# Questions about light speed, time, and dimensions.

1. Jun 21, 2011

### YoungDreamer

I am not a scientist just an observer and enthusiast.

We have 3 spatial dimensions and one dimension of time. What I have been wondering is...
could time have more than one dimension? I mean we percieve time as a continuous line, but can time move in other ways that we cannot percieve, like backwards. If so could this be another dimension of time?
Time can slow down and speed up in certain high gravity scenarios, can time stop?

When we look at distant objects in the universe we see them as they were when the light we are seeing left them, which could be millions of years. So this brings me to my next question, and I'm sure the answer is probably somewhere in Einstiens work, but are time and the speed of light related, aside from the fact that I just mentioned. If its easier to just point out a site that I should read instead of writing out the answers please do so.

2. Jun 21, 2011

### WannabeNewton

Take a look at this: http://en.wikipedia.org/wiki/Time-space#Privileged_character_of_3.2B1_spacetime

When you say can time stop do you mean coordinate time or proper time? Coordinate time can stop simply because the coordinate chart you are using is "bad" or singular at some point or surface. Take, for example, schwarzchild coordinates that describe a very specific class of black holes: at the event horizon of the black hole this coordinate system breaks down - the coordinate time goes to zero. So yes coordinate time can go to zero for the simple reason that a coordinate chart that you constructed is singular. However, I don't think there is ever a situation where matter could experience a vanishing proper time which is the time a clock in its own reference frame measures. Photons, on the other hand, travel on paths that have zero proper time.

Last edited: Jun 21, 2011
3. Jun 21, 2011

### bcrowell

Staff Emeritus
Speed is defined in terms of time.

4. Jun 21, 2011

### Polyrhythmic

The radial coordinate doesn't go to infinity, it goes to 2M, the Schwarzschild radius. What you mean is the dr²-part of the metric, this indeed approaches infinity. Apart from that, I don't think the original poster's level of expertise isn't high enough as to introduce distinctions like coordinate and proper time, we should start it easy!

To answer some of the original poster's questions:

In special and general relativity, which are part of the best description of the physical nature we have, spacetime consists of four dimensions. Different dimensions of time are simply not part of the theory. There are however theories/hypotheses which include higher spacetime dimensions, for example String theory, which can be formulated in 10 dimensions.

Time and the speed of light are kind of intrinsically related in the sense that the whole framework of special relativity is built around the constancy of the speed of light. The theory tells us how different observers experience time and how it all fits together.

5. Jun 21, 2011

### Polyrhythmic

Obviously, but that could be said about any kind of speed. I think that it would make sense to rephrase this question as: What makes the speed of light special and what impact has it on the concept of time?

6. Jun 21, 2011

### WannabeNewton

Yes sorry I meant g_rr.

7. Jun 21, 2011

### YoungDreamer

When you say the speed of light is constant what does that mean specifically, doesn't light bend and change speed in different mediums and in different gravitational fields.

8. Jun 21, 2011

### Polyrhythmic

Light bends in the presence of gravitational fields. Also, the speed of light can be different depending on the medium. When we talk about constant speed of light we mean that it is constant in vacuum.

9. Jun 21, 2011

### YoungDreamer

Or is that irrelevant because it refers to light speed in a vacuum

10. Jun 21, 2011

### YoungDreamer

nevermind you just said that

11. Jun 21, 2011

### YoungDreamer

Is the universe in its entirety a vacuum or just the empty space? Would this affect calculations, since the universe contains so much matter why do we use light speed in a vaccum. Its not always in a vacuum right? Is that accounted for in the calculations?

Sorry i'm such an amateur, I just have a deep passion for these things, I always have, and now i'm looking into going back to school so I can answer my questions myself.

12. Jun 21, 2011

### bcrowell

Staff Emeritus
Cool! I teach at a community college, and my best students are often "reentry" students, i.e., those who are coming back to school.

You will have an easier time making sense out of relativity if you read an organized presentation in a book. Some special relativity books that I like are (from easiest to hardest):

Takeuchi, An Illustrated Guide to Relativity
Mermin, It's About Time: Understanding Einstein's Relativity
Taylor and Wheeler, Spacetime Physics

If you dig into one of these and come up with questions, we can try to answer them for you.

-Ben

13. Jun 21, 2011

### Polyrhythmic

The universe is filled with all kinds of matter, how could it be a vacuum?