How to measure the mass of photon?

[Klaynos]

Yes it may have momentum, but trying to put that into the equation:

 

[math]p = \gamma mv[/math]

 

Fails, as the photons velocity is c, and has no rest mass. In special relativity mass is a lorentz invariant.

 

The particle in a box situation is due to the quantisation of the energy levels, I belive.

[Rocket Man]

i see what your getting at; a photons momentum would come out as:

p=(1/0).0.C

which as you said, fails.

 

my point is, an object heated with a set energy, emits a photon and cools.

the object had extra mass because of the heat energy, so that mass has to go somewhere.

logic would say the mass/energy is now contained in a photon.

I reckon photons follow a different set of equations to describe mass-energy compared to mass itself at speed.

a photon is just energy that, oddly enough, travels at the speed of light.

the energy of a photon is not determined by it's velocity alone, a gamma ray slowed by the same amount as an IR photon will always have a higher energy because of its frequency. since a photon consists of energy, it must have a "sort-of" mass but i highly doubt it follows the same inertial dialation as matter.

[Klaynos]

Mass is NOT conserved.

 

Mass-energy is conserved.

[Rocket Man]

if mass is energy and energy is conserved, mass must therefore be conserved too.

wikipedia has a lot on this topic:

http://en.wikipedia.org/wiki/Mass-energy_equivalence

light is mentioned among the energies which can be said to have mass

so when measuring the mass of a photon, what you're really doing is measuring the energy contained in the photon and measuruing the energy as though it were converted to mass.

supposing you have a kilo of matter in a closed system, convert it to energy, you now have 1 kilo of energy because the system cannot lose energy and mass is energy.

 

a photon does not go with inertial dialition, that's why they dont have infinite momentum, but since they have momentum, they must also have mass which is related only with their speed and frequency, their rest mass is 0 because slowing it down to 0 speed is, in essence, doppler shifting it untill it has 0 energy.

when you slow a photon, the mass in the form of energy transfers to whatever slowed it down so the object that slowed it down now has the photons mass in the form of heat because of the equivalence between mass and energy.

conservation of energy demands that the energy in a photon is finite because it was created from a finite amount of energy.

this finite amount of energy has a non 0 value of mass.

[JaKiri]

if mass is energy and energy is conserved, mass must therefore be conserved too.

 

Energy isn't conserved unless you take mass into account. Mass isn't conserved unless you take energy into account.

 

Mass is not conserved in and of itself.

 

Lets say you have one particle, mass m, moving at another particle of mass m, at 3/5 c. Lets say they form one particle of mass M when they collide. This will, if you go through the maths, be moving at 1/3 c. M will be greater than 2m.

 

Mass is not conserved.

 

wikipedia has a lot on this topic:

http://en.wikipedia.org/wiki/Mass-energy_equivalence

 

Wikipedia has forgotten the gamma factor. Or rather' date=' you've only chosen to read the bits of the article talking about rest mass, and ignored the links to the section on mass dilation.

 

light is mentioned among the energies which can be said to have mass

so when measuring the mass of a photon, what you're really doing is measuring the energy contained in the photon and measuruing the energy as though it were converted to mass.

 

What? No. That's not how you measure the mass of anything.

 

supposing you have a kilo of matter in a closed system, convert it to energy, you now have 1 kilo of energy because the system cannot lose energy and mass is energy.

 

You have the equivilent of a kilogram were it made into energy, but you do not have a kilogram of anything actually in the closed system. It doesn't actually have that mass.

 

a photon does not go with inertial dialition, that's why they dont have infinite momentum, but since they have momentum, they must also have mass which is related only with their speed and frequency, their rest mass is 0 because slowing it down to 0 speed is, in essence, doppler shifting it untill it has 0 energy. when you slow a photon, the mass in the form of energy transfers to whatever slowed it down so the object that slowed it down now has the photons mass in the form of heat because of the equivalence between mass and energy.

 

Inertial mass, and you can't actually slow a photon down per se. They're always travelling at the speed of light on an atomic scale, they just hit things along the way. It's like the difference between my average velocity when walking into town and my average speed.

[Rocket Man]

yes, i see the problem in my argument now, there was a post back there which said that you can have momentum without mass, i must have gotten confused with the momentum side of things.

i'll try to be less stubborn in the future.