Time travel

by Marcus Loane

27^th Feb 2015

I have always been fascinated by the concept of travel through time. Armchair reasoning and what we know from physics can give us clues about whether time travel could be possible and what it might be like.

Time travel to the past

Time travel has been used as a plot device in a lot of fiction, usually in impossible ways. The usual scenario is a subject traveling to the past and making some change that they think will alter the present which they then return to. An example might be to warn their earlier selves to start or end some relationship or prevent the terrible car accident which killed their best friend. However this kind of time travel is not possible because it leads to paradoxes.

The Grandfather paradox

The best illustration of this is the grandfather paradox where the subject goes back in time and kills their grandfather when he was a child. If their grandfather was killed before having children then how could our subject exist to go back in time to kill him? This is a paradox. In fact any time travel to the past where intervention is possible appears to lead to a paradox, a paradox where the remembered past does not match the now altered past. The past you remember is already a part of reality so you cannot erase it or alter it.

There is a solution to this and it is to introduce the concept of parallel time lines. When you go back in time and alter the past, the time line splits at that point and a new timeline is generated containing the change you made. To use the grandfather example, you go back in time and kill your grandfather. There are now two time lines from that point. The first time line is one where your grandfather lives and has children and you are born and then when grown up disappear in a time travel experiment. The second time line has you, a mysterious stranger popping into the universe claiming to be from the future and killing your grandfather. It may be that you cannot return to your original time line now but if you did, nothing would have changed.

The idea of "time lines" actually fits well with multiple universe ideas from physics. You could just as easily name the "time lines" parallel universes. You would be traveling to another universe very similar but not identical to the one you left. In one universe you vanish and in another you appear out of nowhere.

Is time travel to the past really possible?

This question at this time in our technological history is really asking do the laws of physics as we know them allow past directed time travel. The answer is maybe. It involves various proposals involving worm holes, black holes, cosmic strings and lasers. It is highly speculative and even if the equations are solved, the engineering problem could be impractical, involving energies which we are currently incapable of harnessing (like the energy of a star or a whole galaxy being needed.) The physicist Michio Kaku has written some interesting material considering this.

Time travel to the future

Time travel to the future is much simpler. It is allowed by the theory of relativity. Any object traveling at high speeds or close to a strong gravitational source will experience time at a slower rate to elsewhere and will travel into the future. This is not noticed in everyday life. The effects are in fractions of a second unless the speeds traveled are close to the speed of light. It has been verified experimentally using atomic clocks on satellites and airplanes. These clocks have been shown to run more slowly than those on the earth's surface, so when brought back to earth they have experienced less time than earth dwelling clocks. This is not due to some change in the clock mechanism, it is a fundamental feature of the fabric of the universe.

Ernie and Sam

Ernie and Sam are identical twins. Ernie stays on earth and works as a university professor. Sam becomes an astronaut and is appointed to captain the new solar drive spaceship which accelerates up to 80% of the speed of light. He travels for 6 years (as shown on the spaceship's clock) and returns to earth to find that 10 years have passed on earth! He has traveled 4 years into earth's future and his identical twin Ernie is now 4 years older than he is and is showing some grey hairs. This is a prediction of relativity (which has been experimentally confirmed). The numbers are approximate but not far off from the examples I have seen meticulously calculated.

Therefore time travel to the future is very likely to be achieved and you could say the atomic clocks have already done it. The faster you travel, the bigger the time difference. You could travel 1000 years into the future but then everyone you left behind would be dead (unless we had also cracked life extension techniques) and there is no going back. As explained earlier travel back to the past is much more difficult and we may never be able to make it practical even if physics allows it.

In an earlier article of mine I stated that all reality including all times exist at once in a "block multiverse" where time is like another dimension of space. If time travel is achieved in the future, it is already part of the block multiverse.

I wonder should we also consider the possibility that a time traveler from the future could arrive to visit us. One objection to this is that perhaps a time machine would have to be in existence now for someone in the future to use to travel back to now. Another interesting consideration for time travel is where in space you would emerge after for example traveling back in time 6 months. If the machine was on earth, in 6 months time the earth would have moved to the other side of the sun so would you emerge there or into empty space and die? Perhaps all time machines would be spaceships or traversed by spaceships.

An even simpler method to travel into the future would be to go into some type of hibernation and wake up many years later. That is something I might be tempted by, later in life.

Marcus Loane

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