Selected Questions -Answers From All Experts Astronomy Forum (Gravity Wells)
I actually have two questions, I know that the answers may be highly
theoretical because of limited knowledge we have of black holes, but any
answer - even theory - is welcome.
1. If a photon has no mass, how come light is drawn into a black hole?
2. What method of propulsion do black holes use the move themselves
through space? How is their movement in a vacuum achieved?
Thanks for your time mate
You are quite correct in assuming that the answers are 'highly
theoretical', and I don't see how these can be made much simpler than
First, a physical definition. For a term we call "inertia". This is
resistance to change in position or motion.
You can think of it as a kind of "foundational" mass. It is the most
basic, fundamental concept we have by which "mass" can be recognized.
That is, when we perform an experiment and find that 'X' (whatever 'X' is)
exhibits a resistance to change its position or motion.
Keeping that thought in mind, we come to the photon. We already know that
light carries energy, usually expressed in a simple form as:
E = hf
where h is the Planck constant, and f is the frequency of the light.
But what many general readers do not know is that light also carries
momentum. For example, whenever light is emitted from some source there is
a tiny but definite "recoil" effect. (Something like you experience
the stock of a rifle, if you've ever fired on a rifle range).
This recoil betrays the presence of what we call "momentum" to do
This is given by the product of one over its speed (1/c) times the energy,
E. Writing the momentum as p:
p = E x (1/c) = E/c
We already have a good hint that photons have mass-inertia associated
with them from Einstein's equation:
E = m c2 (E = mass x speed of light squared).
Therefore, the mass is:
m = E / c2 (Energy divided by speed squared).
This equation (above) can be combined with the earlier one we obtained
(E = p c) to give:
m = p / c
Thus, the "mass" associated with a photon is its momentum divided by
Other experiments validate this, including one in which a (true) radiometer is
bombarded with light, and turns it vanes - spins them around. This is a
result of the pressure of light exerted on the vane surfaces. (You perhaps
have seen this device in action, though most of them are really toys that
operate on a different principle from the real ones).
Basically, light cannot possibly exert any "pressure" unless it also
carries mass, or inertia.
Having said all this, it is important to recognize that in all these cases
I've noted we are looking at photons MOVING!
Once they cease to move, they no longer exhibit mass.
Thus, what you are really thinking of when you say "if a photon has no
mass", is REST MASS. The mass it exhibits when at rest.
And, of course, this is zero.
Hopefully, you can see from the above, that there is no contradiction. The
light that is prevented from escaping from black holes or drawn into them,
is moving - NOT at rest. Hence, it exhibits all the properties of mass or
Regarding your second question, black holes like the stars they collapsed
from occupy specific regions of space-time. Hence, they are not
"propelled" like rockets or anything.
Rather, as part of a dynamical system (say the galaxy) they will move in
accord with the Newton's law of gravitation - given their distance from
the mass center of the galaxy, and the nature and masses of nearby bodies.
Since space itself is a vacuum, this motion is no more mysterious than the
planets of our Solar System moving "through a vacuum". Vacua provide
impediment to motion, and indeed, every body in the cosmos will move under
the influence of the gravitational forces at work in its neighborhood.
The bottom line is that the presence of gravity means that NO body in the
universe can remain "still" or "motionless".