stupid physics questions

[pogo]

The data isn’t “all over the place”. Since Einstein postulated relativity, as a solution to the problems with Newtonian mechanics, it has been remarkably successful in matching the data. Most importantly it has been able to make testable predictions which the best test of any theory. Gravitational lensing is one of those: Relativity predicts it and observations confirm it. Another is the relativistic effect of gravity on the GPS satellites. Not only does that confirm the theory but if we did not allow for relativity in their signals they would not be accurate.

This is why relativity is so widely accepted. Not because it is mathematically elegant (though it is) but as it makes so many accurate and testable predictions so has been confirmed time and time again. The only comparable theory is quantum mechanics, which has proven to have as many applications on a smaller scale.

[gumboot]

Totally missing the point still.

[gravy]

That's totally missing the point. I'm not trying to invalidate relativity or quantum mechanics. I'm just saying that it'l bullshit to say that they must be right because the data agrees. The data is all over the place and demands supplemental explanations over and over until somebody reframes the whole thing; which is how relativity came about in the first place.

Well yeah. And they're mutually exclusive at extremes.

They're great for predicting behaviour in a lot of circumstances but neither are full models.

In much the same way that maxwell's equations or newtonian mechanics weren't either.

[pogo]

Not full no: no-one has come up with a satisfactory answer to what happens at the centre of a black hole, where you have relativistic effects on a quantum scale. But I think most physicists view this as a low priority. Even if someone comes up with a theory (and there are candidates) you can’t easily test them as you can’t examine the centre of a black hole. But as there are no black holes near us not being able to see inside them has no immediate consequences.

Outside of such singularities though the theories work very well. Quantum mechanics even more so than relativity. As while relativity at sub-light speeds mostly slightly modifies Newtonian mechanics, so clocks run a little slower, orbits precess slowly, the only way to understand much of physics and science today is through quantum mechanics. E.g. the Pauli exclusion principle explains why electrons form into shells, which explains how atoms form bonds and combine into molecules (the basis of chemistry) or how things get their colour.

[gravy]

Models can be wrong and useful still hugely useful. Just within limits.

That we use these models to great effect within those limits is great. Even so it doesn't preclude a more comprehensive and complete model being even more useful to us.

QM and GR aren't axiomatic truths or can't be improved on because Garmin can make a GPS.

[pogo]

QM and GR aren't axiomatic truths or can't be improved on because Garmin can make a GPS.

But nothing in physics is axiomatic. That’s one way maths and physics are different. Maths is axiomatic: everything can be derived from first principles, starting from “what is a number”. Physics is observational and experimental. You observe, design models to describe what you observe, then test them. So yes, it’s always possible a better model will come along. One that better matches observations, explains inconsistent results, or that lets you extend your model into new areas – such as the singularity at the centre of a black hole.

But I don’t think that will invalidate GR or QM, the way SR rendered Newtonian mechanics invalid. Or more precisely SR answered the questions people were already asking about how to reconcile Newtonian mechanics with results such as the Michelson–Morley light speed measurements. People knew there was a need for a new theory and SR, then GR, provided it. QM similarly arose out of problems such as black-body radiation.

In the 100 years since then we have made incredible advances in the science of the large and small. Space, which we then could only peer at dimly through crude optics and through the atmosphere, has been explored and catalogued in way we could not even imagine 100 years ago. The discoveries made on a very small scale have been in some ways even more astonishing. We have come further in the last 100 years than in the previous 1000. But nothing has seriously challenged GM or QM, they have only been confirmed.

[gravy]

SR rendered Newtonian mechanics invalid.

It didn't though. That's like saying laser range finders have rendered rulers invalid.

Newtonian mechanics is an approximation of special relativity that works at low(ish) speeds.

And SR is an approximation of GR in an idealised set of circumstances that could never exist (a flat space-time). But again works brilliantly within certain bounds. And GR? It still has problems.

You're right that both these models on practical level will likely be used in perpetuity depending on the task at hand whatever more complete models come around. They won't be invalidated. But neither will rulers.

[zip accident]

You're right that both these models on practical level will likely be used in perpetuity depending on the task at hand whatever more complete models come around. They won't be invalidated. But neither will rulers.

That might be correct, but it depends on the eventual solution being more complicated than GR and QM; if it's actually a simplification, then it might eliminate them completely, which (the successor being simpler) in my opinion would be a good thing.

[pogo]

Not a simplification. Relativity and QM are mathematically very simple. They are mostly “hard” because of how they challenge our intuition. Relativity says we must ditch our idea of absolute time, or absolute measurements of length or mass. Two different observers in two different frames of reference (one moving relative to the other, or one in space one on the surface of a world) make different but equally valid measurements. QM challenges our ideas of determinism, so much so that even Einstein doubted the implications, famously saying “God does not play dice with the universe”. But the maths is quite simple and elegant, and can hardly be simplified further.

The current search for a more complex, complete solution seems to be focussing on theories involving extra dimensions. These have intuitive appeal as it’s not obvious why we have three spatial dimensions, except it is the only number that allows the universe to function. But we add another dimension to get Spacetime, and yet another to model space as curved by e.g. gravity in GR. Could there be more but they are somehow closed off to us, e.g. of so small extent, smaller even than the things that make up atoms, that they are no visible.

As a sometimes user of higher dimensional mathematics I am skeptical of these approaches. The problem is as the number of dimensions goes up the number of subspaces you can define increases exponentially, so in n dimensions you have 2^n of them, far more once you start combining them. With the 10 or 11 dimensions of some theories that gives you 1024 or 2048 subspaces to start building things with. Essentially things get so complex that all sorts of results might be derived, and it would be unsurprising if you can derive both relativity and QM from such an approach.

[gravy]

You're right that both these models on practical level will likely be used in perpetuity depending on the task at hand whatever more complete models come around. They won't be invalidated. But neither will rulers.

That might be correct, but it depends on the eventual solution being more complicated than GR and QM; if it's actually a simplification, then it might eliminate them completely, which (the successor being simpler) in my opinion would be a good thing.

IMHO the good thing is a more complete theory may mean we have new ways to exploit physical laws.

We've got a fuck load out of QM despite it likely looking completely useless for any practical applications when the important initial discoveries were made. We're really good at applying theoretical knowledge to our benefit. There could be massive opportunities if we understood the fundamentals of the universe a bit better.

[5th Earth]

Not a simplification. Relativity and QM are mathematically very simple.

I don't think I've ever heard anyone call vector calculus "very simple" before.

[pogo]

Not a simplification. Relativity and QM are mathematically very simple.

I don't think I've ever heard anyone call vector calculus "very simple" before.

Special Relativity was one of the topics of the first term of the first year of my maths degree course. It’s changed a lot now as they have to do a lot more remedial stuff because of the dumbing down of A-levels but then it was seen as a good introductory topic for bright students fresh out of school. All of it, so QM + GR, was covered during the course.

I might not have considered it simple at the time, but the stuff I’ve learned since has helped me see how it all comes down to maths not much more complex than that underlying special relativity. It’s the concepts that are difficult. Abandoning things like an absolute time scale (for relativity), or determinism and certainty (QM).

[gumboot]

Is there an intuitive way to understand how a chiminea gets better airflow than an open pit?

[pogo]

Not sure exactly what you mean, but a chimney is a very simple thing which uses the heat from the fire to draw away waste gasses and particles. The chimney guides it, does not give it chance to spread out which might cool it and allow the smoke, soot etc. to enter the room. And the rising heat also draws air up the chimney, which helps air circulate keeping it fresh.

It’s not especially efficient, as a lot of the heat generated goes up in smoke – literally. Modern boilers and heaters often do much more to capture heat. They also benefit from running on much cleaner fuel. Natural gas in particular generates almost no waste, just carbon dioxide and water vapour. So it’s safe to use gas to e.g. cook without any special ventilation.

[zip accident]

Is there an intuitive way to understand how a chiminea gets better airflow than an open pit?

Yeah:

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

He did it. Mostly. Before him there were inglenook fireplaces with wide flues, that made houses drafty.

The Rumford fireplace created a sensation in London when he introduced the idea of restricting the chimney opening to increase the updraught, which was a much more efficient way to heat a room than earlier fireplaces. He and his workers modified fireplaces by inserting bricks into the hearth to make the side walls angled, and added a choke to the chimney to increase the speed of air going up the flue. The effect was to produce a streamlined air flow, so all the smoke would go up into the chimney rather than lingering, entering the room, and often choking the residents. It also had the effect of increasing the efficiency of the fire, and gave extra control of the rate of combustion of the fuel, whether wood or coal. Many fashionable London houses were modified to his instructions, and became smoke-free.