# Does Light Have Mass?

I get confused because i am told it has mass but if it has mass wouldnt the laws be different and we would feel the weight of the sun light when we go outside?

Dale
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Hi Champion, welcome to PF.

There have been experiments designed to answer exactly this question. All of them to date are consistent with the idea that http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html" [Broken].

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light as particle is massless

what do you mean as particles? it take's other forms?

HallsofIvy
Homework Helper
what do you mean as particles? it take's other forms?
Yes, light acts both as a particle and as a wave. In fact, if you get small enough (the size of elementary particles) the very notion of "particle" or "wave" loses meaning.

By the way, your original argument, "we would feel the weight of the sun light when we go outside?" is invalid. If light had a very low weight, we wouldn't feel it. Do you feel the weight of the air?

so if it doesnt have mass why can it not escape from a black hole? why does it experience gravity?

Do you feel the weight of the air?

Wouldn't we lose the "feel" of air for a different reason? I can feel it when I drink from a straw. lol

Light has enough mass to feel the effects of a gravitational field and also exerts its own gravitational attraction, so it must have mass.
But, by definition, it can't have any rest mass.

At rest it would be massless, but it can't be at rest. At the speed of light, it shows the properties one would expect from mass.

HallsofIvy
Homework Helper
No, light does not have mass. In the theory of relativity, "gravity" is a property of space around a massive object. Anything moving around a massive object has its trajectory different from a straight line whether it has mass or not.

I should warn you that you will stir up a nest of hornets if you refer to "mass" as other than "rest mass". Light has energy, not mass.

No, light does not have mass. In the theory of relativity, "gravity" is a property of space around a massive object. Anything moving around a massive object has its trajectory different from a straight line whether it has mass or not.

I should warn you that you will stir up a nest of hornets if you refer to "mass" as other than "rest mass". Light has energy, not mass.

How do hornets have mass? I can't feel them.

So, mass always has energy so that total energy is concerved, but kinetic energy doesn't have mass? Isn't that just arguing semantics?

(Let it be known that I had no ill will when poking the hornet's next with a stick...just bored)

light cannot escape a BH because the escape velocity is >C. alternatively, the photon cannot escape from BH because spacetime is warped to the extent that a straight line cannot extend past the EH, but is rather curved back upon itself.

light does not have measureable mass, per se, but as per E=MC2, the amount of energy of a given photon is equivalent to a fixed amount of mass.

Dale
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2021 Award
How do hornets have mass? I can't feel them. ... (Let it be known that I had no ill will when poking the hornet's next with a stick...just bored)
Hmm, I am quite skeptical of this claim based on my own youthful experiences with bees and wasps.

So, mass always has energy so that total energy is concerved, but kinetic energy doesn't have mass? Isn't that just arguing semantics?
No, it is not semantics. I would highly recommend you look into the standard relativistic concept of the http://en.wikipedia.org/wiki/Four-momentum" [Broken]. In geometric terms you can think of the mass of a particle as being the length (Minkowski norm) of its four-momentum vector, and the energy is just one component of the vector. So the distinction between the two concepts is not just semantic.

The other common usage of the word mass is "relativistic mass". This is the hornet's nest refered to earlier. The concept of "relativistic mass" is deprecated by most modern physicists precisely because there is no distinction (other than semantics) between "relativistic mass" and energy. So it is more clear to use the term energy when refering to "relativistic mass" and reserve the term mass to refer strictly to the invariant rest mass.

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Dale
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2021 Award
Light has enough mass to feel the effects of a gravitational field.
Think about this a little carefully. What does a satellite's orbital path depend on? So in the limit as m->0, what would you expect?

Hmm, I am quite skeptical of this claim based on my own youthful experiences with bees and wasps.

Does this mean pain is analagous to mass? I never really felt them until they announced their presence in the unsavory way they're known to do.

The other common usage of the word mass is "relativistic mass". This is the hornet's nest refered to earlier. The concept of "relativistic mass" is deprecated by most modern physicists precisely because there is no distinction (other than semantics) between "relativistic mass" and energy. So it is more clear to use the term energy when refering to "relativistic mass" and reserve the term mass to refer strictly to the invariant rest mass.[/QUOTE]

Exactly, semantics. The terms hold more meaning and clarity when the term "mass" is limited to invariant rest mass.....

All in fun, I just wanted to see what responses I'd get. Agitated assertions of one's point tend to bring out more definite answers. (An agitated "authority" also tends to talk down to the "moron" with use of simple analogies....which helps this particular moron gain a better understanding) lol

No, light does not have mass. In the theory of relativity, "gravity" is a property of space around a massive object. Anything moving around a massive object has its trajectory different from a straight line whether it has mass or not.

I should warn you that you will stir up a nest of hornets if you refer to "mass" as other than "rest mass". Light has energy, not mass.

Mass is structured energy and as light is also a form of structured energy it will show mass-like characteristics. However the wave structure of light dictates that the mass effect alternates between a photon and an anti-photon and so at the end of the day you end up with zero mass.

Matter, which makes up everything with mass, is very different from light. Arguing about whether light has mass is a dead end because it makes no difference to anything whatever you position you take.

Assigning matter-like properties to light in an attempt to understand it analogously with matter is also pointless. Maxwell's equations tell all.

Can light have a frequency so that E = M?

I am guessing the answer would be yes; energy of any finite frequency can satisfy that equation as long as the time of exposure is sufficient.

Then let me be more clear;

If we assume that a maximum frequency of light exists (denoted by the planck length); What if the observer was to accelerate towards the source of this light of maximum frequency?

Energy of the light must increase with the increase relative velocity, but the light's frequency can't go any higher. Would it be viable to assume that light packets condense into mass and decrease in velocity and in frequency since that is the only way energy can be conserved in this situation?

if it doesn't have mass then why does it bend in gravitational field?

hmmm....light cannot escape a black hole because the light is not bounce back so we can see it? what we see is what is not absorbed?

atyy
See post #8 in the Physics Forums FAQ (in the General Physics forum).

The excuse given in the post for not using the relativistic mass is that it is speed dependent. However, light always travels at the speed of light, so it happens to be the only sort of particle for which you might think the relativistic mass is also invariant, and therefore useful to put in a table. However, looking at the definition of relativistic mass in that post, if v=c, then the relativistic mass contains zero in the denominator. If a particle with rest mass greater than zero travels at the speed of light, its relativistic mass is infinite, which is nonsensical. One way to save the formula is to say that only particles with zero rest mass are allowed to travel at the speed of light. If a particle travels at the speed of light and its rest mass is zero, then its relativistic mass is indefinite (still nonsensical, but not as nonsensical as an infinite mass). I like this idea that light has some sort of indefinite mass, because then it makes sense that it should be attracted by gravity.

Strictly speaking, the special theory of relativity is not compatible with gravity. The indefinite mass of a photon is just a heuristic to see that a proper theory of gravity should predict the deflection of light. However, it is actually possible to construct coherent modifications of the special theory in which gravity doesn't bend light. So ultimately, that gravity bends light is based on experiment, not intellectual necessity.

George Jones
Staff Emeritus
Gold Member
Can light have a frequency so that E = M?

I am guessing the answer would be yes; energy of any finite frequency can satisfy that equation as long as the time of exposure is sufficient.

Then let me be more clear;

If we assume that a maximum frequency of light exists (denoted by the planck length); What if the observer was to accelerate towards the source of this light of maximum frequency?

Energy of the light must increase with the increase relative velocity, but the light's frequency can't go any higher. Would it be viable to assume that light packets condense into mass and decrease in velocity and in frequency since that is the only way energy can be conserved in this situation?

It seems that you're thinking of discrete spacetime. Some physicists have worked on this, but the Physics Forums Rules

prohibit posters from posting their own speculations.
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Dale
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if it doesn't have mass then why does it bend in gravitational field?
I like this idea that light has some sort of indefinite mass, because then it makes sense that it should be attracted by gravity.
so if it doesnt have mass why can it not escape from a black hole? why does it experience gravity?
Light has enough mass to feel the effects of a gravitational field
I don't know why any of you think that something needs mass to be deflected by gravity (aka passive gravitation). Under GR it is clear, but even under Newtonian gravity it should be clear: What is the Newtonian formula for the acceleration of a satellite of mass m located a distance r from a spherical planet of mass M? And what is the limit of the acceleration as m->0?

ma = f = GMm/r²
a = GM/r²

limit as m->0: a = GM/r²

light does not have measureable mass, per se, but as per E=MC2, the amount of energy of a given photon is equivalent to a fixed amount of mass.

Would this mass not then be dependent upon the energy of the individual photon as opposed to fixed?

I don't know why any of you think that something needs mass to be deflected by gravity (aka passive gravitation). Under GR it is clear, but even under Newtonian gravity it should be clear: What is the Newtonian formula for the acceleration of a satellite of mass m located a distance r from a spherical planet of mass M? And what is the limit of the acceleration as m->0?
Well said, DaleSpam.

Is it not a bit iffy cancelling 0's ? It does show that matter doesn't couple to the gravitational field like charge to the electic field, where the force is proportional to the charge.

jtbell
Mentor
jnorman said:
ight does not have measureable mass, per se, but as per E=MC2, the amount of energy of a given photon is equivalent to a fixed amount of mass.
Would this mass not then be dependent upon the energy of the individual photon as opposed to fixed?

There are two different "kinds" of mass here, which have different properties:

https://www.physicsforums.com/showpost.php?p=1842796&postcount=5