# Light revolving around mass like a halo?

1. Feb 16, 2015

### TimeRip496

Does light moves (revolves) around around the mass like how moon orbits around the earth? Or revolution such as that of moon is only possible with something that has mass?

2. Feb 16, 2015

### A.T.

3. Feb 18, 2015

### TimeRip496

When a meteorite fly pass a planet/star, is it possible for it to revolve around that planet/star in an orbit? If yes, does it applies for light? These questions I believe will better help me understand what curvature is. Photon sphere(me not very sure about it)seems to only applies for massive object like black hole, something which is not very general.

4. Feb 18, 2015

### Staff: Mentor

It's a specific application of a very general theory. The faster something is moving, the stronger the gravitational field needed to keep it in orbit. Light moves very quickly indeed, so it needs a very strong gravitational field to hold it in orbit. That means a massive object like a black hole.

5. Feb 18, 2015

### TimeRip496

But isnt gravitational field just bend in spacetime? And that a moving object will move along that spacetime regardless of its speed? Likewise light will move along that bend space time regardlessly. My apologies for such stupid question as I am still starting.

6. Feb 18, 2015

### Staff: Mentor

A very simple experiment will show you that the speed of the object makes a difference. Suppose you are standing at point A and you gently toss a ball at a target at point B; the ball follows a curved path to the target. Then you fire a bullet at the target; the bullet travels in a nearly straight line. So it seems clear that there is something different between the paths followed by the fast-moving bullet and the slow-moving ball.

What's going on here is that gravity is curvature in spacetime, not in just space. When you fire the bullet and throw the ball at the same time (that is, the bullet and the ball leave your hands at the same point in spacetime) the two arrive at the target at different times, and therefore different points in spacetime, even though the target is at the same point in space. Different paths means encountering different amount of curvature on the paths.

7. Feb 18, 2015

### maline

The faster a body is moving, the less time it spends in any given neighborhood, therefore it is less affected by the curvature.
Nugatory's post is very clear if you realize that the paths of freely falling bodies are "straight lines" (called geodesics) through the bent spacetime.
A circular orbit looks like a helix (spiral staircase) if you include the time dimension, and again, because of the curvature, that's "straight". Fast object = less time= more tightly wound spiral, so you need more curvature for that to be a geodesic.

8. Feb 18, 2015

### A.T.

Different speeds in space ->different directions in space-time -> different worldlines in space-time -> different trajectories in space

9. Feb 19, 2015

### TimeRip496

When you say the ball and bullet arrives at tge target at different points in spacetime, do you mean that they will arrive at the same location except at different point of time or both will end up at different location (e.g one end at on mars while the other end up at uranus) as they will never end up in the same location regardless of time?

10. Feb 19, 2015

### maline

He wrote same location, different time.
But keep in mind that " same location" is only meaningful in some particular reference frame, say the Earth's. If the Earth is moving in your frame of reference the two points won't be at the same location. There may even be a frame that has the two collisions at the same time in different locations. "Different spacetime point", on the other hand, is an absolute term.
Edit: whoops- in this scenario the two events are "timelike separated"; you can get from one to the other at the speed of light or less. So no frame will have the events actually be at the same time. Delete that detail.

Last edited: Feb 19, 2015