# Light with mass?

1. Oct 15, 2008

### aman.yash

light with mass????

light moves with greatest velocity and has energy ie in discreate amount called quanta, but according to Einstein's Mass-Energy equation "E=mc^2", a particle has energy then it should have mass, it means that light has mass??????

i am studying in 11th so please answer to me upto my level so that i can understand!!

Last edited: Oct 15, 2008
2. Oct 15, 2008

### mgb_phys

Re: light with mass????

That's not whatE=mc2 means, it means mass can be converted into energy (and energy into mass)
Or you can think of matter as a sort of concentrated form of energy.
Light has energy but no mass, that's why it can travel at the speed of light

3. Oct 16, 2008

### malawi_glenn

Re: light with mass????

The full equation is:

E^2 = (mc^2)^2 + (pc)^2

Where p is the momentum of the particle.

The equation you quoted are for particles at rest, so a photon has energy:
E = pc, since it has no rest mass.

4. Oct 16, 2008

### Hootenanny

Staff Emeritus
Re: light with mass????

5. Oct 16, 2008

### Phrak

Re: light with mass????

And answered in one hundred different ways--where all of which I've read I've found disagreeable. A PF faq would be nice, if done right. But with the typical questioner having little algebra skills...

6. Oct 16, 2008

### atyy

7. Oct 16, 2008

### malawi_glenn

Re: light with mass????

8. Oct 16, 2008

### Naty1

Re: light with mass????

Aman: The concept of mass and energy and light takes a while to absorb and understand. The more you learn the more complex and subtle they become because everything in this universe is related and appears to us as different forms.

Instead of just saying "light has mass", it would be better to think light has an amount of energy which could be converted to mass, but it doesn't exist in both forms simultaneously. It would be misleading to think light has mass (at all times) because no mass can move at the speed of light. Yet both are capable of producing energy and both have gravitational effects.

It might also be useful to know that Einstein said three things have gravitational effects: mass, energy and pressure. So in a sense all three are "equivalent" in that they have gravitational effects. All have forms of energy.

9. Oct 16, 2008

### DrGreg

Re: light with mass????

In relativity, there is more than one definition of "mass". The two main ones you are likely to find are:

- rest mass, or invariant mass, or proper mass: this excludes kinetic energy and all observers agree what the invariant mass of a particle is

- relativistic mass: this includes kinetic energy, and different observers measure a different relativistic mass for the same object.

If the object is stationary relative to the observer then both definitions give the same answer.

There is a convention amongst most modern physicists that "mass" means "invariant mass", but there are some authors, especially those of popular, non-academic books, who use "mass" to mean "relativistic mass". You need to know about both sorts if you want to make sense of a diverse range of different books and websites.

When m is invariant mass, the energy equation is

$$E^2 = (mc^2)^2 + (pc)^2$$​

where p is momentum. This works for all particles, including photons (particles of light). For particles with non-zero invariant mass the following also holds:

$$E = \frac{mc^2}{\sqrt{1 - v^2/c^2}}$$​

When M is relativistic mass, the energy equation is

$$E = Mc^2$$​

for all particles. From which

$$M^2 = m^2 + \frac{p^2}{c^2}$$​

So, photons have no invariant mass but they do have relativistic mass. Therefore most modern physicists say photons have no mass.

It's not quite right to think of converting energy to mass. Mass is a form of energy, just as kinetic energy* is a form of energy, potential energy is a form of energy, heat is a form of energy and so on. What we can do is convert mass energy to and from other forms of energy. The total energy remains unchanged.

(*If you think "mass" means "relativistic mass" then you think kinetic energy is a form of mass!)

10. Oct 16, 2008

### Phrak

Re: light with mass????

A better question is, "When doesn't light have mass?" My box full of photons from last month's round of photon mass questions still has mass. The microwaves in my oven still have mass.

11. Oct 16, 2008

### DrGreg

Re: light with mass????

In my answer I was careful to refer to the mass of "a photon" rather than "light". I think the original questioner needs to walk before they can run!

12. Oct 16, 2008

### bernhard.rothenstein

Re: light with mass????

I think that much confusion arrises from the fact that we avoid the concept of relativistic mass using only the concept of rest mass. When we say that the mass of the photon is equal to zero it would be better to say the mass (rest) of the photon is equal to zero. Doing so we should take into account that some times a new generation reads old books and old generations read new books.

13. Oct 16, 2008

### Count Iblis

Re: light with mass????

Perhaps it would be better to abolish the concept of mass altogether. We know since the discovery of special relativity that it is not an independent physical quantity from energy. Instead of (invariant) mass, we should refer to rest energy.

Mass only becomes an independent quantity in the classical limit
c ----> infinity.

14. Oct 16, 2008

### Fredrik

Staff Emeritus
Re: light with mass????

That's a good point, but I like having a name for the real number m that appears in the eigenvalue equation of four-momentum squared:

$$P_\mu P^\mu|p,\sigma>=-m^2|p,\sigma>$$,

and "rest energy" seems so...I don't know...like something much less fundamental than it is. It is after all one of the concepts used in the definition of a particle.

15. Oct 17, 2008

### Phrak

Re: light with mass????

I can appreciate that Dr. Greg. But I don't think it makes a difference. When it's said that "the photon has zero mass", this should be speaking only of an idealized photon, where none actually exist in nature. It's a useful non-physical construction, like the Dirac delta function as used in quantum mechanics, where you can't do without it.

I'm repeating myself from earlier threads, but for a standing-wave, quantum of light, the momentum is zero, and e=mc^2. But this is just an extreme case. Everything else lays in between.

I've been mulling-over this thread and others like it. There are several questions that come in this category that could be much better answered in the converse. Another, for instance, is the speed of light question, where an answer is better given to the question "When cannot the speed of light be exceded?", rather than trying to make the point of c as a speed limit. There are several categories of things that exceed c. With quantum mechanics included, can you actally claim that any given process doen't include superluminal elements?

I can't disagree with that. It just seems a little odd to me that I get neither disagreement nor agreement, when I've brought things up of this sort, and I don't know why.

Last edited: Oct 17, 2008
16. Oct 17, 2008

### ZapperZ

Staff Emeritus
Re: light with mass????

I think we need to back off a bit here with our own preferences on how to treat the word "mass" with regards to photons and re-read what the OP had asked, which actually is rather straightforward. He/she is connecting the "E=mc^2" with "mass" since photons have energy.

This issue has been sufficiently addressed in the FAQ already. So let's just start with that and let the OP read that first and see if that does not answer his/her question. Only then, if there are other confusion, or if it doesn't explain what he/she wants to know, should we expand on this issue. Otherwise, we are giving a series of possibly confusing and contradicting responses. Or worse still, like what might happen here, we go off on a tangent about how "right" or "wrong" the current concept of "mass", the way SR is being taught, etc.. etc., which does nothing to address the original question.

So hold off on your responses, and wait for the OP to come back and say something. Besides, there are already plenty of threads on the discussion on the nature of "mass". You can always continue with this issue there.

Zz.

Last edited: Oct 17, 2008
17. Oct 17, 2008

### Phrak

Re: light with mass????

Are you still there, aman.yash ?

18. Oct 17, 2008

### DrGreg

Re: Post #15

Phrak: re post #15.

ZapperZ is quite right that this is outside the scope of this thread. Is there another existing thread where this can be discussed further, or if not, would you like to create one?

19. Nov 16, 2008

### Phrak

Re: light with mass????

The OP is long gone, as was evident by post #8. The FAQ "The experimental basis of relativity" is directed at doubters, devoting two lines to the mass of a photon, and two lines or so, in a disclaimer of the original author's opinion of it.

Last edited: Nov 16, 2008