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Can We Travel Faster Than Light? - Page 2

Physicist Thomas Roman of Central Connecticut State University offers the following response:

WORMHOLE. If nature allowed them, wormholes would appear as spherical openings to an otherwise distant part of the cosmos. This doctored photograph shows a wormhole in Times Square that opens onto the Sahara Desert one city block away.

In the pre-Einsteinian conception of the nature of space and time, there is no limit in principle to how fast an object can travel. But in Einstein's special theory of relativity, the notion of causality--of the past completely determining the future--would break down if any type of matter, energy or signal were able to travel faster than light.

In the pre-Einsteinian framework, time has an absolute character. The time of an event--and thus its time ordering--is the same to all observers; velocities add according to ordinary addition. For very small velocities (small compared to the velocity of light), the same holds in relativity, but for large velocities significant modifications occur. Early in the 20th century the Michelson-Morley experiment established that the speed of light is the same to all observers whatever their relative motion.

Therefore the law for adding velocities must be modified. The relative velocity of two objects, one traveling at the same of light and the other traveling at sublight speeds, must equal the speed of light. When both are traveling at sublight speeds, the relative velocity must be less than the speed of light.

One surprising consequence is that time loses its absolute character. The times perceived by observers moving with respect to each other do not coincide. But observers always agree on the ordering of events. If we admit the possibility of faster-than-light speeds, some observers would perceive one event as occurring before another, others would perceive them as occurring simultaneously, and a third group would perceive the reverse order. The time ordering is invariant only when the two events can be linked by a signal traveling at a speed slower than or equal to the speed of light.

In the context of an expanding universe, it is often stated that widely separated points move apart faster than the speed of light. At first sight this would seem impossible. But an expanding universe must be considered within Einstein's general theory of relativity, a generalization of the special theory of relativity. In general relativity, motion relative to the speed of light is defined locally. The separation between two distant points can increase faster than the speed of light as a result of the swelling of the intervening spacetime. Nothing can pass through the space faster than light, but space itself can carry things apart superluminally.

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