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The Question
(Submitted November 02, 1996)
This questions has been bugging me and my chemistry class.
Does light have mass? Most people would think not but here's
why I argue against it. Even though light does not effect
anything it its path like a solid object, it is affected by
gravity. Anything that has mass is affected by gravity. Why do
I say that light has mass? Well, If a black holes gravity
field is so strong that light cannot escape itself, light must
have mass? Am I right? Everyone argues against it.
The Answer
These are interesting issues that you bring up. Whether or not light( or
more accurately photons, the indivisible units in which light can be
emitted or absorbed) has mass, and how it is affected by gravity, puzzled
scientists for many, many years. Figuring it all out is what made Albert
Einstein famous. Bear with me and I'll try to explain both the theory and
the observation.
Back in the 1700s, scientists were still struggling to understand which
theory of light was correct: was it composed of particles or was it made
of waves? Under the theory that light is waves, it was not clear how it
would respond to gravity. But if light was composed of particles, it
would be expected that they would be affected by gravity in the same way
apples and planets are. This expectation grew when it was discovered that
light did not travel infinitely fast, but with a finite measurable
velocity.
Armed with these facts, a paper was published in 1783 by John Michell,
in which he pointed out that a sufficiently massive compact star would
possess a strong enough gravitational field that light could not
escape --- any light emitted from the star's surface would be dragged
back by the star's gravity before it could get very far. The French
scientist Laplace came to a similar conclusion at roughly the same time.
Not much was done over the next hundred years or so with the ideas of
Michell and Laplace. This was mostly true because during that time, the
wave theory of light became the more accepted one. And no one understood
how light, as a wave, could be affected by gravity.
Enter Albert Einstein. In 1915 he proposed the theory of general
relativity. General relativity explained, in a consistent way, how
gravity affects light. We now knew that while photons have no mass,
they do possess momentum (so your statement about light not affecting
matter is incorrect). We also knew that photons are affected by
gravitational fields not because photons have mass, but because
gravitational fields (in particular, strong gravitational fields) change
the shape of space-time. The photons are responding to the curvature in
space-time, not directly to the gravitational field. Space-time is the
four-dimensional "space" we live in -- there are 3 spatial
dimensions (think of X,Y, and Z) and one time dimension.
Let us relate this to light traveling near a star. The strong gravitational
field of the star changes the paths of light rays in space-time from
what they would have been had the star not been present. Specifically,
the path of the light is bent slightly inward toward the surface of the
star. We see this effect all the time when we observe distant stars in our
Universe. As a star contracts, the gravitational field at its surface
gets stronger, thus bending the light more. This makes it more and more
difficult for light from the star to escape, thus it appears to us that
the star is dimmer. Eventually, if the star shrinks to a certain critical
radius, the gravitational field at the surface becomes so strong that the
path of the light is bent so severely inward so that it returns to the star
itself. The light can no longer escape. According to the theory of
relativity, nothing can travel faster than light. Thus, if light
cannot escape, neither can anything else. Everything is dragged back
by the gravitational field. We call the region of space for which this
condition is true a "black hole" (a term first coined by American
scientist John Wheeler in 1969).
Now, being scientists, we do not just accept theories like general
relativity or conclusions like photons have no mass. We constantly test
them, trying to definitively prove or disprove. So far, general relativity
has withstood every test. And try as we might, we can measure no mass for
the photon. We can just put upper limits on what mass it can have. These
upper limits are determined by the sensitivity of the experiment we are
using to try to "weigh the photon". The last number I saw was that a
photon, if it has any mass at all, must be less than 4 x 10-48
grams. For comparison, the electron has a mass of 9 x 10-28 grams.
Hope this answers the questions that you and your Chemistry class have.
Good luck,
Laura Whitlock.
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