h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Neutrinos I believe. It is an interesting point - if a particle with well-defined mass, such as an atom, displays wave-like behaviour by passing through two slits at the same time, would you be able to determine its mass? By, maybe, measuring the force applied to a target by the impact of the wave, say? Or would measurement of mass cause it to resolve back to particle mode, in which case does it mean anything to consider the mass of a wave?

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Well now, measurements by quantum physics? (a simple Yes/No will satisfy ME - as long as you tell the truth!)

I'm no physicist so a link will have to do

http://www.physlink.com/Education/AskExperts/ae694.cfm

Ahah Orcus, That's given a glimmer. All I have to do now is - oh.

> if a particle with well-defined mass, such as an atom, displays wave-like behaviour by passing through two slits at the same time, would you be able to determine its mass?

Its momentum, p. The de Broglie wavelength associated with any particle is

lambda = h / p

where h is Planck's Constant.

"Didn't I read recently that they had now measured a (tiny) mass for photons or was it neutrinos?"


Neutrinos.

Cheers, Martin

No, a photon does not have a mass... but yes, it can have a momentum. The problem with quantum mechanics is that you need to abandon your classical ideas of the Universe which is VERY difficult to do (thus, why quantum is so difficult to grasp). Neutrino's, however, do have a miniscule, yet non-zero mass.

Anyhow, it is possible to have a momentum without a mass because, relativistically, mass increases with velocity.

So...

The enrgy of a photon is E=hc/L where h is plank's constant, c is the speed of light (3e8 m/s) and L is the wavelength. So, E=h(n) where n is frequency (n=c/L)

E=(m-m0)c^2
hn = [m - m0] c^2
For photon the rest mass m0 = 0 .
hn = mc^2 or m = hn/c^2 .

the momenutm is mc = hn/c.

Thus, relativistically, a photon has a mass of hn/c.^2 and momentum of hn/c.