Light's effect on spacetime

[David Lahav]

Hi, I got interested in physics last night and there are a few philosophical questions that came into my mind. I have no formal education in this field but nonetheless, maybe you can answer in a simplistic manner.

1. Particles that has mass affect spacetime and are also affected by spacetime. Particles that don't have mass, like photons do not affect spacetime but are affected by him. Are there particles that affect spacetime but are not affected by spacetime?

2. What is energy? (please no answers like the ability to perform work of something that fills in equations) is it movement? Is there an energy field?

Please feel free to tell me the above is nonsense if it is so.

 

[Drakkith]

Hi, I got interested in physics last night and there are a few philosophical questions that came into my mind.

We don't usually allow philosophical questions here at PF, as they tend to go nowhere, but I'm going to keep this thread open for now because your questions aren't really philosophical. If this does end up getting into a philosophical discussion then the thread may be locked.

1. Particles that has mass affect spacetime and are also affected by spacetime. Particles that don't have mass, like photons do not affect spacetime but are affected by him. Are there particles that affect spacetime but are not affected by spacetime?

That is incorrect. Particles with and without mass affect, and are affected by, spacetime. There are no particles that are not affected by spacetime. Remember that General Relativity is a theory of geometry. All this "affects spacetime" means that the geometry of spacetime is being changed by the presence of mass or energy. Everything is affected by spacetime because everything has to obey geometry and geometrical rules.

2. What is energy? (please no answers like the ability to perform work of something that fills in equations) is it movement? Is there an energy field?

You could say that it's an abstract quantity with several different definitions depending on the context. In the end, "the ability to perform work" is probably the simplest definition that covers most aspects of energy. There is no such thing as an energy field. Fields are mathematical ways of describing the physical properties of something that has values at different points in space. The "something" could be a fundamental force like the electromagnetic force a charged particle would feel or something less fundamental like the velocity of wind in an area. Energy isn't something that can be described by a field. Energy has no properties in and of itself. It is always associated with an object or a fundamental field.

 

[Dale]

I would echo what Drakkith said

Particles that don't have mass, like photons do not affect spacetime

In addition, this specific statement is a fairly common misunderstanding. Anything with energy or momentum will affect spacetime. A massless field, like light, still has energy and momentum and will therefore affect spacetime.

 

[David Lahav]

We don't usually allow philosophical questions here at PF, as they tend to go nowhere, but I'm going to keep this thread open for now because your questions aren't really philosophical. If this does end up getting into a philosophical discussion then the thread may be locked.



That is incorrect. Particles with and without mass affect, and are affected by, spacetime. There are no particles that are not affected by spacetime. Remember that General Relativity is a theory of geometry. All this "affects spacetime" means that the geometry of spacetime is being changed by the presence of mass or energy. Everything is affected by spacetime because everything has to obey geometry and geometrical rules.



You could say that it's an abstract quantity with several different definitions depending on the context. In the end, "the ability to perform work" is probably the simplest definition that covers most aspects of energy. There is no such thing as an energy field. Fields are mathematical ways of describing the physical properties of something that has values at different points in space. The "something" could be a fundamental force like the electromagnetic force a charged particle would feel or something less fundamental like the velocity of wind in an area. Energy isn't something that can be described by a field. Energy has no properties in and of itself. It is always associated with an object or a fundamental field.

Thanks, just want to make sure, photons curve spacetime?

 

[Dale]

Thanks, just want to make sure, photons curve spacetime?

Since GR is a classical theory you would just talk about light rather than photons, but yes. Light curves spacetime. Formally these are known as pp wave spacetimes.