# Does light have a mass?

Discussion in 'Physics & Math' started by GRO$$, Apr 6, 2002. Thread Status: Not open for further replies. 1. ### GRO$$Registered Senior Member

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:bugeye: :bugeye: :bugeye:

They say it don't, but why does my spinny thing turn in the sun?

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Gro$$I know you really, really, really want to believe that light has mass. But it doesn't. Light is comprised of photons. Photons have energy and momentum but no mass. This has been confirmed with the strictest of experimental limits. Confirmed. No mass. Since photons have momentum they can exert a pressure on a surface. This might be the pressure which turns your spinny thing. Or it might have something to do with the energy of the photon. I can't say for sure with your description: spinny thing. btw - You mentioned your frustration regarding my aggression. You might have noticed that my agression is only focused on those that are convinced and are trying to convince others of their wacky theories. You however are not doing that. You are asking good questions even though you might not trust the answers given. That's no big deal as long as you think things through and rationalize your conclusions. Good on ya ! Please Register or Log in to view the hidden image! 4. ### Google AdSenseGuest Advertisement to hide all adverts. 5. ### c'est moiall is energy and entropyRegistered Senior Member Messages: 583 mass is "frozen" energy so why do you insist on photons having mass??? :bugeye: :bugeye: 6. ### Google AdSenseGuest Advertisement to hide all adverts. 7. ### GRO$$Registered Senior Member

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(Q):
I think I see... the spinny tingy i was talking about is a Radiometer...

lucky for me the site also answers my question

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I love how you put that.

8. ### James RJust this guy, you know?Staff Member

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Your spinny thing probably works due to gas flowing around the edges of the vanes. On the other hand, if you've got a really good one, perhaps it works due to light pressure.

Either way, light has no mass.

9. ### GRO$$Registered Senior Member Messages: 304 hehe, did you put that so i would ask? lol, ill ask: whats light pressure? light has no mass, but has momentum and can exert pressure on things? 10. ### James RJust this guy, you know?Staff Member Messages: 30,862 That's right. Photons have momentum, so when they collide with things they give them a bit of a push. 11. ### John Devers(AVATAR)Registered Senior Member Messages: 120 Hi JR, do they give them a bit of a push when they are reflected as well as absorbed and emitted? Is the push the same in both cases? 12. ### GRO$$Registered Senior Member

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Yea... so if something hits you and pushes you back, dosent that mean it weighs something?

13. ### (Q)Encephaloid MartiniValued Senior Member

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Absorbed or reflected, when a photon strikes a surface it transfers its momentum to the surface it strikes. A force or a change of momentum is exerted on the surface in the form of 'radiation pressure.' If the photon is absorbed, the surface acquires the photons momentum. If reflected, the photon rebounds with the same magnitude of momentum but oppositely directed.

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Gro$$: No. 15. ### thedIT GopherRegistered Senior Member Messages: 1,105 GRO$$

Another way of looking at light pressure.

You know the famous E=mc^2. There is a another equation called the invariant conservation of energy. It is

E<sup>2</sup>=p<sup>2</sup>c<sup>2</sup>+1/2m<sup>2</sup>c<sup>4</sup>

If a photon has m=0 that equation becomes,

E=pc

In other words a photon has energy based on it's momentum only. This is light pressure.

Now a trick.

DeBroglie said that a photon has energy E = h &lambda; where h is plancks constant and &lambda; the wavelength.

From E=mc<sup>2</sup> and due to wave particle duality we can equate these

mc<sup>2</sup> = h &lambda;

A photon of wavelength &lambda; behaves like a particle of mass,

m = h &lambda; / c<sup>2</sup>

it is this 'effective' mass that also leads to light prssure.

16. ### thedIT GopherRegistered Senior Member

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dupli post deleted

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GRO$$, Of course light doesn't have mass, but neither do bowling balls. If you drop a bowling ball on your foot, it only accelerates towards your foot because the Earth is curving space/time!!!! And the pain you feel as the bowling ball hits your foot is not the result of mass either, it is because the bowling ball has quantum momentum, or as Thed indicated, "effective" mass!!! On second thought, maybe you don't have mass either. Tom Last edited by a moderator: Apr 8, 2002 18. ### c'est moiall is energy and entropyRegistered Senior Member Messages: 583 "Of course light doesn't have mass, but neither do bowling balls." isn't that ridiculous to say? it seems you are trying to say that there is not such thing as "mass" at all?? :bugeye: 19. ### Joeblow93132Guest c'est moi, I was being sarcastic. The other people on this board know what I mean. Please Register or Log in to view the hidden image! Tom 20. ### GRO$$Registered Senior Member

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Haha, sarcasm for me == confusion

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but i understand what thed pointed out... thanks

21. ### GRO$$Registered Senior Member Messages: 304 P.S. Where can i find more on the invariant conservation of energy equasion? Google only came up with 4 results Please Register or Log in to view the hidden image! is there another name for it? 22. ### Joeblow93132Guest GRO$$,

Sarcasm to the side, I'll explain what I meant in my previous post.

Photons DO have mass. Here are the proofs:

1. Atoms recoil as they emit photons, just like a gun.

2. Photons are influenced by gravitational fields(their paths bend when passing massive objects).

3. Photons exert force on objects they impact(like the plates of the radiometer you were talking about)

The reason everyone on this thread is telling you that photons do not have mass is that , because if they did, Einstein would be wrong. And since many people on this board have inflateable Einstein dolls under their beds, they could never accept the posibility that Einstein was wrong.

Under Einstein's relativity theory, a particles with a mass could never reach light speed, since at light speed, it's mass would become infinite(Einstein believed that as a particle traveled faster, it's mass would increase as well). Since a photon travels at light speed, the people on this board believe that it can't have mass.

I, on the otherhand, believe that photons DO have mass, and that Einstein was wrong. Afterall, if it looks like chicken, smells like chicken, and tastes like chicken, then it is likely chicken.

Tom

Last edited by a moderator: Apr 9, 2002
23. ### thedIT GopherRegistered Senior Member

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GRO

Try invariant kinetic energy and invariant momentum-energy. Different ways of expressing the same thing. Invariant KE is probably the most excepted term.

Also have a read of <a href="http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html">john Baez's take on this </a>

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