physics question

[teuchter]

Recent measurements suggests the universe is not spinning - it appears to be isotropic.

Is it not the case that it would be meaningless to say the universe is spinning anyway? Spinning relative to what?

 

[NoXion]

Is it not the case that it would be meaningless to say the universe is spinning anyway? Spinning relative to what?

Its axis?

 

[2hats]

i wonder how they take the measurement..

Temperature and polarisation measurements of the cosmic microwave background (CMB).

Is it not the case that it would be meaningless to say the universe is spinning anyway? Spinning relative to what?

The CMB.

 

[teuchter]

But the CMB is part of the universe, is it not?

 

[NoXion]

I would have thought the fact that the universe is very obviously not revolving around some kind of axis, would be enough to say that universe is not spinning.

 

[teuchter]

I would have thought the fact that the universe is very obviously not revolving around some kind of axis, would be enough to say that universe is not spinning.

So how would you tell if it was?

 

[NoXion]

So how would you tell if it was?

There would be a movement of galaxies and superclusters away from the axis of rotation, with the rate of acceleration increasing with distance from said axis. Seems like it would be incredibly obvious but we don't see it.

 

[teuchter]

But what would things be rotating relative to, to create this effect? I don't see how the whole universe can rotate because by definition there's not anything outside of it as a frame of reference.

 

[2hats]

A rotating frame is non inertial. It is accelerating relative to the axis.

 

[NoXion]

But what would things be rotating relative to, to create this effect? I don't see how the whole universe can rotate because by definition there's not anything outside of it as a frame of reference.

What 2hats said. To put it another way, if the universe was rotating, it would be rotating around a line within the universe. There's no need for any external references.

 

[DotCommunist]

I think the answer to your question is "no" but check out Mach's principle.



This does my head in a bit.

excellent youtbe rabbit hole. Everything I ever knew about how wings work was a lie

 

[moon]

There would be a movement of galaxies and superclusters away from the axis of rotation, with the rate of acceleration increasing with distance from said axis. Seems like it would be incredibly obvious but we don't see it.

Like a centrfugal force ?

 

[NoXion]

Like a centrfugal force ?

Exactly.

 

[teuchter]

What 2hats said. To put it another way, if the universe was rotating, it would be rotating around a line within the universe. There's no need for any external references.

If the line is part of the universe, and the line is not rotating then all that is happening is that part of the universe is rotating relative to another part of the universe, surely.

 

[NoXion]

If the line is part of the universe, and the line is not rotating then all that is happening is that part of the universe is rotating relative to another part of the universe, surely.

The line, AKA the axis, is just a particular part of the reference frame. It's just a name for that particular part of the model. The axis neither spins nor rotates, it's an abstraction, like an equator. The objects within a rotating universe however are real, and they would be experiencing acceleration orthogonal to the axis, and the magnitude would be dependent on their position relative to it.

 

[2hats]

But the CMB is part of the universe, is it not?

Yes it is. But don't forget you are looking back in time at, not spatially across, an object. If the universe were rotating you would spot some signal, a particular distribution, in the CMB (ie structure in it) that we do not observe.

 

[SpookyFrank]

could ask google but you lot have a crack. Is my understanding here correct. Spin imparts stability. That's why we have rifling in guns, spin bowlers and fletching on arrows.

When you spin the bucket of water on a rope around and it stays in this is centrifugal force yes. But you are stationary relative to the larger gravitational mass, the massive planet. Is it then the case that by imparting spin to an object you stabilise it because it is now benefiting from that same 'relative to the more massive object' by spinning?

Spin bowling is supposed to create an unstable flight path.

Pace bowlers spin the ball along an axis broadly perpendicular to the direction of travel, this allows for an aerodynamic effect called swing resulting from asymmetry of the ball. Swing arises from the same quirk of fluid dynamics which allows aircraft wings to generate lift.

Your actuall spin bowlers spin the ball along an axis approximately parallel to the direction of travel which will have an aerodynamic effect on the flight path of the ball, bending it away from the standard 2-dimensional parabolic arc, as well as causing it to change direction as it impacts the pitch.

 

[eskdave]

could ask google but you lot have a crack. Is my understanding here correct. Spin imparts stability. That's why we have rifling in guns, spin bowlers and fletching on arrows.

When you spin the bucket of water on a rope around and it stays in this is centrifugal force yes. But you are stationary relative to the larger gravitational mass, the massive planet. Is it then the case that by imparting spin to an object you stabilise it because it is now benefiting from that same 'relative to the more massive object' by spinning?

yes

 

[eskdave]

yes

but

 

[SpookyFrank]

So how would you tell if it was?

All the tea in teacups would slosh up one side.

 

[moon]

I have some physics questions..
I will start with... what is in the centre of the earth? Is it crystal?

 

[SpookyFrank]

I have some physics questions..
I will start with... what is in the centre of the earth? Is it crystal?

That's not really a physics question, but anyway. The Earth's inner core is basically solid iron, surrounded by a liquid outer core of iron and nickel whose motion relative to the rest of Earth is what creates that lovely magnetic field that stops cosmic rays from slicing all our DNA into nucleotide confetti.

 

[2hats]

I have some physics questions..
I will start with... what is in the centre of the earth? Is it crystal?

Many crystals of iron with an ever varying alignment (seismology suggests).

 

[moon]

Ok thanks, so do all planets have a crystalline core? Does the earths core differ from that of other planets because the earth is alive i.e. it has seismic activity?

 

[NoXion]

Ok thanks, so do all planets have a crystalline core? Does the earths core differ from that of other planets because the earth is alive i.e. it has seismic activity?

All rocky/metallic planets the size and density of Earth are large enough to retain a semi-molten core after formation. Planets with unusual properties, such as Venus due to its sluggish rate of rotation, might possibly be lacking the crystalline structures, or if present they'll likely be different to Earth's. I'd probably have to ask a planetary scientist or dig into the literature to be more certain.

 

[2hats]

Ok thanks, so do all planets have a crystalline core?

Most likely not but we don't have the seismological data for any planet other Earth to clearly determine such.

Does the earths core differ from that of other planets because the earth is alive i.e. it has seismic activity?

From some, yes, but there are almost certainly similar out there somewhere. Even in our own solar system we see evidence of seismic activity on the Moon, Mars, Venus. Most likely Mercury and Io also experience such. But only on Earth is this driven by plate tectonics (as best we know). Io, Moon, Mercury are gravitational tidally/thermally driven, Venus probably through radioactive isotopic decay heating and Mars perhaps similar (an upcoming mission will carry out more thorough seismic observations of Mars over the next few years to better understand what is going on there).

 

[moon]

ok thanks, so this is a weird question but its still related to ^^^^^ so there is something called the fabric of space, yes? as defined by Einstein... I think.. how does it relate to planetary bodies? Does it sort of sink into the planet, or is it bent (I think this is the theory).
Basically I want to know how a planet affects the fabric of space and whether its crystalline core exerts any type of influence on the nature of space fabric.. or what ever it is, the stuff that the planets are suspended in.

 

[2hats]

You are asking about the fabric of space-time, I think?

That's just a device, a metaphor to explain what is happening in greatly oversimplified terms. There is no fabric. It is just an attempt to communicate what is happening (like whirling a weight on a piece of string above your head to illustrate the orbit of a satellite; there is no string), because actual general relativity is hard to understand being (often) counterintuitive to our everyday experience (like quantum mechanics).

Space-time is distorted by all matter. That is what general relativity tells us.

I guess you could be asking about the (a)ether but such, as described by Einstein, does not exist. Modern physics doesn't accept the existence of an ether, though some theoretical physicists flirt with quite different notions of it from time to time (none have been accepted).

 

[moon]

Ok thanks, I will read up on the basics of space-time, I think what I was trying to get at was whether a planet represents negative space in the 'fabric' of space, sort of like a black hole but with actual matter in it... This is a assuming that a black hole is considered to be negative space...
And how this relates to gravity.
You don't have to answer my questions if they seem too crazy etc lol

 

[Santino]

Is the centre of Jupiter a diamond the size of our moon?