The Possibilities of Time Travel

How many times have you said, “If I could do this or that again that you would do it differently?” I know I do and I said this statement from time to time when something does not go according to plans. Then I start thinking about what if I can build a time machine and go back in time just simply to change a decision that I might have made to cause something to go wrong. Many physicist believe time travel is possible but the main problem with time travel is that it will take a lot of energy to make it happen especially if they want to sent something as large as a human forward or backward in time. But It is very easy to do this with sub-atomic particles in an accelerator as we will see later.

We have seen many movies of time travel such as the sci-fi classic “Time Machine”, the TV series of the 60s, “The Time Tunnel” and more recent movies “The Time Traveler’s Wife” and “Back To the Future” movie trilogy. These were all good movies in their times but they never quite explain why time traveling needs so much power to sent something back and forth across the time continuum or even how it was done. In these movies you always saw a lot of fancy equipments with lights, dials and gauges to dramatize the power of time traveling and the person would disappear in a blink of an eye. Today physicist can explain how it is possible to travel through time based on what they learned from particle physics and black holes because of Einstein’s  papers on relativity.

From a physicist’s viewpoint time is one of the four dimensions in our physical world. Everything that exists in the universe has length, width, height, and the fourth dimension called time. When we are moving around in the world we always move in these four dimensions and everything in the universe does the same thing down to the atoms and sub-atomic particles that all matter is made of. Time is in essence the existence of something in the universe and it is another dimension in length. Each one of us will be around for 70 to 100 years, the pyramids will be around for about couple thousand years more, and the earth and sun will exist for a few billion years more.

Physicists have known for a while that time slows down near massive objects. It was clarified in Einstein’s paper on Special Relativity when he first published it in 1916 that mass put a drag on the flow of time. Think of time, as water flowing in a river, the speed of the flowing water will slow down around large boulders in the river. This phenomenon is happening all the time on earth when tourists are standing near the giant Pyramid of Giza in Egypt. This is the most massive structure on the planet with an estimated mass of 40 million tons. Time slows down near the pyramid due to it large mass but the effect is small. However, if you were to exaggerate this effect, an observer looking at the pyramid would see people moving slower near the pyramid with respect to himself and if an observer standing near the pyramid were to look out toward the desert he would see people moving along quicker with respect to himself. In this exaggerated scenario, if he stood there long enough he would emerged a few minutes, hours or even a day into the future depending on how long he stood there while time away from the pyramid is zooming by faster than his time.

In fact there is proof of this effect taking place literally every second of the day just above our heads. The precise clocks on the 31 global positioning satellites (GPS) circling the earth experience this effect. Time is moving faster up there with respect to time on earth because the satellites are further away from the massive body of the earth. The effect is very small, but it throws the clocks off by about one-billionth of second everyday. This effect is enough to throw the positions measured on the earth’s surface off by 6 miles a day from the satellites’ perspective. Fortunately, there is a built-in correction program on each satellite to correct this time error.

Physicist know with this fact that the effect of time dilation near a massive object can be greatly amplified if we could fly a spacecraft near the most massive object in the universe, a black hole. This is Mother Nature’s time machine. In order for this to work everything must be done correctly. If this could be done the astronauts in the spacecraft must approach the black hole at the right speed and trajectory to avoid being drawn into it. If done correctly, the astronauts in the spacecraft would circle the black hole for a period time experiencing this slower passing of time while everyone else away from the black hole would experienced time moving at a twice the speed with respect to the astronauts in the spacecraft. If they stayed near the black hole for one year the people back on earth would see two years go by with respect to the astronauts in the ship. Obviously, this would not be a practical way to travel into the future because it will take too much time and energy to achieve any significant time travel into the future. However, there is a straightforward approach to traveling into the future and that is speed.

Another aspect from Einstein’s paper on Special Relativity states that time slows down to an observer approaching the speed of light. This is a fact proven by particle physicists at the CERN particle accelerator facility in Geneva, Switzerland. It is here where sub-atomic particles are accelerated to speeds near the speed of light in a tube buried underground in a circular tunnel 16.8 miles in length. To study a very short-lived sub-atomic particle called a Pi-meson, which has a life span lasting only 25 billionth of a second, the particles are accelerated to 99.99% the speed of light. About a trillion of these particles are placed in the circular accelerator and are accelerated from 0 to 60,000 miles per hour in a few seconds with powerful magnets. The particles will continue to accelerate until they are traveling at 99.99% the speed of light. At this speed the particles are traveling around the 16.8-mile circular accelerator 10,000 times per second and the life span of the particles last 30 times longer due to the time dilation effect at speeds near the speed of light.

This same scenario can be done with a train traveling close to the speed of light on earth. This would be a very difficult task to do. But if we could this is probably how it could be done. Imagine about 200 to 300 passengers board a train for a trip into the future. This is a one-way trip you cannot go back. I will explain why later. The doors close and the train begins to accelerate slowly on a 25,000-mile track circling the earth. It continues to accelerate until it reach a speed close to the speed of light and once there it will be circling the earth seven times a second at that speed. To an observer outside the train if he is able to see inside the train, the passengers will appear to be moving very slowly due to the time dilation effect. If this train continued at this speed going around and around and finally come to a stop after one week, one hundred years would have gone by for the people not on the train. The passengers on train will only see one week go by. They will be 100 years into the future once they step off the train. The problem with this scenario is that it would require a lot of power, energy, advance technology and manpower to accomplish this but it would work if it could be done.

This scenario could be done in space as well with a very massive spaceship. The problem here is that it would require a lot of fuel and again manpower. Also the ship will have to literally travel out of the galaxy to achieve the same effect because it will take the ship almost four years just to reach 90% of the speed of light . By that time it would be just passing the nearest star, Alpha Centuari, about four light-years from here. The other obvious problem is that this would be a one-way trip. You will not be coming back on this one.

Cosmologists and physicists said there is one thing you cannot do in time travel. If you had a time machine you cannot go back in time to the past. This is what everybody wants to do with a time machine if they had one. But it is impossible and here is the reason why. You cannot have "effect" before "cause" or you cannot see the effect before cause. It doesn’t make sense. Here is an example, imagine if a scientist had assembled a gun to shoot himself in the past. Now let’s say he invented a time machine to open a portal about a minute back in time to shoot himself before he assemble that gun. He shoots his past self and his past self dies before he assembles the gun. Who fired the shot? It doesn’t make sense. It is a paradox. This is the way all events progress in the universe, cause then effect, not the other way around. Another way of looking at it is that the future is the “effect” and the present and past is the “cause”. Unfortunately, you will not be able to go back in time to go dinosaur hunting.

References:

1. Wikipedia

2. Information from Stephen Hawking's "Into The Universe" series on the Science Channel

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