Undergrad Theory of Relativity: Light, Mass & Gravitational Lensing

[richard lareva]

If light is pure energy, then how can gravitational lensing work? And if light has some mass, how can it change directions instantly when it is traveling through a prism? My high school science teacher had no answer 45 years ago.

 

[Orodruin]

If light is pure energy,

It is not. There is no such thing as "pure energy".

Gravitational lensing works by light signals taking what amounts to the straightest possible path in a curved space-time. This works exactly the same for massive test particles except that their straight paths have slightly different properties.

Also note that mass is not what gravity couples to in general relativity. In GR, gravity couples to energy, momentum, and stress. Only in the non-relativistic limit where the large majority of all energy is due to an object's mass is the mass contribution to the energy the dominant source and the other sources can be neglected.

 

[richard lareva]

It is not. There is no such thing as "pure energy".

Gravitational lensing works by light signals taking what amounts to the straightest possible path in a curved space-time. This works exactly the same for massive test particles except that their straight paths have slightly different properties.

Also note that mass is not what gravity couples to in general relativity. In GR, gravity couples to energy, momentum, and stress. Only in the non-relativistic limit where the large majority of all energy is due to an object's mass is the mass contribution to the energy the dominant source and the other sources can be neglected.

Thank you for this answer. Obviously, My high school understanding of physics from back in the 70's is way outdated. I have been reading every science magazine I could find for 50 years and I have not gained as much as I thought I did. Time to go back to the basics.

 

[Herman Trivilino]

If light is pure energy, then how can gravitational lensing work?

Many authors do say that light is pure energy. What they mean is that its energy is all kinetic because unlike other types of matter it can have no rest energy. That is a loose way of speaking that leads to misconceptions. Energy is a property of naturally-occurring things, and light is one of those things.

And if light has some mass, how can it change directions instantly when it is traveling through a prism? My high school science teacher had no answer 45 years ago.

Mass is not a requirement for participation in gravity. The idea that you have to have mass to be effected by gravity is part of the Newtonian approximation. Most of a high school physics class and a freshman-level introductory college physics course is spent studying that approximation.

The instantaneous change in direction of a light beam entering a prism or lens is part of an approximation known as the ray model. Again, it's the model taught in those classes mentioned above.

These models are used with great success by many physicists, engineers, technicians, and other professionals. For example, the ray model works perfectly well for opticians, so that's what they use.

Physics is a process of developing a series of more and more refined models. How well they describe Nature's behavior is the test of their utility. None of those models are perfect in the sense that they each have limits of validity.

 

[Comeback City]

If light is pure energy, then how can gravitational lensing work?

I also had a question regarding the bending of light in GR here if you want to check it out.

And if light has some mass, how can it change directions instantly when it is traveling through a prism?

Light doesn't have mass.