# What exactly is meant when people say that a Light Cone is tilting ?

1. Jun 8, 2009

### Benjamin113

What exactly is meant when people say that a Light Cone is "tilting"?

I understand the general idea of a light cone when it comes to how it's used to represent light particles. However, I do not understand what is meant when one states that in Relativity, "Light cones cannot be tilted so that they are parallel."

Would anyone care to explain this to me?

2. Jun 9, 2009

### Fredrik

Staff Emeritus
Re: What exactly is meant when people say that a Light Cone is "tilting"?

That sentence doesn't make any sense to me either. Is that an exact quote from a book?

Do you understand what it means to say that a Lorentz transformation "tilts" the time axis or a simultaneity line?

3. Jun 10, 2009

### Benjamin113

Re: What exactly is meant when people say that a Light Cone is "tilting"?

Yes, and that does make more sense. I believe now that the term "tilting" was meant to have a more...figurative...meaning than the light cone literally tilting.

4. Jun 10, 2009

### JesseM

Re: What exactly is meant when people say that a Light Cone is "tilting"?

In coordinate systems in general relativity, light cones in a diagram using these coordinates may be tilted...for example, here is a diagram showing worldlines of particles and photons near the event horizon of a black hole in Eddington-Finkelstein coordinates (the diagram is from the textbook Gravitation by Misner/Thorne/Wheeler), you can see that if we draw in the future light cones of various events on these worldlines, they look more tilted as you approach the horizon (the grey column, the vertical axis being time):

http://www.valdostamuseum.org/hamsmith/DFblackIn.gif [Broken]

http://www.etsu.edu/physics/plntrm/relat/blackhl.htm has some similar diagrams at the bottom, one showing more clearly how for an event exactly on the horizon, the light cone has tilted over enough so it becomes impossible for anything in the future light cone to be outside the horizon:

http://www.etsu.edu/physics/plntrm/relat/eventho2.gif

Still, I don't understand what it would mean to say light cones "cannot be tilted so that they are parallel". Can you give some more context for that statement? Were they talking about general relativity or special relativity, for example?

Last edited by a moderator: May 4, 2017
5. Jun 13, 2009

### Benjamin113

Re: What exactly is meant when people say that a Light Cone is "tilting"?

Yes, it was actually an internet (or, to be more precise, Wikipedia) article on light cones.
http://en.wikipedia.org/wiki/Light_cone

6. Jun 13, 2009

### Benjamin113

Re: What exactly is meant when people say that a Light Cone is "tilting"?

By the way, thanks for the diagrams.

7. Jun 13, 2009

### JesseM

Re: What exactly is meant when people say that a Light Cone is "tilting"?

OK, so the full paragraph of the wikipedia article is:
Does this reference to the Weyl tensor make sense to people well-versed in GR? The article doesn't cite a source...