The Learn Physics Thread - Space and Time - The Bear and Badger

SpaceGazelle

First up there are no stupid questions. Many of the questions that feel dumb turn out to be the most profound and interesting.

Secondly, I'll post when I get time, because there's quite a bit to get through, so we'll take our time and make sure everybody understands stuff before moving on.

Thirdly, I'll make mistakes. I'll assume you know stuff and you should ask if I forget to mention something that's needed to make sense of it all.

We need to have a quick history lesson to get us all up to speed and I'll only include the relevant stuff.

A BRIEF HISTORY OF PHYSICS

Before the 16th century not much happened physics-wise with one notable exception. We got advancements in maths and some cool stuff but nothing that need concern us right now.

The one notable exception is this: The Greeks noticed that if you were on a sailboat going at constant speed in smooth water and you dropped a coin it still landed at your feet. It's as if you weren't really moving at all. This was largely glossed over at the time but the impacts are profound. It hints that maybe any experiment you do will have the same result if you're either standing still or moving at constant velocity. Maybe the boat isn't moving but the water underneath is?

Other than that the world was simple. We lived at the centre of the of Universe on a world that was static and everything else moved around us. This seems reasonable as we don't feel like we're moving and besides, God created Us and The Earth and we're the most important things in it, so everything else better bloody move around us. So the Sun and all the planets rotate around our magnificence as do the stars, which seem to be points of light on a black sphere that rotates around the Earth like everything else. The comets were a little more baffling, they'd just appear and disappear and everyone was a little scared of them.

*Einstein later revisited this idea, the idea that you could and should think of yourself not moving and everything else, including all the other observers are moving relative to you.

Then the 16th century happened, or more to the point Nicolaus Copernicus happened. In 1532 Copernicus started putting together his Heliocentric theory that if you just imagine all the planets (including the Earth) went around the Sun it made all the calculations a bit easier. Understandably, he was a little nervous about publishing it and told a few close friends instead. News quickly spread anyway and news got to the Catholic Church and eventually, in 1533, the theory was presented to The Pope, who actually took it pretty well. Copernicus was smart and never explicitly said the Sun was at the centre, just that if you briefly imagined it at the centre it made all the calculations work out so it was a useful tool.

Next Galileo happened. His first great discovery was The Law of Inertia. Before that it was thought that the natural state of a body was to be at rest, which made sense at the time because everything stops moving eventually if you just leave alone, or so it was thought. Galileo said this wasn't the case and that a moving object will keep moving at constant speed unless acted on by a force. Some of you might recognise this as Newton's First Law of motion but Galileo discovered it first. Newton just added it as one of the three laws of motion.

You see Galileo had heard of Copernicus and his Sun-centred model and believed it, but needed a way to explain it. Before his law of inertia it was believed things just stopped, and the planets and everything else except the Earth was being constantly pushed by angels or the hand of god or whatever. The Earth was deffo not being pushed otherwise we'd FEEL it. The Law of inertia allowed for things not to be constantly being pushed, including the Earth, so maybe the Earth is orbiting the Sun and the reason we don't feel it is because there is no constant pushing.

Alarm bells started ringing in The Catholic Church because this Sun-centred business was getting a bit too real. Before it was just a tool to make calculations easier but now it was an affront.

The other great thing Galileo did was look through a telescope. Galileo was a genius and all it took for him to build a telescope was to hear about the invention of a telescope. He heard about a patent that had been registered in The Netherlands in 1608 and went about making himself one. He realised he could use this invention to push the Sun-centred idea and he was right. He looked at Jupiter and saw it had it's own moons. To my mind this is the greatest scientific discovery of all time and the thing that kickstarted all the rest. He took his time, made lots of observations over many nights and saw that the moons were absolutely going around Jupiter. This was the proof he needed, that this biblical rule that stated everything must go around the Earth was silly, and he delighted.

The Catholic Church on the other hand were horrified. It's one thing having a nice theory to aid calculations but this was actual bloody evidence. He was hauled in front of The Inquisition and the only thing that saved his life was that he immediately recanted, said he was talking nonsense and was made to say so in public. He spent the rest of his life under house arrest pondering why the world was so mean. Another astronomer touting similar ideas, ~~Joseph~~ Giordano Bruno, was less of a pussy, refused to recant as was burnt at the stake.

Right we'll leave it there for now. Next time a bit more history and then we can start proper.

poprock

I’d never heard of Joseph Bruno. I’m away to look him up now.

poprock

Well that got confusing quickly. It’s Giordano Bruno, not Joseph. Joe Bruno was a US senator who got done for corruption.

davyK

Nice..subbed.

SpaceGazelle

Here you go.

https://en.wikipedia.org/wiki/Giordano_Bruno

He was also called Giordano and Filippo.

SpaceGazelle

I think the catholic church named him Joseph posthumously, to cleanse his sins or sommat.

SpaceGazelle

Oh I forgot to do a summary.

The two big takeaways are this:

1. The idea that you can't tell if you're going at a constant velocity on a boat/train/plane if you shut your eyes or put the blinds down. You can still throw balls straight up in the air and they'll come back to you, not behind you. In fact all the physics experiments still work.

2. The law of inertia. The idea that you don't need to be constantly pushed to maintain a constant velocity. We kind of know this now from space but the idea was weird at the time.

SpaceGazelle

poprock wrote:

Well that got confusing quickly. It’s Giordano Bruno, not Joseph. Joe Bruno was a US senator who got done for corruption.

Edited.

b0r1s

Good start SG. Keep it coming.

SpaceGazelle

Will do. I've edited it for typos, changed the Joseph thing and added a sentence about comets which I forgot to mention. I'll probably need to this every time I do a long post.

yourfavouriteuncle

This is the kind of physics I can understand. Stories, basically.

In.

b0r1s

Watch Cosmos on Disney+ it’s all done in the context of stories.

n0face

I enjoyed this recently

Pseudoscot

A good read is the Neil Stephenson Baroque Cycle Series. It is fictional (and a rollercoaster of a yarn) but inside it is the change in perception of science during the late 17th and early 18th centuries. Newton and Liebniz had a massive falling out about the terminology to use even though they were saying very similar things.

It can be quite dense at times but it is fun.

Lurch666

Hopefully you can explain why according to relativity gravity actually goes up (or something like that).

SpaceGazelle

It's more the gravity isn't a force, but we'll get to that later.

yourfavouriteuncle

Steady please! I’m the one who’ll be baby stepping in here so let’s not start nattering about stuff I don’t yet even know exists. Tyvm.

Blue Swirl

SpaceGazelle wrote:

1. The the idea that you can't tell if you're going at a constant velocity on a boat/train/plane if you shut your eyes or put the blinds down. You can still throw balls straight up in the air and they'll come back to you, not behind you. In fact all the physics experiments still work.

We call this in the trade “a frame of reference”.

I’m going to hang out because physics is proper bat shit at higher levels. It’s tons of fun.

Blue Swirl

SpaceGazelle

Ok, so where were we? We've got coins dropping at our feet on boats going at a constant velocity, which was more formally stated by Galileo in the Law of Inertia. And that's really it so far.

It's my favourite law and is perfectly correct, but Galileo had thought about it in the wrong way and the reason was this:

He thought inertia was a magical and invisible property that a moving body has that a non-moving body does not.

You see, Galileo and Newton after him believed in the concept of absolute space. Everybody did at the time and it's a natural assumption to make, and the basic idea of absolute space is this:

A ball moving through an otherwise empty universe still has a speed.

Now, like Einstein did, you might be wondering a speed relative to what? Galileo and Newton argued it was moving relative to space itself. This actually makes a bit of sense for the time. We only need to imagine that the Universe has an edge, a boundary. Maybe it's the black sphere all the stars are glued to?

With such a universal boundary we can measure a speed for the ball in an otherwise empty Universe because the ball is moving relative to this boundary, and we can say the boundary is always at rest because the Universe doesn't move, things just move through it.

But Newton believed in an infinite Universe and so didn't have the luxury of this boundary, so he had to invent an invisible coordinate system system instead. You couldn't see it but is was there. Incidentally, to measure the speed of the ball Newton knew he also needed the concept of absolute time.

The idea of absolute time is this:

The Universe has a clock and that's the only true clock.

Sure, we can invent our own clocks and invent seconds and minutes and so on, but our clocks only work because of the Universal clock. If the Universe's clock stops ticking then so do ours. The Universe's clock controls all the clocks and the only true time is Universal time - absolute time. And it always plods on at a steady pace.

As we'll see Einstein simply threw absolute time and absolute space in the bin. That's not say say inertia doesn't really exist, it absolutely does, but Einstein realised that inertia, like speed, can only be measured relative to any given observer.

ANYWAY, that's a rather long-winded explanation of how Galileo perceived inertia. He thought that you're either at rest with absolute space - this invisible coordinate system in the background, or you're moving through it. The things that are moving have this magical property called inertia that he couldn't quite explain, but it was the difference between moving things and stationary things.

So Galileo is under house arrest feeling sorry for himself but he's discovered The Law of Inertia. He just hasn't figured it out it's a relative concept. If two cars are traveling towards each other, say one at 20mph and the other at 40mph, neither driver should give two hoots about absolute space and should be more concerned they're about to have a 60mph crash.

The Catholic Church are on the back foot. Although initially they could handle it as a mere theory, suddenly every fucker was making telescopes and looking at Jupiter. Plus the Protestants were kicking off, and they even hated the theory. Even thinking about a Sun-centred universe was a dangerous business and they had a lot of money riding on the fact that people believed in the bible. It wasn't until the 19th century that they officially adopted the Copernican view, but better late than never.

The genie was out of the bottle though, so although they condemned it they stopped burning people alive. As long as people still believed in god they could relax a bit and allow people to have their crazy views.

Now we can introduce Kepler. Kepler was a religious man that loved the Copernican view. He saw the beauty in it and was enthralled. Kepler's god was a more dynamic, playful god. A god that like chucking the planets around and watching them spin forever under their own inertia. He thought the idea that the natural state of an object was to be at rest was boring. This Law of Inertia business was delightful and he wanted to figure out how god made the planets go around the Sun in such a sublime way.

He came up with the three laws of planetary motion and we needn't dwell on them in detail. Basically he figured out that orbits needn't be these perfect circles, they could be ellipses. not only that he figured they didn't need to orbit at a constant speed. The could get nearer the Sun, be whipped around at high speed and get nearly flung out, only to do a long slow arc and eventually come back to the Sun only to be whipped around again. Kepler knew the Sun must be some kind of attractor - that the planets were drawn to it. God was truly playful! This version of god was exciting.

He got in trouble with the church but stood his ground and they didn't burn him. Slap on the wrist.

Right that'll do for today. we just need to introduce Newton and can then get down to the bits I really want to explain.

Summary:

Kepler believed in the notion of absolute space and absolute time. He thought you could say something was deffo moving because it was moving relative to space itself - this invisible coordinate system. If you're at rest with respect to this coordinate system you don't have inertia and if you're moving relative to it you do. He was wrong about these invisible coordinates. Einstein had no truck with it. If it's invisible then it has no purpose anyway so why not bin it?

SpaceGazelle

Btw, Einstein only realized he could throw absolute time away once he'd realized you could throw absolute space in the bin. We'll look at why he could do both later.

Diluted Dante

Ok, but what about PhysX? Why don't I need a card for that any more?

Lurch666

Just blew my mind.
So if you have a moving object but nothing to measure it against it's not moving (relatively speaking)?

GooberTheHat

Diluted Dante wrote:

Ok, but what about PhysX? Why don't I need a card for that any more?

Has Musk bought it yet?

Wookienopants

This thread is great, loving your explanations.

SpaceGazelle

Lurch666 wrote:

Just blew my mind. So if you have a moving object but nothing to measure it against it's not moving (relatively speaking)?

Correct. There is no experiment you can do that would could determine otherwise. We can see this on a train moving at constant speed. Close all the blinds so you can't see outside and it's like you're in a living room or a science lab. All the experiments give the same results. It's only when you're accelerating things change. Then all the science equipment goes sliding off the table and so on.

We'll cover acceleration later.

SpaceGazelle

It's easier to think about this: You're alone in an empty infinite universe. It's just black. Einstein realised the concept of speed made no sense. He realised the concept of position makes no sense. This upset Newton (who had thought about it) and it made him uncomfortable. Newton said that you are moving - relative to space, you just have no way of knowing or how fast you're going.

Einstein thought this was silly because he realised it was an unnecessary complication that Newton had invented just because Newton was uncomfortable with the idea.

We'll cover this kind of thought experiment a lot later.

poprock

I love the speed you’re getting through this with.

Makes me realise the only reason it took so long to get through at school was because we had to stop and prove each bit before moving on - first with experiments, and then with maths as we got to the harder bits.

SpaceGazelle

Einstein didn't like maths either. He used these ingenious thought experiments instead and got the right answer just from thinking.

Lurch666

When I'm arguing with evolution deniers or flat earthers the thing they miss is that we didn't evolve to have the cognitive ability to understand this sort of unintuitive thinking.
Just asserting something must be true because it appears to be true denies the brilliance of the people before us who figured this out.

Your explanation of relative movement went against what I thought I knew but instead of arguing against the facts I accept that what I knew is wrong.

Really enjoying this and thanks for taking the time to explain this.

SpaceGazelle

Lurch666 wrote:

When I'm arguing with evolution deniers or flat earthers the thing they miss is that we didn't evolve to have the cognitive ability to understand this sort of unintuitive thinking. Just asserting something must be true because it appears to be true denies the brilliance of the people before us who figured this out. Your explanation of relative movement went against what I thought I knew but instead of arguing against the facts I accept that what I knew is wrong. Really enjoying this and thanks for taking the time to explain this.

It's a common problem even amongst scientists. Newton was a great man, perhaps the cleverest scientist of all but he was weighed down by religion. He thought of the universe as a cathedral made by the almighty. He knew relative speed was a thing but thought it that was just a human glimpse - just part of a bigger picture we would never be able to see - absolute space. Absolute space is god's creation and is immovable. We would only be able to see relative speed, this puny glimpse of a bigger creation. He regarded relative speed as vulgar because it wasn't godlike enough but he knew he had to work within it, because that's the price we pay for not being a god.

Einstein just went, "Fuck this, it's nonsense. Absolute space does not exist and I'll bloody well prove it". He didn't actually say that but he almost certainly thought it and went on to prove it.

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