Talk:Interstellar travel/Archive for 2006

Time travel and faster-than-light travel

I was told by a physicist (and also read elsewhere on Wikipedia, don't remember where) that both time travel and faster-than-light travel are now generally agreed to be possible, under bizarre conditions that don't violate the laws of physics. At least, they don't violate the modern theories of physics, which have come a long way since Einstein. I was also told that the amounts of energy involved are so absurd as to render these completely impractical.

Can anyone else confirm this? Xezlec 20:08, 1 April 2006 (UTC)

Time travel and faster-than-light travel are equivalent under special and general relativity; if you can accomplish one, you have the means to accomplish the other. There are several proposed methods of doing this. Whether they are "possible" or not depends on your assumptions. Several spacetime geometries allow you to move a patch of spacetime at FTL speeds relative to the universe around you, but these involve negative mass. Similarly, you can create shortcuts (wormholes being the most famous) that have properties that allow time travel and very rapid transport, but stabilizing them requires negative mass for most geometries studied. You can also spin a very dense object (a Tipler Cylinder or a Kerr black hole) in a way that might allow time travel, but this requires conditions extreme enough that it's not likely to be done as a result of human actions any time soon. There may be other methods of time travel and FTL travel that I'm not aware of, but the ones I'd heard of were variants on the ones above. The more interesting ones attempted to reduce the amount of matter involved by using very thin, dense shells. It's a neat topic, but still one that scentists are reluctant to make blanket statements about. The more conservative view is that anything that can produce closed timelike curves (allowing time travel) is forbidden by the laws of nature (either as a consequence of the ones we know about, or as a consequence of the features of a theory of everything unifying gravity, the other forces, and quantum mechanics). The less conservative view is that it may be possible given conditions that involve assumptions (like the existence of negative mass) that we don't know the validity of yet. --Christopher Thomas 22:14, 1 April 2006 (UTC)

Cool, thanks. I believe that's the same basic story I had heard. So, maybe someone should remove or revise the line that says "Current theories of physics indicate that it is impossible to travel faster than light". Would changing "impossible" to "impossible or astronomically difficult" be an option? I leave this decision up to the experts. Xezlec 20:21, 29 May 2006 (UTC)

FTL travel requires negative mass? Not imaginary mass?

The exotic matter page briefly summarizes both, and doesn't mention FTL speeds in the negative mass subsection, but does refer to tachyons in the imaginary mass part.

$E={\frac {mc^{2}}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}}.$

For a $v$ to be greater than $c$, $m$ must be an imaginary number (not a negative number) to keep $E$ real... --HantaVirus 13:17, 26 July 2006 (UTC)

under sublight

in the FTL section sublight it states

"NASA is studying methods of extracting energy from empty space. The Casimir force has been proposed to be a force coming from the vacuum energy of virtual particles in empty space. The Casimir force has been measured and proved to be a real phenomemon and recently, tiny amounts of energy have been extracted by devices that work on the Casimir force. Many have put forth the idea that the vacuum energy of empty space and the energy of virtual particles in space is much bigger than Casimir calculated as a result of the "false bottom" effect. There is no proof of a false bottom and a much deeper energy well filled with virtual particles nor has there been proof that empty space contains virtually limitless amounts of energy and gravitiational mass as a result of the equivalence of mass and energy posited by Einstein. Devices that generate energy from the Casimir effect may power nanocircuitry on long voyages on board starships, but they will never provide a source of power for propulsion."

doesnt that seem to need some ciation —Preceding unsigned comment added by 141.154.238.62 (talk) 15:54, 8 August 2006 (UTC)

SI

Why isn't the International System of Units (SI) upheld in this article?

Wikipedia is international. —Preceding unsigned comment added by 83.108.240.129 (talk) at 10:28, 7 October 2006 (UTC)