# Q & A: What is light?

Q:
Q.1 It seems light has been travelling in waves... but the Q is does it have weight? and if it does, How come it doesnt hit us to death ? Its momentum must be high ? Q.2 How does there is light in waves ? is light is a matter? The main Q is WHAT IS LIGHT?
- Darmandran (age 18)
Kluang, Johor, Malaysia
A:
Hi Darmandran,

These are very very good questions which are central to our understanding of the world we live in -- not in the least bit because we perceive most things by the light they give off, and therefore we are very interested in how this all works.

Q1: Yes indeed, light travels as waves in the electric and magnetic fields. It has lots of wavelike properties -- it diffracts, it interferes, it resonates in appropriately sized cavities, its frequency doppler shifts, and it can be polarized. But light travels as particles, too! If we shine light very feebly on a screen, only one spot on the screen will get "hit" by a particle of light at a time! And the energy of the particle of light depends only on the color of the light, not how much light there is.

It's not clear what the "mass of a wave" means, but it is more sensible to ask what the mass of a particle is. Here, light has another surprise for us -- each particle -- a photon -- has a mass of zero! * (or as close to zero as anyone has ever measured). My old electricity and magnetism book quotes the upper limit of the mass of a photon at 4x10^(-48) grams (that's a four with 48 zeros in front of it and then a decimal point!). The current experimental limit is probably much more stringent. This gives light all kinds of good properties: 1) it travels at the speed of light, which is the speed limit in the univese; 2) it propagates infinitely far without attenuation. The other forces like the strong and weak nuclear forces have short ranges because the mediating particles are massive. Gravity seems to propagate infinitely far without attenuation and we suspect the existence of massless "gravitons." to do it.

Now weight and mass are different things. Weight is the force an object feels in a gravitational field. Light is actually bent from a straight line ever so slightly by a gravitational field, so you might say it has "weight." A more correct way of saying it is that the light still travels in a "straight" line, it's just that the space itself is curved and "straight" lines near heavy objects are curvier than straight lines farther away from the heavy objects.

Even without worrying about gravitational fields (which are very complicated if you want to know exactly how light behaves in strong ones), even without worrying about weight, light can in fact "hit us to death." Although it is not an issue of momentum (light does in fact have momentum, but very little, for ordinary light sources), but it can carry quite a lot of energy. On a sunny day, sunlight deposits approximately 1 kW per square meter, which will quickly sunburn me at least. Even stronger light sources can be used as weapons (for example, very strong lasers). But the effect is to melt or vaporize the target and not to impart momentum (which is a very very much smaller effect and even very difficult to measure because it is so small).

I can't quite figure out your Q2. Light isn't really the same kind of "matter" we usually think of -- it's hard to put some together and make a ball or something out of it because it doesn't stay put -- it travels away at the speed of light, and also because it doesn't interact with itself. Light waves pass right through each other unaffected! (except for a very small quantum mechanical effect, that is).

Electrical and magnetic forces exist because of the exchange of photons between charged particles. Photons are what makes electrons stay inside their atoms, and what makes atoms stick together in molecules and what makes molecules arrange themselves in larger structures.

Traveling light waves of low frequency are very useful as radio and TV broadcast waves, and at higher frequencies for microwaves, visible light, ultraviolet, and X-rays, which all have multitudes of uses.

Tom

* One thing you might find confusing is that light has no mass, but it certainly does have energy, and yet there's Einstein's famous equation E=mc^2 saying that mass and energy are just a constant times each other. The confusion arises from two different uses of the word "mass". Often it's now used to mean rest mass, which corresponds to the energy a particle has when it's standing still. Light can't stand still and has zero rest mass. Einstein used mass to mean the number you multiply the velocity by to get the momentum, and light does have mass in that sense. It takes a little care to keep these different uses of the word from getting mixed up. And yes, light has weight not only in that it follows curved paths in space, but also in that it's a SOURCE of gravity. You would be attracted to a box full of light energy just as you would to a box of particles with rest mass.

Mike W.

(published on 10/22/2007)

## Follow-Up #1: mass, weight of light

Q:
Mike, Am I understanding you correctly when you say that photons, although massless, behave as if they have mass? You give the illustration of a "box full of light" as having a gravitational pull. So is it weight, or mass that reacts to gravitity, or both?
- W.A.
Orlando Florida
A:
It's just a matter of wording whether you choose to say that light has no rest mass or to say that it does have a mass in the following ways:

1. An individual photon has momentum, which can be expressed as a mass m times the velocity, c.

2. It has energy which can be expressed as that same m times c2.

3. It exerts a gravitational pull, which is proportional to that same m.

4. A collection of photons going different directions (and hence with zero net momentum) in a box behaves mechanically and gravitationally like an object with rest mass equal to the sum of those m's for the photons. The sum of the rest masses for the photons is zero, but the box does not behave like an object with rest mass of zero.

I'm not sure what else there is to say about it, except to discuss which names it's convenient to use.

Mike W.

(published on 09/28/2010)