Sorry that it’s been forever since I posted. I started this post a couple weeks ago but hit a dead end trying to explain it, set it aside, then got busy with other things. This post was a tough one and I’m still doing my best to wrap my head around it but I thought it was incredibly interesting and trying to explain something is often the best way to understand it yourself. However, I have no background in physics whatsoever and the purpose of this post is to make ideas accessible to everyday people so if I make any egregious technical errors or simplify a concept to much, please feel free to correct me in the comments.
This topic requires a little bit of background. Back in the days of the ancient Greeks, it was believed that the natural state for all things to be in was resting. Something only moved if it was pushed and, left to it’s own devices, it would go back to being still. This was the belief for a long, long time because no one ever really bothered to test if it was true or not.
Eventually, Galileo did an experiment which showed that that was not the case and that when a force acts on an object, it’s effect is to speed it up or slow it down. Not just get it moving in the first place. When you push a ball, it does not stop on it’s own, but rather is stopped by friction on the ground and air resistance. If those and other external forces didn’t exist, it would continue to move forever. Newton expanded on that idea by publishing his laws of motion and gravity. For simplicity’s sake, I’m not going to recite them in this post but you can look them up here.
Newton’s laws have a lot of implications but the most important one for our purposes is that the idea of an absolute state of rest is incorrect. Common sense would lead us to believe that there is an absolute state of rest. Look at your chair or computer. It’s not moving and thus appears to be at rest. But while it looks like it’s just sitting there, it’s actually hurdling 67,000 mph (along with the rest of the planet) around the sun. And the sun is moving at it’s own breakneck speed around the center of the galaxy. So if you walk down the street at a brisk pace, are you walking at 5 mph or at 67,005 mph? Either one is technically correct and it demonstrates that the idea of space is relative, not absolute. Another example (shamelessly stolen from the book where I am getting most of my information, A Brief History of Time by Steven Hawking) is that of a train travelling north at 60 mph. Since the universe has no definitive boundaries, it would be equally correct to say that the earth is standing still and the train is moving north at 60 mph or that the train is standing still and the earth is moving down at 60 mph. Or both.
This was a major milestone in understanding the universe, however people still thought of time as being absolute and unrelated to a “relative space”. Time marched forward, one second at a time and there was nothing anyone could do about it. However, people were still scratching their heads about something else: light. Did light have a constant speed? Well, in 1865, James Clerk Maxwell predicted that it did. So how fast does it go? The way that you determine speed is by dividing the distance it travels by the time it takes to get there. But space is not absolute, so no one could agree on how far it had traveled, even if they could agree on how long it took to get there.
In 1905, Albert Einstein, in a paper in response to this quandry, proposed his famous equation, E = mc^2. This essentially said that as an object goes faster, it gets heavier and thus requires more energy to propel it forward. Eventually, it will get to a point where it is infinitely heavy and requires an infinite amount of energy to move it and can not move any faster. However, since light has no mass, it can go faster then that speed (at 299,792,458 m/s to be exact) and nothing can move faster than light. This equation gave light a constant speed that was not affected by distance. To find the time it takes to get from one place to another, you divide the speed by the distance traveled. If no one can agree on the distance, since space is relative, it follows that no one can agree on the time it took to get there. This leads us to the conclusion that time is ALSO relative to the observer and is not a constant force which had been believed for thousands of years. This also integrated space and time so much that they can no longer be seen as separate and are now referred to as space-time.
I hope that that was a simple enough explanation. It’s hard to simplify it too much and still hit all the main points. Feel free to leave comments and let me know what you thought! Also, if you have any topics that you thought sounded interesting but were just too lazy to look up, let me know and I will try to get them explained for you!
Hawking, Stephen. A Brief History of Time. New York: Bantam, 1996
Wikipedia. “Newton’s Laws of Motion.” Accessed March 31, 2013. http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion
Curious about Astronomy? “At What Speed Does the Eath Move Around the Sun?” Accessed March 31, 2013. http://curious.astro.cornell.edu/question.php?number=356team
Wikipedia. “Mass-Energy Equivalence.” Accessed March 31, 2013. http://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence
Wikipedia. “Speed of Light.” Accessed March 31, 2013. http://en.wikipedia.org/wiki/Speed_of_light