Time Travel to the Past, Possibilities or Impossibility
In our struggle to come to terms with our mental and physical reality, nothing vexes us more than the nature of time. For us mortals, time has been an enigma; a puzzle which holds tantalizing rewards of incomparable nature. If only we could go back to see where we came from or go forward in time to see what awaits us. To travel in time – could there be a more thrilling or exciting adventure than that?
Einstein’s Theory of Special Relativity allows us to time travel into the future, but what about the past? We are capable of looking into the past if that is what you want. In fact, all our telescopes do exactly just that. If we observe our nearby Alpha Centauri star system, we are looking at it not as it is today, but as it had looked more than 4 years ago. The further we look away, the further back in time we look.
We may then like to see past events on Earth. Stand 5 feet in front of a mirror, the image that you see of yourself now is not an image of the present but is an image of you 10 nanoseconds ago. We are actually looking into the past. This time delay is due to the fact that even light takes time to travel small distances.
Princeton physicist Richard Gott used the idea of the occurrence of time delays to conceptualize his time machine which he dubbed, the “time mirror.” His idea involves sending a powerful light collector to a black hole say a hundred light years from Earth. At the very edge of the black hole, light could slingshot 180o in an arc due to the immense gravitational forces at its edge. The light collector could help to direct and power the light rays back to Earth, creating a gravitational mirror 200 years into the past.
However, creating such an operational device would be technically very difficult. At such vast distances, light rays from the Earth will be very negligible. Remember that intensity decreases as a function of the square of the distance from earth to the black hole: I D^2. It would be impossible to view anything with significant detail. In addition, we would need to filter out all the other light rays coming from all sources, not to mention boosting the signal for its return journey back to Earth. Also, such a device would not be able to see the past prior to its construction. It would take at least a hundred years traveling at the speed of light to transport it there. It will need another hundred years for the first signals to be received back on Earth. For instance, even if one was completed and launched today. We would only be receiving signals from the past 200 years later in 2206, and seeing images of what it was like today in 2006.
We have seen that Einstein’s Theory of General Relativity allows for the dilation of time in the vicinity of gravitational forces, but can a singularity such as a black hole really allow for time travel? In 1965, New Zealand physicist Roy Kerr obtained a solution to Einstein’s equations of relativity for black holes possessing angular momentum. The angular momentum of such a rotating singularity would form not one, but two event horizons. You could possibly fly into the hole without touching the singularity by matching the speed and direction of the ship with the hole’s rotational velocity and emerge into another region of space-time. It is even possible to choose how far back in time you wish to travel, simply by adjusting the trajectory of your flight path through the ring.
However, some scientists believe that such a singularity would spew an intense flux of high-energy particles into the tunnel, not only killing the time-traveler, but also possibly sealing off the tunnel itself. In addition, the navigable aperture of such a singularity would make the journey a precarious one to say the least. Even if there was some way to overcome these problems, it will still only be a one-way passage through time. In addition, we still face the problem of actually locating such a black hole and actually traveling there.
Wormholes provide another possible approach to achieving time travel. A ‘wormhole’ is a theoretical object that is constant with relativity and is predicted by it. It can be imagined as a tunnel connecting two regions of space-time from each other. We can think of the wormhole as a piece of cheese. An ant can get from one side of the cheese to the other side more quickly by crawling through one of the holes through the cheese rather than crawling along the surface of the cheese.
Now we can use the wormhole to traverse through space, but what about time? A wormhole could allow time travel by accelerating one end relative to another. This can be done by bringing a massive spaceship close to the mouth and simply let the mouth get attracted by the gravity of the ship. In this way, you could force the mouth to be accelerated at speeds close to light. Thus the two ends of the wormhole can be desynchronized. The wormhole now connects two parts of the universe of different times.
However, wormholes are inherently unstable and tend to collapse in on itself. Mathematical physicist Paul Davis calculated that a hypothetical kind of matter know as exotic matter would be needed to stabilize the wormhole by counteracting the massive gravity forces associated with the wormhole. Such a substance is required to have the strange properties of being repelled, rather than attracted by gravity, as such, will possess negative energy density and negative mass. We have already found quantum phenomena such as the Casimir effect that produces a negative energy density. It might one day be possible to use such quantum effects to stabilize a wormhole.
If physically traveling to the past proves such a difficult task, let us explore simply communicating with the past. In the 1970s, some scientists postulated the idea of particles that can travel faster than the speed of light, and yet are compatible with special relativity. Such faster than light particles are know as Tachyons . Einstein’s relativity theory does not allow for the acceleration of a massive particle to the speed of light. However, it is suggested that tachyons move faster than the speed of light from the instance that they are created, hence avoiding the problem of acceleration through the light barrier. These theoretical, yet plausible particles may be a possible key to sending messages into the past.
We know that as we approach the speed of light, time slows down. Traveling at the speed of light, time would seem to stand still for those light photons. Hence, by being able to beat light beam back and forth, we could conceivably use tachyons to send signals into our past. However, Gott found that these particles tend to self-destruct as they moved at higher speeds due to their particular nature. As a result, tachyons could probably send data back through time but only over subatomic distances.
The quest to conquer time has spanned for several hundred years, but it is only been since Einstein that there has been any legitimacy to the science behind the efforts of time travel. Since then, various physicists have proposed various ways in which time travel could be made possible without violating the known laws of physics such as black holes and wormholes. However, many of these proposals require traveling immense distances which would require traveling at near light speeds which is beyond our current technological capabilities. Recent breakthroughs involving cosmic strings or creating a Bose-Einstein condensate may hold new keys to smashing the final frontier, but it is improbable to have a feasible working time machine anytime soon. However it has been shown that time travel, given sufficient technological capability and resources, is indeed a feasible proposition.
Time travel appeals, irresistibly, to the romantic soul of anyone who is human. It has crossed over from the fantasy world of science fiction to the esteemed journals of physics. Once we unlock the gates of time, there can be no turning back. Our universe will be irrevocably altered. The question is not if, but when. Only time will tell.