stupid physics questions

[gravy]

So there's this:

http://physics.stackexchange.com/questi ... ruh-effect

QFT predicts particles in a space for observers with accelerating inertial frames but non accelerating observers seeing nothing in the same space.

gumboot: I think it just doesn't fit together and that's that. People seem pretty happy throwing around terms like black hole but isn't it really just short hand for "we don't what the fuck it is because GR maths don't work at that point"

[KlaxonCow]

People knew Newtonian physics was incomplete long before Einstein came along, both for theoretical reasons and due to measurements not agreeing with predictions. That’s why his discoveries were so important, they essentially provided answers to all the questions physicists had been asking for decades.

The analogy still holds.

There is no quantum theory of gravity. All the other forces - the strong and weak forces, electromagnetism - have been unified, but gravity can't reconciled with them. And not in a trivial way either. Gravity is significantly weaker than the other forces in a big way. Why is that?

Galaxies ought not to look as they do. There isn't sufficient visible mass to explain it. Okay, the answer is that there must be some kind of "invisible mass" - dark matter - to fill in this gravitational gap. But the thing is, the word "dark" is just the fancy scientific way of saying "we haven't got a fucking clue what this stuff might be".

Before observations were taken, the question was how fast the universe was slowing down in its expansion. Not whether it was slowing down at all, as that was taken as read - the universe was expanding but gravity should be slowing that expansion down - but merely how fast it was happening. So we could work out if we were in a universe that would eventually slow down, reverse direction and go into a "big crunch", or a universe where gravity wasn't enough to halt the expansion and it would expand forever until its eventual heat death.

Then the measurements were taken. The expansion was speeding up, not slowing down. Completely counter to every expectation.

Okay, again, the "fill in the gaps" explanation for this, for the time being, is "dark energy". But, again, the word "dark" there really means "no fucking clue, mate".

And between "dark matter" and "dark energy", that's some 95% of the entire universe that we don't know anything about. I mean, you smile, put on the brave face and continue with the grand enterprise of science to eventually work out what's going on. Of course you do. But that is, to put it mildly, a pretty bloody disastrous result.

Regards unification, general relativity is geometric, while quantum mechanics is statistical. It's not just these two views - "top down" and "bottom up", so to speak - haven't been unified, they're fundamentally incompatible approaches. The real first question, before unification is even possible, is how the hell do you even fit these things together at all?

We know that there's a problem here. We know that a quantum theory of gravity is missing here. We know that theory and observation are not matching up properly, and let's be clear that our bandage job on this - dark matter and dark energy - both have the word "dark" in them to represent that we don't fucking know what the shit any of it is, or why it's there, or where it comes from, or anything particularly useful about it at all.

We equally know, right now, that Einstein is incomplete too. Both for theoretical reasons and observations not agreeing with predictions.

And, indeed, if someone comes along who can explain all this with a neat little theory, then their discoveries will also be greatly important for starting to provide answers to questions physicists have been asking for decades.

Same difference. The analogy still holds.

[gravy]

We've been through this with pogo earlier.

He won't have it despite it being obvious neither GR or QM are complete.

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.

[gumboot]

I think it just doesn't fit together and that's that. People seem pretty happy throwing around terms like black hole but isn't it really just short hand for "we don't what the fuck it is because GR maths don't work at that point"

Yeah, it was easier when it was a strictly a relativity question and a black hole was just the case where escape velocity exceeded the speed of light.

When you start counting particles over those scales, I thought everyone except pogo acknowledged that the models don't join up in the middle. Don't they?

[pogo]

We've been through this with pogo earlier.

He won't have it despite it being obvious neither GR or QM are complete.

It may be just we approach it from different philosophical angles, but my view is the “problems” with GR and QM are nothing like those with classical physics. By the time Einstein discovered SR physicists were already looking for alternatives because of the problems it had, such as reconciling the constant speed of light, as measured by e.g. Michelson and Morely. QM similarly arose in part to address the problems with black body radiation.

But there is nothing we can point to today that invalidates GR or QM. There are things we don’t yet know what we are, like dark matter. But there are numerous possible explanations for that. It could be just regular matter we have not been able to detect. It could be some exotic particle or particles that we don’t yet have a way to make or discover. It could be the laws of gravity are subtly different on a large physical or time scale, which might mean GR needs modifying, though this seems least likely.

Physics is meant to be an experimental science. You observe something that physics does not explain, come up with theory, then try and confirm it by experiment and further observations. The problem is GR and QM explain our universe very well. There is nothing we can point to which contradicts it. Dark matter might, if we could be sure what it is. Dark energy similarly. Maybe what goes on at the heart of a black hole, except we cannot even see them never mind peer inside.

At the same time there are many different theories trying to unify GR with other things – with the other fundamental forces or with QM. The problem is there are many different theories, none of them obviously correct. This again may be a philosophical view (a variant of Occam's razor perhaps), but you would think that if there were a unifying theory it would be simple and obvious, like relativity was once Einstein discovered it. That there are so many complex competing theories suggests they are perhaps barking up the wrong tree.

[zip accident]

Not mine, at all:

http://www.giantitp.com/forums/showthre ... t-abstract

I don't pretend to understand the question, maybe it's not even stupid.

[gumboot]

Physics is meant to be an experimental science. You observe something that physics does not explain, come up with theory, then try and confirm it by experiment and further observations.

Is it really experimental science when the only observations we can make of large-scale phenomena are of things that are already out there? Particularly when our idea of what's out there is so heavily coloured by our current estimation of how physics works.

[pogo]

We can do more observations. The recent history of astronomy has largely consisted of gathering more and more data, from bigger and better telescopes and from a growing fleet of satellites. E.g. observation of the cosmic microwave background was used as evidence of the Big Bang theory. We can’t do an experiment to reproduce the Big Bang but can look for evidence of it such as the CMB.

Experiments and observation are key components of the scientific method. You have a hypothesis. You design an experiment, which might be in the lab or in the real world, or even based on astronomical observations. You do your experiment, and use the results to either confirm or refute your hypothesis.

Without that you are not really doing science; not really doing physics. You are maybe doing theoretical physics which is quite different, closer to mathematics than the rest of physics. Some people treat theoretical physics not as part of physics but as part of mathematics. And you can see why. When people are doing e.g. string theory in 26 dimensions it has little to do with the physics of the real world and is more an abstract mathematical construction.

[gravy]

So the discovery of the positron was more maths than physics?

Or special relativity. Einstein's impetus was mostly mathematical inconsistency in accepted models rather than experiment.

A lot of people wouldn't agree with that.

[gumboot]

I think everything is dominated by maths, now, isn't it? It's no longer a matter of quality optics showing you what's there in a way that's completely tangle ("it's like looking, but closer"). Now you have to infer much more from models of how things are expected to work and reconstitute observations in much more abstract, and generally mathematical, ways.

Which is something I thought to revisit when I saw such-and-such cloaking technology on some Sci fi thing. Really, when you consider the complexity of a sensor that might detect something as small as a space ship at substantial distances in unknown directions, manipulation of signals to render oneself undetectable a could be a far blunter instrument than what you typically get on TV where you can see discreet starts through the hull up close. It doesn't seem so implausible as that.

[pogo]

So the discovery of the positron was more maths than physics?

Or special relativity. Einstein's impetus was mostly mathematical inconsistency in accepted models rather than experiment.

A lot of people wouldn't agree with that.

The impetus for SR was experimental. People knew light had a speed but expected it to vary due to the rotation and movement of Earth through space, or through the luminiferous ether that was thought to pervade space and carry light. But experiments showed that the speed of light was constant. SR also made predictions people could test.

GR is perhaps a better example; when devised it was a mathematical construct, designed to unify gravity and spacetime. There was not any experimental result it was trying to explain, as it has few consequences on the Earth’s surface. This meant initially it was not as widely accepted as SR. But it was confirmed by observations, such as of gravitational light deflection, and has since been confirmed in many other observations and experiments.

A good modern example of theoretical physics is that is not good physics are closed timelike curves. These arise from solutions to the equations of GR, and would seem to enable time travel, but are dismissed by most mainstream physicists on a variety of grounds. Some people without a physics background think the fact they are described within physics means time travel might be possible.

And a lot of theoretical physics like that: mathematically attractive in some way or other but which does not make any testable predictions. There‘s the core, of generally accepted mainstream physics, which has been confirmed by experiment/observation. And there’s everything else. What you call it is less important than understanding that it is not mainstream, not generally accepted.

[gravy]

So the discovery of the positron was more maths than physics?

Or special relativity. Einstein's impetus was mostly mathematical inconsistency in accepted models rather than experiment.

A lot of people wouldn't agree with that.

The impetus for SR was experimental. People knew light had a speed but expected it to vary due to the rotation and movement of Earth through space, or through the luminiferous ether that was thought to pervade space and carry light. But experiments showed that the speed of light was constant. SR also made predictions people could test.

GR is perhaps a better example; when devised it was a mathematical construct, designed to unify gravity and spacetime. There was not any experimental result it was trying to explain, as it has few consequences on the Earth’s surface. This meant initially it was not as widely accepted as SR. But it was confirmed by observations, such as of gravitational light deflection, and has since been confirmed in many other observations and experiments.

A good modern example of theoretical physics is that is not good physics are closed timelike curves. These arise from solutions to the equations of GR, and would seem to enable time travel, but are dismissed by most mainstream physicists on a variety of grounds. Some people without a physics background think the fact they are described within physics means time travel might be possible.

And a lot of theoretical physics like that: mathematically attractive in some way or other but which does not make any testable predictions. There‘s the core, of generally accepted mainstream physics, which has been confirmed by experiment/observation. And there’s everything else. What you call it is less important than understanding that it is not mainstream, not generally accepted.

Einstein talked a bit about his path to special relativity and was quite clear that michelson / morely wasn't an impetus for him. He said the start of it was a thought experiment when he was in his teens about what a light wave would look like if you were travelling alongside at the same speed and thought it wouldn't make any sense if was frozen.

It's true you need both experiment and theory to drive physics forward. But one isn't subservient to the other and knocking string theory for not having testable outcomes is utterly tedious. It's a work in progress that may or may not come to something important but if no one thinks about then it definitely won't.

Back to SR / GR being incomplete though, you do realise that was Einstein's view as well?

[pogo]

Back to SR / GR being incomplete though, you do realise that was Einstein's view as well?

But that is not unusual in science. Very often discoveries no matter how profound and significant still leave questions unanswered. With GR Einstein felt the need to introduce the cosmological constant to account for the universe being static, but was unhappy that there was no justification for it. It turned out to be not needed as the universe is not static but expanding, due to the Big Bang. But that was not widely accepted until much later.

It depends what you mean by incomplete. Except for Einstein’s own development of GR to extend SR no-one has found any flaws in his theories, despite a century of applying them to everything from particle accelerators to the large-scale universe. There are things that we cannot describe yet with GR, such as at the singularity of a black hole, but it may be such limits of spacetime are inherently indescribable with the physics of spacetime, just as the time “before” the Big Bang is, and we should not lose too much sleep worrying about it.

[gravy]

I wouldn't lose sleep about it but some people do. Which is good as that's the only way knowledge will ever progress.

Einstein spent 30 years of his life trying to unify gravity with the other fundamental forces. He failed but what are you saying? He was mistaken to do so?

[gumboot]

Pain-delirium posting, here... Woke up with this problem (or non-problem) stuck in my head.

If you travel towards something at such a rate as to have notable time-dilating effect, does that cause that target to appear to be increasing or decreasing in density as a consequence of universal expansion?

Well everything increases, I guess, so it's more does it increase faster or slower than the observer?


I think the problem comes from trying to do some dimensional analysis on the nutritional information panel on some whiteboard makers in a dream... I at least know that that part doesn't make sense.