I just thought that, given the gravitational mass, and the way that nebulous gases come together to form suns that they might just sort of stay in the same general area? It's probably one of those facts that people can miss. Like the fact that raisins are dried grapes.

ZMM wrote:

I just thought that, given the gravitational mass, and the way that nebulous gases come together to form suns that they might just sort of stay in the same general area? It's probably one of those facts that people can miss. Like the fact that raisins are dried grapes.

What did you think they were?

I knew that raisins were dried grapes. Prunes are dried plums. Dried dates are dates.

Bump

Ok, as promised. I've more time now so the thread should move along for a while, and it's good for everyone to slowly ponder stuff once the book is closed as it were. Most of the hard stuff I did at uni sank in later.

We should have a recap anyway. We've talked about this concept of absolute space a lot because understanding why it's wrong is really the only thing to understand to 'get' relativity, but it's hard. Our biases are strong. When we say the universe and the space within it is expanding, we mean the measurement between objects is getting bigger. To say they're expanding into something makes no sense because space is just a measurement, and that's all it is. 

Space, both physically and conceptually does not exist outside of the bounds of a measurement. Space IS the measurement - a number. Everything in physics is a number with a tag attached to it. A mass number, a distance number etc. These tags are linked by equations. If we plug in the numbers and we always get the same on both sides of the equals sign for all observers, this becomes a law of nature. If we spot an error in a particular circumstance we have to change or slightly modify the equation.

This is all physics does. It tries to find the equations. Only the good physics students seem to appreciate this limitation, the bad ones think they know stuff. So don't be put off if this this thread seems difficult to grasp, it just shows you're potentially one of the better students and you should've taken physics at uni.

So how do we approach the subject knowing we'll never really understand anything? We should relax and get used to it. It'll get easier as we go on. So later, when we talk about matter "curving" space we needn't worry about how or why, we just try and figure the equation. Nobody knows how matter interacts with space - how it grips it to curve it. The question probably has no meaning because space isn't a tangible thing anyway, it's just another number in an equation.

Whether you think it is or not, the scientific approach is to treat the universe as if it were a simulation. Mass curves space according to some equation and it does so via the equation, like a bit of code in a program. Who wrote the equation and why the equation isn't some other equation instead is not our problem. There's no reason one photon passes through a window and another is reflected off it, it's just a probability and you can never work out the outcome in advance. Welcome to post-classical physics, which began at the start of the 20th century, where we are now.

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So here we are, and out of Maxwell's equations pops out this constant, and almost unbelievably, it's a speed. How can a relative thing be a constant? This is the question everyone set out to answer and only Einstein got it right. He did this by taking the most obvious route and just assuming it was a constant. It says nothing in Maxwell's equations about the speed of the object that's emitting the light. It's simply not one of the terms in any of the equations but this was underappreciated for a bit. It was properly explored by some maths people who actually proved (using the equals sign) that the speed of an emitting bulb had no effect on the speed of the light that was emitted.

This seems weird but we have an equivalence with the speed of sound, so everyone assumed that light required a medium to travel through and it was this medium that gave the light it's constant speed. They called this the Luminiferous Ether. Many appreciated it might not be directly detectable but that's ok because neither was absolute space and everyone believed in that, even the clever ones. In fact we should argue they're the same thing.

And the good news is that this idea is testable. Set a torch up on a table, the light passes through this absolute space stuff at a constant rate, and then just run away from the torch measuring the speed of the beam, run towards it and do the same, and you should have two different speeds! The only problem being that light was very fast and the difference was hard to measure.

Along come Michelson and Morley and their quite brilliant set of increasingly accurate light-measuring machines. So good was the idea that it's pretty much copied to measure gravitational waves today. They pointed it at the Sun, used the fact the Earth orbits on a slight ellipse (so it moves slightly towards the Sun and away from it over the year) and they found they couldn't detect a difference. Remarkably, it's probable Einstein had never heard of the results and derived special relativity from his own thought experiments. He was a great thinker but not so much of a talker and only referenced the experiments much later, in response to people disbelieving his paper.

We'll go over his possible thought experiments next time, probably tomorrow now this thread is getting going again. It's taking slightly longer than I thought for each post because I REALLY want you to understand the bit about how the equations don't really explain anything yet are also all we have. Think coded simulation and we only need worry about discovering the code, not why the code. There's never any why.

There is never a why in science unless you're the religious sort. WHY does a mountain exist? Meaningless question. HOW do mountains form? Now we're onto something and we can look at it scientifically.

SpaceGazelle wrote:

There is never a why in science unless you're the religious sort.

This is a bit of a nonsense statement unless you define what you mean a bit more tightly. "Why did that apple fall on my head?" could mean multiple different things. Consciousness and perception are areas of interesting scientific endeavour. Why questions can often keep in interesting directions, that yield How answers.

IMHO, as a non-religious person who doesn't punch nuns

The why point was never about religion. It was more of a way of thinking about scientific perception. How can I state space is just a number? Because scientifically it is. Asking why it appears it's just a number is not within the remit of this thread, but that shouldn't deter anyone from thinking it's just a number because that's how nature seems to work. The hard part is convincing people that's how physicists think and work. I can only try and explain what they know, not what we don't. And it's purely mathematical when it comes to physics.

Yeah, it seems sementically picky to point these things out but we need to be in the right mindset moving forward. It saves the time in asking things like "Why is the speed of light a constant?". That is not known. We could say the universe wouldn't behave like it does if it wasn't a constant, but that doesn't answer a why question. 

o/
Another student finally caught up.

Define understand.
What is sentience? What is intelligence?
Is a cat or a dog sentient and intelligent? What about a chimpanzee or bonobo?

Who's to say sentience or intelligence cannot occur in an AI in the near future?

You get interesting behaviour from complexity, putting lots of simple things together into one thing. But they have to be connected, so computer science uses a graph data structure for AI. From small matrices you can get incredibly complex and interesting behaviour, and because they're not hand-coded they can automatically find very complex patterns, patterns our complex brains struggle to understand. 

It feels like magic but so does life in the universe. One of the hurdles of physics is it deals in very simple things, like a single atom or the expansion of space. The next breakthroughs in science will come from understanding tremendous complexity and AI is very suited for that.

I mean, me typing this is just what atoms do after 13 billion years.

A bit like fractals. A simple math basis but endlessly repeating no matter how far you zoom out, forming ever more complex structures.

So here we are in 1905. The maths people have looked at Maxwell's equations and proved that the speed of light isn't influenced by the speed of the emitting bulb. Everyone agrees with this because a mathematical proof is the ultimate proof of anything and nobody is arguing about that. It's just that everyone thinks it must be because of a medium - this mysterious absolute space stuff.

They could argue that this actually seems to indicate that absolute space is a thing after all and we can now detect it indirectly. If you measure a beam of light coming towards you at exactly c then you'd know that you were at rest with respect to absolute space because c always travels through space at c. If you measured it at 10 meters per second less, you'd know you were moving at 10 ms with respect to absolute space in the same direction as the beam.

But Einstein hates this idea. For a start you can't detect this invisible stuff directly and also because he's really sold on the idea that all non-accelerating frames are equal, that speed is purely a point of view. And this means that all the equations should be true in all inertial frames, including Maxwell's equations in his.

He was rather perturbed to think that his headlights might stop working just because someone else observed him flying past them at c. He wasn't moving, they were, and just because they think he is shouldn't break his shit.

Maxwell's equations should be perfectly valid in his inertial frame and that includes the part about light speed being a constant. Nobody was breaking his headlights just by looking at him. In fact all light for all inertial observers should be c because that's what Maxwell's equations say it should be. We find this hard to grasp because we're so used to velocities stacking that it breaks our minds a little bit. Einstein said he wished he'd called the theory of invariance, not relativity. Light speed is invariant to everything because it's a universal constant. We just need to change our perceptions of space and time instead, and there were no equations stating they should be constant. We just assumed they were.

If he could show that all observers in all inertial frames measured light to be c, he could finally prove that an inertial frame is definitively not moving and that speed was purely a relative concept! If he can do this then we can throw absolute space in the bin forever.

Fortunately for us, he managed this by doing yet another thought experiment. I'll try and find a good vid on the proof and we'll talk about it after. As long as you know Pythagoras it'll be ok.