# B Mass of light

1. Mar 25, 2016

If light is made of particles (particle theory of light), why doesn't it have any mass at all ?

2. Mar 25, 2016

### phinds

Light is NOT made of what you probably mean when you say "particles". Light is a quantum object that has particle characteristics (but QM particles, not classical particles) if you measure for them and wave characteristics if you measure for them but it is NOT a "particle" OR a "wave", it is a quantum object.

That is, you are trying to apply a classical concept to quantum mechanics and it doesn't work that way.

3. Mar 25, 2016

### PeroK

Who says particles must have mass?

4. Mar 25, 2016

### DrStupid

And light can have mass.

5. Mar 25, 2016

### phinds

Oh? You want to expand on that?

6. Mar 25, 2016

Light isn't made up of photons ?

7. Mar 25, 2016

How can something be without mass ?

8. Mar 25, 2016

And tell me how ?

9. Mar 25, 2016

### phinds

No, I did not say that. Light IS made of photons but photons are not "particles" as you probably think of them. Please re-read post #2.

10. Mar 25, 2016

### Staff: Mentor

Please somebody post a link to the FAQ about rest mass and photons (I'm being lazy)...

11. Mar 25, 2016

### ZapperZ

Staff Emeritus
12. Mar 25, 2016

### phinds

+1 on that !

13. Mar 26, 2016

### Orodruin

Staff Emeritus
Mass is a property, there is absolutely no reason to assume this property must be non zero for all objects. Why would you think it must have mass?

14. Mar 26, 2016

I don't know.I think I always find relation between microscopic and macroscopic world.Or maybe because I haven't seen things without mass in my surroundings.

15. Mar 26, 2016

### phinds

You've probably never seen an electron or a quark either. Do you think they don't exist? The very limited range in which humans evolved makes us TERRIBLE at having any "common sense" regarding quantum mechanics (the very small) and cosmology (the very large).

16. Mar 26, 2016

### PeroK

In any case, would it really have been so crazy if, say, it had turned out that the electron was massless? And all the mass in matter came from protons and neutrons? That's not the case, of course, but I can't see any way to look at the macro world and conclude that all elementary particles must have mass.

17. Mar 26, 2016

I haven't really thought much about whether light had mass or not.So when I read about it in my textbook,It surprised me.I thought maybe light had very negligble amount of mass as it is made of photons.But It was my fault.I considered photons to be "particles". That was wrong.
Thank you for clearing my stupid doubt

18. Mar 26, 2016

No.The very reason why we have common sense makes us terrible in quantum mechanics.And our common sense is very much dominated by what we see around us.THAT is the problem.These electrons or quarks are just those things that mathematically fits our observations and we agree with it as we trust in mathematics.These atoms are all just the product of human's imagination that correctly fits the logical notion of describing things.But we still cannot really be sure whether these quarks exist or not.It just fits the logical and mathematical theory and we build more on it.

19. Mar 26, 2016

I am sorry.I shouldn't cause people this much pain.I'll search for things in insight section next time.But the thing is I didn't know that insight section existed

20. Mar 26, 2016

### DrStupid

For a simple example lets take two plane waves, each with the energy E/2 which travel with an angle of $\alpha$ relative to each other. That means for the individual momentums

$p_1 \cdot p_2 = \cos \left( \alpha \right) \cdot \left| {p_1 } \right| \cdot \left| {p_2 } \right|$

$\left| {p_1 } \right| = \left| {p_2 } \right| = \frac{E}{{2 \cdot c}}$

With the total momentum

$p^2 = \left( {p_1 + p_2 } \right)^2 = p_1^2 + 2 \cdot p_1 \cdot p_2 + p_2^2 = \left[ {1 + \cos \left( \alpha \right)} \right] \cdot \frac{{E^2 }}{{2 \cdot c^2 }}$

the mass of the resulting wave is

$m = \sqrt {\frac{{E^2 }}{{c^4 }} - \frac{{p^2 }}{{c^2 }}} = \frac{E}{{c^2 }} \cdot \sin \left( {\frac{\alpha }{2}} \right)$