# One question about h*f=m*c*c

1. Sep 13, 2009

### Zong xiaobo

f is the frequency of a particle.
In the theory of relativity, energy equals m*c*c.
In the theory of Photoelectric effect, energy equals h*f
If the particle keeps still.
m is the rest mass of the particle.
In this case the frequency is 0 and h*f is 0
However m*c*c isn't 0.
It's a little strange.

2. Sep 13, 2009

### kof9595995

I think E=hf is just for particles with 0 rest mass such as photons. So m*c*c is 0.

3. Sep 13, 2009

### diazona

Well... actually, you can say $E = hf$ for massive particles too (although the formula is usually used for massless particles, true) The reason this works is that the formula for energy is actually
$$E^2 = p^2c^2 + m^2c^4$$
If you set $p=0$, then you get $E = mc^2$ - but only then. The popular version of Einstein's formula applies only to particles at rest.

In this case $E = hf$ is used to define the frequency of a particle.

I refer you to Wikipedia: http://en.wikipedia.org/wiki/Matter_wave for more information.

4. Sep 13, 2009

### Halcyon-on

Well, the mass can be alternatively defined as the rest frequency of a wave. $$f_0 = m c^2 / h$$. De broglie postulated that every massive particle has an internal clock whose frequency is fixed by the mass, and actually this intrinsic periodicity of the particles has been observed in a recent experiment [ http://www.ensmp.fr/aflb/AFLB-301/aflb301m416.pdf [Broken] ]

Last edited by a moderator: May 4, 2017
5. Sep 13, 2009

### map19

From the full formula given by diazona you can see that when mc^2 is zero but the particle still has energy it must all be in E = pc.
Thus the particles with no rest mass have momentum, frequency, and energy. having no rest mass they must travel at c.

6. Sep 14, 2009

### Zong xiaobo

I think when the mc^2 is zero, p must be zero. So the E=pc must be zero.

7. Sep 14, 2009

### Zong xiaobo

It seemed have answered my question. I will read the paper first.

Last edited by a moderator: May 4, 2017
8. Sep 14, 2009

### Halcyon-on

9. Sep 15, 2009

### Zong xiaobo

10. Sep 16, 2009

### map19

Zong Xiaobo -I think when the mc^2 is zero, p must be zero. So the E=pc must be zero.

No ! A photon transports energy, it has momentum p = hf/c
You know that light has momentum everytime you sit on the beach.
E = hfc/c = hf.

11. Sep 16, 2009

### Zong xiaobo

I think when a photon travel at a speed of c. The invariant mass of a photon isn't zero.
So mc^2 isn't zero.

12. Sep 16, 2009

### map19

That statement is not only not logical - if the invariant mass is able to vary, then it's not invariant - but it's just wrong in physics.

13. Sep 17, 2009

### Zong xiaobo

14. Sep 17, 2009

### Zong xiaobo

It seemed that I misunderstand what you said.
when you mentioned mc^2, m is the rest mass.
I always consider m as the relativistic mass.

15. Sep 17, 2009

### ZapperZ

Staff Emeritus
You might want to read an entry in the FAQ thread in the General Physics forum.

Zz.

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