Time stays. We pass.


 

Is time travel possible?
What is time?
Is time real at all?
Did it have a beginning?
Will time have an end?
What is eternity?
The mere asking of these questions stretches our minds, and the continual search for answers provides useful insights along the way.



Time

The concept of time is central to all the world's religions.
Most cultures have a grammar with past and future tenses.
Most societies have some kind of demarcation such as seconds and minutes, and yesterday and tomorrow.

The study of time became scientific during the time of Galileo and Newton, but a comprehensive explanation was given in the twentieth century by Einstein, who declared, in effect, time is simply what a clock reads.

 
Causality

For physicists, the question as to whether or not time travel is possible is a most important one, if only to discover whether we live in a causal or acausal universe.

Causality is related to time think of the words 'sequence' and 'consequence'; and the question really is whether an effect can precede the cause.

Do the laws of physics permit time travel, even in principle?

 
Time tavel

Does the idea of time travel have to be confined to myths, science fiction, Hollywood movies, or even speculation by theoretical physicists?

Thanks to Einstein we may at least approach the notion of Time Travel.

 
Einstein

As Albert Einstein showed back in 1905 with his special theory of relativity, time slows down for objects moving close to the speed of light, at least from the viewpoint of a stationary observer.

You want to visit the earth 1,000 years from now? Just travel to a star 500 light-years away and return, going both ways at 99.995% the speed of light. When you return, the earth will be 1,000 years older, but you'll have aged only 10 years. You will be able to meet your descendants, and collect a lot of interest from the bank, but your library books will be overdue.

 
Back in time

Is there any way of going backward in time? Einstein's general theory of relativity, which he published in 1915, showed that space and time are curved, and that the curvature can be large in the neighbourhood of very massive objects. If an object is dense enough, the curvature can become nearly infinite.

So much for geometry. There are also questions of the topology of our Universe.

Would it be possible perhaps to open a tunnel that connects distant regions of space-time as though they were next door?

 
Wormholes

Physicists call this tunnel a wormhole, in an analogy to the shortcut a worm eats from one side of a curved apple to the other.

In 1988, Kip Thorne, a physicist at Caltech, and several colleagues, suggested that you could use such a wormhole to travel into the past.

Here's how you do it: move one mouth of the wormhole through space and return it to its original position, while leaving the other one fixed. Then jump in through the moving end. This end ages less, so it connects back to an earlier time on the fixed end. When you pop out through the fixed end an instant later, you'll find that you've emerged in your own past.

 
Negative density

The problem with wormholes is that the openings tend to snap shut a fraction of a second after they're created. The only way to keep them open, as far as we know, is with matter that has negative density, i.e. stuff that weighs less than nothing. This may sound impossible, but the Dutch physicist Hendrik Casimir theorized in 1948 that holding two plates of electrically conducting material very close together in a vacuum actually does create a region of negative density that exerts an inward pressure on the plates. The force predicted by Casimir has been verified in the laboratory.

"...mere trifles like negative energy... should not deter any mathematical physicist worthy of his salt."
Arthur C. Clarke

Dr. Serguei Krasnikov of Starlab has shown that the same mechanism can support a macroscopic wormhole for an arbitrarily long period.

Wormholes could conceivably occur naturally.

 
The Grandfather paradox

The worry for physicists is that Time Machines raise the possibility of paradoxes, familiar to science fiction fans. For example, a time traveller could go back in time and accidentally (or even deliberately) cause the death of her granny or grandad, so that neither the time traveller's mother nor herself could ever be born, and thus she would have been unable to kill either of them etc. etc.

 
Science fiction

Time Travel has long been a theme in myth and legend.

The Prophet Mohammed is carried by a mare into heaven. After a long visit, He returns to Earth just in time to catch a jar of water the horse had kicked over before starting its ascent.

A medieval monk is entranced for a minute by the song of a magical bird. When the bird stops singing, the monk discovers that several hundred years have passed.

King Arthur's daughter Gweneth slept for 500 years under Merlin's spell.

The book 'Time Machines' by Paul J. Nahin reviews hundreds of works of fiction all of which in some way concern the theme of time travel, from the best known ones such as H.G. Wells' famous novel The Time Machine (1895), to the rather romantic tale by Young entitled 'Dandelion Girl', singled out by one of Starlab's physicists.