by Brian Dodson

Created on: January 15, 2010   Last Updated: January 16, 2010

Is time travel theoretically feasible?

In a trivial sense it is clear that time travel is possible – all of us are currently travelling into the future at a rate of one day per day. The real question here is can we change the rate at which we travel in time?

Again the answer is well known to be yes. This was established by Einstein in his formulation of the special theory of relativity in 1905. The young Einstein (not a graduate student, by the way – he graduated in 1900 from the ETH in Zurich) established the theory of time dilation. 

Time dilation predicts that time passes more slowly for an observer in motion than it does for a stationary observer. This prediction has been tested an enormous number of times, and has always proven to be accurate. 

Some of the most precise technology we currently possess, including the GPS system and the Large Hadron Collider, depend fundamentally on the accuracy of the theory of time dilation. 

A clear example is given by observing the decay of muons. A muon can be thought of as a heavy electron, and has a decay time of 2.2 microseconds. To measure the rest frame decay time, muons are stopped in a detection apparatus, and the time before they decay is measured. 

Light travels about 0.4 miles in 2.2 microseconds. Yet muons from cosmic rays, which are generated in the upper atmosphere, are routinely detected at ground level. As muons generated by cosmic rays are born in the upper atmosphere (about 10 miles high), they cannot survive long enough to make it to the surface. Yet they do. 

The answer is that they are moving rapidly enough to experience significant time dilation. The muon velocity required to agree with observations is at least 98% of the speed of light. At this speed, the muons are traveling into the future at a rate of about 5 days per day experienced in their rest frame. 

The same effect applies to humans, of course. At present, there are long term cosmonauts assigned to the International Space Station who have moved a few hundredths of a second into the future compared to their personal experience. 

Time dilation also appears in the general theory of relativity, in that time passes slower for an observer in a stronger gravitational field. That is, time passes slower for those of us on Earth than it would for an astronaut on Mars, where the gravitational pull is considerably weaker. 

This effect

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