MadSci Network: Physics

Re: What type of matter is light?

Date: Fri Oct 30 12:29:34 1998
Posted By: Pauline Barmby, grad student, Harvard University Astronomy Dept.
Area of science: Physics
ID: 909186864.Ph

Jennifer - this turns out to be a pretty profound question, so I'll just
sketch out an answer here.

Light is not matter at all - it is a form of energy. More specficially,
it's a form of electromagnetic radiation. The ordinary matter that
we are made of consists of particles called electrons, neutrons, 
and protons. These particles have mass and, in the case of protons
and electrons, electric charge. Light can be tought of as being
made of particles called photons, which have no mass and no electric
charge. Some other forms of electromagnetic radiation include radio waves,
X-rays, and gamma rays.

(Side note: the "radiation" that comes from radioactive atoms is
sometimes matter and sometimes energy. The confusing name was given
before people understood radioactivity.)

But that's not the end of the story. It turns out that matter can
be turned into energy and vice versa. This is what happens in nuclear
reactions, and is what Einstein's famous formula E=mc^2 refers to.

There is lots more to be said on this topic, and you can find
information about it in any physics textbook. A book with a bunch
of good, short essays on this topic is "The World Treasury of Physics,
Mathematics, and Astronomy", and almost any popular physics book
will talk about matter and energy to some extent. An interesting book
with a different slant on the topic is "Catching the Light: the entwined
history of light and mind" by Zajonc.


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