Ask a physicist anything. (7)

[mzungu]

What is the weight of a photon that travels at C if at rest it weighs zero?

Weight? This is a very vague term. What is your weight? Before you answer consider this:

You are in space or on the moon, or on a neutron star, or on earth. See what I mean?

 

[Tinker Grey]

Quarks don't exist except in specific bound states (see color confinement), so it's a question of the life expectancy of the bound state, not of the quark itself. The second longest-lived bound state of quarks is the neutron, which has a lifetime of about 15 minutes (longer if it is in a stable nucleus). The longest is the proton, which has a lifetime so long that we don't know just how long it is. Basically, the fact that protons were produced in the early universe in excess over anti-protons forces protons to be unstable, but that lifetime has to be at least 10^34 years by current experimental evidence (it may be much longer, we don't know yet).

Thanks.

Is the proton the longest lived particle? Would it decay into something else or simply cease to exist?

 

[Chalnoth]

Thanks.

Is the proton the longest lived particle? Would it decay into something else or simply cease to exist?

It's probably the longest-lived unstable particle. The electron, neutrino, photon, and some others are probably perfectly stable.

The proton is expected to decay into other particles, by the way. The decay may be into some set of pions and other particles, but is likely to eventually lead to a positron, one or more neutrinos, and some photons.

 

[cupid dave]

Weight? This is a very vague term. What is your weight? Before you answer consider this:

You are in space or on the moon, or on a neutron star, or on earth. See what I mean?

True, the weight can vary depending how close a mass may be to other mass.
However, all mass has some weight since virtual gravitons are constantly travelling away from it.

With this point in mind, the more traditional question would be to ask if photons had mass.

 

[mzungu]

True, the weight can vary depending how close a mass may be to other mass.
However, all mass has some weight since virtual gravitons are constantly travelling away from it.

With this point in mind, the more traditional question would be to ask if photons had mass.

 

[Tinker Grey]

It's probably the longest-lived unstable particle. The electron, neutrino, photon, and some others are probably perfectly stable.

The proton is expected to decay into other particles, by the way. The decay may be into some set of pions and other particles, but is likely to eventually lead to a positron, one or more neutrinos, and some photons.

Perfectly stable? So it would be safe to say that electons have existed unchanged as far back as we can speculate and will not change?

When did electrons come into existence?

 

[Chalnoth]

Perfectly stable? So it would be safe to say that electons have existed unchanged as far back as we can speculate and will not change?

When did electrons come into existence?

Well, no. It's just that they were likely produced as a byproduct of the production of protons. When the protons decay, the number of electrons and positrons will again be in balance.

 

[Wiccan_Child]

Mass, not weight. And the mass of a photon is always zero. This is why they travel at c.

Do you think it would be massively disruptive for relativity if photons turned out to have a phenomenally tiny, but still non-zero, mass?

 

[mzungu]

Do you think it would be massively disruptive for relativity if photons turned out to have a phenomenally tiny, but still non-zero, mass?

If photons have no mass then how does light have a push in a vacuum

 

[Chalnoth]

Do you think it would be massively disruptive for relativity if photons turned out to have a phenomenally tiny, but still non-zero, mass?

Well, certainly not for relativity. But it might have some implications for cosmology, where we observe photons that have been traveling for as much as 13.7 billion years, such that if photons have a small but non-zero mass it might impact some of our observations of the most distant universe.

 

[Chalnoth]

If photons have no mass then how does light have a push in a vacuum

Photons have no mass, but they do carry momentum. The momentum carried by light can be directly calculated from classical electricity and magnetism.

 

[mzungu]

Photons have no mass, but they do carry momentum. The momentum carried by light can be directly calculated from classical electricity and magnetism.

momentum without mass How can massless particles be affected by gravity?

 

[Wiccan_Child]

momentum without mass How can massless particles be affected by gravity?

Because massive particles warp space around them, and light, following straight lines, 'falls' down the dips and curves caused by these heavy particles.

 

[Chalnoth]

momentum without mass How can massless particles be affected by gravity?

Mass is only the "charge" for the gravitational force in Newtonian gravity. In General Relativity, it is the stress-energy tensor that acts as the source of gravitational fields. The stress-energy tensor includes terms that correspond to energy density, momentum density, pressure, and twisting shears.

Now, with normal matter, the only appreciable term in the stress-energy tensor is the energy density term, and that is dominated by the rest mass of the particles in question. So this is why approximating gravity as only responding to mass is a pretty good approximation.

This isn't true, for example, of neutron stars, which have monstrous pressures so that the pressures start to become noticeable when compared to the energy density due to the rest mass of the neutron star.

But light isn't like this at all. Having no mass whatsoever, a photon has a momentum equal to its energy. And so it still manages to create its own gravitational field. Now, typically the energy and momentum of even quite a lot of light are completely inconsequential compared to the rest-mass energy of normal matter, so we don't usually notice the gravitational effect of light (thought it was important in the very early universe).

As for how light waves respond to gravitational fields, well, they respond as all other matter responds: by following the curvature of space-time.

 

[mzungu]

Mass is only the "charge" for the gravitational force in Newtonian gravity. In General Relativity, it is the stress-energy tensor that acts as the source of gravitational fields. The stress-energy tensor includes terms that correspond to energy density, momentum density, pressure, and twisting shears.

Now, with normal matter, the only appreciable term in the stress-energy tensor is the energy density term, and that is dominated by the rest mass of the particles in question. So this is why approximating gravity as only responding to mass is a pretty good approximation.

This isn't true, for example, of neutron stars, which have monstrous pressures so that the pressures start to become noticeable when compared to the energy density due to the rest mass of the neutron star.

But light isn't like this at all. Having no mass whatsoever, a photon has a momentum equal to its energy. And so it still manages to create its own gravitational field. Now, typically the energy and momentum of even quite a lot of light are completely inconsequential compared to the rest-mass energy of normal matter, so we don't usually notice the gravitational effect of light (thought it was important in the very early universe).

As for how light waves respond to gravitational fields, well, they respond as all other matter responds: by following the curvature of space-time.

Does the frequency of light respond differently to a given gravitational field? Also absorbed photons release energy or are converted to energy through which process since they are massless they cannot have friction? Why do black surfaces absorb more light than white surfaces?

I remember the slit experiment in school and wondered how photons can be particles yet have no friction nor can be split?

 

[Chalnoth]

Does the frequency of light respond differently to a given gravitational field?

No, not at all. All light responds the same. They follow what are known as "null geodesics", which basically means that in natural units, light takes a path where it crosses as much space as time. That is to say:

dx = c dt

Also absorbed photons release energy or are converted to energy through which process since they are massless they cannot have friction? Why do black surfaces absorb more light than white surfaces?

Well, light sorta does experience friction, and can definitely cause it. Basically, light can experience a sort of friction when it interacts with conductive matter. Sea water, for example, conducts electricity, so that when light waves enter sea water, the oscillating electromagnetic field sets up currents in the water. This causes the light to dump its energy over time.

For metals, light loses its energy almost instantly. For sea water, it takes a number of meters before all of the energy is lost.

In the other direction, light can cause friction for massive, charged particles. If you have a big, charged particle (like a proton) traveling through a gas of photons, those photons will periodically bounce off of the proton, slowing it down relative to the gas.

I remember the slit experiment in school and wondered how photons can be particles yet have no friction nor can be split?

I don't quite know what you mean here.

 

[Mr. Pedantic]

How does having more mass slow the expansion of the universe?

 

[Wiccan_Child]

How does having more mass slow the expansion of the universe?

More mass means more gravity is 'pulling' on spacetime, counteracting its expansion.

 

[mzungu]

OH MY GOOD LORD!!!!!!! I dedicate this video to all our CREATIONIST friends (Miss Vermont is the only one voting for the Democrats the rest are Republicans)"math is a theory and it is not what the Bible tells us" :

Miss USA 2011 — Should Math Be Taught In Schools? - YouTube

 

[mzungu]

Does America have any education in her schools any more?

Miss USA - Should Gravity be Taught in Schools? - YouTube