# Can energy exist without mass?

ZapperZ
Staff Emeritus
As far as I know all energy is associated with some sort of mass, whether that be rest mass or some other type of mass such as relativistic mass. But as I am not well educated on this, I could very easily be completely wrong.
This is wrong. Photons have no rest mass, and it is misleading to insist that it has a "relativistic mass". Look at the expression for relativistic mass. It requires the existence of a non-zero rest mass!

One should also stop using "relativistic mass".

Zz.

Drakkith
Staff Emeritus
This is wrong. Photons have no rest mass, and it is misleading to insist that it has a "relativistic mass". Look at the expression for relativistic mass. It requires the existence of a non-zero rest mass!

One should also stop using "relativistic mass".

Zz.
Why is wikipedia saying that photons have mass then? Not rest mass, no. But look at my quote. Are they just wrong or is this some kinda gray area or misunderstood area?

i think that mater is the sourse of mass particles and mass les particles. In mass particles mater has a moviment circular and in mass les particles a linear. If supose plank mass as created by mater or energy will have:
M = G*M^2 / ( R *C^2) Only in this case mass and mater are the same and have the same dimentions. Here M plank and Rplank.
This hypothesis will be invalid only if is 100% sure sure that antimater has not repeling gravity force.
Sorry that a layman has dare to debate with physicist .

russ_watters
Mentor
It is strange that we keep seeing people using "E=mc^2" to justify the argument that photons must have mass (which is wrong), but we don't see people using, say Hooke's Law F=kx to justify that force must have distance. After all, mathematically, these two equations are identical in structure. We also don't hear people arguing that potential difference is equivalent to current based on V=IR. Or better yet, why not say that energy and frequency of oscillation are the "same thing" (E=hf).
And ironically at the same time, we also get a lot of people saying based on f=ma that force should always result in acceleration!

Just a general reminder to people that just because an equation has a parameter in it you are interested in (like force), that doesn't mean that that equation necessarily applies everywhere you see that parameter. And vice versa.

Andrew Mason
Homework Helper
Could be! Though it is quite weird to think of something having momentum but no mass, yet light is a constant (no pun intended :P) reminder.
As has been stated above, a photon has no mass or inertia. But it has momentum. And it can transport mass/inertia through space from one atom to another. This realization by Einstein was expressed in his famous equation: E = mc^2 (or, as he put it, m = L/c^2). The m is not the mass of the photon itself. Rather it is the quantity of inertia that the photon takes away from the atom that emits it and which it then transfers to the atom that absorbs it.

AM

ZapperZ
Staff Emeritus
Why is wikipedia saying that photons have mass then? Not rest mass, no. But look at my quote. Are they just wrong or is this some kinda gray area or misunderstood area?
Here's the http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html" [Broken]:

$$m = \gamma m_0$$

where $m_0$ is the rest mass, and $\gamma$ is the usual relativistic factor.

What is m when the rest mass is ZERO? What do you believe more? Wikipedia, or the physics? What is the credential of the person (or persons) who wrote that Wikipedia entry that you trust so much?

Light has MOMENTUM. The apparent "inertial" reaction that light has is due to this momentum, not due to "mass", relativistic or not. There is no "gray or misunderstood area" here.

Zz.

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russ_watters
Mentor
Why is wikipedia saying that photons have mass then? Not rest mass, no. But look at my quote. Are they just wrong or is this some kinda gray area or misunderstood area?

...or better yet, read the article specific to the question: the photon article. It says in plain english a dozen times that the photon has no mass!

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...or better yet, read the article specific to the question: the photon article. It says in plain english a dozen times that the photon has no mass!
Photons have mass because they have momentum and if they had no momentum they wouldnt exist because photons cant exist without momentum so they must have mass

ZapperZ
Staff Emeritus
Photons have mass because they have momentum and if they had no momentum they wouldnt exist because photons cant exist without momentum so they must have mass
This is wrong. You need to read the PF FAQ in the General Physics forum first before making such statements.

Zz.

Photons have mass because they have momentum and if they had no momentum they wouldnt exist because photons cant exist without momentum so they must have mass
Only according to classical Physics P(Momentum)=Mass*Velocity! Not according to modern Physics where all u need is energy to carry it!!:)

Drakkith
Staff Emeritus

...or better yet, read the article specific to the question: the photon article. It says in plain english a dozen times that the photon has no mass!
What article at the end of the point? (Did I misunderstand what a "Point" is in wikipedia? I didn't see anything linked after the part I quoted.)

Alright, I wholeheartily agree that photons have no mass. I've read up a little bit more on it all and it appears to me that there is simply a lot of confusion over the concept of mass. This quote from Einstein sums it up pretty well in my opinion.

It is not good to introduce the concept of the mass of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the 'rest mass' m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion.
Gotta go do some stuff here at work, I'll be back later.

Drakkith
Staff Emeritus