Does Light Have Mass? Answering Your Questions

[sclancy]

does light have mass? i thought it did because it couldn't escape a black hole. i need to know. thanks.

 

[quasar987]

If you just want to know, the answer is 'no'.

For a small proof, see

https://www.physicsforums.com/showpost.php?p=534072&postcount=5

Relativity gurus will do a better job than me at explaining why they cannot escape black holes, but the short answer is that one does not need mass to be affected by the gravitationnal field, only energy. And photons have energy proportionnal to their frequency: E=hf.

 

[pervect]

does light have mass? i thought it did because it couldn't escape a black hole. i need to know. thanks.

This is a frequently asked question, which is addressed in the sci.physics "frequently asked questions" list.

See http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html

The short answer is no.

 

[rbj]

wouldn't it be more accurate to say that the short answer is "light has no rest mass" m_0 because if it had a rest mass, the relativistic mass which is

m = \frac{m_0}{\sqrt{1 - \frac{v^2}{c^2}}}

would be infinite at the the speed of light v = c.

since a photon has energy

E = \hbar \omega

and energy has an equivalent mass

E = m c^2

does not each photon have an inertial mass that is

m = \frac{E}{c^2} = \frac{\hbar \omega}{c^2}?

at least it has momentum that is

m c = \frac{E}{c} = \frac{\hbar \omega}{c}.

is that not true?

 

[Crosson]

wouldn't it be more accurate to say that the short answer is "light has no rest mass"

And then tell a long story about what rest mass is...

To me, mass is assumed to mean rest mass. Relativistic mass is a totally unnecessary concept.

 

[The Bob]

does light have mass? i thought it did because it couldn't escape a black hole. i need to know. thanks.

Light does not have mass. Light is pure energy (if I am correct). A black hole will pull anything in due to its gravitational field. Light is not faster than light and nothing is. As light can be pulled in, anything can be.

The Bob (2004 ©)

 

[pervect]

wouldn't it be more accurate to say that the short answer is "light has no rest mass" m_0 because if it had a rest mass, the relativistic mass which is

<snip>

The problem is that that's not a short answer! People who want the whole, full, answer (and not the short answer) can read the FAQ entry I quoted (it's only a click away) which gets into everything you said and a bit more.

 

[RandallB]

\hbar

since a photon has energy

E = \hbar \omega

I didn't find \hbar in the links.
Could you referr me somewhere that discribes \hbar a bit.
How and where it's used, name, value, units, maybe some history.
Thanks
RB

 

[quasar987]

\hbar is Planck's konstant divided by 2pi. It is used in quantum mechanics. But \hbar is not the subject of this post, so if you wish to talk some more about it, please create a whole new thread. Thx.

 

[rbj]

The problem is that that's not a short answer! People who want the whole, full, answer (and not the short answer) can read the FAQ entry I quoted (it's only a click away) which gets into everything you said and a bit more.

fine, i guess. paraphrasing Einstein: "An explanation should be as simple as possible, but no simpler." i think to simply say "light has no mass" implies that "light particles, photons, have no mass" which sort of implies (since momentum goes to zero if mass or velocity does) that "photons have no momentum" which simply is false.

short or whole or full, the answer should be sufficiently accurate to not lead someone to erroneous conclusions of substance that do matter. it is not a difference of semantics.

perhaps a better explanation for the OP is that there is no such thing as graviational force considering general relativity. the reason that light (or anything else) does not escape the black hole is due to the severe curvature of space-time due to the concentration of mass (or energy, it's about the same) in the black hole. similarly to the thought experiment of Einstein regarding the Equivalence Principle, the one with the accelerated frame of reference in space (at 9.8 m/s^2) and the non-accelerated frame on the surface of the Earth, in both cases, a beam of light will be bent, from the POV of the observers in both frames, to the same degree. one is due to the acceleration of the frame of reference, the other is due to gravity.

 

[rbj]

\hbar is Planck's konstant divided by 2pi. It is used in quantum mechanics.

also used in the photoelectric effect. historically, it is first used in the photoelectric effect to relate the energy of these particles of light to the frequency of radiation.

But \hbar is not the subject of this post, so if you wish to talk some more about it, please create a whole new thread. Thx.

the (relativistic) mass of a photon is m = \frac{\hbar \omega}{c^2} where \hbar is the reduced Planck's constant, \omega is the angular frequency of the equivalent wave of the photon of light, and c is the speed of light.

it is germane to the subject of this thread.

 

[dextercioby]

Planck used "h" when claiming that the energy of the em field vibrating mode was proportional to the frequency of the radiation.The character \hbar was introduced by Dirac (some people call it Dirac's constant) around 1926.

Daniel.