# What happens to a photon as it experiences an increasing amount of force opposite to its momentum? Could it be frozen?

Image that a photon is emitted from a sphere, moving orthogonally away from the sphere's surface. As the weight of this sphere increased, how would the increasing gravitational force affect the motion of the photon? Imagine that the mass of the sphere was sufficiently high to form a black hole - but the photon is emitted outside the black hole's event horizon - what happens to the photon then?

• It gets redshifted, therefore it loses momentum. Other than that, it just moves forward in a straight line. – Wood Oct 18 '16 at 9:08
• So as the force approach infinity, would the only thing that occured be the photon losing energy? – Honwang Oct 18 '16 at 9:14
• Additionally, where does this energy go? – Honwang Oct 18 '16 at 9:14
• I think it just goes to gravitational potential energy, just like any massive particle. – Wood Oct 18 '16 at 9:33

• @Honwang: more or less; the gravitational frequency shift when increasing the distance from the center of mass from $r_0\to r$ is given by $1+z = \sqrt{(1-r_s/r)/(1-r_s/r_0)}$, which goes to infinity at the Schwarzschild radius – Christoph Oct 18 '16 at 16:38