Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I have been reading Hawking's 'A Brief History of Time' and it has gotten me thinking about Einstein's theory of relativity, in that it assumes that an object must have infinite mass if it is to be traveling at the speed of light (please correct if I'm wrong in my beginner's knowledge of physics).

But, do light waves have any sort of measurable mass? Or in that same vein, do sound waves? Is it somehow possible?

share|improve this question
If photons have mass the mass must be madse up of atoms. Thus I would like to see the atomic number of a photon. –  user10046 Jun 22 '12 at 1:37
@JamesFreeman Simply wrong. The constituents of atoms (protons, neutrons and electrons) have mass and they are the bits that atoms are made of. Muons have mass and are not made of atoms nor are they a part of an atom. –  dmckee Jun 22 '12 at 1:58
In the exact context you are asking the answer is no I have been waiting for a question like this. Light has relativistic mass. In my severely limited opinion the only way to make this mass particle-like in the sense you are asking would be to slow down the speed of the wave without absorbing any of the momentum. Unfortunately ability to measure this mass can not be realized untill you convert the energy.(we have not even begun to understand that process or have any physics that support it) –  Argus Jun 22 '12 at 2:41
+1 like the question it follows a certain logic. Old school relativity at it's finest –  Argus Jun 22 '12 at 2:51
Comments to the question (v2): The question formulation fails to clearly distinguish between various notions of mass, such as, e.g., rest mass and relativistic mass. For more information, see also this Phys.SE post. –  Qmechanic Jul 13 '13 at 18:49
add comment

2 Answers

up vote 8 down vote accepted

The particles of light waves - the photons - have the rest mass $m_0$ equal to zero. However, at the speed of light, $v=c$, the total mass $$ m= \frac{m_0}{\sqrt{1-v^2/c^2}} $$ is increased to an indeterminate form, $0/0$, which should be evaluated as a finite number. The photons - and everything else - carry the total mass that is proportional to the total energy via the famous $E=mc^2$ relation.

Yes, this mass may be measured. For example, uranium nuclear power plants burn the uranium and reduce its mass by 0.1 percent or so because the waste products (the nuclei) are actually a little bit lighter. This energy may be completely transformed to the radiation coming from light bulbs - and the light from these light bulbs carry 0.1 percent of the uranium mass away. This mass is a source of gravitational field and adds inertia to boxes with this light etc.

Sound is different. The speed of sound is much smaller than the speed of light.

While "phonons" in low-temperature condensed matter physics - particles of sound - are analogous to photons in many respects, and $E=mc^2$ still applies, the same is not true for sound waves in the air etc. Because the temperature of the air is nonzero, the "ground state" - the lowest-energy state at fixed conditions, with the minimum number of "sound quanta" or "phonons" - is not really unique. Instead, there are many states of the air "without any sound" which correspond to chaotic configurations of the air molecules. So one can't consistently divide the energy of the air to the energy of its ground state and the energy of the phonons.

But of course, if you produce some loud sounds, they will carry lots of energy in the air and the mass of the air will inevitably increase by $m=E/c^2$ which is, well, not too high because $c^2$ is a large number.

share|improve this answer
Lubos's answer is very good and very complete. Just to make some of his numbers concrete towards the end there: The energy density in a sound wave from a jet engine is $E=\frac{\textrm{intensity}}{\textrm{speed of sound}}=(100W/m^2)/(340m/s)=0.29 J/m^3$. That corresponds to an increase in density of the air of $3.22\times10^{-18} kg/m^3$ within the sound wave from the energy of the noise. Pretty tiny! –  kharybdis Feb 4 '11 at 17:16
I'd add just one thing to this very good answer, which is that the word "mass" in physics nowadays generally refers to the rest mass. In older texts, one often saw references to the "total mass" Lubos describes; the most common term for it was "relativistic mass." But we don't usually use that terminology anymore, so we don't generally say that an object's mass increases with its speed. (Just to be clear, the point I'm making is purely a vocabulary point -- Lubos has the physics exactly right.) –  Ted Bunn Feb 4 '11 at 18:19
@espais In fact any particle travelling at the speed of light has to have a rest mass of zero. –  Gordon Feb 4 '11 at 22:32
add comment

an object must have infinite mass if it is to be traveling at the speed of light

No, that's not true at all. An object's mass is a fixed property that doesn't change, regardless of what speed it travels at. But its energy does change. The energy increases with increasing speed, according to the formula

$$E = \frac{mc^2}{\sqrt{1 - v^2/c^2}}$$

Here $m$ is the mass, an inherent property of the object, and $E$ is the energy. What you're probably thinking about is the fact that an object that has a nonzero mass ($m > 0$) can never travel at the speed of light, because it would require an infinite amount of energy for it to do so. But zero-mass particles like the photon (the quantum of light) can travel at the speed of light without requiring infinite energy. (In fact, they can't travel at any slower speed.)

Some people use the term "mass" to mean the quantity that I'm calling energy (in different units), and the term "rest mass" when they want to refer to what I call the mass. In that case, one would say that a material object traveling at the speed of light would have infinite mass. I refer you to another answer I've written for more information on the historical context of the terms.

But, do light waves have any sort of measurable mass? Or in that same vein, do sound waves?

No, but they do have energy. For light waves, the energy is related to the frequency $\omega$ of the wave,

$$E = n\hbar\omega$$

($n$ is the number of photons), and for sound waves, the energy is related to the amplitude (particle displacement) $\xi$ and the frequency $\omega$,

$$E = A\rho \xi^2\omega^2$$

where $A$ is the cross-sectional area of the sound wave.

You could calculate an "equivalent mass" as $m_\text{eq} = E/c^2$, which would tell you the amount of mass it would take to have the same energy (at rest) as a given light or sound wave. If it were possible to convert the energy of the wave into mass directly, $m_\text{eq}$ would be the amount of mass you'd get. But that's the only sense in which a wave has mass.

share|improve this answer
add comment

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.