# Does time travel possible?

1. Jan 7, 2010

### anvesh111

If time travel is possible,is there any problem of universe revolving which would affect time travel,because as,Gödel was the first person who found solution to Einstein equations involving time travel,but he disappointed when astronomers told that universe is expanding,but the net spin of the universe is zero.
i mean ,does revolving and expanding of universe affect the time travel how?

2. Jan 8, 2010

### Galteeth

The field equations for general relativity show that rotation of the universe around a symmetrical axis would lead to closed time like curves.

I was interested in this question of a rotating universe. It seems that the CMBR shows the universe is not rotating.

3. Jan 8, 2010

### aib

Really, but if the CMBR is rotating along with the universe and also us, it would all seem stationary, I think something like that can only be detected from an external point of view, and is thus impossible to either confirm or deny that concept.

4. Jan 8, 2010

### Wallace

The Godel metric represents a different spacetime to the expanding FRW solution, so the two can't really be compared. Aib's post above is instructive, "but if the CMBR is rotating along with the universe and also us, it would all seem stationary"; that is exactly right, in order to define that something is rotating, you need to say what it is rotating with respect to. This is the same as any motion, if everything moves together then nothing is really moving in a relative sense, and relative motion is all there is.

On the other hand, the 'rotation' in the Godel metric is of a different nature. It is more like the rotation in a the Kerr solution for a rotating black hole (although the Godel solution is not asymptotically flat or vaccum like the Kerr metric, so there are important differences). Unlike the homogenous and isotropic FRW metric, the Godel metric contains special points, in this case a special line about which the space rotates. The Godel metric is not isotropic, you could actually observe the rotation by noting that along the axis of the spin, everything looks 'normal' but across the plane of rotation you would observe objects to be smeared out.

There are various ways (the key word is 'Killing Vector' if anyone wants to look deeper at the maths) to determine the true co-ordinate independant nature of a given spacetime, and in the Godel metric this analysis tells you that there is true rotation; physical experiments and observations could therefore detect that the space is rotating. On the other hand this same analysis tells you that the FRW metric (that of our expanding universe) is not rotating, and that no experiment of observation would say that it is.

5. Jan 8, 2010

### Ich

There are no special points, as the Gödel metric is homogeneous. There's a special direction at every point, however.

6. Jan 8, 2010

### Galteeth

http://www.absoluteastronomy.com/topics/George_Smoot

According to this entry on George Smoot,

"He then took up an interest in cosmic microwave background radiation (CMB), previously discovered by Arno Allan Penzias and Robert Woodrow Wilson in 1964. There were, at that time, several open questions about this topic, relating directly to fundamental questions about the structure of the universe. Certain models predicted the universe as a whole was rotating, which would have an effect on the CMB: its temperature would depend on the direction of observation. With the help of Alvarez and Richard A. Muller, Smoot developed a differential radiometer which measured the difference in temperature of the CMB between two directions 60 degrees apart. The instrument, which was mounted on a Lockheed U-2 plane, made it possible to determine that the overall rotation of the universe was zero, which was within the limits of accuracy of the instrument. It did, however, detect a variation in the temperature of the CMB of a different sort. That the CMB appears to be at a higher temperature on one side of the sky than on the opposite side, referred to as a dipole pattern, has been explained as a Doppler effect of the Earth's motion relative to the area of CMB emission, which is called the last scattering surface. Such a Doppler effect arises because the Sun, and in fact the Milky Way as a whole, is not stationary, but rather is moving at nearly 600 km/s with respect to the last scattering surface. This is probably due to the gravitational attraction between our galaxy and a concentration of mass like the Great Attractor."

7. Jan 8, 2010

### Galteeth

To answer the topic question, yes, time travel does possible, but only in one direction.

8. Jan 8, 2010

### Wallace

Ah yes, you're right. Thanks, looks like I've been mistaken about that metric for a while. Quite an interesting thing to try and get your head around really...

9. Jan 8, 2010

### Ich

Yeah, I gave up trying long ago.

10. Jan 8, 2010

### Chronos

GR permits reverse time travel for imaginary matter. But, imaginary matter is still ... imaginary.