How can some particles (photons) have no mass?

Question

It seems that a thing with no mass ceases to be a thing. Also, if they has no mass, how do we know they exist? If it is something along the lines of "they exist as energy and not as matter" as I suspect the answer might be, then why call them particles, and classify them in the same category as things with mass (like protons)?

Answer 1

Technically, photons have no *rest mass*. Their relativistic mass, however, is E/(c^2), where E is the photon energy. If you trapped a photon is a box lined with perfect mirrors, the box would increase in *its* rest mass by this amount.

Yes, a photon is a particle. The consequence of having no rest mass is that photons cannot be brought to a rest. In fact, in vacuum, they must always travel at a fixed speed (the speed of light) as viewed by all observers, regardless of their own speed. That's the fundamental axiom of special relativity that people stuck in Newton Land cannot grok. Two complementary consequences of relativity are that nothing with mass can achieve the speed of light (it would require infinite energy), and all (rest) massless particles *must* travel at the speed of light. It can be understood in terms of mathematical limits. Think of a particle with mass approaching zero as it's velocity approaches the speed of light; Infinity times zero can equal a finite energy.

Answer 2

These are interesting issues that you bring up. Whether or not photons (or light, photons beaing the indivisible units in which light can be emitted or absorbed) has mass, and how it is affected by gravity, puzzled scientists for many, many years. Figuring it all out is what made Albert Einstein famous. Bear with me and I'll try to explain both the theory and the observation.

Back in the 1700s, scientists were still struggling to understand which theory of light was correct: was it composed of particles or was it made of waves? Under the theory that light is waves, it was not clear how it would respond to gravity. But if light was composed of particles, it would be expected that they would be affected by gravity in the same way apples and planets are. This expectation grew when it was discovered that light did not travel infinitely fast, but with a finite measurable velocity.

Armed with these facts, a paper was published in 1783 by John Michell, in which he pointed out that a sufficiently massive compact star would possess a strong enough gravitational field that light could not escape --- any light emitted from the star's surface would be dragged back by the star's gravity before it could get very far. The French scientist Laplace came to a similar conclusion at roughly the same time.

Not much was done over the next hundred years or so with the ideas of Michell and Laplace. This was mostly true because during that time, the wave theory of light became the more accepted one. And no one understood how light, as a wave, could be affected by gravity.

Enter Albert Einstein. In 1915 he proposed the theory of general relativity. General relativity explained, in a consistent way, how gravity affects light. We now knew that while photons have no mass, they do possess momentum (so your statement about light not affecting matter is incorrect). We also knew that photons are affected by gravitational fields not because photons have mass, but because gravitational fields (in particular, strong gravitational fields) change the shape of space-time. The photons are responding to the curvature in space-time, not directly to the gravitational field. Space-time is the four-dimensional "space" we live in -- there are 3 spatial dimensions (think of X,Y, and Z) and one time dimension.

Let us relate this to light traveling near a star. The strong gravitational field of the star changes the paths of light rays in space-time from what they would have been had the star not been present. Specifically, the path of the light is bent slightly inward toward the surface of the star. We see this effect all the time when we observe distant stars in our Universe. As a star contracts, the gravitational field at its surface gets stronger, thus bending the light more. This makes it more and more difficult for light from the star to escape, thus it appears to us that the star is dimmer. Eventually, if the star shrinks to a certain critical radius, the gravitational field at the surface becomes so strong that the path of the light is bent so severely inward so that it returns to the star itself. The light can no longer escape. According to the theory of relativity, nothing can travel faster than light. Thus, if light cannot escape, neither can anything else. Everything is dragged back by the gravitational field. We call the region of space for which this condition is true a "black hole" (a term first coined by American scientist John Wheeler in 1969).

Now, being scientists, we do not just accept theories like general relativity or conclusions like photons have no mass. We constantly test them, trying to definitively prove or disprove. So far, general relativity has withstood every test. And try as we might, we can measure no mass for the photon. We can just put upper limits on what mass it can have. These upper limits are determined by the sensitivity of the experiment we are using to try to "weigh the photon". The last number I saw was that a photon, if it has any mass at all, must be less than 4 x 10-48 grams. For comparison, the electron has a mass of 9 x 10-28 grams.

Answer 3

Mass is defined by atoms. These consist of protons, neutrons, and electrons.

The electrons orbit the proton/neutron core at various orbits also called valence bands. The farther out the orbit, the greater the energy.

At some points, an electron might migrate from a farther out valence band to a nearer one. To compensate for the change in energy, the electron emits a wave of energy called a photon.

Answer 4

You are correct photons do not have mass, but you were incorrect naming them particles. Photons are not particulate, but rather a type of Electro Magnetic radiation. The question your are asking has a lot to do with wave/particle duality. Photons function like waves how they travel, but travel in a bundle of energy of which the unit is a photon. The issue that was originally proposed, and the one you seem to be proposing is, how can it not be matter if it occupies space. Well, that is the trick to wave particle duality, photons don't have to occupy space to exist, because they travel in the form of waves. It's a reasonably complicated dualism, but basically photons are NOT PARTICLES, thats the key. Electrons, protons, neutrons, Alpha Particles, Beta Particles, those are particulate. Gamma, and X-ray, visible light photons are not particulate. Photons are simply the smallest form of the energy that is the EM spectrum.

Answer 5

It's not that they have no mass it's that it is such a small number that it is ignored. For instance in atomic mass they add the mass of protons and neutrons but the Mass of the electrons is not included because it is insignificant compared to the other molecules.

Answer 6

Everything has mass. But it is considered no mass if the mass is negligible, means too small. A particle is assumed to has no mass to make questions in research easier to be solved.

Answer 7

The photons do have mass, but it has no "rest mass" since it is never at rest.

Answer 8

Dude, science is BS. We have never actually seen these particles and are not even close to describing them accuratly.