# Stupid Question

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?

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CompuChip
Homework Helper
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.

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.
Okay...

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?

@CompuChip

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.

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.
No mass... nothing to suck in.

That's a good point.

JesseM
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?
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.

JesseM

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.
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.

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.
Has this been measured, or is this an assumption based on SR?

@Dave9600

c- the speed of all EM radiations is the speed of all particles having zero rest mass and yes, photon travels with c from the moment it is created to the moment it is annihilated. There is no acceleration or deceleration. One of the many nonsensical [to me] things of relativity.

@Dave9600

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.

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.
Sorry. = )

How can photons have a rest mass if they are moving at the speed of light the instant they are created?

"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.

Gravity affecting photons is not related to the photons' momentum.

"How can photons have a rest mass if they are moving at the speed of light the instant they are created? "

Photons have 0 rest mass. Rest mass is a misnomer, I think. I prefer the term invariant mass. If something moves at the speed of light, it has 0 invariant mass.

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"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.
The shortest path to what?

HallsofIvy
Homework Helper
The shortest path between two nearby points- in the same sense that a straight line is, in Euclidean geometry, the shortest path between two points.

The shortest path between two nearby points- in the same sense that a straight line is, in Euclidean geometry, the shortest path between two points.
But what is the other point?

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.

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.
But the other point... what is the photon travelling towards and why?

Fredrik
Staff Emeritus
Gold Member
Isn't about a third of all threads in the relativity forum about the mass of the photon? Another third seems to be about the twin paradox. Isn't it time that someone makes a couple of sticky threads about these two questions?

Fredrik
Staff Emeritus
Gold Member
But the other point... what is the photon travelling towards and why?
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.

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.
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?

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Dale
Mentor
what is causing photons to move in a straight path?
Conservation of momentum.

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But a straight path implies direction towards something.
A straight path implies direction towards something just as much as a curved path would.

What is the photon travelling towards?
I don't understand what you're asking here. The photon continues to travel until it runs into mass.

Or, what is causing photons to move in a straight path?
An analogy here could be an electric current, it always looks for the path of least resistance, be it a straight path or curved.

JesseM