Quantum Mechanics and Gravity
Name: Joe N.
I was wondering about the energy involving gravity.
According to quantum mechanics, if I understand correctly, gravity can
be described in terms of a virtual particle called a graviton which is
exchanged between every type of fermion.
I doubt that I can do better than your teacher, but I will try. Maybe
hearing it in somewhat different terms will help. These ideas are subtle
and are not all well understood or agreed upon by everyone. I will give you
what I believe are the generally agreed upon ideas.
My comments follow your questions below.
It is exchanged between all particles with mass or energy (the same thing by
Einstein's well-known equation E = m c^2 and produces the gravitational
force just as the exchange of photons produces electrical forces.
Does this bouncing back and
forth of the graviton require energy? It would seem to me that after a
certain period of time a particle would run out of energy to expel
It doesn't "run out of energy", but it has to "borrow" energy to exist at
all. However, since a graviton has zero mass (we believe), the graviton can
exist a long time if it's kinetic energy is very small. This is by the
Heisenberg Uncertainty Principle (HUP) which says DE x Dt >= hbar. So if
the energy is very small, the uncertainty in that energy (DE) is very small
and the uncertainty in the time (Dt) can be very large. So the graviton can
exist for a VERY long time if its energy is VERY small. This is why gravity
(like the coulomb force) extends to infinity, though it weakens like 1/R^2
as the distance R increases. Incidentally, particles do not "expel gravity"
as far as I know. In fact, I do not know what those words mean
I also do not understand how each particle is gravitationally
affected by every other particle in the universe. I might also conclude
that entropy would play some part..?.. My teacher tried to explain it to
me, but she did not do a very good job. If you could explicate in more
simple terms, that would be great. Thank You
Yes every particle with mass attracts every other particle in the universe.
When you say you do not understand this simple statement, I guess it's
because you find it hard to believe (perhaps because there are so many
particles in the universe). It is nonetheless true, as far as we know.
However, this comment should probably be modified since we now know there is
"dark energy" which apparently produces negative gravity which is causing
the speed of the expansion of the universe to increase! WEIRD!!
If you would like to learn more of these matters, I would recommend the book
"The Fabric of the Cosmos" by Brian Greene. He tells it like it is, so it is
not always easy going, but there is no mathematics and I believe someone
with the interest you have shown would get a lot out of this book. There
are other good books in this area (I especially like "Inflation" by Alan
Guth); you might like to visit a Borders or Barnes and Noble bookstore to
see which you like.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012