# Bending time?

1. Jan 4, 2012

### joffess

Hi, I'm kind of a noob when it comes to physics so I just have to ask. Me and my friends are kind of in a pickle, so here it goes:

IF you run around a tree at the speed of light, thus bending time (correct me if I'm wrong) is it possible to touch yourself in the back?

2. Jan 4, 2012

### Matterwave

Your body would tear itself to shreds far before you reached the speed of light.

3. Jan 4, 2012

### joffess

Yeah I know, but If you could run at the speed of light?

4. Jan 4, 2012

### world line

As i know, theoretically if u want to to touch some thing in the back u should run at speed more than light.

5. Jan 4, 2012

### joffess

Could you elaborate?

6. Jan 5, 2012

### khil_phys

If you want to touch your back, your front will have to complete the circle and reach the same place before your back does. It is as if you are separating the front from the back, and before the back reaches a certain point, you will have to make the front reach that point while completing a circle. Going faster than light is the way to do that.

7. Jan 5, 2012

### joffess

So basically, you stretch your body?

8. Jan 5, 2012

### hamson

According to size shrinkage effect，the tree will see a lines around himself.
These problems of the transformation of frame is very important

9. Jan 5, 2012

### nitsuj

They would not be able to touch their back. It would be close from the perspective of the dude running.

But that is just observation, the actual event of their hand touching their back would never happen.

Last edited: Jan 5, 2012
10. Jan 5, 2012

### Staff: Mentor

I just reached around the banister and touched my back without any running involved.

I think what you really asking about is if a point-particle could intersect its own worldline, which is called a closed curve. Massive particles have timelike worldlines, meaning that they are strictly less than c. There are no closed timelike curves in flat spacetime.

11. Jan 5, 2012

### nitsuj

Is that regardless of how slow the acceleration is?

Is it because of length contraction?

12. Jan 6, 2012

### khil_phys

No. It does not mean that. What I was trying to say doesn't involve stretching. Your front will have to run mush faster than your back. You will have to vary the velocities of the two parts of your body.

13. Jan 6, 2012

### maverick_starstrider

You have mass, you can't go the speed of light. Furthermore, going around a tree is circular motion which requires constant acceleration which means you're not in an inertial frame and special relativity doesn't apply, you have to move to general relativity. Also, Lorentz contraction is an effect on the observers side, you, in your own rest frame don't notice anything, it's not like, from your perspective, you're being stretched or anything. This is generally true of special relativity (which doesn't apply to this situation) all relativistic effects are relative to an observer, YOU never notice anything funny going on, it's only when you look around you that you see funny things, you are not stretched, you do not feel a force slowing you to below the speed of light, etc.

14. Jan 6, 2012

### pervect

Staff Emeritus
"Bending time" isn't very precise, alas. The usual sense in which one says that SR and GR bends time is "gravitational time dilation".

You can analyze the problem of running around a tree with special relativity. If you do this, you'll find there is no gravity involved in the problem. (At least the problem as I understand it). Hence, there is no gravitational time dilation in such an analysis.

If you try to perform an analysis in the rotating frame, you will get some effects that are very similar to gravitational time dilation - effects that you used to ascribe to simple doppler shift in the non-rotating frame. These effects will be in the direction of the "centrifugal force", though, no along your line of motion.

You will see some subtle effects in the behavior of time in the direction of your travel - basically, if you had a lot of people running around a tree, they wouldn't all be able to synchrdonize their clocks, as they would if they weren't running. This is called the Sagnac effect in the literature - unfortunately, it is notorious for confuising people new to relativity, as does the entire topic of dealing with rotating frames.