stupid physics questions

[Dio]

What decreases with increasing frequency is amplitude. For a given energy, a low-frequency wave has a larger amplitude than a high-frequency wave.

This may also be about right. Do higher energy waves travel faster?

Don't think so. The speed depends on the medium; for something like sound waves in air it's a constant. For water waves there are different behaviours for deep and shallow water; shallow wave velocity depends on the depth, while for waves in deep water it's effectively constant. The definition of 'deep' is frequency-dependent IIRC.

This is what causes the wave breaking effect; the wave slows as it approaches the shore, lowering its frequency and raising its amplitude, because the energy has to remain constant.

This isn't quite right, though it may be a terminology thing rather than a misunderstanding of the underlying physics. What happens is that as the speed slows, the frequency (the time betweeen wavecrests) stays the same but the wavelength (the distance between wavecrests) shortens, so the amplitude has to go up, and the wave breaks. The speed is related to the depth of the water, I don't remember whether I ever knew what the exact relationship is.

Yes, sorry, you're right thtere. Pulse compression is compression of wavelength not frequency and I may have bollocksed up my terminology talking about frequencies when I should have been talking about wavelengths.

[zip accident]

Do higher energy waves travel faster?

Don't think so. The speed depends on the medium; for something like sound waves in air it's a constant. For water waves there are different behaviours for deep and shallow water; shallow wave velocity depends on the depth, while for waves in deep water it's effectively constant. The definition of 'deep' is frequency-dependent IIRC.

I don't think this is right. The North Sea is choppier than the Atlantic, because the North Sea is shallower. The relation between depth and speed might be the square or cube (or maybe fourth or fifth) root of the depth causing the speed. Depth probably averages over the surrounding area or something (say the area of half the depth?).

Pressure waves in gas are supersonic. Tsunami are fast, much faster than tides, are they also supersonic (for their medium, which is the water/air interface, in which the speed of waves is much slower than sounds in air)?

[pogo]

I think I once read that one possibly explanation for Dark Matter was its just normal matter displaced over an axis we cant see (ie, a 5th dimension, or a 4th non-time dimension, however your counting).
The fundimental forces like gravity wouild work evenly in all dimensions, so even though we couldnt see this matter, on very large scales it would still effect galaxy formation.

But that's exactly the sort problem I mean: entirely theoretical physics which we can't and haven't observed (dark matter) is hypothesised to involve extra dimensions, which then explains away the fact that we can't see it.

I don't buy it. The justifications for four dimensions are pretty strong. Not just that we can observe and measure them, and that everything we know takes place within them, but the mathematics of the universe only works in 4D. The inverse square law for example, which allows stable orbits, only occurs in 4D.

[zip accident]

I think I once read that one possibly explanation for Dark Matter was its just normal matter displaced over an axis we cant see (ie, a 5th dimension, or a 4th non-time dimension, however your counting).
The fundimental forces like gravity wouild work evenly in all dimensions, so even though we couldnt see this matter, on very large scales it would still effect galaxy formation.

But that's exactly the sort problem I mean: entirely theoretical physics which we can't and haven't observed (dark matter) is hypothesised to involve extra dimensions, which then explains away the fact that we can't see it.

I don't buy it. The justifications for four dimensions are pretty strong. Not just that we can observe and measure them, and that everything we know takes place within them, but the mathematics of the universe only works in 4D. The inverse square law for example, which allows stable orbits, only occurs in 4D.

How would you explain curved space-time if there are only three geometric dimensions?

If the dimensions decrease with scale, then at huge distances gravity might fall off a bit faster than otherwise predicted, I don't know whether that fits the facts or not, but it's probably testable.

Not sure that would work though, a dimension is either there or it isnt. You cant have half a dimension, and I cant picture one "just stopping" at some scale.

The idea that they curve in on themselves with different internal size's I guess could work though.

I think I'd tend to go with the curvature of space getting more extreme, until the "new" dimension appeared out of that.

It may well be a silly idea.

In part it was because with Relativity and Quantum Mechanics being incompatible, the point where the change of paradigm between the two comes about has to be the scale at which we normally live, so something odd must happen here, except it doesn't seem odd, because it's normal to us. So, what is it?

[Dio]

Pressure waves in gas are supersonic.

These are very different kinds of waves, they're not lossless for one thing.

I presume you mean shore waves rather than tides? I don't know about the behaviour you discuss.

[pogo]

How would you explain curved space-time if there are only three geometric dimensions?

You can either say the universe is 4D but the paths of particles are bent according to the laws of general relativity. Or you can say the universe is 4D but curved in a higher dimensional space, so the curved paths of particles are due to them following straight paths (geodesics) in this space.

But it doesn't matter which model you prefer as the outcome is the same, the laws of physics are still constrained to 4D and our universe is essentially four dimensional. There could be extra dimensions outside our four but we can't detect them or measure them: any measurement would involve breaking the laws of physics as we know them. Someone could come up with new laws which allow for multiple dimensions but we're still waiting.

[zip accident]

I presume you mean shore waves rather than tides? I don't know about the behaviour you discuss.

No, I actually mean tides. They have a frequency of about 12.5 hours (depending upon the height of the tide for the location (heights vary a lot in height between locations, these differences between locations don't appreciably affect the times of the tides), higher tides are faster), and they have a speed around the coastline that can be determined by the difference in high tide times at different locations, of course the direction of the tide might be out to sea, except in the channels, but the times around the coast give an approximation. I don't know what the speed of the tide is, but that Fukushima tsunami was expected in Hawaii within a day or two, that's faster, and the frequency of a tsunami is shorter than a tide.

[Darkflame]

Can you have a citation for "the maths/physics" only works in 4D because its the first Ive ever heard that.
4D is the minimum, but that doesnt mean it breaks down higher.

any measurement would involve breaking the laws of physics as we know them.

I dont think this is true.
A object with sufficiant mass displaced along another spacial dimension should be completely messurable with even normal newtonian gravational mechcanics.

This just requires as an assumption that gravity works in the same accross all dimensions - it wouldn't change how gravity effects our normal 4.

Of course, we would probably have to be insanely lucky to find a good observation of this effect, given how empty space is, and how weak gravity is.

But that's exactly the sort problem I mean: entirely theoretical physics which we can't and haven't observed (dark matter) is hypothesised to involve extra dimensions, which then explains away the fact that we can't see it.

I do acknowledge this though; they are just there to explain current diviations in the observations from the predications.
Whatever you feel is most likely more or less goes by your instinct untill we have more data.

It could be a 5th dimension, or it could be particals that have mass but nearly no interaction whatsoever, or it could be we just have the rules of gravity at large scales wrong.

It seems all are still open possibilities.

[pogo]

Can you have a citation for "the maths/physics" only works in 4D because its the first Ive ever heard that.
4D is the minimum, but that doesnt mean it breaks down higher.

http://en.wikipedia.org/wiki/Spacetime# ... _spacetime

any measurement would involve breaking the laws of physics as we know them.

I dont think this is true.
A object with sufficiant mass displaced along another spacial dimension should be completely messurable with even normal newtonian gravational mechcanics.

I was thinking of things like conservation laws, of e.g. energy, charge and momentum. If things could disappear off into and appear from higher dimensions these laws wouldn't hold. So we're constrained to 4D even if that 4D is warped in 5D. The above links has many more arguments.

It's always possible that someone will come up with some new laws of physics, which is relativistic and four dimensional up to a limit but beyond that limit diverges, much as (three dimensional) Newtonian physics is valid for non-relativistic speeds. The problem is what is that limit? The existing laws describe the known universe very well, there's not a hole in our understanding that needs filling, as there was with relativity.

[zip accident]

there's not a hole in our understanding that needs filling, as there was with relativity.

Yes there is.

Quantum Mechanics is not compatible with Relativity. At atomic scales, QM rules. At Galactic scales Relativity rules. At out scale, Newtonian Mechanics are a better approximation than either. That is your hole.

<edit>

On tsunami:

http://www.bbc.co.uk/news/uk-england-norfolk-15910264

That is exactly the same sort of thing as a tsunami, but it's moving much slower. Five hours to get down the coast of some of England? a tsunami would do all of that in a couple of hours or less.

[pogo]

Quantum Mechanics is not compatible with Relativity. At atomic scales, QM rules. At Galactic scales Relativity rules. At out scale, Newtonian Mechanics are a better approximation than either. That is your hole.

Not true. If QM and relativity were incompatible it would be possible to identify where they were incompatible and so where they made different predictions. Then you could do an experiment to see which of their predictions were accurate and so which was correct. But as far as we know both are correct and they are perfectly compatible. What they are not though is unified: both relativity and QM seem to describe the whole universe, but neither can be used to predict or derive the other. Nor is there a single theory from which both QM and relativity can be derived.

Science would like to find such a theory. One justification is that many other things (electricity + magnetism, time + space, waves + particles, electromagnetism + the weak nuclear force) have been unified, alway offering greater insights and understanding. If we can unify QM and relativity then it will effectively solve physics: all phenomena will be described by one set of rules. But just because it would be nice to find such a theory doesn't mean it exists. It could be there is no such theory of everything, and we have to make do with the separate rules we have today.

[Darkflame]

Can you have a citation for "the maths/physics" only works in 4D because its the first Ive ever heard that.
4D is the minimum, but that doesnt mean it breaks down higher.

http://en.wikipedia.org/wiki/Spacetime# ... _spacetime

Interesting....still have to read a lot more though.
Orbits work fine in 2D so I'm not sure why they wouldnt it 4 or more.

I must admit I am instinctively skeptical of seemingly arbitary limits. From a brief reading the essential arguement seems to be that the falloff we expirence with gravity is inverse square due to the number of dimensions. This seems intuative but I'm not sure that has to be the case - and if it is, I'm not sure why orbits wouldn't work with higher falloffs. (the speeds would need to be different, of course).

I was thinking of things like conservation laws, of e.g. energy, charge and momentum. If things could disappear off into and appear from higher dimensions these laws wouldn't hold.

I simply assumed they would hold, merely accross the extra dimensions? ie. the total accross all dimensions must be maintained.
Just as if you could time travel, you would bring extra mass into the past, but from a 4D standapoint nothing has been created or destroyed.

It's always possible that someone will come up with some new laws of physics, which is relativistic and four dimensional up to a limit but beyond that limit diverges, much as (three dimensional) Newtonian physics is valid for non-relativistic speeds. The problem is what is that limit? The existing laws describe the known universe very well, there's not a hole in our understanding that needs filling, as there was with relativity.

Well, there is a hole in order to tie it into observations of the universe at present - this is why we have concepts like Dark Matter and Dark Energy. Its hard to argue they are anything other then "patchs" untill we have more direct evidence for them.

[pogo]

Orbits work fine in 2D so I'm not sure why they wouldnt it 4 or more.

Orbits don't work in 2D. The inverse square law, that e.g. gravity decreases with the reciprocal of the square of distance from the source (a mass in the case of gravity), only arises in 3D. It arises as the area of the surface of a sphere is 2D, so proportional to the square of the distance/radius. And only an inverse square law allows stable orbits.

[Ollie]

Not sure that would work though, a dimension is either there or it isnt. You cant have half a dimension, and I cant picture one "just stopping" at some scale.

You can have half a dimension if it's a fractal dimension. Just saying.

[zip accident]

Quantum Mechanics is not compatible with Relativity. At atomic scales, QM rules. At Galactic scales Relativity rules. At our scale, Newtonian Mechanics are a better approximation than either. That is your hole.

Not true. If QM and relativity were incompatible it would be possible to identify where they were incompatible and so where they made different predictions. Then you could do an experiment to see which of their predictions were accurate and so which was correct. But as far as we know both are correct and they are perfectly compatible.

This is rubbish. At galactic scales, quantum mechanists make no predictions, because they know their theory, if applied, would give the wrong answers, the same is true of relativity at atomic scales. They both appear to be about as good as we can get at the scales where they do give answers that match up with reality, but they are neither of them adequate at the other end of the scale.