Albert Einstein is most well known for his general theory of relativity. Today we are going to tackle as best we can, this theory of relativity. To do this, we have phoned in the best source we could find: Chico the 7th. Chico was Einstein's dog. Although Chico disliked Einsteins work, (and would frequently bite the mailman in efforts to cut off Einsteins correspondence) he nevertheless learned quite a bit from his master. This knowledge of relativity (in dog lingo) he passed down to his offspring. Chico the 7th has had no formal college training, but has attended several lectures on the subject, and has been working on a theory of why cats are so annoying (corollary, why cats' bones tend to disappear when they sleep). But I digress.
Chico, it's good to hear from you. What can you tell us about all this relativity nonsense?
Well, it's not nonsense, if you go in for this sort of thing. :) Now lets see, where to begin. The theory of relativity has been around since before Einstein. He just improved it. Simply put, any law of nature should be the same at all times, that is, any person observing a tree falling will see just that, a tree falling. To this we will add the dimension of time. Suppose one sees a tree falling, and then 5 minutes later, another person sees a different tree falling in the same way. By comparing these two instances, we can record and observe the results, and begin to calculate the energy involved. It is in this way scientists can make testable predictions on the laws.
Okay, I can picture that. I spend a lot of time outside and around trees. :)
Now lets look at the theory of general relativity. Einstein did most of his most famous work with the general (and special) theory of relativity. The theory of special relativity states that these laws of physics (like the tree falling) should be the same in every inertial frame of reference.
What's that mean?
It means assuming where you are standing is a non-moving entity. Technically, you would be standing in a forest, on earth. Now the earth is moving around the sun, but for our purposes, we assume the earth is standing still in reference to the tree. So, special relativity states that the laws must remain the same (the tree will fall toward the earth and emit a loud THWUMP). This allows special cases, like a non-inertial frame of reference to have a different set of laws. For example, the light from a distant star is traveling at the speed of light. How can we still see it? Because even though we seem to be standing still, the earth is traveling pretty fast, the sun is moving about our galaxy, and the light is traveling not away or across the earth, but directly towards the earth. So, we can see particles traveling at the speed of light.
You're very smart for a dog, did you know that?
Thanks fluffy. Like I said, I attended a few lectures, and the subject fascinates me. As I was saying, the theory of special relativity allows for special cases to be made. Now then, in special cases, physics professors would historically attack the problem (*gobble snarf*) using a coordinate transformation, first to an inertial reference frame (back to our tree problem), calculate it, and move it back to the special case (absolute inertia). Our star problem above would be a little more difficult. But since it would use strictly non-inertial reference frames, it is not constricted to our regular laws of nature.
Thanks...I think. Now lets move on to the question we have been dying to ask. According to Einstein, is time travel and faster-than-light travel possible?
This is a question I am often asked. This question deals with a branch of relativity called time dilation. Time dilation has been proven and tested with astronauts and the ISS. Time dilation, simply put, is an actual difference of elapsed time between two separate events relative to gravitational pull. A dog on earth with a pocket watch, and a dog in space with the same pocket watch may observe that after a day apart, their watches would be about 42 minutes off from one another. This is not due to a mechanical failure of the watch, rather the nature of space time itself. Here's another example: place a cat in a spaceship for a little over 2 months. Place another cat on earth. At the end of the time period, the cat on earth will have waited spent 81 days watching tv and taking naps. The cat in space will have only spent 80 days watching tv and taking naps. This 1 day difference is due to the space time/gravitational difference between earth and the first layer of space around our planet. furthermore, the further away from gravity you go, the slower time will appear to pass for you. The same theory holds true for velocity. The faster you are going, the slower time will pass for you. The ISS space station, for example, is moving much faster than most of our commutes to work. They are also further away from the gravitational pull of the earth. They age more slowly, (e.g. 0.007 seconds less than we do for every 6 months they are in space). The difference isn't earth shattering, but it is a marked change.
I see. So you can alter the speed you travel through time using speed, and force.
That is correct. Some scientists theorize that if you are able to move fast enough (many times the speed of light), then time will slow to a crawl, then a stop, then move backwards. At a practical level, scientists hope one day to create a strong enough propulsion system to send humans to the far reaches of the galaxy without the dramatic aging. A human may travel 8 years in space, but for them, only 1 year may have passed. Likewise, you can also travel very slowly in space, and near a dense object with a heavy gravitational pull. This would speed up your clock, and slow everyone else clock. For example, if you traveled to the center of the earth, you may emerge having spent 5 years there, only to find the earth has only spent 1 year. However, the effect of reversing it becomes a much more difficult problem to solve.
So you cannot travel backward in time?
According to our current understanding of time dilation, we can travel forward into the future, but cannot travel backwards. We have proved that traveling ahead is possible, we just lack the technology to jump forward years or decades. Einstein also proposed a universal speed limit on the universe. He believes the speed of light is the fastest possible particle. In other words, the star trek enterprise would be stuck at warp 1. :(
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