WARNING: Ahead is a very long-winded and text heavy post. Proceed with free time. You have been warned.
As some of you know, I’m a physics teacher! So I figured I would be remiss if I didn’t talk about some cool science on here! I know most people have probably taken an intro physics class at some point in their lives, but as someone who teaches those classes, I know they don’t always cover the most interesting topics. Therefore, I’m going to do my best to talk about something that a lot of people hear about, but nobody learns unless they study more advanced physics: Special Relativity!
In case you’re wondering, Einstein’s Special Theory of Relativity is the thing that says that time and space do funny things when an object travels close to the speed of light. Not to be confused with his theory of General Relativity which has to do with gravity and is much more confusing. I don’t pretend to be anywhere near as knowledgeable about that one.
|According to General Relativity, planets like to have picnics together on big, groovy space-blankets (Source)
So, in order to talk about Special Relativity, we need to define what relativity means.
Let’s begin with a fairly simple thought experiment:
Imagine you are in a featureless white room. There are no distinguishable features on the walls or floor, so you really can’t tell if you’re moving (I think they have those in photography studios?) If you begin walking forward and throw a ball straight up, it would come back to your hand. It would appear that you and the ball did not move forward at all. Now, if someone were to observe you doing this, they would tell you that you and the object BOTH moved forward, which is why it landed in your hand. So both of you have very different versions of what just happened. This is because of the concept of reference frames.You and the ball share a reference frame, and so it seems that you are in one position the whole time. The observer is their own reference frame, in which you and the ball both moved forward while they stood still.
This is the case for all systems in the universe, and they will behave similarly. If you throw a ball from a moving car, the ball will have the speed you threw it with, plus the speed of the car it originated in. This isn’t so with light. As it turns out, the speed of light is the same no matter what, and everything we can see in the world is actually just the light particles from an object hitting our eyes. This means that when things start moving close to the speed of the light we observe coming off of them, things get very interesting.
Consider an object moving towards you from very far away (like a spaceship). The light from this object travels at a finite pace, so when we see it, we are actually seeing it in the past. Now if this object is moving towards you at some large fraction of light (let’s say 90%), that means that by the time you saw that object very far away, it would be much closer.
|No, that’s not why they put these warnings there. (Source)
But the universe can’t be inconsistent like this. If you think about it, this means you would always see the object lagging behind where it actually was at all points, and that would mean that if something moving fast were to hit you, you would perceive it as being very far away but still quite clearly causing you pain! If this seems to go against your intuition, it should. This is not what happens. If possible, the truth is even stranger.
Put on your thinking caps kiddos, it’s about to get abstract in here!
What happens in this situation is different for each reference frame involved. When discussing relativity, we have a saying you may have heard: “Moving clocks run slow”. For you, standing here on Earth, it would seem that everyone on board that spaceship is moving in slow motion. To see why, let’s do another thought experiment:
Let’s assume that on board the spaceship is a clock. But this is no ordinary clock, this clock measures time by sending out a pulse of light straight up to a mirror and receiving it back every second. Now, to the observer on the ship, the pulse of light goes straight up and down, and so nothing is wrong with their perception of time, just like the ball and white room experiment. However, to the observer looking at the ship from outside (not moving with it), they see something different. We know the light hits the mirror and comes back down to the sensor but as it does so, the ship is moving. This means that the light no longer goes up and down, but rather diagonally. Basic geometry tells us that the path the light takes is longer in this frame than in the spaceship frame. Since the speed of the light has to be the same, that means that to the observer outside the ship, their clock takes longer than one second to tick. If you’re confused, here’s an illustration:
There’s an additional consequence to this weirdness. For this to work, that means that the distances these frames believe themselves to be travelling have to change in order to keep the time difference consistent. The result is a weird spacial “smooshing”. But I suppose if you want to sound smart when you talk about this stuff you could call these phenomena “time dilation” and “length contraction”.
|An example of relativistic “smooshing” (Source)
Now I know, this sounds all made up, so I’m going to try and explain it with a common phenomena that we have actually observed here on Earth.
There is a particle that is well known to physics called the muon. Muons travel at about 99.8% the speed of light and they originate in our upper atmosphere, thousands of meters above sea level. It’s a well established fact in physics that muons decay in about 2 microseconds (that’s 0.000002 seconds). In that time with the speed they travel, muons can only make it about 600 m before they decay. So how could we possibly observe them thousands of meters from where they come into existence????
You guessed it, relativity. These particles move damn close to the speed of light. That means to us on Earth, their “clock” runs slow. We know that they decay in 2 microseconds in their own reference frame, yet we observe them to be in existence for about 30 microseconds because of time dilation. Likewise, the muon experiences its travel in 2 microseconds, but because of length contraction, it appears that the Earth’s surface is only about 600 m away, instead of several thousand.
This is an example of how time and space both have to distort as a consequence of each other. If only time dilation occurred, the muon would reach Earth in our frame, but not in its own frame, which is a serious causal dilemma. Because we know that time does dilate, and I believe I’ve made a strong case that it does, length then must contract or else causality is broken. A nonexistent muon would reach Earth!
So there you have it. There are a ton of other strange paradoxes that arise from relativity like the twin paradox
and the pole-barn paradox
that are worth looking into if you’re interested. Who knows, they may even be cause for a future post! The universe is a strangely beautiful place, and its a shame that we don’t get a chance to teach the cool stuff in grade school!
This has been an incredibly quick and very imperfect lesson on Special Relativity. I tried to leave out all the math, which does not lend itself well to this kind of discussion so if you have questions, I’m not surprised. I’d be more than happy to discuss things in the comments!
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