Paradoxes of Time Travel

In several previous posts, I have discussed various paradoxes that time travel could cause if it were possible. Travels to the future controlled from the past do not cause paradoxes. That they are possible is obvious, since we all travel to the future at a rate of twenty-four hours each day.

It is also possible to travel to the future if one is in a coma and later awakens; or through hibernation, if it were possible, which it is not at the moment, but it may become so someday; or by traveling through space at relativistic speeds, which would result in a time dilation that, upon returning to the starting point, translates into a journey to the future. This last procedure is not possible right now, but it could become so someday, apart from the technical difficulties of achieving it.

Probability of the existence of extraterrestrial life

In a previous post I talked about the probability of the existence of extraterrestrial intelligence and mentioned the difficulty of its calculation, as we don’t know of any planet where they exist, apart from Earth, and to calculate the probability of an event one must know the number of favorable cases and the number of possible cases. For extraterrestrial life, we ​​don’t know either.

In another post I detailed the conditions that should be necessary if life similar to ours were to be possible on a planet similar to Earth. These conditions are many, which reduces the probability that we will find life on some extrasolar planet located in our vicinity. In fact, among the almost 10,000 planets detected so far (of which just over half have been confirmed), 65 are at a distance from their star that could be favorable for life (the Goldilocks zone), but only three of them orbit around stars similar to the Sun (of the stellar class G).

Another conversation with ChatGPT

As I said in the previous post, last January I did two sessions with ChatGPT from OpenAI. In the second, my questions were directed to the following scientific topics:

• The special theory of relativity, the limit of the speed of light in a vacuum, and the possibility of traveling in time.

Throughout the session I was asking the following questions:

1. Can a particle travel at a speed greater than light?

Answer: According to the theory of relativity, as first described by Albert Einstein, nothing with mass can travel at the speed of light...

Superluminal interstellar travel

James H. Schmitz

The human imagination knows no limits. Einstein can tell us that the speed of light sets an insurmountable limit for objects with mass, but deep down we don't believe it. There has to be a way to break that limit! Otherwise, how could we reach the stars during the short span of our lives, return to Earth and tell here what we have seen?

People in our time, especially our civilization, are determined that we must get everything we want. I want to be immortal, so I must be; if I can't, at least my children or my grandchildren. (We don’t usually go beyond our grandchildren…) I want to travel to the center of the galaxy; if I can't, someone will. I want to do whatever I want with my life, so I will… no matter what!

Traveling at the speed of light

Image of
"2001, a Space Odyssey"

Actually, the title of this post is wrong. Traveling at the speed of light is impossible, because it would take infinite energy to accelerate to that speed a body with rest mass greater than zero. What I am going to talk about here is travel at relativistic speeds (close to the speed of light), which means more than 10% of the speed of light (i.e. 30,000 kilometer per second).

Can time travel paradoxes be avoided?

Cover of Fantastic SciFi with a
Porges story (The Shadowsmith)

In a science fiction story written in 1962 by Arthur Porges, entitled The rescuer, the inventors of a time machine discover that a man has entered the machine to travel back in time. To stop him, they destroy the machine with the man inside. When they are tried for murder and destruction of valuable property, they explain:

This man had taken with him a repeating rifle and five thousand rounds of ammunition. His intention was to arrive at Golgotha in time to rescue Jesus Christ from the Roman soldiers. In short, to prevent the crucifixion. And with a modern rifle, who can say he wouldn’t succeed? And then what?... What of the effect on the future, the entire stream of history, secular as well as religious?

The story is an excellent example of the paradox of predestination mentioned in a previous article, with several more that make us doubt the possibility of time travel. But is there no way to avoid the paradoxes? Is it possible to devise a theory that would remove them making time travel feasible, at least in principle? There have been several attempts to achieve this.

Will time travel be possible?

Hibernation in 2001, a Space Odyssey

The obvious answer to this question, such as it is formulated, is yes, of course it’s possible! We all travel in time at the rate of 24 hours a day.

Naturally, this is not what comes to mind when this question is asked. What is usually meant is this: will we be able someday to make sudden jumps in time, either forward (to the future) or backward (to the past)?

There are several schemes for traveling to the future. If they aren’t possible just now, one day they could become so. For example, perhaps human beings will be frozen and remain in suspended animation, to wake up and resume their ordinary lives a hundred years later. Or they could take passage on a spacecraft, make a trip at relativistic speed, and return to their starting point a century later, whilst for the travelers the elapsed time would have been just one year. In both cases, from the point of view of the persons in question, this would have been a trip forward in time, but in reality no sudden leap would have happened, for time would have kept going on for the hibernated body, though the mind would not be aware, and also for the relativistic traveler, although in this case time would have been accelerated.

But when we speak of time travel, we do not refer to these cases, which are possible in principle. We mean disappearing from the present and appearing in the past or in the future. Will we be able to do this someday?