# Weight of light and Gravity Speed

My math teacher was talking to me about how NASA weighed light. If it doesn't have any mass, how is this possible? Also, he was saying that they are beginning to study a speed faster than the speed of light. Called Gravity Speed. Which has no lag, and is instantaneous. Do any of you know about these?

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Originally posted by Guard
My math teacher was talking to me about how NASA weighed light. If it doesn't have any mass, how is this possible? Also, he was saying that they are beginning to study a speed faster than the speed of light. Called Gravity Speed. Which has no lag, and is instantaneous. Do any of you know about these?

And your math teacher told you the derivative of x is 0.

Integral
Staff Emeritus
Gold Member
Here is a concept, read some of the other threads on the forum.

Gravity is not instantaneous, it propagates at c.

Originally posted by Guard
My math teacher was talking to me about how NASA weighed light. If it doesn't have any mass, how is this possible? Also, he was saying that they are beginning to study a speed faster than the speed of light. Called Gravity Speed. Which has no lag, and is instantaneous. Do any of you know about these?
Perhaps you misunderstood him. Maybe he was talking about the gravitational aspects of light? Do you recall exactly what he meant by that? Perhaps be meant the same thing Alan Guth did when he wrote the following. From his class notes on The Early Universe
We are perhaps not used to thinking of electromagnetic radiation as having mass, but it is well-known that radiation has energy density. If the energy density is denoted by u, then special relativity implies that the electromagnetic radiation has a mass density $$\rho$$ given by

$$\rho = u/c^{2}$$

To my knowledge nobody has ever actually "weighed" electrogmagnetic radiation in any way, but the theoretical evidence in favor of Eq. (7.3) is overwhelming - light does have mass. (Nonetheless, the photon has zero rest mass, ...)
The full page can be read at
http://www.geocities.com/physics_world/guth.gif