Speech #6 - Work with Words: “Einstein Made Easy”
I was not a sports star in school. My legs are short. (Yes, I know they reach the ground!) I am right-eyed and left-handed. I liked reading better than playing ball. I did that so much I skipped a grade. After that, I was a year younger than all the other kids were, sure to be picked last for any team. Since I couldn’t brag about my great plays, I boasted about how smart I was instead. This is not a good idea. I may have been book smart, but not people smart.
While I have learned not to tell people how high my S.A.T. scores were, I still, by habit, use big, long words, just to show my learning. My last speech had many, but the point of this project is to be clear by using shorter words. Therefore I will try to explain a tricky topic clearly, using only short well-known words. I could call my speech “Einstein for Dummies” but I hate titles like that. My old boasting habit would never, ever let me buy a book in that series, no matter how useful or well written it was. Worse, to address such a speech to you would be a put down. Besides, they would probably sue me for using their trademark.
Einstein had two major-league ideas. I will talk about the first one, which he wrote about in 1905, called special relativity. That is the only five dollar word I will use in this talk, but if you want to use what you will learn, it helps to know the real name. The real world is very complex. To figure it out bit-by-bit is simpler than trying to do it all at once. That is the meaning of special in this idea. Einstein first worked on how things behaved that weren’t speeding up or slowing down. This is a special case that is simpler to figure out.
Einstein’s idea was to pretend that all people would always notice that light was always going past them at the same speed, 186,000 miles per second, no matter how fast they were moving or which way the light was going. He then thought clearly through all that this meant and came to results that amaze. Why did he choose to pretend that? The earth spins once per day. It is also moving around the sun. Adding these up means we are not always moving on the same course or at the same speed. Yet each time light’s speed was clocked, no matter where or when it was timed, the answer came out the same. He wanted to make his idea fit that fact.
At slow speeds and with normal objects it isn’t like that. If someone hits you with a snowball thrown at 20 miles an hour, it might hurt a little bit. If you are riding in the back pickup truck going sixty, and someone throws a snowball from the curb at the same 20 miles an hour that hits you in front, it could knock you down or worse, because it would hit you at close to 80 miles an hour.
With light it doesn’t work like that. The green light from the next corner passes the snowball thrower at 186,000 miles per second. It passes the snowball at 186,000 miles per second, even though the snowball is moving away from the signal at 20 miles an hour. It passes the pickup truck at 186,000 miles per second, even though you are moving towards the light source at 60 miles per hour. For these all to be true something strange has to happen.
To measure speeds we need two things, a known distance, like a ruler, and a timer. To the person on the curb the stopwatch on the pickup truck would seem to be running very slightly slow. If the pickup truck moved faster, the clock would appear even slower. At 148,800 miles per second, which is 80% of the speed of light, we would see its clock only tick off 36 seconds in a minute. Brian Greene shows why in his book about string physics. Imagine a light clock. It has two perfect mirrors and bit of light bouncing up and down between them. The clock tells time by counting the number of times the light bounces back and forth. Suppose the mirrors were three feet apart, sitting in the back of that super fast pickup. Standing on the curb, the snowball thrower would notice that the light had to travel four feet forward while it was travelling the three feet from one mirror to another. Using A squared plus B squared equals C squared, he figures that the light is going 5 feet between bounces, so they think it takes one and two-thirds seconds to tick off a second. We, on the pickup see that is just has to go 3 feet, so one second goes by per second, like always, staying in time with all other clocks we have with us.
It gets really strange when we look at the light clock’s twin on the curb. From our point of view, they are moving backwards at 80% of the speed of light, so we see the light in their clock moving 5 feet per bounce. To the pickup riders, the curbside clock seems slow. This is a logic problem. Both clocks can’t be slower than the other. To find out which was really slower, we would have to slow down the pickup, which would take us out of the special case we agreed to think about. Einstein’s next major league idea, general relativity, tried to cover that problem, but that is another speech. After all, it took him ten more years to come up with that.
The change in the speed of the clock doesn’t make the speed of light constant, although it helps. Suppose there were two light clocks on the pickup, with the second one tipped on its side, so the light raced forward and back. To us on the pickup, the two clocks would stay in synch, so they have to for the curbside viewers as well. From their point of view, to bounce from the back mirror to the front mirror, the light would have to travel fifteen feet (the three feet between the mirrors plus the twelve feet the truck moves forward during the light’s flight. While the trip back would be really quick, the time of the forward journey alone would take longer than the ten feet light would have to travel on a round-trip on the first clock. To fix this, the only other tool to use is the distance between the mirrors. If lined up with way the pickup truck was going, it would seem shorter to someone on the curb. From the pickup truck, a curbside ruler pointed the way the truck was moving would seem shorter too. There is that logic problem again.
Now all this would break down if you could go faster than the speed of light. After all, how could light pass you at 186,000 miles per second if you were going faster than that? Einstein’s idea prevents this, as there are other results that follow from the same logic that show that besides clocks slowing down and rulers shrinking, mass grows as you go faster. Thus it would take more power than there is to push you up to the speed of light, with nothing left to push you any faster. This may be sad news for those who hope Star Trek or Star Wars could come true.