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)
