GooberTheHat wrote:

But as with motion, acceleration only exists if there is a frame of reference with which to measure it against?

Supposing each astronaut has a belt with a slinky spring tethered to it. The astronaut need not look at the other person, they only need look at their spring. Whoever has a stretched spring is the one that is accelerating, and for now we'll just except that.

We'll look at acceleration even more closely when we look at gravity, but we can't get ahead of ourselves. For now we'll just say there's a force on one and not the other, and the force is doing the accelerating. Eventually we'll look at how you can accelerate without even a force - which was Newtons mistake with gravity. But we need to be more pragmatic and worry about these things later, as Einstein did.

The Daddy wrote:

Just watching Cox on bbc 4, his genuine joy at watching a proper demo of the bowling ball/feather thing is great - like, a full on physicist who knows all this stuff inside out is still thrilled to see it in action. This episode is worth a watch on iPlayer if you’re following this thread. (Brian cox’s adventures in space and time)

Watched that last night as well.

The hard "splash" of the Soyuz capsule must be quite something to experience.

First time I've ever been likened to Einstein.

This thread can continue.

Yeah, it's an odd thing. People assume special relativity is complex but it's really not. Once you accept absolute space is not a thing everything falls into place, including E = MC^2. You just need to be brave enough to accept it and see where it takes you. Which is what we'll try to do!

That's progress. One builds on the previous ideas.

This isn't going to happen with QM btw.

I dunno there were quite a few physicists in the 19th who were like hmm this absolute space thing, doesn’t fit a load of observations etc… maybe there’s some kind of substance, invisible to all our instruments, we could call it luminiferous aether?

It was that grappling with the results, the consequences of absolute space / time, in a way that seemed like a big cop out (god coordinates, aether, aliens) that had Einstein so het up

Funkstain wrote:

I dunno there were quite a few physicists in the 19th who were like hmm this absolute space thing, doesn’t fit a load of observations etc… maybe there’s some kind of substance, invisible to all our instruments, we could call it luminiferous aether? It was that grappling with the results, the consequences of absolute space / time, in a way that seemed like a big cop out (god coordinates, aether, aliens) that had Einstein so het up

Not quite sure what you're saying. The aether IS the concept of absolute space. It's just that Newton knew it was invisible and could never be found. He was way ahead of the people looking for it, and he didn't even know about c being a constant.

SpaceGazelle wrote:

Funkstain wrote:

I dunno there were quite a few physicists in the 19th who were like hmm this absolute space thing, doesn’t fit a load of observations etc… maybe there’s some kind of substance, invisible to all our instruments, we could call it luminiferous aether? It was that grappling with the results, the consequences of absolute space / time, in a way that seemed like a big cop out (god coordinates, aether, aliens) that had Einstein so het up

Not quite sure what you're saying. The aether IS the concept of absolute space. It's just that Newton knew it was invisible and could never be found. He was way ahead of the people looking for it, and he didn't even know about c being a constant.

Sure I'm agreeing with you - Newton even called it Aether I think? - just that the experiments they could run in the C19th (michelson/morley being the primo example, no?) needed this aether shit to work but made it look more and more made up (like, it isn't God any more, it's this invisible substance that physically exists for sure), to match the observations, and this added to the pressure in Einstein's brilliant head. I wonder if it had just been Newton's thoughts would it have been enough? the experiments provided the need for a better explanation I think, I was wondering if you were going to get into the practical side of physics?

Ok. So back to history and F = ma.

This is a physics thread so we really should look an equation or two and not be scared of them. Equations are both good and bad because they trivialise understanding. They're good because they enable people to calculate stuff quickly but they're bad because they let undergrads pass exams without having to really understand anything.

So what are equations anyway? The clue is in the name and it's this equals sign, and it's tremendously important.

Let's start simply with 5 = 5. We can cope with this. The handy thing about equations is that as long as we do the same thing to the left hand side (LHS) as the RHS, they're still gonna be equal. So we can multiply both sides by 3 and it's fine, 15 = 15.  

We can do this with F = ma. 10F = 10ma. Yes both sides are 10 times bigger but it's still true. Pointless, but the equation still holds. But in physics we can't just think about numbers and we have to think about concepts instead, in this case mass and acceleration.

So, what is F = ma? Well, it's the definition of a force. As defined by Newton. 

So what does this mean? Let's say we have a 1kg ball and we measure that ball accelerating at (one meter per second) per second. Acceleration is just the rate of change of velocity, so after one second it's going at one meter per second, A second later it's going at two meters per second, and so on.

Newton says that the ball is undergoing some force, and because he's just defined a force to be equal to m * a, we can say this force equals 1kg * 1 meter per second per second. But that's a mouthful, so let's invent some new unit and call that 1 Newton.

So a one Newton force accelerates a 1kg mass by 1 meter per second per second. Overnight Newton has just defined this new concept of a force.

But we need to be careful. We haven't really explained what a force is, but whatever this mysterious thing is we can tell it's there because balls are accelerating. Nobody really knows what a force is, but that's ok because nobody really knows what time or energy are either, and we can't dwell on these things or we'll never get anywhere.

But there is a problem and it's worth pointing out now because it'll make the transition to General Relativity easier later on.

And it's this:

In order to fully define this force thingy we need to fully define what mass and acceleration are too. After all, we're saying force IS m * a.

How do we define mass? No problem you might say, we can weigh it. Things that are more massive weigh more, and as long as we can agree on what a kilogram is it'll be fine. So for no particular reason let's define a kilo to be a copper cube of a certain width. We distribute these cubes out and tell everyone - that's a kilogram. Everyone agrees these cubes weigh the same. If anything else weighs the same when put on a scale then that's a kilo too. So now we can have kilos of apples and so on.

But the problem here is that the scales are really measuring a force - Newton's gravity. We're defining a mass in terms of a force (weight), and we're defining a force in terms of a mass (ma). It took nearly 300ys for someone (Einstein) to point this out.

It's like a mathematician trying to define 10 by saying 10 = 10 * 1. Maths people have their own problems, namely trying to define what numbers really are but we'll leave that to them and not worry about it too much.

Anyway, this has been a minor diversion and next time we'll clarify what we do know so far, and then we only need to get one more piece of the puzzle - light, and we can figure out who's right about absolute space. We don't need F = ma for that.

I like that last post SG.

My applied maths education covered the equations of motion and resolving forces acting on objects sitting on slopes into horizontal and vertical components but that was pretty much as far as it got.

RamSteelwood wrote:

Yeah that was it basically. Here's a formula and some parameters, work out the result. Don't worry about how or why

This is exactly it, but we shouldn't beat ourselves up too badly. The general idea in physics is to distil complex phenomena into simple ideas, ideally one equation, and we'll worry about what the terms of the equation really are at some point in the future. 

In this way we can unify things like temperature and sound into the theory of motion and then we just need to worry about one thing - this motion business. Soon, just by thinking about motion, we'll be able to unify space and time too into a spacetime thingy.

But it is vital to understand that just because we define something (by giving it a name) doesn't mean we really understand it. The students that appreciate this can hopefully go on to become professors and teach their students that nobody truly understands anything!

As long as whatever representation of reality we use works it doesn't matter.

We don't really understand reality , just the model of it that we are using. And even then in some models we just have to accept things - like complex numbers and quantum entanglement.

Who is still following this thread? Only two people have asked a question. This is not a good sign and the fear is I've not taught this too well. I know people might be casually reading it but do people understand it or are invested in it? For anyone that is following, do you understand the arguments made by Einstein vs Newton regarding absolute space? Yays or nays would be welcome.