# I Question about massless things

1. Jan 10, 2017

Are there any other things/objects in our universe that are massless other than Gluons and Photons? I guess energy is massless, but it's not so much a thing, more-so a property.

2. Jan 10, 2017

### Staff: Mentor

Indeed, energy is a property of objects and systems of objects, it is not something in and of itself. But it does have mass. Or, rather, arranging a system such that it has more energy than before will mean it has more mass as well.

3. Jan 10, 2017

### mathman

Mass and energy are equivalent. When you say something is mass-less, it usually means that the rest mass is zero. This applies to photons and gluons and gravitons (if they exist).

4. Jan 10, 2017

### Khashishi

Graviton, if it exists at all.

5. Jan 10, 2017

### rkolter

Don't forget Weyl fermions.

6. Jan 10, 2017

Last edited: Jan 10, 2017
7. Jan 11, 2017

### Khashishi

Those aren't physical objects.

8. Jan 13, 2017

### rkolter

The OP didn't really qualify what a Thing or an Object was. If you want a macro-sized object, it would have to be an object made of massless particles, like a laser beam.

9. Jan 13, 2017

### Austin Z W

Laser is an acronym for light amplification by stimulated emission of radiation. It is made of photons of a specific wavelength, but photons none the less and it does have mass just as all photons do, however small it is.

10. Jan 13, 2017

### Staff: Mentor

The mass of the beam is non-zero, but the mass of each photon is zero.

11. Jan 13, 2017

### Austin Z W

Each photon has energy which means it has mass. The mass is usually said to be zero in most cases and equations because it is such a small amount of mass.

12. Jan 13, 2017

### Staff: Mentor

That is not correct. The mass, energy, and momentum of a particle are related by $E^2=(mc^2)^2+(pc)^2$. For a photon we have $E=pc$ so a bit of algebra will quickly show that the mass of a photon is precisely exactly zero.