Sunday, January 14, 2007
Time Travel - A BBC Horizon Video (49 mins)
UPDATE: February 24 2007: The BBC video 'Time Travel' is no longer available and has been replaced by:
"The 2003 BBC documentary "The World's First Time Machine", directed by Ben Bowie and featuring Ronald Mallett (Professor of Physics, University of Connecticut), premiered in the USA on The Learning Channel on December 3, 2003." - see below.
"BBC Horizon's Time Trip (originally broadcast on BBC Two, Thursday 18 December 2003 at 9pm) is a thrilling journey deep into the strangeness of cutting-edge physics - a place where beautiful, baffling ideas are sometimes indistinguishable from the utterly crazy.
On this journey, we meet a time-travelling pizza, a brilliant mathematician in a ski mask and even God. The journey ends with a strange and dark conclusion - one which calls into question our very existence.
Ever since Einstein showed it was theoretically possible, the quest to travel through time has drawn eccentric amateurs and brilliant scientists in almost equal numbers. The amateurs include Aage Nost, who demonstrates his time machine in front of the cameras. The professionals include the likes of Professor Frank Tipler of Tulane University. His time machine sounds good - but it would weigh half the mass of the galaxy.
There is, however, one way that time travel to the past could be possible..."
From 'Time Trip - questions and answers'
How widely accepted is the theory that we can travel in time?
According to Professor Paul Davies "Scientists have no doubt whatever that it is possible to build a time machine to visit the future". Since the publication of Einstein's Special Theory of Relativity in 1905, few, if any, scientists would dispute that time travel to the future is perfectly possible.
According to this theory, time runs slower for a moving person than for someone who is stationary. This has been proven by experiments using very accurate atomic clocks. In theory, a traveller on a super high-speed rocket ship could fly far out into the Universe and then come back to Earth at a time hundreds or thousands of years in its future.
Another consequence of special relativity is that gravity slows time down. So, another way of time travelling to the future would be to go and sit next to a black hole or a neutron star, both of which are very massive and have huge gravitational fields. When you went back to Earth, it would have aged more than you.
Time travel to the past is more problematic, but there is nothing in the known laws of physics to prevent it. It is accepted that if you could travel faster than light, you could travel to the past. However, it is impossible to accelerate anything to a speed faster than light because you would need an infinite amount of energy.
But hope for prospective time travellers comes from Einstein's General Theory of Relativity, considered to be the best theory of time and space that we have.
In 1948 Kurt Godel (info/profile) worked with general relativity to produce equations suggesting the possibility of time travel to the past. He showed that a rotating universe, consistent with Einstein's theory, would allow you travel back in time. Godel knew that his model was unlike the real universe we inhabit and also that even if we did live in such a universe, time travel would be practically unachievable because you would need a hugely powerful rocket in which to cover astronomical distances. Despite this, Godel's work was firm evidence that time travel to the past is, at least in theory, possible.
Since then, numerous other scientists have come up with other solutions of general relativity that allow time travel to the past. Most rely on the prediction of the existence of 'closed time-like curves'. According to these scientists, there are ways of distorting space-time to make it curved in such a way that shortcuts through space-time exist allowing you to effectively travel faster than light and journey back into the past.
Not all scientists like this idea and there are some scientists, like Professor Stephen Hawking, who insist that there must be something that prevents it. In 1990, Hawking proposed a Chronology Protection Conjecture* which says that the laws of physics disallow time machines. Basically, such scientists argue that nature will conspire to prevent the building of a time machine - one possibility is that runaway surges in quantum energy would generate massive gravitational fields and turn any time machine into mush. There are no clear answers to the issue because quantum physics and gravity do not sit well together and there is not yet a unified theory of quantum gravity.
Hawking and others have serious problems with the fact that time travel to the past would violate causality and this would have serious implications for our understanding of how the Universe works. A final answer to whether we really can travel back in time may have to wait until scientists find a way to bring quantum mechanics and general relativity together.
*See Stephen Hawking's Space and Time Warps:
"In science fiction, space and time warps are a commonplace. They are used for rapid journeys around the galaxy, or for travel through time. But today's science fiction, is often tomorrow's science fact. So what are the chances for space and time warps. The idea that space and time can be curved, or warped, is fairly recent. For more than two thousand years, the axioms of Euclidean geometry, were considered to be self evident. As those of you that were forced to learn Euclidean geometry at school may remember, one of the consequences of these axioms is, that the angles of a triangle, add up to a hundred and 80 degrees. However, in the last century, people began to realize that other forms of geometry were possible..."
With a brilliant idea and equations based on Einstein's relativity theories, Ronald Mallett from the University of Connecticut has devised an experiment to observe a time traveling neutron in a circulating light beam. While his team still needs funding for the project, Mallett calculates that the possibility of time travel using this method could be verified within a decade.
..."Einstein showed that mass and energy are the same thing," said Mallett, who published his first research on time travel in 2000 in Physics Letters**. "The time machine we’ve designed uses light in the form of circulating lasers to warp or loop time instead of using massive objects."
The gravitational field due to the circulating flow of electromagnetic radiation of a unidirectional ring laser is found by solving the linearized Einstein field equations at any interior point of the laser ring. The general relativistic spin equations are then used to study the behavior of a massive spinning neutral particle at the center of the ring laser. It is found that the particle exhibits the phenomenon known as inertial frame-dragging.
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