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 has also been measured precisely, although our limited access to gravitational fields of varying strengths currently prevents us from obtaining confirmation as definite as we have for time dilation caused by relative motion.
So, we clearly have the tested and proven ability to travel into the future at any rate greater than one day per day. At present, however, we do not know how to travel more slowly into the future, or how to travel into the past, or even if these concepts truly make sense in our universe.
Despite this, considerable serious scientific research has gone into elucidating the concept of unrestricted time travel.
This research does not lead to a firm conclusion at present. There are spacetimes compatible with general relativity that include closed causal curves, which are a signature of a universe in which time travel into the past is possible.
There are models of wormholes which provide a path into the past. The present consensus of theoretical analysis of quantum mechanical models of wormhole time travel suggests that producing inconsistencies is not a possibility. That is, the grandfather paradox (a son going back in time and killing his grandfather) is inconsistent with quantum mechanics.
It is not clear in the context of general relativity if time travel is allowed while restricting our models to the types of matter and energy with which we are currently familiar. Several possibilities require the existence of matter or energy having negative energy.
Although there are theoretical possibilities for such types of stress-energy-mass sources, at present these are quite speculative in nature.
Recently there have been general relativistic spacetimes discovered that contain only normal matter and energy, and yet also contain closed causal curves – that allow time travel. These spacetimes are currently attracting a great deal of attention and analysis.
Within the multiverse concept, there is yet another possible meaning for time travel. In this case, in travelling back in time a time traveler would begin a new time track rather than influencing his original time track.
Roughly speaking, the act of time travel would involve creating another universe. Within the multiverse concept, however, creating another universe does not cost any energy, so this is a possibility.
The big question, however, is what will happen to these theoretical models when a proper quantum theory of gravity is developed. We know a few things about such a theory, and it appears that a spacetime with closed causal curves may turn out to be unstable – that is, that they may automatically change into a spacetime in which such closed curves do not occur.
This is not yet a firm result – there is much that no one knows about the interaction of quantum mechanics and gravity.
It is even possible (and probably likely) that our current view of the universe is incomplete not only in detail, but in concept. In that case, all bets are off. Do paradoxes instruct us, or is reality happy with paradox?
At this point in time, we don’t have the slightest idea. It is possible that time travel is literally impossible. It is also possible that Doctor Who style time travel is possible.
What is clear is that whole generations of theoretical physicists have been and will continue to be fascinated by the problems and opportunities presented by real time travel.