SR says that as an object reaches the speed of light its mass approaches infinity. How can light reach the speed of light without having an infinite mass?
Okay...Simple, it doesn't reach it :)
Light doesn't accelerate from a velocity below the speed of light. From the moment a photon gets created until the moment it is destroyed, it travels at the speed of light (in the medium it is in, of course).
Something which travels slower than c must have mass and cannot reach the speed of light. Something which travels at the speed of light must be massless and cannot reach speeds below c.
Yes, they are moving at c from the moment they're created, and they don't decelerate when destroyed, they just get absorbed by some other particle.I guess I don't understand how something can be created / destroyed without acceleration / deceleration. Are you saying that photons are moving at C the instant they are created, and instantly decelerate to C=0 when destroyed?
Even in Newtonian gravity the rate at which an object accelerates in a gravitational field is totally independent of its own mass (remember F = ma, so ma=GMm/r^2 which implies a=GM/r^2). In general relativity gravity is not really a force but curvature of spacetime, and photons always follow geodesics which are the closest things to "straight lines" in curved spacetime, but which may look like curved paths from a purely spatial point of view.Why are photons affected by gravity of black holes if they have no mass? F=GMm/r2... So, if photons are massless, then they shouldn't be affected but on the contrary, they are sucked in.
Sorry. = )No No No.... I didn't mean that. I said if photons have zero mass, then they cann't be affected by gravitation. Actually photons have zero REST mass. But in motion, they too have mass.
The shortest path to what?"I said if photons have zero mass, then they cann't be affected by gravitation."
This is not correct. Gravity is not viewed as a force, but as the curvature of space-time. Photons will follow geodesics (the shortest path on a curved space) in spacetime.
Since gravity is the warping of spacetime, it will affect the paths of photons.
That question is the reason I think it's better to use the other definition of a geodesic: It's the straightest path.But the other point... what is the photon travelling towards and why?
But a straight path implies direction towards something. What is the photon travelling towards? Or, what is causing photons to move in a straight path?That question is the reason I think it's better to use the other definition of a geodesic: It's the straightest path.
The definition of a geodesic as the shortest/longest path is appropriate when two points on the path are known. The definition as the straightest path is appropriate when one point and the tangent vector (i.e. the velocity) at that point is known.
A straight path implies direction towards something just as much as a curved path would.But a straight path implies direction towards something.
I don't understand what you're asking here. The photon continues to travel until it runs into mass.What is the photon travelling towards?
An analogy here could be an electric current, it always looks for the path of least resistance, be it a straight path or curved.Or, what is causing photons to move in a straight path?
One important thing to realize, though, is that a photon doesn't follow a geodesic path which is the shortest one in curved space, instead it follows a geodesic path in spacetime. For a slower-than-light-object the geodesic path actually maximizes the proper time (time as measured by a clock following that worldline), rather than minimizing the spatial distance. Not having studied GR I'm not actually sure how a geodesic in spacetime is defined for a photon (whose path always has zero proper time), but presumably it's still different than a geodesic in space would be.For where the points are on the geodesic, think of it as trying to go the shortest distance on the curve as possible, your starting point is wherever you want on the photons path.
When you warp the space a photon is traveling in, you create a new shortest path for it.