Expert: Philip Stahl - 11/9/2010

Question
IF LIGHT CAN'T ESCAPE A BLACK HOLE THEN DOESN'T LIGHT HAVE TO HAVE WEIGHT FOR GRAVITY TO GRAB ON TO IN ORDER FOR IT NOT TO BE ABLE TO ESCAPE THE DRAVITATIONAL PULL OF THE BLACK HOLE?

Answer
Hello,

Actually, since Einstein's seminal paper (on the inertia of energy) we know that photons (at least those moving) must have mass- though extremely small. To grasp this conceptually at a basic level, let the momentum of a photon be:

p = mc

where m is the putative photon mass, and c the velocity of light.

Then from Einstein's mass-energy equivalence equation:

E = c p = c (mc) = mc^2

and the mass is:

m = E/ c^2

which is exactly the result obtained by Einstein in his paper (though he used the symbol l for energy) to obtain the inertia associated with an energy quantum.

You can read many more details, including the current estimates for the photon mass, here:

http://www.desy.de/user/projects/Physics/ParticleAndNuclear/photon_mass.html

Note (excerpt):

"An upper limit to the photon mass can be inferred through satellite measurements of planetary magnetic fields.  The Charge Composition Explorer spacecraft was used to derive an upper limit of 6 × 10^-16 eV with high certainty"

Using appropriate conversions (based on 1 eV or electron volt = 1.6 x 10^-19 Joule) one obtains a mass of about: 1.06 x 10^-51 kg, or 1.06 x 10^-48 g. You can compare this with 9.1 x 10^-28 g for the electron. In other words, extremely minute!