Advanced Physics Made Simple

Special Relativity

       Introduced by Albert Einstein in 1905, this theory was developed to explain the experiment of Michelson and Morley:

The Michelson Morley Experiment

Think of a baseball pitcher throwing a baseball at 50 mi/hr from the pitchers mound. Then the ball crosses homeplate at 50 mi/hr.  Now suppose the pitcher stands in the back of a truck travelling at 50 mi/hr and throws the ball at 50 mi/hr.  Then the ball will cross homeplate at 100 mi/hr.  This is just natural.

In the late 19th century, Michelson and Morley designed an experiment that replaced the baseball with light, and the truck was replaced by the entire Earth.  But what they found was not the obvious solution. They found that light travelled at a constant speed. If the pitcher shines a flashlight at the batter, it goes at the same speed as if the pitcher shines the flashlight from the back of a moving truck.

 Minkowski Spacetime

        Actually, Minkowski developed this approach to special relativity after Einstein, but it makes the physics simple.  You may have heard that time is a dimension just like space.  IT IS NOT LIKE SPACE.  Lengths measured in space are positive, and they are always positive. Lengths measured in the time dimension are called IMAGINARY, which means that a given length, multiplied by itself, is always negative.  ( Since time is a negative dimension and space is a positive dimension, it is possible to have a total length of zero - this is the case for light)

        The results of this simple physical idea imply, after long mathematical proofs,

1. TIME DILATION -  A person who moves through space experiences a shorter period of time. In other words, if you sit on Earth for one year while your friend travels to Pluto and back, then your friend will claim that only six months had passed.  This is not an illusion, nor is it a fault in the mechanism of a clock, it is a real effect. One way to think of this is if you walk along a straight line between your house and your neighbours house, and your friend walks from your house to the grocery store two miles away, and then to your neighbours house, you both start and end your trips at the same places, but your friend has travelled four miles further.

2.  LENGTH CONTRACTION - If a car moves past you, it will seem shorter than it actually is.

3. MASS INCREASE - A moving object weighs more than an object which isn't moving. It can be shown that if an object travels at the speed of light, it must either weigh nothing (which is only true for light) or it is infinitely heavy. This implies nothing can travel at the speed of light.

Tachyons

      You have probably heard this term in science fiction books and movies without knowing exactly what it was.  If you read the section on special relativity, you know that nothing can travel at the speed of light except for light itself. Obviously there are things which travel slower than the speed of light, because our entire world fits in the category. What about faster than light?

     There is nothing in the theory of relativity which prevents an object from travelling faster than the speed of light, although such objects can never slow down to below the speed of light or they would violate special relativity. Any particle which travels faster than the speed of light is called a TACHYON.   Such objects must obey certain rules:

1.  The mass of a tachyon is imaginary. Recall that this means that if you multiply the mass by itself, the result is negative.

2. Tachyons go faster when they lose energy.  I must admit I can't think of a case when a tachyon would lose energy, since a collision with normal matter is forbidden.
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