# Light as a Particle

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Hi there

My question is: "If light can a particle, does it have mass. If so how can something that has mass travel at the speed of light?"

Please be gentle, I am no expert. Just pondering on something I thought of the other day. Also if any answers could be given in layman's terms I would be very grateful -

- thanks for taking the time to read

* can be a particle

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No, the photon does not have mass, by the common physics definition of mass. That's why it travels at c.

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No, the photon does not have mass, by the common physics definition of mass. That's why it travels at c.

Thanks swansont. Something I've learned from your answer that I find interesting is that a particle of light (photon) doesn't have mass by the "common physics definition of mass", which suggests to me that there is another definition of mass. Is this correct? And does this have anything to do with Planck length? Or am I way off base?

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Thanks swansont. Something I've learned from your answer that I find interesting is that a particle of light (photon) doesn't have mass by the "common physics definition of mass", which suggests to me that there is another definition of mass. Is this correct? And does this have anything to do with Planck length? Or am I way off base?

Soem people take E=mc^2 and use that to define mass as E/c^2; it's simply a proxy for total energy. There are a number of threads discussing it, with a few being current, so there's really no reason to start a new discussion on it here.

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I'm not sure I understand that but I appreciate you taking the time to answer. Cheers. I'll have a look and try and understand the threads. Thanks again for your answers

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Hi there

My question is: "If light can a particle, does it have mass. If so how can something that has mass travel at the speed of light?"

Please be gentle, I am no expert. Just pondering on something I thought of the other day. Also if any answers could be given in layman's terms I would be very grateful -

- thanks for taking the time to read

* can be a particle

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

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Fantastic! Thank you Juanrga

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Hi there

My question is: "If light can a particle, does it have mass. If so how can something that has mass travel at the speed of light?"

A complete treatment can be found in the physis FAQ at - http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

It really depends on what one means by the term "mass". Particle physicists and quantum physicists tend to use the term to mean "proper mass" aka "rest mass." Others use the term to mean inertial mass. When it comes to more general systems the inertial mass is more meaningful. proper mass can't be applied to open and extened systems such as a body which is radiating energy, thus having a variable mass. Inertial mass is defined as the p in p = mv. For a particle traveling at the speed of light this relationship still holds. We now have v = c so that p = mc. For photons E = pc which implies that m = E/c2 = hf/c2.

I see cosmologists and GRists use the term to mean inertial mass as well as active and gravitational mass.

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It really depends on what one means by the term "mass". Particle physicists and quantum physicists tend to use the term to mean "proper mass" aka "rest mass."

And solid state physics, astrophysicists, and many other physicists; also most of chemists and biologists... Although the correct name is "invariant mass" or simply mass, not "proper mass", neither "rest mass". "Rest mass" is specially a misnomer for light particles, because photons are never at rest.

I see cosmologists and GRists use the term to mean inertial mass as well as active and gravitational mass.

Your concept of "inertial mass" (aka the old concept of "relativistic mass") is almost not used today in science. Cosmologists and general relativists use what you call "rest mass":

A No-Nonsense Introduction to General Relativity by a well-known cosmologist.

Or just check what general relativists Taylor & Wheeler write in their Spacetime Physics (2nd Ed.):

The concept of 'relativistic mass' is subject to misunderstanding. That's why we don't use it. First, it applies the name mass - belonging to the magnitude of a 4-vector - to a very different concept, the time component of a 4-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of space-time itself.
Edited by juanrga
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Hi there

My question is: "If light can a particle, does it have mass.

Light is a wave, too. If light were not a particle, we could not explain photoelectric effect properly?

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Light is a wave, too. If light were not a particle, we could not explain photoelectric effect properly?

Wave/Particle duality is very interesting to me. I would like to learn so much more about it than I know at the moment. Thanks for your reply.

A complete treatment can be found in the physis FAQ at - http://math.ucr.edu/...hoton_mass.html

It really depends on what one means by the term "mass". Particle physicists and quantum physicists tend to use the term to mean "proper mass" aka "rest mass." Others use the term to mean inertial mass. When it comes to more general systems the inertial mass is more meaningful. proper mass can't be applied to open and extened systems such as a body which is radiating energy, thus having a variable mass. Inertial mass is defined as the p in p = mv. For a particle traveling at the speed of light this relationship still holds. We now have v = c so that p = mc. For photons E = pc which implies that m = E/c2 = hf/c2.

I see cosmologists and GRists use the term to mean inertial mass as well as active and gravitational mass.

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You're most welcome. Here is a more detailed treatment on inertial mass

http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htm

There is more here too

http://home.comcast.net/~peter.m.brown/sr/invariant_mass.htm

That page describes proper mass (sometimes called invariant mass) of macroscopic systems

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You're most welcome. Here is a more detailed treatment on inertial mass

http://home.comcast....ertial_mass.htm

There is more here too

http://home.comcast....ariant_mass.htm

That page describes proper mass (sometimes called invariant mass) of macroscopic systems

This is brilliant thank you! - this will keep me busy for a while

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• 2 weeks later...

This is brilliant thank you! - this will keep me busy for a while

Did you find those pages helpful?

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Did you find those pages helpful?

Hi pmb

Thanks for those links. I had a good look at them and realised how much I didn't know that I didn't know!

But having said that it did start to cement some of what I had read on these forums. I think just seeing things written again and again helps make sense of it and it starts sinking in (that's the hope anyway!)

It's a great place to learn things and to get help from others who can help.

Cheers again!

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Hi pmb

Thanks for those links. I had a good look at them and realised how much I didn't know that I didn't know!

But having said that it did start to cement some of what I had read on these forums. I think just seeing things written again and again helps make sense of it and it starts sinking in (that's the hope anyway!)

It's a great place to learn things and to get help from others who can help.

Cheers again!

That's awesome. There are a lot of smart people here whom you'll find very helpful in your journey into physics. They're here to help so make full use of them. Best wishes.

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That's awesome. There are a lot of smart people here whom you'll find very helpful in your journey into physics. They're here to help so make full use of them. Best wishes.

There certainly are. Thank you, will do

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