What Defines a Massless Particle Like a Photon?

[hexrd]

"Massless" Particle?

What is meant by a massless particle? I know that a photon is massless and travels at the speed of light. Then how is it that a black hole ( or gravity) can affect it? I thought that gravity affects only objects with mass.

 

[selfAdjoint]

No, this should be in a FAQ because it's such a common misconception. In general relativity gravity works on ENERGY and MOMENTUM. Mass is a form of energy, but only one form; any form will work, even kinetic energy!

And light does carry energy and momentum even though its quantum, the photon, is massless. This has ben observed experimentally.

 

[Fredrik]

In a certain sense, light always takes the "straightest possible" path. (Massive particles also do this). But the presence of a massive object curves spacetime, and that changes those "straightest possible" paths.

When a ray of light passes a massive object, it will take a path that isn't a straight line in the 3-dimensional slice of spacetime that we think of as "space, at this moment in time". But the path is a straight line (technically a "geodesic") in spacetime.

 

[Chirapat]

It is not that a photon has no mass. A photon has no “rest-mass” but it does have a unified entity called “mass-energy”, which has the same properties as rest mass does, as far as gravitation is concerned.

Consider a pair of electron and positron. They have rest mass, which other masses surrounding them can feel and react to. If this pair of electron and positron were to collide each other and turn themselves into photons, their previous rest mass did not just simply vanish but became mass-energy of the photons. The surrounding masses would still feel the gravitational effect of “mass-energy” of the photons as if it still were rest mass of a pair of electron and positron.

What is meant by the sentence “A photon has no rest-mass” is that there is no frame in which a photon appears at rest. So, it’s impossible to talk about rest-mass of a photon. A photon can only travel at a speed of light and what is measurable is only its mass-energy quantity.

 

[Olias]

What is meant by a massless particle? I know that a photon is massless and travels at the speed of light. Then how is it that a black hole ( or gravity) can affect it? I thought that gravity affects only objects with mass.

Photons are information relaying Particles, they are the "measuring devise/bits", of Matter/Energy.

To measure the 'measurer', means one has to use another 'measure-bit', chasing photons in order to measure their make-up will mean one has to stop a photon in its tracks,(therby assigning it with mass), without interfering anyway whatsoever, then one has to choose an energy that can relay the information of a single photon 100%, and the best choice one finds is...Photons!