predict, it is still necessary to check this prediction by doing an experiment.

It is almost certainly impossible to do any experiment that would establish the photon rest mass to be exactly zero.  The best we can hope to do is place limits on it.  A non-zero rest mass would introduce a small damping factor in the inverse square Coulomb law of electrostatic forces.  That means the electrostatic force would be weaker over very large distances.

Likewise, the behavior of static magnetic fields would be modified.  An upper limit to the photon mass can be inferred through satellite measurements of planetary magnetic fields.  The Charge Composition Explorer spacecraft was used to derive an upper limit of 6 × 10−16 eV with high certainty.  This was slightly improved in 1998 by Roderic Lakes in a laboratory experiment that looked for anomalous forces on a Cavendish balance.  The new limit is 7 × 10−17 eV.  Studies of galactic magnetic fields suggest a much better limit of less than 3 × 10−27 eV, but there is some doubt about the validity of this method.

#### stu

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• Posts: 23
##### Re: mass of a photon
« Reply #29 on: 07/09/2012 09:07:09 »
If the photon has no mass. How can it react on surfaces whith force. In the case of vanes of mill under a vacuum (crookes radiometer)
Quote
or in the case of this link

http://www.newscientist.com/mobile/article/dn16158-photon-force-harnessed-to-do-some-light-work.html [nofollow]

and second part of question

after reacting on these mechanisims what is energy is used or diminished from the photon?

« Last Edit: 07/09/2012 09:12:15 by stu »

#### Emc2

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##### Re: mass of a photon
« Reply #30 on: 07/09/2012 10:43:28 »
If the photon has no mass. How can it react on surfaces whith force. In the case of vanes of mill under a vacuum (crookes radiometer)
Quote
or in the case of this link

http://www.newscientist.com/mobile/article/dn16158-photon-force-harnessed-to-do-some-light-work.html

and second part of question

after reacting on these mechanisims what is energy is used or diminished from the photon?

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

Optical tweezers are capable of manipulating nanometer and micrometer-sized dielectric particles by exerting extremely small forces via a highly focused laser beam. The beam is typically focused by sending it through a microscope objective. The narrowest point of the focused beam, known as the beam waist, contains a very strong electric field gradient. It turns out that dielectric particles are attracted along the gradient to the region of strongest electric field, which is the center of the beam. The laser light also tends to apply a force on particles in the beam along the direction of beam propagation. It is easy to understand why if one considers conservation of momentum. Photons that are absorbed or scattered by the tiny dielectric particle in its path impart momentum to the dielectric particle. This is known as the scattering force and results in the particle being displaced slightly downstream from the exact position of the beam waist, as seen in the figure.

#### Emc2

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##### Re: mass of a photon
« Reply #31 on: 07/09/2012 10:46:57 »
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

I am not sure about your assertion that a photon cannot have a gravitational influence.  General Relativity is much more complicated that just saying that mass is required for gravity.  Photon have energy - energy and energy flux are parts of the calculations to determine curvature.  Now I cannot believe we could possibly measure that gravitational influence at present - but I am not sure that it is zero.

However - one fly in ointment, a photon is a exemplary quantum mechanical object, our theory of gravity is GR which is not quantum.

I got the definition of mass from this link.

http://imagine.gsfc.nasa.gov/docs/dict_jp.html#mass        ( the goddard space centers site )

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.

#### evan_au

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##### Re: mass of a photon
« Reply #32 on: 07/09/2012 22:09:25 »
Quote
a Photon does not affect other particles gravitationally, cause it has zero mass....
... photons do not have mass, but they do have momentum.

Eddington showed that photons are diverted by the Sun's gravity.
This produces a change in the direction of their velocity, which represents a change in the direction of their momentum.
Because momentum is conserved in interactions, a change in the momentum of the photon must be balanced by a change in the momentum of the Sun (as small as that may be).
Or you could look at it in a Newtonian sense - the photon feels a gravitational force towards the Sun, so the Sun feels an equal and opposite gravitational force towards the photon...
This suggests that the photon does affect other objects gravitationally, does it not?

Some of the advantages quoted for the newer momentum-based relativistic notation is that momentum is conserved in all inertial reference frames, and applies equally to all particles regardless of whether they travel at the speed of light or not...

#### Emc2

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##### Re: mass of a photon
« Reply #33 on: 08/09/2012 05:37:05 »
Quote
a Photon does not affect other particles gravitationally, cause it has zero mass....
... photons do not have mass, but they do have momentum.

We also knew that photons are affected by gravitational fields not because photons have mass, but because gravitational fields (in particular, strong gravitational fields) change the shape of space-time

In 1915 Albert Einstein proposed the theory of general relativity. General relativity explained, in a consistent way, how gravity affects light. We now knew that while photons have no mass, they do possess momentum (so your statement about light not affecting matter is incorrect). We also knew that photons are affected by gravitational fields not because photons have mass, but because gravitational fields (in particular, strong gravitational fields) change the shape of space-time. The photons are responding to the curvature in space-time, not directly to the gravitational field. Space-time is the four-dimensional "space" we live in -- there are 3 spatial dimensions (think of X,Y, and Z) and one time dimension.

Let us relate this to light traveling near a star. The strong gravitational field of the star changes the paths of light rays in space-time from what they would have been had the star not been present. Specifically, the path of the light is bent slightly inward toward the surface of the star. We see this effect all the time when we observe distant stars in our Universe. As a star contracts, the gravitational field at its surface gets stronger, thus bending the light more. This makes it more and more difficult for light from the star to escape, thus it appears to us that the star is dimmer. Eventually, if the star shrinks to a certain critical radius, the gravitational field at the surface becomes so strong that the path of the light is bent so severely inward so that it returns to the star itself. The light can no longer escape. According to the theory of relativity, nothing can travel faster than light. Thus, if light cannot escape, neither can anything else. Everything is dragged back by the gravitational field. We call the region of space for which this condition is true a "black hole" (a term first coined by American scientist John Wheeler in 1969).

Now, being scientists, we do not just accept theories like general relativity or conclusions like photons have no mass. We constantly test them, trying to definitively prove or disprove. So far, general relativity has withstood every test. And try as we might, we can measure no mass for the photon. We can just put upper limits on what mass it can have. These upper limits are determined by the sensitivity of the experiment we are using to try to "weigh the photon". The last number I saw was that a photon, if it has any mass at all, must be less than 4 x 10-48 grams. For comparison, the electron has a mass of 9 x 10-28 grams.
« Last Edit: 08/09/2012 05:41:14 by Emc2 »

#### lightarrow

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##### Re: mass of a photon
« Reply #34 on: 08/09/2012 13:29:59 »
Surely this equation could be rewritten E = mc^2 + pc. Unless these are not exponential functions.
Then, the equation 5^2 = 4^2 + 3^2 could be rewritten as 5 = 4 + 3?

#### lightarrow

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##### Re: mass of a photon
« Reply #35 on: 08/09/2012 13:58:00 »

#### waytogo

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##### Re: mass of a photon
« Reply #36 on: 09/09/2012 21:57:11 »
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. 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.

This (green text). Interesting question, I still no have a clue in that point.

Also, how can 'born' a photon without any mass @ 300.000 km/s and without any acceleration as well?
« Last Edit: 09/09/2012 22:04:14 by waytogo »

#### JP

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##### Re: mass of a photon
« Reply #37 on: 09/09/2012 22:11:46 »
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. 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.

This (green text). Interesting question, I still no have a clue in that point.

Also, how can 'born' a photon without any mass @ 300.000 km/s and without any acceleration as well?

I don't think anyone's able to answer that scientifically.  Right now, we know that this is how the universe appears to work as a result of our measurements, but no one's been able to actually show a reason why the universe works this way.

#### lightarrow

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##### Re: mass of a photon
« Reply #38 on: 10/09/2012 11:10:10 »
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. 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.

This (green text). Interesting question, I still no have a clue in that point.

Also, how can 'born' a photon without any mass @ 300.000 km/s and without any acceleration as well?
Are you assuming a corpuscolar behaviour for the photon in this situation? That is, are you assuming that the photon behaves as a little bullett, which starts from the source and moves towards the target being spatially localized in between?
« Last Edit: 10/09/2012 11:19:23 by lightarrow »

#### waytogo

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##### Re: mass of a photon
« Reply #39 on: 10/09/2012 11:11:31 »

a photon is basically pure energy, that it why it can go from particle to wave and visa versa..

(red text) Hi, can you provide an example?

#### Emc2

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##### Re: mass of a photon
« Reply #40 on: 11/09/2012 07:20:11 »

a photon is basically pure energy, that it why it can go from particle to wave and visa versa..

(red text) Hi, can you provide an example?

http://en.wikipedia.org/wiki/Double-slit_experiment
This result establishes the principle known as wave–particle duality. Additionally, the detection of individual photons is observed to be inherently probabilistic, which is inexplicable using classical mechanics.[3]

#### yor_on

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##### Re: Why is the photon regarded as a massless particle?
« Reply #41 on: 12/09/2012 07:17:11 »
There might be one way more too look at it?

'Gravity' is the metric of Space defining it. I become flabbergasted trying to imagining all those 'geodesics', paths of smallest or no 'resistance that becomes the easiest choice for all uniformly moving things. To see the point here you have to imagine all those objects, all able to 'distort' space and all moving relative each other, in different directions simultaneously. It must be very dynamically balanced system SpaceTime. And the photon is just one of those 'objects moving'. What they all comes down too, from the photon to matter, is that they all can be tracked back to 'energy'. So the geodesics gives 'energy' the paths, and they all must change with relative position, simultaneously for all involved. The alternative, as we never see a photon 'move', only when it 'comes to be' as in the recoil from the system it 'leaves' and in its annihilation, is to assume some logic allowing those two to exist without it needing for anything to be there in between. But if so, it still behaves exactly as all other 'moving objects.

Anyway, photons-matter, expressions of 'energy'.

#### hubble_bubble

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##### Re: Why is the photon regarded as a massless particle?
« Reply #42 on: 14/09/2012 03:50:33 »
It is interesting to note that at the Planck scale, if we consider 1 Planck length to be an absolute, that light travels this discrete distance in 1 Planck time. If we don't worry about the macroscopic scale this all makes sense. Consider a shrunken universe contained within say 10 cubic planck lengths. Each cube will contain shrunken particles, of which only photons can jump to the next cube instantaneously. Every other particle is trapped in a particular cube until its momentum moves it to an edge. In this scenario all particles within each cube will see light at the same speed. To move faster necessarily contracts the distance traveled due to dilation. The distance is experienced as less from an external observer.

If a photon starts 10 cubes away, it will reach each cube within a set interval and all particles within a particular cube will 'experience' it at the same instant. This can be scaled up and still works. So it doesn't matter if we measure in Planck scale or metres.

What if the length contraction also applies to the photon moving through this frame? As if spacetime in and around a moving object does contract physically. This could relate the effects of momentum to those of gravity. Ultimately a black hole contracts all spacetime into a singularity (theoretically).
« Last Edit: 14/09/2012 03:53:32 by hubble_bubble »

#### hubble_bubble

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##### Re: Why is the photon regarded as a massless particle?
« Reply #43 on: 14/09/2012 03:56:11 »
Does anyone know of any work comparing time dilation and length contraction whilst leaving a gravitational field to those approaching light speed?

#### hubble_bubble

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##### Re: mass of a photon
« Reply #44 on: 14/09/2012 05:03:58 »
Surely this equation could be rewritten E = mc^2 + pc. Unless these are not exponential functions.
Then, the equation 5^2 = 4^2 + 3^2 could be rewritten as 5 = 4 + 3?

#### imatfaal

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##### Re: Why is the photon regarded as a massless particle?
« Reply #45 on: 16/09/2012 23:29:33 »
EMC2 assertions concerning the claim that the photon is pure energy have been moved to New Theories

http://www.thenakedscientists.com/forum/index.php?topic=45584.0

#### imatfaal

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##### Re: Why is the photon regarded as a massless particle?
« Reply #46 on: 16/09/2012 23:35:32 »
Does anyone know of any work comparing time dilation and length contraction whilst leaving a gravitational field to those approaching light speed?

Sorry HB - just seen this question.  Firstly - I am pretty sure that you only get length contraction in high relative velocity cases.  Secondly - I guess you are looking for qualitative differences between the time dilation caused in the two different circumstances; I just don't know.  Frankly I cannot think how time a/o time dilation could be different (aprt from quantitatively).  And I do know that you can add the two effects to get a single result - this is done all the time in the GPS satellites; high speed makes the clocks slower, and higher grav potential makes the clocks tick faster, but you can just add the two effects and get the actual correction.

At present I cannot imagine how there would be two types of effect (rather than two causes) and in the one case I know of where they both apply they are simply added - so I would guess there is no difference.

#### yor_on

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##### Re: Why is the photon regarded as a massless particle?
« Reply #47 on: 20/09/2012 07:08:32 »
A pretty nice question that one, and yes, they differ. If we imagine Earth as 'moving' at one gravity, the time dilation expressed by that 'gravity' will differ from that of a spaceship moving at a constant one gravity, the faster it moves relative, for example. lights blue shift (measured locally in that frame), or Earth. The equivalence found with gravity is in its 'intrinsic properties' expressed, using a constant uniform acceleration, not about 'time'. And the same goes for length contractions.

#### hubble_bubble

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##### Re: Why is the photon regarded as a massless particle?
« Reply #48 on: 25/09/2012 03:13:26 »
A pretty nice question that one, and yes, they differ. If we imagine Earth as 'moving' at one gravity, the time dilation expressed by that 'gravity' will differ from that of a spaceship moving at a constant one gravity, the faster it moves relative, for example. lights blue shift (measured locally in that frame), or Earth. The equivalence found with gravity is in its 'intrinsic properties' expressed, using a constant uniform acceleration, not about 'time'. And the same goes for length contractions.

Also a photon does not always theoretically travel at c. It is assumed that light cannot escape an event horizon. Can this be still considered a vacuum? If not then what is inside the event horizon may not be a singularity. As we know light travels slower through a medium. Does this imply that intense gravity acts like a physical medium?

#### lightarrow

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##### Re: Why is the photon regarded as a massless particle?
« Reply #49 on: 25/09/2012 11:49:54 »
Also a photon does not always theoretically travel at c. It is assumed that light cannot escape an event horizon.
It doesn't matter if it can't escape: light in the void always travels at c by definition.

#### The Naked Scientists Forum

##### Re: Why is the photon regarded as a massless particle?
« Reply #49 on: 25/09/2012 11:49:54 »