# Hf=mc2 what is this equation

1. Nov 28, 2005

Hi all you physicists out there.

I am just a physics enthusiast and have a question. I have seen this equation (hf = mc2) bandied about at a lot of places. Recently I read Mark Perakh's following passage in an essay where he discredits this equation (the essay is a criticism on Gerald Schreoder). Here's the passage

What do you guys say? Is Perakh right or is hf=mc2 is a valid physical construct?

Cheers.

2. Nov 28, 2005

### pervect

Staff Emeritus
Take a look at

http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

The big question is whether Schroder is guilty of simply poor word choice or whether he is about to make some further unfounded leap. One cannot tell from the material presented which is the case, though it is very common for a semantic confusion over the issue of "rest mass vs invariant mass' to lead to further, more serious, errors.

3. Nov 28, 2005

hf=mc2

Thanks for the reply. Ignoring for a second what Schroeder is going to do with the construct, I just wanted to know if Perakh is right that hf=mc2 cannot be derived from E=hf and E=mc2.

Thanks

4. Nov 28, 2005

### rbj

i take it that you're saying that Perakh is saying that Schroeder is saying that

$$E = m c^2$$

and

$$E = h \nu$$

and equates them to say

$$h \nu = m c^2$$ .

the issue simply is what does Schroder mean by $m$ and by $E$? if he means "rest mass" and "rest energy" it's wrong. if he means total relativistic mass and energy, he right. if fact, in my 30 year old physics book, this very equating of the two E's is what is done to derive the deBroglie wave velocity and the wave equation and eventually make a stab at Schrodinger's Eq.

r