stupid physics questions

[Absolute_Zero]

My alimentary canal suggested I post this as a marker. Besides, for this and other contributions... Arkani-Hamed is a name that should be better known.

https://www.simonsfoundation.org/quanta ... m-physics/

[Whiteball]

A Capella Science - Bohemian Gravity!
http://youtu.be/2rjbtsX7twc

[pogo]

Physics: What We Do and Don’t Know

Only covers two areas, elementary particle physics and cosmology, but a good choice as they include the most significant unknowns in physics today. A good read and relatively accessible.

[zip accident]

Physics: What We Do and Don’t Know

Only covers two areas, elementary particle physics and cosmology, but a good choice as they include the most significant unknowns in physics today. A good read and relatively accessible.

Thanks for posting that link.

[zip accident]

Thinking about gravity waves, it seems to me probable that if there are any, they'll be low frequency, like tsunami, and if that's the case we can't spot them with earthbound experiments or equipment. What we need to watch for are apparent perturbations of the distances of the orbits of objects in the solar system. I'm not sure whether we already have the data, if not it ought to be relatively easy to arrange to get it, though if the distances moved are metres, surface irregularities will be a problem.

[Mr Dom]

Yeah, gravity waves would be fairly low frequency.
Supernovae should throw off some great waves with a short period too, but you never know when one is coming...
But there are smaller waves thrown off by things binary stars, and some binary orbits are of the order of hours to days (the fastest known is ~3 minutes I think).

The problem is that the effect of a gravity wave is extremely small, changing 1m by something really incredibly small like 10^-21m even for a big supernova wave, but reading this signal against noise caused by the trivial stuff like the vibrations of the atoms in the mirrors you use is pretty tricky. There are plans for space-based experiments that measure over distances of 10,000km or more, but they have other issues to deal with.

But every few years the sensors get an order of magnitude better, and I don't think we have got into the range where we _should_ definitely have found something yet.

[zip accident]

What I was thinking of, was that with low frequency waves at the speed of light, the wavelengths will be really really long. For one minute frequency waves, the wavelength would be about 10,000,000 miles, and for a frequency of an hour sixty times that, and the solar system is just about big enough to deal with that. Waves with a frequency of a year you'd need something bigger still.

[pogo]

I think a more fundamental problem is good sources of gravitational waves will be incredibly rare. The problem is that anything emitting strong gravitational waves, such as a very quickly orbiting binary star system, is losing energy rapidly through those waves. This means that before long the orbit will decay, they will collide, and it will no longer be a good source of such waves. If there ever were any formed in our galaxy they decayed long long ago. We in theory have more chance of finding one in a distant galaxy, because looking out in space we're also looking back in time. But even then we're looking for something that only existed for a very short time, perhaps a few years, within the billions of years of the history of the galaxy.

[pogo]

Why does warm water freeze more quickly than cold (the Mpemba effect)? Some clever scientists in Singapore have the answer:

https://medium.com/editors-picks/d8a2f611e853

[zip accident]

What's wrong with the rubber sheet analogy of gravity?

It's trying to use a force twice in the same equation.

It's so late it's early, and I'm being kept awake by a thought, I'll get it down, then I can forget it and sleep, so ...

The first problem with the rubber sheet analogy of gravity bending spacetime is that it uses gravity to explain gravity, but that's okay, use magnetism or a static charge instead, and bingo all's well that ends well. Except it isn't. The really big problem with the rubber sheet model, is that it uses the same force for two things that aren't really related. Firstly, it's used to deform the sheet. Then it's used to pull the masses toward the sheet, so they are attracted by depressions in the sheet. These are different things, and the same force can't be expected to do both.

So, now that's written, maybe I can sleep, tomorrow/today I'll be interested if anyone has any relevant thoughts.

[Absolute_Zero]

Like most analogies, if taken too literally they become misleading.

Rather than type reams of mistake-laden text, I'll point to a good place to pose "stupid physics questions"... and the specific answer to your query.
http://physics.stackexchange.com/questi ... me-gravity

[Darkflame]

What's wrong with the rubber sheet analogy of gravity?

It's trying to use a force twice in the same equation.

It's so late it's early, and I'm being kept awake by a thought, I'll get it down, then I can forget it and sleep, so ...

The first problem with the rubber sheet analogy of gravity bending spacetime is that it uses gravity to explain gravity, but that's okay, use magnetism or a static charge instead, and bingo all's well that ends well. Except it isn't. The really big problem with the rubber sheet model, is that it uses the same force for two things that aren't really related. Firstly, it's used to deform the sheet. Then it's used to pull the masses toward the sheet, so they are attracted by depressions in the sheet. These are different things, and the same force can't be expected to do both.

So, now that's written, maybe I can sleep, tomorrow/today I'll be interested if anyone has any relevant thoughts.

+1
No part of the rubber sheet analogy helps me know why objects move towards the "dense" space at the middle from standing still.
I can, oddly enough, see how a curvature in spacetime can make linear movement look like acceleration though.

[pogo]

I don't see the problem with it. In one sense it is only an analogy. You can describe gravity without resorting to rubber sheets if you want, and that's what was done for hundreds of years. Rubber sheets are perhaps a crude analogy as if you tried to set up an experiment with such sheets and e.g. similar size orbiting masses you'd quickly run into problems with energy loss as the sheet dissipated energy as it stretched and rebounded. It work better with a rigid curved surface and static fields, such as a large central mass and small masses orbiting about it. It's easy then to see how masses closer in need to orbit more quickly to counteract the steeper slope/overcome the greater gravity.

It's real strength though is modelling the behaviour of massless particles such as photons. As when we say space is curved we don't just mean it's curved for masses but curved for everything, according to general relativity. So light approaching a massive object is bent. And this has been seen and confirmed repeatedly, around our sun during solar eclipses, around distant stars and galaxies in a phenomenon called gravitational lensing. As far as the light's concerned it travels in a straight line, a geodesic, but because space curves locally the light is diverted around the mass due to the laws of physics.

[zip accident]

Rather than type reams of mistake-laden text, I'll point to a good place to pose "stupid physics questions"... and the specific answer to your query.
http://physics.stackexchange.com/questi ... me-gravity

Thanks for that, however, those answers don't in fact cover what I was talking about.

Like most analogies, if taken too literally they become misleading.

No, a good and exact analogy does explain. It's poor analogies that don't. There are a lot of poor analogies about the place, generally made by people who don't understand what they are trying to explain, and some really good ones, generally made by people who do understand things, but sometimes just by raw luck.

[pogo]

I'll go off on a bit of tangent here but it makes much more sense once you realise everything is moving at the speed of light on geodesics. Everything. Light moves at the speed of light, obviously. But if you consider a mass it also moves, except in time.

Spacetime, the realisation of special relativity we live in, is a four dimensional space with three space dimensions and one time dimension. You have things like light in it that move at 'the speed of light', which simply means all their travel is in the space dimensions. And you have massive objects like us which are relatively fixed in space but are moving in time.

This means everything has a path it traces out in space, whether it's space-like (e.g. light) or time-like (e.g. us, or the Earth). And these paths are always geodesics.

A geodesic is simply a line that is straight to the person/object following it, but may be curved in some enclosing space. The best example is the equator, or any great circle on the Earth. Follow one in a straight line and you'll end up back where you are having travelled in a circle.

The geodesics in spacetime are the paths of objects under the influence of gravity. So they are parabolas, ellipses, hyperbolas of simple orbits and more complex paths between multiple masses. They are also the path of e.g. something dropped that falls under gravity. That's because as well as being curved in 3D spacetime is curved in the fourth time dimension, so an object moving only in time (a stationary object) will change to move in space because of the curvature of space and time.


So perhaps a rubber sheet is a good model for moving objects but not so much for stationary objects. Because you can trace out the geodesics on the rubber sheet for orbits and deflections. But those for objects that accelerate from rest are quite different, involving space and time, so aren't easily represented by a 2D rubber sheet.