if a photon of light is supose to have no mass, why is it that gravity can alter the path of a light beam?

Question

if a photon of light is massless, how can the force of gravity alter the path of light? eg: light passing in the vicinity of a black hole would bend at an angle which is proportional to the black hole gravitational field. does'nt something needs to have mass to be affected by the force of gravity?

Answer 1

Gravity bends spacetime. A photon simply follows a straight path in a curved spacetime universe.

Inside a black hole, gravity is so strong that it bends space time back on itself. Photons caught in a black hole keep going straight ahead from their point of view but the dimensions of space are curled back on themselves so they turn in (circle, ellipse, spirals?) from our point of view.

Note to a previous answerer: relativistic mass applies to objects with mass approaching the speed of light: their rest mass is increased. A photon has no mass, and so no relativistic mass.

Answer 2

if a photon of light is massless, how can the force of gravity alter the path of light? eg: light passing in the vicinity of a black hole would bend at an angle which is proportional to the black hole gravitational field. does'nt something needs to have mass to be affected by the force of gravity?

Answer 3

In general relativity, by the definition of Minkowski space-time, light follows a straight 4D line. I believe this is the source of the myth that light has no mass. If light has no mass in GR, it is because "mass" doesn't have the same meaning in GR as the meaning given to it by Newton. In 3D Newtonian space, light bends when passing thru the gravitational field of a star. This changes the direction of the light's momentum vector. Subtracting the momentum before bending from the momentum after bending gives a non-zero momentum-difference vector pointing at the star. If momentum is to be conserved, an equal and opposite momentum must be imparted to the star. In other words, the star must feel a gravitational attraction to the light. Therefor, light has gravitational mass. Newton defined inertial mass by the formula, f = ma, which works well for particles with a rest mass, even at relativistic speeds. This formula works for photons, too, as long as the force and acceleration are lateral only. It doesnt work, at all, for a photon if the force has a non-lateral component, because the photon can't accelerate forward or backward. We need new definitions of mass and force. I am no mathematician, so I may be getting in over my head, but I believe the correct definitions are: m = p/v (where p is momentum and v is velocity), and f = dp/dt (where dp/dt is the rate of change of momentum). Those definitions are in perfect agreement with f = ma where particles with a rest mass are concerned. They have the distinct advantage, however, of applicability to photons and neutrinos. I suspect that the inertial mass and gravitational mass of the photon are equivalent. I defer to the mathematicians to verify whether these formulas contradict general relativity, bearing in mind that Minkowski tacitly altered the meanings of such concepts as "mass".

Answer 4

There are 2 types of mass in relativity. Invarient, or rest mass, and relativistic mass, which depends on the frame of reference. Photons have 0 rest mass but do have a relativistic mass, which allows them to be acted upon by other forces. Its basically just a fancy way of saying that a photon can never be at rest. and to correct the above poster, i hate to tell you that gravity is a force. On earth it operates at 9.8 m/s. And quantum field theory predicts the existence of the graviton, an elementary particle that mediates gravity in a quantum field, which i might add is also massless. But what does that mean? Only that a graviton can never be at rest, just like the photon. Note to the poster trying to correct my relativistic mass statement. the relativistic mass of a photon is a function of its wavelength. see below link...i will say the tradititonal sense of mass is not what we are talking about.

Answer 5

No, there is no requirement for a particle to have mass to be deflected by gravity. Strong gravitational fields distort spacetime, which actually causes the shortest path through the gravitational field to be curved, not straight; thus, light follows a curved path through those areas and gravitational lensing occurs.

Answer 6

omg does no one know what a black hole is........ill take that as a no

listen up

a black hole is a sun that has reached such a massive size that light cannot escape it this is because light is magnetic much the same as electricity..you follow me ok

the reason you cannot see a black hole is for the reason above this is why you can only see its effects this is the gravity pullin things in and folding small amounts of space, and so is time therefore you may feel like you have been consumed in seconds but on earth it would have been generations or years, obiously dependant on the size of the sun itself also heat is greatly affected but if you ever got as close as earth you would have already been crushed to a point of zero size and little mass

Answer 7

Because gravity is not a force. It is a curvature of space-time. What we perceive as a force causing a planet to move in a curved path is actually the planet moving in a straight line in curved space-time. So light also just moves in a straight line in curved space-time, which appears to us as a curved path since we cannot perceive the curvature of space-time. Pretty esoteric stuff. Not intuitively obvious at all.

Answer 8

because gravity alters the shape of the universe and the photon follows that path

Answer 9

Well
You can download Photon here http://bitly.com/1k4c40G
Try it out

Answer 10

lights absorbs in black hole not because of this gravitational property but because of another important property of absorbing...absorption and gravitation r two different things....u also absorb some light ths y ur visible but ur absorption is not because of ur gravitational force