Light has mass?

James R,

Pardon my suspicious insinuation. I've been too long in other fora where skeptical combativeness is the common currency. I must remind myself to adopt a more contemplative style of posting appropriate to here.

As for leaping to the conclusion that you cut'n'pasted, I made a snap judgement that the level of sophistication of your post seemed atypical and I erroneously assumed

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it must have come from an outside source.

I guess I'll linger here longer since there's sophisticates afoot.

 

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i suggest some of you guys check out this site, it might answer some of your questions, it is kind of long though

http://compbio.caltech.edu/~sjs/tew1.html

 

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Hi!

Why is it so?

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I kind of know that photons have no mass before reading this thread but I am now confused!

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Dreamsa, James R,

I'm not expert on quantum field theory either, but I happen to know that the range of a force is inversily proportional to the mass of the force-carrying particle. For photons (mass = 0) this results in an infinite range, for strong nuclear force (carried by pions) this results in 10^(-10) m or something in that neighbourhood.

Bye!

Crisp

 

Photons do have relativistic mass because of e=mc2...

 

Mass is the quantity of matter that an object contains. (typical unit: grams)

Momentum is Mass times Velocity (p=mv) (unit: gram-cm/sec)When an object strikes another object it may transfer some of its momentum. Think golf club launching golf ball.

A photon (or particle of light) has no resting mass, it is not made of matter. It is just a packet of energy.

How then do photons transfer momentum to a solar sail and "push" a solar ship along?

According to Relativity, E=Mc^2. This means that matter and energy are interchangeable. The people of Hiroshima were vaporized by the conversion of a few grams of matter (U235) to MANY ergs of energy.

So... If momentum is mass x velocity AND energy is interchangeable with mass then..
a photon carry momentum!
(For some important reasons the p=mv equation can't be used at relativistic speeds. This works for photons: p=planck's constant/wavelength)

 

If this is true, and we 'know' it is to some extent, then the question becomes...

What is the mechanism by which momentum is transfered from the photon, which has no rest mass, only relativistic mass, to the solar sail?

It is somewhat easy to understand how a photon can force an electron from one orbital to another through the transfer of energy from the photon, thereby decreasing the photon's frequency, to the electron. But how does a massless particle accelerate a particle having mass, such as the atoms in the solar sail, in a particular direction?

Also, you don't have to have a solar sail to see this effect. You can use a radiometer. They are actually kind of cool.

 

James R sums it up with this quote:

You cannot stop a photon and still have it exist; stop it and it deposits its energy and vanishes.

And another quote from Mr. G:

Quantum Electrodynamics. Electrons, particles with mass, respond to the absorption of massless photons.

Therein lies your answers.

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yo!

photons dont have mass but they do have relativistic mass. yes when they dont move they don't have mass but when they move that's when they become to have mass.