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Q & A: Is sunlight matter?

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Q:
Is sunlight matter?
- Anonymous
A:
We try not to get hung up on questions like whether or not to call light 'matter' or not. Light has energy and momentum, it exerts a gravitational pull on other things, etc. so it has a lot of the properties of the more ordinary stuff that gets called matter. On the other hand, you can view light as consisting of particles called photons, and a photon has no rest mass. That's the mass a particle has as viewed by somebody who says the particle isn't moving. Light is moving with respect to everybody. So in that way it differs from ordinary matter.


Other particles- mostly protons, electrons, and maybe a small amount of heavier stuff - constantly boil off of the sun and fly into space at large speeds. This is called the "solar wind" and is responsible for aurora as it falls to earth. This stuff is most definitely matter, but alas, it is not sunlight.

Tom and Mike

(republished on 07/29/06)

Follow-Up #1: Is light matter?

Q:
What is about sunlight ,is it matter or not ,why is E=mc2 related to it when it is not practically proven
- Ilyas zargar (age 17)
Srinagar, jammu and kashmir,india
A:
"Matter" is just a vague word, used differently by different people. Without further clarification, the question of whether sunlight is "matter" has no meaning. Therefore we can't answer it. What is it you actually want to know about sunlight?

What do you mean by "E=mc2... is not practically proven"? It's about as "proven" as any fact we know about the universe. That includes many, many experimental demonstrations

"Our disputes are about the sensible world, and not one of paper."

(Galileo, Dialog Concerning the Two Great Systems of the World)

Mike W.


(published on 06/23/11)

Follow-Up #2: gravity from energy

Q:
If light exerts a gravitational pull, does that mean that energy do the same? If a system acquires more energy (heat, chemical, kinetic, potential, etc.) does that mean that the same system exerts a greater gravitational pull? That would explain why fermions acquire more mass instead of more velocity at relativistic velocities.
- Anonymous
A:
The answer to your question is yes. In most cases the effect is extremely small.
I don't see how it "explains" the increase in inertial mass with increased velocity, although it certainly fits in the same picture.

Mike W.

(published on 06/24/11)

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