Mad about time II. Time travel to the past.

We live in a vertex of past and future time cones. We have seen the past and we know that we will see a part of the future. We even know that is possible to travel to the future but the question is: can we travel to the past?

In a previous entry we talked about time travel to the future and Einstein’s relativity theory. One important think in that theory is that the light speed is a constant (named c). Speed is a physics quantity defined as the distance travelled by somebody in a time interval. Light speed must be a constant but not the time.

Now think about time travel to the past. How has to be our speed in relation the light speed? We know that if our speed is close to c, time travel to future start to became possible but how about time travel to the past?

If we could have and speed higher to c, what would happen?

If something starts at some time, let us say at 6 in the afternoon (maybe moonrise) and is far away from us about 384 000 km arrives to our eyes in 1.28 seconds. I mean, moon is in the sky but we can see its light 1.28 seconds after. The same happens with the sun but the time is 8 minutes as the distance Earth-Sun is higher (about 150 000 000 km).

If we think in everything around us, we are immersing in a world of light signals that travel different distances; with that in mind the light future cone can be defined. Inside this cone the events are accessible to us but not outside. For example, it is impossible for us to go to the moon and come back in less than a second and we have a future cone for every instant in our life. So we have the opportunity to travel only at some possible close distances in a second and if we want to go more far away, we have to wait more time.

Speed light is thus a limit and says ‘nothing can travel at c value or more’ However some recent theories suggest that this prohibition is only for our speed equal to c but it says nothing for higher or shorter values.

In the same sense as the future cone we have a past cone that involves all the events that could reach to us. At 6 in the afternoon with 1.28 seconds the moon is in our past cone and sun is in some place in our past cone 8 minutes before. Places that are more far away are more and more in our past cone very far away from our present as well.

If we could go out from the light cone, it would be possible to travel to the past or to the future at any place.

Is there a way to do that?

Maybe yes.

The theory of relativity says that we can see that time is perturbed if our speed is close to c but it also says that time changes in the presence of massive things as planets or stars. So time is perturbed close to Earth and Sun and even close to us, but we cannot notice this change.

In the final stage of a star, some interesting celestial objects appear as novas, pulsars, supernovas and black holes.

If a star has 6.6 times the mass of the sun, its dead involves the iron burning, the explosion of the external layers and the contraction of the star. This contraction cannot be stopped by the star gas internal pressure. For this reason its gravity starts to attract everything around it even light.

When something, let us say a clock, is close to a black hole times begin to run more and more slower for people observing outside. Black hole is like a machine to the future.

Theoretical studies have shown the existence of a kind of black hole named wormhole. These wormholes could be a connexion between two distant places in the universe because they change its geometry (included time). Scientists believe that is possible to travel in time using wormholes.

But talking about the progress of real trips, how about the actual possibility for them? Well some people are mad about time travel and not only in science-fiction. Are there real machines or transport for that? Will there be?

Next time I tell you…