# Can we travel through time?

Time travel has bounced around the mind of just about anyone who has ever felt regret, fear, or just simple curiosity about it, and the idea of it has spawned a whole branch of speculation from physicists, Sci-Fi authors and moviemakers, philosophers, and dreamers.  It would give us the potential to reroute or fix problems like war, extinctions, and climate change.  We have yet to travel through time, but can it be done?

Theoretically, yes it can.

I.  Everything’s Relative

First, we need to understand the nature of time.

If you are standing on a train going 100 mph, and you throw a ball forward at 30 mph, to you, the ball appears to travel at 30 mph.  If someone else was standing beside the train and watched you throw the ball, however, they would observe the ball traveling at the train’s speed (100) plus the speed of the ball (30), or 130 mph.  This addition of speeds would be the case no matter how fast the train was traveling, right?

Actually, no.  In the late 19th century, scientists discovered that the universe has a “speed limit”: nothing can travel faster than the speed of light—not even if you turned a flashlight on while riding a train that was traveling the speed of light.

Enter Albert Einstein.  In 1905, Einstein released a monumentally inspiring paper on what he called “Special Relativity”.  In it he uses a basic law of motion, that speed is dependent on distance and time (speed = distance / time), measured in distance (e.g. miles) per time (e.g. hours), to prove that if it is impossible for anything to travel faster than the speed of light, time must slow down as speed increases.  Time is not a constant flow.

What does this mean for time travel?  If you can travel at a speed near the speed of light, you will age slower as seen from someone standing on Earth.  You could bend down to tie your shoe in five seconds, but the person on Earth would see you moving so slowly that the act of tying your shoe would take years.  In a matter of seconds, you could travel years into the future.

(…Twins?)

But what about backwards in time?

II.  Wormholes Aren’t Just for Worms

If you could travel faster than light, you would be able to reach things before light did, i.e. you would be moving backwards in time.  According to Einstein and the laws of physics, however, moving at a rate faster than the speed of light is impossible.  But what if you could travel “around” space, instead of through it, go between point A and B without traveling the distance between them?

Although scientists cannot be certain that they exist, some believe that backwards and forwards time travel is possible going from point A to B using something called a “wormhole”.  The Universe can be imagined as composed of something called “the fabric of spacetime”, and anything with mass creates a depression in spacetime, which we know as gravity.  Think of a planet or a star as a bowling ball on a trampoline.  If two of these depressions in spacetime, which might be millions of light-years apart, were to connect because of the depth of their depressions, it is possible that a wormhole would be created, through which we could pass, effectively traveling the millions of light-years between A and B within a lifetime.  In the image below, the green arrow goes through the wormhole, while the following the curve would represent traditional space travel.

Some theorists believe that if you looked back through the wormhole, you would see yourself at a younger age, thus having traveled back in time.

III.  Is It Likely?

Some say yes; some say no.  Stephen Hawking, one of the world’s foremost authorities on theoretical physics, says that if time travel were possible, we would necessarily be flooded by thousands of time travelers from the future, and it’s likely that we would already have the technology.  But others argue that one could not travel back in time further than when the time machine was created, or that time travelers would also necessarily have the technology to hide themselves from us.

Other questions arise, such as what would happen if you went back in time and murdered your grandfather, thus preventing you from ever being born?  Is it impossible to murder your grandfather, would fate intervene by jamming your gun, for example?  Or would you open the door to a parallel universe, in which you don’t exist?

Make up your own mind, and dream on!

Photo Credits:

commons.wikimedia.org/wiki/File:Wormhole-demo.png

thewhizzer.blogspot.com/

http://www.flickr.com/photos/bbaunach/1055569383/