# Time Travel?

*Most recent answer: 10/22/2007*

Q:

I cannot believe what you told that ten year old girl about travel at the speed of light. Maybe if you guys actually read recent theoretical physics books you would see that travel at speeds exceeding the speed of light is possible. I recommend that you read "Time Travel in Einstein’s Universe" by Richard Gott to get the whole theorem

- Matt (age 15)

Emerald Ridge High School, Puyallup, WA

- Matt (age 15)

Emerald Ridge High School, Puyallup, WA

A:

The laws of physics as we know them do not allow any object to pass at
greater than the speed of light by any other nearby object. We doubt
that Prof. Gott would disagree that this law of Special Relativity
applies to all objects big enough to be described without quantum
gravity.

As for another question, whether there are anomalies in the structure of spacetime which might allow travel to remote parts of the past or future, the theoretical arguments are not yet resolved. Gott himself has supplied a reasonable summary of the current state of knowledge:

"Time travel to the past is more difficult but it is a theoretical possibility that seems to be allowed by Einstein’s theory of general relativity. There, space and time are curved, and you can have solutions to those equations that are sufficiently twisted that allow you to circle back and visit an event in your own past. This is in the same way that Magellan’s crew left from Europe, went steadily west, and circled the globe. There are solutions like this to Einstein’s equations of general relativity: wormhole solutions, some that I found involve cosmic strings, and so forth. However, the question is whether these possibilities can be realized in our Universe or not. To understand that, we may need to understand the laws of quantum gravity. That is why gravity is particularly interesting project. We take solutions to Einstein’s equations quite seriously because they have been tested. For example, light bending around the Sun and so forth. Therefore, the time travel to the past is a topic under current investigation. "

That idea has nothing to do with the issue of simply going faster locally, but involves large-scale topological features of our spacetime. (We suspect that due to causal anomalies no such travel of any coherent information-bearer is possible, but as Gott says neither the experiments nor the relevant quantum gravity theory are available now.) It’s unclear why you think that any of this contradicts our answers, or why you think we are unaware of such speculations, which have been well-known at least since the publication of a famous book by Kip Thorne on the topic.

To what ’theorem’ were you referring?

Mike W.

As for another question, whether there are anomalies in the structure of spacetime which might allow travel to remote parts of the past or future, the theoretical arguments are not yet resolved. Gott himself has supplied a reasonable summary of the current state of knowledge:

"Time travel to the past is more difficult but it is a theoretical possibility that seems to be allowed by Einstein’s theory of general relativity. There, space and time are curved, and you can have solutions to those equations that are sufficiently twisted that allow you to circle back and visit an event in your own past. This is in the same way that Magellan’s crew left from Europe, went steadily west, and circled the globe. There are solutions like this to Einstein’s equations of general relativity: wormhole solutions, some that I found involve cosmic strings, and so forth. However, the question is whether these possibilities can be realized in our Universe or not. To understand that, we may need to understand the laws of quantum gravity. That is why gravity is particularly interesting project. We take solutions to Einstein’s equations quite seriously because they have been tested. For example, light bending around the Sun and so forth. Therefore, the time travel to the past is a topic under current investigation. "

That idea has nothing to do with the issue of simply going faster locally, but involves large-scale topological features of our spacetime. (We suspect that due to causal anomalies no such travel of any coherent information-bearer is possible, but as Gott says neither the experiments nor the relevant quantum gravity theory are available now.) It’s unclear why you think that any of this contradicts our answers, or why you think we are unaware of such speculations, which have been well-known at least since the publication of a famous book by Kip Thorne on the topic.

To what ’theorem’ were you referring?

Mike W.

*(published on 10/22/2007)*