Mass of a photon

[5614]

well i know the photon has zero rest mass... but i was wondering about the other mass's, like relativistic mass and inertial mass etc.

 

i mean, i know for a photon you use:

e2 = p2c2 + m2p4

 

and not:

e = mc2

 

( ^ where all those numbers are the previous letter to the power of the number ^ )

 

but can someone just like do a 3 line post like:

 

photon mass:

rest mass: 0 g

relativistic mass: x g

inertial mass: y g

 

where g = grams, i assume that's what its measure in, or you can use micro-grams, or nano-grams or whatever is needed, i mean, there's no point in saying 0.00000000x KG you may as well put it in nano-grams.

 

and maybe you could include after that how you go each one... i mean, i can rearrange an equation to get m = whatever it does, but i dont know the energy of a photon etc, obviously if it includes frequency you can say frequency, and then maybe the frequency of some of the colours in the visible light spectrum, i cant remember them off by heart!

 

thanks

[Martin]

5614,

are you happy to be told energies in ELECTRON-VOLTS?

this is the most convenient unit for talking about photon energies, for most people.

 

I think the idea of "mass" can be very confusing when applied to photons so I will let other people talk about the mass of photons, and I will just talk about the energies of different photons.

 

if you have a box lined with ideal mirrors and some photons bouncing around in this box then the fact that there is some light inside the box will give the box a little bit extra inertia (make it harder to accelerate from rest) and give the box a bit extra gravitational mass. But I dont want to talk about that. Let us just get straight about the energies.

 

 

Photons have all different energies. A green photon can be 2.5 eV

and a red photon can be 2.0 eV

and a blue photon can be 3.0 eV

 

I think that is right. that is how I remember. maybe Severian will say different numbers but what i say is at least approximately right.

 

of course invisible heat radiation infrared photons have LESS energy, and microwave photons have even less, and Xray photons have much more.

Xray photons are like 1000 eV

it depends simply on the voltage of the Xray tube. if it is very high voltage like 10,000 volts then you can get very high energy photons like 10,000 eV.

 

so you can have photons all different energies

 

(and the energy of the photon influences how much inertia it adds to the box, but still it can be bad to talk about the mass of photons, it can lead to confusion because the word mass does not have a stable meaning even among physicists)

 

well i know the photon has zero rest mass... but i was wondering about the other mass's' date=' like relativistic mass and inertial mass etc.

 

i mean, i know for a photon you use:

e2 = p2c2 + m2p4

 

and not:

e = mc2

 

[i']( ^ where all those numbers are the previous letter to the power of the number ^ )[/i]

 

but can someone just like do a 3 line post like:

 

photon mass:

rest mass: 0 g

relativistic mass: x g

inertial mass: y g

 

where g = grams, i assume that's what its measure in, or you can use micro-grams, or nano-grams or whatever is needed, i mean, there's no point in saying 0.00000000x KG you may as well put it in nano-grams.

 

and maybe you could include after that how you go each one... i mean, i can rearrange an equation to get m = whatever it does, but i dont know the energy of a photon etc, obviously if it includes frequency you can say frequency, and then maybe the frequency of some of the colours in the visible light spectrum, i cant remember them off by heart!

 

thanks

[ed84c]

electron volts= E of one e at 1V

[goodyhi11]

try to observe the electron with a photo whose position can be well known. Then the photon's associated wave has high frequency and short wavelength and therefore (since E=hf), the photon has high energy.

[5614]

[edit]sorry for the similar post, goodyhi11, it wasnt there when i loaded the page, obviously i spen more than 2 minutes typing this and reading martin's post etc.

 

yes i am familiar with electron-volts and although i cant say i knew all of martin's post i did know the basis of it.

 

the thing is that when it comes to energy of a photon its not as complicated and confusing as when you ask about the mass of a photon! so martin's post is great as a general post in this thread, but, this sounds rude but i cant think how to say it, can we not get side-tracked and get back onto photon's mass please, like, it's all useful and all interesting but not what i asked, if you see what i mean!

 

just to add to martin! the energy is proportional to the frequency, shown in the equation e=hf.... you probably know that, just though't i'd add it!

 

now, back to photon mass!

[ed84c]

well if you observe the electron then you know the electrons direction (i.e. towards your eye) so prosumably (as per heisenbergs UP) you cant know the position?

[Martin]

... energy of a photon its not as complicated and confusing as when you ask about the mass of a photon! so martin's post is great as a general post in this thread' date=' but, [u']this sounds rude but i cant think how to say it, can we not get side-tracked and get back onto photon's mass please[/u], like, it's all useful and all interesting but not what i asked, if you see what i mean!

 

OK the next thing is to learn the mass equivalent of one eevee!

 

A photon has no mass (as many perhaps most working physicists now use the word)

 

but it can contribute mass to a box that it is in

 

in the NIST constants website you can find the kilogram equivalent mass of one eevee

 

I will go get it

 

1.782662 E-36 kilograms

 

then to find the massequivalent of a green photon of 2.5 eevee you just multiply by 2.5

[Martin]

5614, you are all right now I think?

 

if you have some green photons, each of 2.5 eV, in a box

then they each contribute some mass, which is

2.5 times 1.782662E-36 kilograms

 

so the box has some extra mass which is the number of photons times the individual energy times 1.782662E-36 kilograms.

 

that is how much extra (kilogram or gram or whatever unit of) mass the box has

 

but each photon DOES NOT HAVE A MASS because the mass of something which cannot be accelerated and cannot sit still IS NOT DEFINED.

 

the box has a mass because it can sit at rest, and it can be accelerated from rest, and so its mass is defined. part of the mass comes from the energy inside it

 

but the photon has no mass because it is not defined.

 

like it or not, this is how many (perhaps most) working physicists have come to use the word mass (synonymous with a body's inertia at rest)

 

the term "relativistic mass" appears less and less often (although it may still get used in popularizations for wide audience and in the media etc.)

 

the proton has one and only one mass (what used to be called its "rest" mass)

the photon has no mass (although its energy can and does have a mass-equivalent)

[5614]

good good to all you said except:

 

the photon has no mass (although its energy can and does have a mass-equivalent)

 

and what is this mass equivalent?

[Klaynos]

good good to all you said except:

 

 

 

and what is this mass equivalent?

 

E=hf=mc^2 <--- the mass in that is NOT REST MASS

 

E=mc^2 is not the full equation, it is actually the relativistic mass, so you can work out the amount of mass theoretically associated with the energy of a photon using that I belive.

 

So a rough bit or rearangement...

 

E=hf=hc/w=mc^2 <-- note w = wavelength

m=h/(wc)

 

(E=(mc^2)/squart(1-(v^2/c^2)) <-- total energy of a particle using the rest mass, the more correct form of the equation)

 

Everything I've said here might be completely wrong.

 

E^2 = p^2c^2 for a photon, but p = mv so

 

E=mvc, m=E/(c^2)=hf/(c^2)=hc/(c^2w)=h/(cw)

 

OK well I've just got the same equation twice in a row doing it slightly differnt ways, it's at this point I give up and go to the pub saying something like "I might be right but who really knows..."

[JaKiri]

electron volts= E of one e at 1V

 

I think you mean 'energy given to a charge of 1e by a potential of 1 volt'.

[ed84c]

sorry, my bad english again