Sunday, July 10, 2011

Strangest Facts of Physics: Time Travel to the Past

Is it Possible? 

Last time we looked at time travel to the future, which is indeed real, but how about time travel to the past? Is it possible?

This is where opinions of scientists split up. Some believe that time travel to the past is possible, though we don't have the technology yet, whereas others believe that it isn't possible due to various paradoxes.

Nevertheless, time travel to the past, at least theoretically seems to be possible. And that is the strange fact of today.

Seeing the Past 

So let's look at some ways to travel back to the past. First of all, we see past all around us, as the light from any object takes time to travel to our eyes. Even though light is extraordinary fast, it takes very long periods of time to travel from distant stars. This means, that if we were to look through a telescope, we would see distant stars as they were billions of years ago. In a sense, this is a form of time travel, as we can see the past. But is there any real way to not only see the past but also travel to it?

Black Holes

The possibility might hide deep in the far reaches of the space. Mysterious massive objects known as black holes might hold the key to time travel. But what is a black hole? When a very massive star runs out of fuel to burn, it collapses. If the star is massive enough, there is simply nothing that can hold the gravitational collapse. Thus such a star collapses rapidly down to a singularity (infinitely dense and small region of space). As the gravitational pull of such a singularity is extraordinary strong, even light can't escape the gravitational pull near the star.

From special relativity we know that the faster an object moves the slower time runs for it. If an object reaches light speed, time completely stops running for it. Now following this logic, some scientists believe, that if something moved faster than light, time would run backwards. So this gives an idea how we could travel through time. If we were to fall down to a black hole, at some point we would end up travelling faster than light.

However, there's one "little" problem. The gravitation of the black hole would simply tear our bodies apart. So travelling back to time using this kind of black holes is out of the question.

But don't worry, there might be another type of black holes. In 1962 New Zealand mathematician Roy Kerr, proposed a working theory of a rotating black hole. According to Kerr such a black hole would be spinning, thus wouldn't collapse to a singularity. In theory, it would be possible to travel near such a black hole without getting torn apart. So at least in theory, it might be possible to travel back to past, although we have no proof of such black holes existing.

Roy Kerr:


Every sci-fi fan probably knows what wormholes are and how they work. But could they exist in reality? Could we be travelling back in time like those guys in Star Trek?

First let's look at how wormholes actually work. The idea is that a wormhole is a tunnel connecting two points in space-time. A wormhole that connects two points in space-time in a specific way, so that one point is in the past, might be used for time travel.

The problem is that, even though such wormholes seem theoretically possible, in reality they would be very unstable and probably wouldn't exist for a long time.

Closed Timelike Curves

It's amazing, but ideas of time travel to the past span back more than half a century ago. Back in the 1940s Austrian mathematician Kurt Gödel  was working with the solutions of Einstein's general theory of relativity. In 1949 Gödel discovered interesting solutions of the equations of the general theory of relativity. These solutions showed that there might exists objects called closed timelike curves, which would allow time travel. A CTC is a world line that a particle or a person can follow which ends at the same space-time point (the same position and time) as it started.

Kurt Gödel:

Unfortunately, CTC's seem to exist only if space-time has a special form (it should have holes sort of like a doughnut shape).

Cosmic Strings 

Some scientists believe that strange objects called cosmic strings (not to be confused with strings in string theory) might exist. These massive and extraordinary thin objects might help in time travel. If two such strings could be brought close together close timelike curves might be created.

As always, this has some problems. First of all, these strings might not even exist. Furthermore, energy required to manipulate such strings to create CTC's would be enormous.


So as you can see there are many great problems with time travel to the past. Naturally, there are scientists who do not believe in time travel to the past. To defend their position they often use famous paradoxes, which show that time travel might not be logical. The most important and popular paradox is the so called grandfather paradox. It goes like this: let us say you go to the past and for some reason decide to kill your grandfather before you were born. Would it be possible? I mean if you'd kill your grandfather in the past, you would never been born and thus wouldn't travel back in time and kill your grandfather, which is not logical. Of course some say, that there would always be something stopping you from killing your grandfather (which doesn't sound realistic to me). Others believe, that you would kill your grandfather in a parallel universe, which wouldn't cause a paradox.


Even though many scientists don't believe in time travel to the past, it is still theoretically possible. We don't have a sufficient technology to test it, however such a technology might be available in the future. And even though time travel to the past seems unlikely, only time will tell...

Thanks for reading! If you enjoyed my articles, feel free to comment and follow.

For more about time travel check out these books:


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