Problems with Time Travel
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My Site for Time Travel
There may be no other concept that captures the imagination more than the idea of time travel -- the ability to travel to any point in the past or future. What could be cooler? You could jump into your time machine to go back and see major events in history and talk to the people who were there! Who would you travel back to see? Julius Caesar? Leonardo da Vinci? Elvis? You could go back and meet yourself at an earlier age, go forward and see how you look in the future... It's these possibilities that have made time travel the subject of so many science fiction books and movies.
Why time travel is possible Physicists have found the law of nature which prevents time travel paradoxes, and thereby permits time travel. It turns out to be the same law that makes sure light travels in straight lines, and which underpins the most straightforward version of quantum theory, developed half a century ago by Richard Feynman.
Building such a device would be very difficult -- it would involve manipulating black holes, each with many times the mass of our Sun. But they could conceivably occur naturally, either on this scale or on a microscopic scale.
A black hole isn't really a hole at all, but that's the easiest way to think of its effects on the rest of the universe. Take a star that's at least thirty times larger than our sun and make it explode (called a supernova). Stars do that at the end of their lifetime, sometimes leaving a remnant of the violent explosion. The nature of the remnant depends on its mass. If the remnant is less than 1.4 solar masses, it will become a white dwarf, a kind of hot dead star that isn't bright enough to visibly shine. If the remnant is roughly 1.4 solar masses, it will collapse. The protons and electrons will be squished together, and their elementary quarky particles will recombine to form neutrons. What you would get is small (by stellar terms) sphere of neutrons with perhaps a thin film of electrons and other stuff at its surface. That's why it's called a neutron star. The neutrons don't mind being near each other; but if you get them close enough to each other, they resist being pushed any closer. The neutrons of a neutron star are, indeed, pressed quite close to one another and exert a certain pressure on each other. This pressure prevents the further collapse of the neutrons star. If the remnant is larger than 3 solar masses, it becomes a black hole (well, 2 or 3 depending on who's giving you the number). It is about 30 solar masses before a star becomes a supernova.