How can something be massless?

[VelociBlade]

I am confused about a problem that has arose from my reading of the forums.

If something is massless, then how can it exist? I was taught that all objects in the universe have mass, however small, so how can photons and gluons have no mass?

 

[Meir Achuz]

I am confused... I was taught that all objects in the universe have mass, however small,

You were taught wrong.

 

[Thaakisfox]

The fact that photons are massless, rises from the Gauge invariance of electrodynamics, which means that the Lagrangian can be a function of the Field tensor only.
Usually if we want a Gauge invariant field theory, then the mediator of the field must be massless.
Mass is actually not quite easy to define...

 

[mathman]

I am confused about a problem that has arose from my reading of the forums.

If something is massless, then how can it exist? I was taught that all objects in the universe have mass, however small, so how can photons and gluons have no mass?

Photons and gluons have no rest mass, but moving at the speed of light allows them to have energy equivalent.

 

[LennoxLewis]

It is a hard concept to explain.My teacher was jokingly talking about Newton's infamous low, F = m * a, which he explained as: "F is the force applied, a is the acceleration as a consequence as of that... and well, then there's this proportionality constant that we call m".

And really, that's all that is mass. The ratio of force to acceleration. Light (or any gauge boson) cannot be accelerated by a force. It does travel slower in different media and can seem to make a curve when in fact the space is curved, but neither case has anything to do with "force".
Hope that helps.

 

[Thaakisfox]

Light (or any gauge boson) cannot be accelerated by a force.
Hope that helps.

Actually that's not quite true. In the general case the four-force is not orthogonal to the four-velocity, so there is four-work. Light(photons) is a special case, since in electrodynamics its true, that four-force, doesn't do four-work.
But in a general field-theory it isnt.
And if there is four-work, then the rest mass of the given particle changes. For example, if the force is some four-gradient of a scalar field, then the this field changes the rest mass of the particle, and the particle will have different mass throughout the field. This is basically the Higgs-Mechanism, if the field is the Higgs field, which is constant everywhere, so actually the mass of the mediator, the Higgs boson doesn't change throughout the field.

 

[DeepSeeded]

It is ok for something to be proven mathematically to have no mass at rest, because it still has mass until it comes to a rest. If a photon ever came to a rest, then we would have something to talk about!

It is true that it is impossible for something to exist without mass. A photon has mass because it has energy, it has energy because it moves.

 

[Thaakisfox]

No that's not right. A photon does not have mass. The only thing it has is energy, which is not mass. If it would have mass, then it would have a mass shell.
It may have something like "equivalent" mass for energy, but that is NOT mass.

 

[LennoxLewis]

Actually that's not quite true. In the general case the four-force is not orthogonal to the four-velocity, so there is four-work. Light(photons) is a special case, since in electrodynamics its true, that four-force, doesn't do four-work.
But in a general field-theory it isnt.
And if there is four-work, then the rest mass of the given particle changes. For example, if the force is some four-gradient of a scalar field, then the this field changes the rest mass of the particle, and the particle will have different mass throughout the field. This is basically the Higgs-Mechanism, if the field is the Higgs field, which is constant everywhere, so actually the mass of the mediator, the Higgs boson doesn't change throughout the field.

I stand corrected. I know about the Higgs mechanism, but doesn't this apply only to massive particles? If there is four-work, then the particle is no longer massless, right?

 

[Thaakisfox]

The Higgs mechanism, is through which particles gain their mass, so it doesn't apply only to massive particles.
Yes, if there is four work then the particle is no longer massless. So for example if a "particle" doesn't have rest mass, and it is but into some field, which does four-work, then the particle will gain mass

 

[DeepSeeded]

No that's not right. A photon does not have mass. The only thing it has is energy, which is not mass. If it would have mass, then it would have a mass shell.
It may have something like "equivalent" mass for energy, but that is NOT mass.

But mass and energy are the same thing, as an object gains energy of any form it is proven that its mass increases.

If an object, even a massless object, has speed it attains mass, so a photon must have mass.

 

[malawi_glenn]

But mass and energy are the same thing, as an object gains energy of any form it is proven that its mass increases.

If an object, even a massless object, has speed it attains mass, so a photon must have mass.

seriously, what are you talking about?

 

[DeepSeeded]

E=MC^2 and C^2 is a constant, so E=M.

E=1/2MV^2 so Velocity is proportional to M, a photon has velocity so it has energy so it has mass.

If that doesn't convince you, a photon also has momentum, and momentum is VM. If M were 0 it would not have any momentum.

 

[nuby]

E=m*c^2 isn't the same as E=m . Maybe you could say light and mass are two different states of energy? I picture mass as a 'potential' form of energy, that requires lots of energy to release. (correct me if this doesn't sound right).

 

[DeepSeeded]

E=m*c^2 isn't the same as E=m

Sure it is, the c^2 is just there because our SI units, chosen independently, are not going to line up perfectly, for example, Joules and grams were both decided independently and 1 Joule is not going to turn out to be equal to exactly 1 gram.

c itself comes from two constants that were measured through lab experiments, these constants are there only to bridge the difference between our SI units.

 

[daschaich]

E=MC^2 and C^2 is a constant, so E=M.

E=1/2MV^2 so Velocity is proportional to M, a photon has velocity so it has energy so it has mass.

If that doesn't convince you, a photon also has momentum, and momentum is VM. If M were 0 it would not have any momentum.

E=mc^2 is only for a particle of mass m at rest, and the speed of light sure ain't zero. For a particle with speed v, basic special relativity gives
E = \frac{mc^2}{\sqrt{1 - v^2 / c^2}}
(checking the v = 0 limit is easy).

Now, the speed of light is v = c. Plug this into the formula above, and you get zero in the denominator. The only way to salvage the situation is to have m = 0 in the numerator. Now the expression is merely an ill-defined 0 / 0. (The appropriate formula for massless particles is E = h\nu, where h is Planck's constant and the Greek letter \nu (nu) is the particle's frequency.)

Photons are massless. So are any other particles that travel at the speed of light. And, going the other direction, all massless particles must travel at the speed of light.

If thinking about massless particles is giving you conniptions, recall that light is a wave, and try thinking about massless waves for a while. (Once that gives you conniptions, try going back to thinking about massless particles. And study some more special relativity in the mean time.)

 

[daschaich]

If that doesn't convince you, a photon also has momentum, and momentum is VM. If M were 0 it would not have any momentum.

To follow up on my previous post, the photon's energy is E = h\nu and its energy and momentum are related by E = pc (where p = |\textbf p| is the magnitude of the momentum three-vector). Therefore the momentum of a massless particle is
p = \frac{E}{c} = \frac{h\nu}{c} = \frac{h}{\lambda},
where \lambda is the particle's wavelength. (The direction of \textbf p is the direction of the particle's propagation.)

You can get a deeper understanding of this from Wikipedia, or any modern physics or special relativity book.

 

[0xDEADBEEF]

How funny it seems, that it is supposed to be more natural for things to have mass than not to have it, since no one knows where it comes from anyway.
Next someone will pull out Zeno again, and say infinities don't exist.
Just because you can't imagine it, doesn't mean it can't exist.

 

[malawi_glenn]

E=MC^2 and C^2 is a constant, so E=M.

E=1/2MV^2 so Velocity is proportional to M, a photon has velocity so it has energy so it has mass.

If that doesn't convince you, a photon also has momentum, and momentum is VM. If M were 0 it would not have any momentum.

Now I see that you have not done Special Theory of relativity in collage.

The Energy and momentum relation is NOT E = mc^2 but

E^2 = p^2 + m^2

and hence p = E for massless particles, i.e p = mv is for newtionian dynamics, but now you were invoking Einstein, so play by those rules please, no cheating ;-)

daschaich posts are also illuminating, E = gamma * m, if v = c, gamma -> infinity, which is non sense here.

 

[nuby]

The big question is, what does the speed of light squared actually represent? Is it two different velocities, if so, what are they for?

 

[daschaich]

The big question is, what does the speed of light squared actually represent? Is it two different velocities, if so, what are they for?

As DeepSeeded points out, it's a unit conversion factor. Not that big a deal. I don't understand your second sentence.

 

[malawi_glenn]

The big question is, what does the speed of light squared actually represent? Is it two different velocities, if so, what are they for?

It is not a big question, in SI units it is a conversion factor to get the correct units, but units are arbitrary, you can choose any gauge you want I would say.

 

[nuby]

If you have a single (real) proton. How do you turn it into pure energy?

 

[malawi_glenn]

If you have a single (real) proton. How do you turn it into pure energy?

Define "pure" energy...

 

[daschaich]

If you have a single (real) proton. How do you turn it into pure energy?

We're discussing photons here, protons are rather different. How do you feel photons differ from "pure energy"?

 

[nuby]

1.50e-10 Joules, and the mass goes away.

 

[malawi_glenn]

1.50e-10 Joules, and the mass goes away.

i) is there an requirement for your definition that energy is not attached to mass? if, why? is a photon "pure" energy? Why should your definition be standard? who cares?

ii) easy, let the proton interact with an antiproton -> two photons of energy 1.50e-10 Joules will be created.

Why is it ok that a photon flies away with 1.5 e-10 joules but not a proton? :S
"Instead of mass", a photon has angular frequency -> I claim this is not pure energy, I want energy not attached to angular frequency of Electric- and Magnetic field.

Can you solve it?

 

[DeepSeeded]

E=mc^2 is only for a particle of mass m at rest, and the speed of light sure ain't zero. For a particle with speed v, basic special relativity gives
E = \frac{mc^2}{\sqrt{1 - v^2 / c^2}}
(checking the v = 0 limit is easy).

Now, the speed of light is v = c. Plug this into the formula above, and you get zero in the denominator. The only way to salvage the situation is to have m = 0 in the numerator. Now the expression is merely an ill-defined 0 / 0. (The appropriate formula for massless particles is E = h\nu, where h is Planck's constant and the Greek letter \nu (nu) is the particle's frequency.)

Photons are massless. So are any other particles that travel at the speed of light. And, going the other direction, all massless particles must travel at the speed of light.

If thinking about massless particles is giving you conniptions, recall that light is a wave, and try thinking about massless waves for a while. (Once that gives you conniptions, try going back to thinking about massless particles. And study some more special relativity in the mean time.)

Very good point, I guess I was wrong. I have not taken relativity yet, though I do remember that equation.

Now I have to wonder, if light doesn't have mass... what does? Everything is made of forces, electric, magnetic, strong, weak, etc. If none of these forces have mass then well nothing does.

 

[malawi_glenn]

Very good point, I guess I was wrong. I have not taken relativity yet, though I do remember that equation.

Now I have to wonder, if light doesn't have mass... what does? Everything is made of forces, electric, magnetic, strong, weak, etc. If none of these forces have mass then well nothing does.

What kind of nonsense claim is that - that everything is made up of forces? :S

What you have in the standard model is that you have particles and interactions.

Electrons have mass and cannot move at the speed of light in vacuum, but always below that speed. Quarks have mass, and so on.

 

[DeepSeeded]

Electrons have mass .

I thought electrons were just the result of a resonance phenomenon of energy, I never thought of them being made of some sort of substance... I guess I am over my head, this post was originally on the classical physics forum.. now I am in quantum physics

 

[malawi_glenn]

I thought electrons were just the result of a resonance phenomenon of energy, I never thought of them being made of some sort of substance... I guess I am over my head, this post was originally on the classical physics forum.. now I am in quantum physics

I think you miss an important thing here: electrons HAVE mass, they are not "made up of mass".

 

[DeepSeeded]

I think you miss an important thing here: electrons HAVE mass, they are not "made up of mass".

I think of everything being made of up of energy or force and either mass is just an illusion or a conscience measurement of energy.

am I so far off? I have not studied the Higgs yet, getting there.

 

[malawi_glenn]

You should perhaps wait til you can study quantum field theory at graduate level..

Also take string theory into consideration.

 

[daschaich]

I think of everything being made of up of energy or force and either mass is just an illusion or a conscience measurement of energy.

am I so far off? ...

Yes. Let's leave the philosophical speculations until after you get the math under your belt. Everyday intuition is not a reliable guide.

 

[VelociBlade]

I'm confused again. Daschaich said that light is a wave, not a partice. Don't photons have wave-particle duality? Also, what energy is Malawi talking about in post #27? Photons have no electric charge, right?

 

[daschaich]

... Daschaich said that light is a wave, not a partice...

I never said that! I said if thinking about light as particles is confusing, you could make use of wave-particle duality to try thinking about it as a wave. Various equivalent approaches can be easier than others, depending on the problem.

 

[malawi_glenn]

i) I leave it Daschaich to respond what he said about light beeing a wave and not a particle.

But I can tell you this: we physicsists often use the word "particle" sloppy. We mean that a particle is not a particle in the classical sense (i.e. a tiny cannon ball / sphere) but we say that a particle is a particle is particle. And this particle has properties of a wave and properties of classical particle.

ii) I did not refer to anything about electric charge or anything, I was discussing how bizarre the concept of "pure" energy is. And the guy who asked me said that he wanted energy not in the form of mass. But then I said that I wanted energy not in the form of a oscillating field.

The energy I was referring to is perhaps energy in form of photons. Colliding an antiproton with a proton will result in two photons.

 

[VelociBlade]

Daschaich: You never mentioned wave-particle duality. Also, I still don't get the part about massless particles, or how something could exist in reality as a non-kinetic wave. Could you explain this?

Malawi: What do you mean, "a particle is a particle is particle"? Also, could someone explain the posts on the first two segments of this post in terms I can understand? I'm a sophomore and I originally posted this on the classical physics forums, I had no idea the problem would involve quantum physics and the like.

 

[daschaich]

Daschaich: You never mentioned wave-particle duality...

I did not use that exact phrase, but I did speak of light as being a wave, and light as being a particle. This is wave-particle duality (glossing over what the definition of "be" is).

...Also, I still don't get the part about massless particles, or how something could exist in reality as a non-kinetic wave. Could you explain this?...

What do you mean by "non-kinetic wave"? It's not a phrase I recall seeing before, and certainly not one I would use to describe something moving at the speed of light.

...Also, could someone explain the posts on the first two segments of this post in terms I can understand?...

Which posts where now?

Though quantum physics and special relativity can be difficult to learn and parse, that is the only way your question, "how can something be massless?" can be addressed.

You don't have to understand everything in the universe immediately, nor can you. Don't view this as a failure, but rather a challenge and motivation to explore those subjects more fully in coming years. Then one day someone will say "photons and gluons have no mass due to gauge invariance" and you'll reply, "yes, of course". But that day is still well off in the future, so don't worry if that imaginary conversation doesn't make any sense to you now.

 

[Shackleford]

Look up the photoelectric effect. That will probably clear up some things for you.

 

[malawi_glenn]

Daschaich: You never mentioned wave-particle duality. Also, I still don't get the part about massless particles, or how something could exist in reality as a non-kinetic wave. Could you explain this?

Malawi: What do you mean, "a particle is a particle is particle"? Also, could someone explain the posts on the first two segments of this post in terms I can understand? I'm a sophomore and I originally posted this on the classical physics forums, I had no idea the problem would involve quantum physics and the like.

An electron is an electron.

"non-kinetic wave" what is that?, never heard of that concept..

Why does things have to have mass? I can't see the physical argument.

 

[nuby]

Is spin is related to mass? Edit: just noticed W-Z bosons have mass.

 

[malawi_glenn]

W,Z bosons are spin-1 and has mass

However, Weinberg-Witten theorem tells you that you can't construct a Quantum Field theory which is lorentz invariant with massless particle with spin greater than 1.

 

[Shackleford]

An electron is an electron.

"non-kinetic wave" what is that?, never heard of that concept..

Why does things have to have mass? I can't see the physical argument.

Well, I guess because someone views a "thing" as a "particle" or rigid collection of particles. Of course, most "things" have inertia, of which mass is a quantitative measure.

 

[VelociBlade]

What I meant by "non-kinetic wave" was that I thought waves could only exist through kinetic energy, such as ocean surface waves, sound waves, etc. I've never really understood the concept of light waves, because of people saying it's either a wave, a particle, or both. But everything is made of something, right? After all isn't mass required for something to exist? Also, I'm trying to understand now because, with the current financial crisis in California, I might not get a chance to later.

 

[Shackleford]

What I meant by "non-kinetic wave" was that I thought waves could only exist through kinetic energy, such as ocean surface waves, sound waves, etc. I've never really understood the concept of light waves, because of people saying it's either a wave, a particle, or both. But everything is made of something, right? After all isn't mass required for something to exist? Also, I'm trying to understand now because, with the current financial crisis in California, I might not get a chance to later.

Well, a wave is simply a disturbance that is propagated. An elastic/mechanical wave is a disturbance that is propagated through a medium. An electromagnetic wave (any of the radiation in the EM spectrum) is a wave consisting of alternating electric and magnetic fields that are normal to each other. There is an archive to which I will post the link that should help you in your understanding of the descriptions (wave and particle) of the properties light exhibits. Don't confine your thinking of the wave-particle duality of light to comparing it to some other tangible thing in the particle sense. A wave is simply a disturbance that does carry energy.

http://www.physforum.com/index.php?showtopic=3310&st=285

 

[daschaich]

The energy of a wave is
E = h\nu
where the Greek letter \nu (nu) is its frequency. Shackleford mentioned the photoelectric effect, which would be a good place to learn some of this stuff.

 

[YorkLarry]

I think the the idea is no rest mass. A massless entity such as a photon cannot be brought to rest.

 

[daschaich]

A massless entity such as a photon cannot be brought to rest.

This is correct, and can be a helpful way of thinking about some problems.

I think the the idea is no rest mass.

Introducing "rest mass" as opposed to "relativistic mass" is not helpful, and tends to cause confusion and errors. There is only one mass, and massless particles have no mass. For a good discussion of this issue, I refer you to Lev Okun's article, "The Concept of Mass", which I have posted online http://daschaich.homelinux.net/okun.pdf .

 

[pzona]

But everything is made of something, right? After all isn't mass required for something to exist?

Everything is made of something, but that something is not always matter. Mass is a property of matter, so therefore not everything must have mass. I don't claim to fully understand it either, but this is a rough version of what my physics teacher told me in high school, and it made things much clearer for me when trying to understand ideas like light.