| Special Relativity The contributions that Albert Einstein made to theoretical physics were numerous, and they deeply impacted how we think about time, space, matter and energy. In the following paragraphs I will try to give the reader a basic understanding of one of Einstein’s most influential theories and some of the astounding consequences of it. The name of the theory contains the word “special” to help distinguish it from a more broad theory of relativity proposed by Galileo Galilei. Galileo pointed out that it is very difficult to measure the “true” speed of an object. He showed that observed speed depends on the speed of one’s frame of reference. For example, a computer might seem to be lying still compared to its table top, yet it is actually hurtling (along with the planet that it is on) at a tremendous speed through space. Similarly, a passenger inside of a boat might look at a stationary mast outside of her window and determine that it must be the mast that is moving, and not herself, because of the way that the mast travels by her window and thus past her frame of reference. Galileo promulgated that it is necessary to have an external frame of reference to determine relative speeds. The concept of an objective, external frame of reference necessitates that space and time are absolute, universal constants. Einstein used a theory that he had about light, to help update Galileo’s assumptions, and destroy the conventional conceptions regarding absolute space and time. Before Einstein, scientists knew that light traveled at a constant and finite speed- 186,000 miles per second (designated by the letter “c”). What they didn’t know is what medium it traveled through. Because sound waves travel through air, and ocean waves travel through bodies of water, physicists supposed that light traveled through a hypothetical substance that they called “ether”. In 1887 Albert Michelson and Edward Morley performed an experiment that influenced Einstein to question the existence of this imaginary substance. The Michelson- Morley experiment attempted to measure this “ether” rushing by the earth, just like you can sense air passing by you when riding a bike. Before they performed the experiment they assumed that the tremendous velocity of the earth would add to the velocity of light if light was shot in the same direction as the earth’s movement. Conversely they assumed that light shot in the opposite direction would be noticeably slower, because of a concept called “additive velocities.” For example, if you are carrying a loaded pistol with you in a race car and are traveling at 200mph, then you, the bullets and the car all have forward velocity of 200mph. As your intuition might suggest to you, if you shoot the gun ahead of you on the race track, the bullet will have its normal velocity plus the 200 mph added to it from the speed of the car (relative to an observer standing next to the track or course). But if you were to shoot the gun behind you, the stationary observer would notice that the speed of the bullet was less than normal. The bullet would be traveling 200mph less than the normal velocity of a bullet fired by a stationary observer. The same thing was expected of light, but it turns out that light is not affected by additive velocities. The Michelson Morley experiment detected the same exact speed of light in both directions, and the scientific community did not know what to think. Einstein interpreted this finding in an innovative way. He figured that the speed of light must be absolute, and that nothing can travel faster than it. If the speed of light was the speed limit of the universe, then how could you explain velocities relative to it? Well, Einstein reasoned that time slows down for rapidly moving objects. In fact, the amount that time slows down for a moving object increases with its speed. So, the reason that the light that was shot off, in the same direction as the velocity of the earth, was not additively faster is because time on the Earth was actually slower than time would be for a stationary observer. This is kind of non-intuitive, but consider a real life experiment… To test this theory, scientists synchronized two atomic clocks, they left one at an airport, and placed another in a supersonic jet. After the jet flies around at high speeds, and lands, scientists then compare the two clocks. The clock that was in the jet is slightly behind the clock that was left on the ground. Again, this is because time slows down for objects traveling at high speeds. It is interesting to note that physicists have determined that because light travels at “c” light particles (photons) do not experience time and therefore do not age. Another example of this phenomenon is illustrated by Einstein’s “Twins Paradox.” This is the story of two twins (born, of course, on the same day). One twin goes into space and travels in his space ship at near light speeds. He doesn’t notice that time passes slower for him than it does for his brother at home- but it does. He can’t notice this difference though because when time slows down, everything slows down: biological aging, atomic clocks, radioactive half-life, phenomenological consciousness… So by the time the astronaut twin comes back to the earth, he finds that much more time has passed on the earth, than passed in his spaceship. When he meets his twin, he might be surprised to see that his twin looks much older than he does. This concept of relative time was not ignored by science fiction writers or by real life time machine builders. Building a vehicle that is sufficiently fast to serve as a time machine is very difficult and as we will see in the next essay, it becomes progressively harder to increase your speed- especially near the speed of light. Also, it is theoretically impossible to achieve the speed of light – a sad truth for time machine builders. In reality though, we are all time travelers. Every time we go for a jog, or step in a car, train, bus or plane, our experience of time slows (ever so slightly) compared to a stationary observer. To learn more click here for information about Einstein’s “energy-mass equivalency theory.” Before Relativity Before writing his article “On the Electrodynamics of Moving Bodies,” now known to the world as special relativity, Einstein published three other important papers: 1) An article on the “photoelectric effect” helped to further define the quantum (the fundamental unit of light) and its dual nature as both a wave and a particle, thus adding to our knowledge about how light propagates, and how it interacts with matter (an area first explored in 1900 by Max Plank). 2) He explored the physics of “brownian motion,” how air molecules travel and interact with one another. The equations that he derived in this paper, for example, help scientists make more accurate predictions about how smoke dissipates with time; they also explain the seemingly random movements of very small amounts of pollen (he explained that pollen is seen to vibrate under a microscope because of the way that it is bombarded by air molecules). 3) He also wrote a very influential paper examining how molecules bond and interact. All of these papers, along with his theory of special relativity, were completed in 1905- quite a productive year for Einstein. These three findings were monumental, but the development of special relativity is probably one of Einstein’s most widely recognized achievements. |