12 September 2009
Sometimes, when people get together, the conversation will steer towards some of the very weird things predicted by Albert Einstein and his papers on relativity. Things like travelling at the speed of light, and how time travel into the future is possible.
It’s a fun subject to speculate about, even if you’re not a certifiable science geek like me. (My wife would definitely agree on the certifiable part- but my daughter would be the one to go so far as to have me committed and put in a white rubber room!)
Understanding the details of relativity is something that requires a level of mathematics far beyond mine. Einstein himself needed help from the mathematicians to work out the details! Didn’t know that did you! (The basic equations however are actually simple algebra and easy to work with)
Understanding the basics though is not hard. I don’t mean understanding what the predictions are. I mean understanding WHY time must run slower as you approach light speed. If you already do understand it, read on anyway, you might find my way of thinking about it easier to understand. In any event, if you do not understand WHY it MUST be true, then let me try and explain it.
I should warn you, that once you DO “get it”, you will drive your friends (and wife and daughter) crazy with your desire to explain it to them! It’s a fundamental way the universe works and it’s truly amazing. I have held off on this story because, I felt I wasn’t really qualified to write it. My science is Meteorology. Which is just Atmospheric Physics. I know only a little of the fundamentals of Relativity.
I do understand this part however, and I rarely see it explained in an easy to understand way by the giant brains. Seems to me that it really should be.
So here’s my version of what Einstein called a Gedanken. A thought experiment. You cannot explain this in one paragraph. That does not mean it’s hard, it just means you’ll likely never see it on TV!
How will you know if you understand it?
You will probably say something along the lines of “Holy Cow I get it! So that’s what Einstein was thinking!”
Let’s begin. Nothing hard here, but this will be the longest post I’ve written. I truly hope that you (perhaps, for the first time) understand what Einstein really figured out in 1903, and just how strange our Universe is!
Light is a constant. Scientists write it with the symbol “c”. c is the speed of light in a vacuum. About 300,000 km per second. At this speed you can circle the Earth 8 times in a second. You can reach the Moon in 1.4 seconds. The sun will take about 8 minutes. Jupiter is 43 minutes away at light speed. The nearest star will take you 4 years. Want to cross the Milky Way galaxy? It will take you around 100,000 years travelling at light speed c.
People knew light was a constant before Einstein. He was just the first to understand what the consequences were.
Light ALWAYS travels at c. Always. 300,000 km/sec or to be very precise:
the speed of light = 299 792 458 m / s or “c”
If I stand up in a convertible going 50 miles per hour, and throw a cricket ball at 30 mph. How fast is it going?
The answer of course depends on who is measuring!
I see it going away from me at the speed I threw it. 30 mph. The guy at the side of the road see’s the ball going 80 mph. (50 mph for the car and 30 for the ball). That’s easy. No big deal. It just depends on the frame of reference.
An Astronaut in space might say that the ball is going 780 mph! If I am in Huntsville, Alabama, then just sitting on the ground, I’m rotating eastward with the Earth at 700 mph. If I threw the ball. while driving toward the East at 50 mph, then just add them up. 700 + 50+30. The ball is going 780 mph.
What if instead of a cricket ball. I threw a beam of light while driving at 50 mph??
Would the guy on the side of the road measure the speed of my light beam at
c + 50 mph??
I would measure it going away from me at “c” and so would everyone else in the universe. No matter how fast they are going. Light speed is a UNIVERSAL CONSTANT.
Yes, I’m overstating the obvious, but read on!
From watching Saturday morning cartoons, you may know that George Jetson lives in the 24th century, and that he’s not the brightest bulb in the pack! Lets pretend, that his boy Elroy is trying to explain that light speed is a constant to George and Jane- his wife.
George does an experiment. He drops his wife at the space mall high above the Earth. He then drives by her very fast. I mean fast. Like 200,000 km per second. This is two thirds light speed c. As he passes Jane, he turns on his headlight. He then times how long it takes for his light beam to reach the Pizza Hut exactly 300,000 km away.
What does George measure?
Easy. Light travels 300,000 km/sec, so George sees that it took exactly one second for his headlight beam to go 300,000 km in front of him, to reach his favorite pizza place. It didn’t matter that he was going almost at the speed of light. Light speed is always the same. The light beam moved away from George at 300,000 km per second. George exclaims Einstein was right! Light always travels at the same speed.
Jane, standing in the space mall car park, disagrees. She tells George, that after one second, the light beam did indeed reach the pizza hut. However, George she says. It was not 300,000 km ahead of your car. You were right behind the light beam. You were going too fast George!
To make her point, she pulls out her super GPS Light speed gun and shows George that it measured the light beam going away from his car at only 100,000 km per second. Not 300,000 km/sec. You see George, you were already going 200,000 km per second when you turned on the beam. The light beam did go at speed c. You were flying along just behind it and it was moving slowly out ahead of you.
George tells Jane that she is nuts!
Jane, he says, I turned on the headlight and exactly one second later, that light beam was 300,000 miles in front of me! It had to be Jane, because light travels 300,000 km every second.
She says “I know it travels that fast George, but you were going 200,000 km per second and you were right behind it, so you could not have seen it get that far in front of you in one second”. “Maybe your watch is broken George!”
So they do it again. This time George is going just a little less than light speed as he passes Jane. He turns on the light and Jane sees the beam shoot out at 300,000km per second. Since George is going almost that fast, she sees the light beam moving very slowly ahead of George’s space car. She agrees the light beam was moving at the speed Einstein said.
George stops and comes back. He has a totally different story. Jane, he says, “I turned on my video camera, look at this!” The video shows George turning on the light beam. One second later, he measures how far way from him it is and gets 300,000 km. See Jane, light moves at 300,000 km every second, so it had to be that far away from me!
Who is right? Jane or George?
Einstein thought about this for weeks and months. He had never heard of the Jetsons, but he pondered this same problem. What Jane saw makes sense. The beam of light was traveling at light speed c, but George was going just below that speed so he was almost catching up to his light beam. George knows that light always moves away from the observer at 300,000 km per second. NO MATTER WHAT REFERENCE FRAME. Just like when I threw my beam of light in the convertible.
Light truly is a constant. It does not matter what speed you are traveling, it always moves away from you and everyone else at the same speed-c.
Einstein realized there was only one solution that could explain what George saw, AND what Jane saw, and that they could both be right.
Was George’s clock broken??
Except it wasn’t.
It was just running much slower!
What was one second to George, travelling near the speed of light, was several seconds to jane. If Jane could see into his car at the moment he turned on the headlight and started counting to one, she would see George moving in slow motion! She would watch as the light beam moved slowly ahead of his fast moving car, and then as it finally got 300,000km ahead of him, she would see George’s clock click to one second!
The only way to explain what George saw and what Jane saw, and for them both to be right, is that as you go very fast and approach light speed, TIME SLOWS DOWN FOR YOU. You would not notice it. Everything would seem normal. When you stopped and went back to pick up Jane at the space mall, you would discover that what was 20 minutes to you was 40 years to her!
Time is relative. Light is not.
This is what Einstein discovered that no one else had. This is why time really does slow down for someone travelling very close to light speed. If you are only going 80% of light speed, the difference is very minor. It only becomes important at speeds very close to c.
The clocks in orbit that are used to run your GPS are moving at about 7km sec. These clocks have to be precise to a millionth of a second or the GPS system would not be accurate. They run slow because they are moving this fast and the relativity equations must be used, or your GPS will not be accurate!
Time really is relative. It’s not some weird thing you can only understand with advanced mathematics. IT HAS TO BE THAT WAY IF LIGHT SPEED IS ALWAYS THE SAME!
and it is!
I hope you “got it”. If not, reread it a few times. It took me a few minutes when it was first explained to me. Reality is not what we think it is. The Universe is a very strange place. It just looks ok because we go slow all the time! Imagine what Albert Einstein felt when he realised that he was the first human being to ever know this! He knew that for the first time, in all of human history, someone had figured it out, and he was only one who knew it.
That truly is what Science is all about.