[atondwal]

Photons have zero rest mass, but light has an energy of $h\nu$, where $\nu$ is the frequency, so this gets added on to the mass of our thing.

Oversimplified explanation: Photons are massless so they can go as fast as they want without needing kinetic energy, but even massless things can store energy (think of the massless springs you studied in high school physics). Energy is the same thing as mass, but we still refer to photons as massless, in the same way that we think of an (imaginary) massless spring as such; we don't like to think of potential energy as _being the object_, just somehow _there_.

This makes more sense if you study physics, in no small part due to the fact that we make it all rigorous (though not to the satisfaction of the mathematicians, I guess...).

[DanWaterworth]

Does this mean that if you move an object away from Earth's gravitational influence it will increase in mass because it has more potential energy?

What about if you heated something up, does it gain mass then?

How about sound? Does the sound of my voice cause imperceptible ripples of increased mass as it vibrates everything that it touches?

Is a charged battery heavier than an empty one?

Is glass heavier with light travelling through it?

[simcop2387]

> Does this mean that if you move an object away from Earth's gravitational influence it will increase in mass because it has more potential energy?

Gravitational potential energy is a little different. It's the energy that the object would have if it fell to earth, it doesn't actually exist until it starts falling. (correct this, i'm sure i'm wrong)

> What about if you heated something up, does it gain mass then?

Yes, but it's incredibly miniscule to the point that it's not worth looking at in most situations

> How about sound? Does the sound of my voice cause imperceptible ripples of increased mass as it vibrates everything that it touches?

This one I think does but it's going to be so small that you also won't notice it. What you would be able to notice though are the subtle changes in density at the peaks and troughs of the wave.

> Is a charged battery heavier than an empty one?

Maybe. This depends on the chemistry of the battery more than anything. Some of them will oxidize as it gives out energy and give off the oxygen when charging. While the energy will contribute slightly to the mass because of the chemcical bonds, the battery will change mass more because of the lost/gained atoms than any other cause.

> Is glass heavier with light travelling through it?

I really don't know how to answer this one. The photon never really becomes part of the glass as far as I'm aware, but some will be absorbed and raise the temperature so maybe?

[DanWaterworth]

The wikipedia page on the subject seems to say that potential energy does increase mass [1].

[1] https://en.wikipedia.org/wiki/Mass-energy_equivalence#Meanin...

[simcop2387]

Interesting. I wonder how that reconciles with having multiple gravitational bodies then.

[ubercow13]

Yes, yes, yes, yes. Not sure about the last one as I'm not sure about the way photons travel through glass.

[TeMPOraL]

Think about the opposite - nuclear fission. The energy released when nuke goes off comes from the difference of mass between original atoms and products of the split. This difference is the energy stored in strong nuclear force, binding protons and neutrons together.

[Zergy]

I belive I kind of understand what you are saying.

I am confused as to whether or not the photons actually stop moving. Since they halt them by shutting off the transparency it seems that the photons would get trapped and end up bouncing around being absorbed slowly.

Is that why the effect is temporary? The light eventually gets absorbed and disperced though the crystal?

[icebraining]

(Irrelevant aside) Unfortunately HN doesn't have MathML, but you can use unicode: ℎν.

[TeMPOraL]

Actually it could be a good idea to have MathML or TeX on HN; we tend to use math-like expressions here quite a lot.