Time travel has fascinated people since the first stone age man forgot first stone age woman's birthday. Since then, science has fueled speculation in science fiction, and vice-versa. So what, exactly, is modern science's latest and best theories on the possibility of time travel?
In answering this question, we need to be careful about whether we are talking about the true nature of time (the answer to which is 'we haven't a clue') or how time can be measured, or more specifically 'perceived to pass'.
'Perceived to pass', being subjective, gives us some literary room to move. This 'relativises' the passage of time to a subjective experience. Hence, we have 'time travel' resulting from suspended animation, and the Twin paradox, in which high-speed relativity ages one person slower than others.
Literature also plays on the relationship between memory and time - an individual's orientation on the timeline is based on a memory of the past and an ability to foresee the future.
This is physics' idea of how to know which direction you are travelling in time: is entropy greater or less than zero?
The Second Law of Thermodynamics is the one that says energy is continuously becoming more and more dispersed in the universe. If the total amount of the universe stays constant, and it was all concentrated in one place at the Big Bang, it will ultimately become evenly distributed throughout the cosmos, making life impossible. You see, life rides the waves of uneven distribution of energy.
this is great, because we get a certain amount of licence to give a subjective definition to the passage of time. This leads to the corollaries of Relativity, such as the Twin Paradox.
Our ideas of physics today started from the revolution, known as the 'New Physics', which came about because of Albert Einstein's 'annus mirabilis' in 1905. In this year he published 4 seminal papers which completely changed how we view the universe.
One of the 1905 papers was 'Special Relativity'. Einstein went on to produce 'General Relativity' in 1915, which is considered by many to be the greatest scientific work since Newton's Principia mathematica (1687).
Special Relativity introduces the concept that time and distance can dilate (change) when something or someone is travelling at high speeds. In classical mechanics (those using Newton's Laws of Motion), space is considered to be a fixed, unchanging frame with three dimensions. In this frame, time is constant and the same everywhere (universal). No, no, says Einstein, time is relative to the observer: either his or her speed, or position relative to large gravity sources. This is because of something he called 'space-time'. A star or planet has a gravitational field because it distorts the space it occupies, like putting a heavy ball in the middle of a rubber sheet. not only does this cause things to fall towards it and fly into orbits, but it causes changes to the way time is perceived to pass!
If someone were to fly towards a Black Hole, they would appear to us to slow down, and eventually become stuck forever near the surface (the event horizon), because their time has slowed down so much compared to ours.
|Speed (c = speed of light)||Time dilation: % of Earth time experienced by astronauts||1 astronaut year experienced on Earth as:|
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