Fraser Cain: After bending your brains with the Big Bang, we took pity on you and gave you a couple of weeks to recover with some lighter material: getting started in astronomy and meteor showers. Now weâ€re going to crank it up again with some concepts key to all things astronomical: relativity.

 

So if you don’t understand it yet, our hope is by the end of today’s show, you’ll be able to finally wrap your head around relativity. Alright Pamela, explain!

Dr. Pamela Gay: Relativity is one of those things that actually came out of the ether, and I’ve always wanted to be able to say that about something.

 

Back in the 1800s, scientists thought the entire universe (or at least the universe as we knew it then) was permeated by this substance called ether, and light propagated through the ether the same way sound propagates through air.

 

The problem with this is if it’s true then as the planet Earth moves forward through the ether and light is let off of a flashlight, candle (or whatever source of light you want), then the velocity of the Earth gets added to the velocity of light. This means you have light travelling at one speed in one direction, but if you look at it at some right angle to that, you get light going at a different speed.

Fraser: This is a pretty normal or understandable theory they came up with, because waves moving through water, the wave moves and propagates through the water, or sound waves that propagate through air. The waves will speed up or slow down depending on what things are moving.

Pamela: It all seemed to make sense. If you have a wave, you must have something that’s getting shaken. So since light was determined to be a wave experimentally, they figured there must be something the wave is “waving” in., so they used this ether concept.

 

Now, if I’m standing on a boat and I throw a baseball (being a non-baseball throwing type of person) at a whopping 30mi/h toward the person in front of me, and our boat is going 20mi/h relative to the shore, someone standing on the shore is going to see the baseball I just threw going 50mi/h, because the velocity of the boat got added to the velocity of the baseball.

 

Now, if I turn around and throw the baseball toward the back of the boat, they’re going to see that baseball going a very sad 10mi/h because in this case, the velocity of the boat is subtracted off the velocity of the baseball.

 

Everything is relative. In this case, it’s relative to the land.

 

A couple of really smart guys by the names of Albert Michaelson and Edward Morely said light should act like that baseball on the boat, and the velocities should add. Now, if I throw a baseball at a wall on that boat, and wait for it to bounce back to me, my friend on the land will see it going at a different speed in both directions because it’s reflecting and as it goes toward the wall the wall is moving away form it. As it comes back toward me, I’m moving toward the ball because of the motion of the boat.

Fraser: So far that makes sense: all intuitive.

Pamela: Now, if I turn that wall and myself at a right angle, so I’m basically throwing the ball at the person on the land, but there’s a wall in between, in this case the motion of the boat doesn’t affect the velocity of the baseball. The baseball’s going that same sad 30mi/h no matter what. So if I throw the ball perpendicular to my motion, nothing weird happens. If I throw the baseball in the direction of the motion, weirdness happens because the wall, ball and everything is moving and we have these weird relative velocities kicking in.

 

So if it works with baseballs, it should work with light. Michaelson and Morely set up an experiment in 1887 where they had two sets of mirrors. One set was perpendicular to the other; they were at right angles to one another. They very carefully measured the amount of time it took light to reflect back and forth from the starting point to the mirror and come back, in these two directions.

 

They figured the velocity of the motion of the Earth should affect things. They watched and watched and they waited to see the velocity of the Earth adding to or subtracting from the velocity of the light as the Earth moved through the ether and the light propagated through the ether, and nothing happened.

Fraser: What do you mean nothing happened?

Pamela: Everything kept going as the light moved at the exact same speed, no matter what happened. So itâ€s sort of like throwing a baseball and it goes, “yeah Iâ€m going to go 30mi/hr regardless of what the boat is doingâ€?.

Fraser: So even though they shouldâ€ve been able to detect the light going faster one way than it was going the other way, it travelled the same speed – even though theoretically it was getting a boost from its reflection.

Pamela: Yeah. So, imagine if, when youâ€re standing on land, you throw a ball at 30mi/hr. You go stand on a boat going 20mi/hr, let go of the ball and it changes velocities going, “unh-unh, Iâ€m not going to go faster than 30mi/hr and you canâ€t make me.â€? Thatâ€s what the light seemed to be doing.

 

This experimental result required people to re-examine how they looked at the world. Some people started suggesting that maybe time and space change to compensate for this and make light always appear to do the same thing. It was actually George Fitzgerald and Fredrick Lorenz that suggested time and space contract to keep light always going at the same speed.

 

In this case, as I move, the space around me changes. The lengths contract and in this case I end up seeing the speed of light as always the same thing. They didnâ€t have a full physical understanding of what was going on, and it took Albert Einstein to come up with that in 1905.

 

He did a series of gedonkin experiments (German for thought experiments). He imagined people on trains (because back then people were on trains and it was something they were very familiar with).

 

Imagine youâ€re standing in a field and Iâ€m on a train. If we both have two sets of mirrors, we can bounce light back and forth between those mirrors because itâ€s light and its cheap – we have light bulbs. Itâ€s possible to measure the amount of time it takes for the light to go from the bottom mirror to the top mirror if we have the mirrors one at eye-level and one at foot-level.

Fraser: Hold on. Youâ€re shining the light inside the train, from one mirror to another mirror?

Pamela: Yes. One mirrorâ€s at the floor, and the otherâ€s at eye-level. The light is bouncing back and forth, basically tracing out the length of my body. You have the same thin with you in your field.

Fraser: Right.

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