A dumb question

[Shadowy Man]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

But the Earth is only absorbing heat from one direction. There may be a power equilibrium but not a directional one.

The red-shift/blue-shift effect applies to incoming photons too, not just outgoing ones -- the solar heating that hits the morning side of the daytime hemisphere is a little bit bluer than the solar heating that hits the afternoon side.

but both the morning and afternoon are absorbing energy from the sun (be it a little blue- or red-shifted as you say) while the night side is not. That’s the asymmetry to which I refer.

Do you know of a mechanism by which that asymmetry affects the spin of the object? I can see how it would push the object away from the sun, but I'd expect that to be neutral with respect to the object's rotational angular momentum.

sorry. I was speaking more generally to the point Steve bank was making about equilibrium with the environment and that even though there maybe a power equilibrium it doesn’t mean there isn’t a momentum transfer asymmetry. I wasn’t speaking specifically to the impact on the spin of Earth.

When the earth then reradiates into space it’s more equally distributed.

I think that will depend on the object's size, temperature, thermal conductivity, shape, albedo, atmospheric CO2, yada yada. One aspect of the distribution that's well-studied is the Yarkovsky effect -- it's hotter in the afternoon than in the morning because of the time lag between absorption and reradiation, so the reradiation tends to push the object along in its orbit and make it spiral outward. It's measurable on small asteroids.

Sure, that’s why I said “more equally” and not “equal”. It’s difficult to imagine that those effects would conspire to make the reradiation only on the dayside. So whatever the emission distribution would be it would not be the same as the absorption.

 

[Loren Pechtel]

The thing to ask isn't whether it'll stop spinning, but rather, what force is acting on it to stop spinning?

PERPLEXITY DAY...

Conservation of Angular Momentum​

Despite the energy loss through thermal radiation, the ball's angular momentum must be conserved in the absence of external torques. ...

The thermal radiation applies an external torque. Photons leaving a point on the spinning object's surface in the direction of the spin are blue-shifted; photons leaving in the direction opposite the spin are red-shifted; the momentum of a photon is proportional to its frequency.

I don't think that works--either the object cools and reaches 0K, or it's heated by incoming energy that should be just as distorted as the outgoing energy.

 

[Loren Pechtel]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

But the Earth is only absorbing heat from one direction. There may be a power equilibrium but not a directional one.

And Earth's orbit is rising because of this. AFIAK not enough to matter.

 

[Loren Pechtel]

When a photon leaves a laser is there an equal and opposite reaction?

Photon drive says yes. Laser-pumped lightsail says yes. Admittedly, neither has ever been built.

 

[Shadowy Man]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

But the Earth is only absorbing heat from one direction. There may be a power equilibrium but not a directional one.

And Earth's orbit is rising because of this. AFIAK not enough to matter.

Most of the effects mentioned here are “not enough to matter”. The basic question was answered early and now we seem to be on the fringes of effects on extremely long timescales.

 

[Loren Pechtel]

(your link derives this same result via what to me seems an unnecessarily tortured method, whose only justification appears to be their dislike of considering energetic mass and rest mass as though they were both mass, but as the SI unit for both is the kg, it is hard for me to understand that reluctance).

A spherical black-body at constant temperature radiates photons at a characteristic mean frequency in all directions, but as @Bomb#20 points out, the photons from a rotating sphere are blue or red shifted by the rotation, depending on which side of the axis they are coming from. So the average momentum of the photons differs from one side to the other, causing a reduction in angular momentum.

It ain't much, but it ain't zero.

If you treat energetic mass as rest mass you get some paradox situations.

Maximum mass neutron star whips past Sag A* just outside it's Roche limit. Let's take two observers: One stands in the distance and watches--treat energetic mass as rest mass and the neutron star should go down the rabbit hole. The second observer rides the neutron star. They see nothing that should cause it to go down the rabbit hole.

Thus our two observers see radically different end states from the flyby. The only way you can resolve this is to ignore energetic mass in this calculation and that means you have to ignore it in a whole bunch of other places, also.

 

[Loren Pechtel]

Unless I'm mistaken, tidal forces are proportional to the square of gravity, or to the fourth power of distance. If they were proportional to gravity, Sun would be a bigger factor in Earth's tide than the moon is.

Third, not fourth. They are proportional to the change in gravity.

 

[steve_bank]

A theoretical isotropic radiator is a massless infinitely small point radiator that radiates equally in all directions.

We treat distant stars as point radiators.

All objects above absolute zero are radiating thermal photons What changes with temperature is the peak wavelengths and the distribution.

A perfect black body at a temperature perfectly matches the BB curve for the temperature.

Shiny polished metal surfaces are poor radiators.

Put two metal pates in space as heat radiators next to each other at the same temperature. The outer surfaces will radiate into space cooling the pates. The two faces of the pates facing each other will radiate and absorb equally from each other for a net energy loss of zero.

The net reaction forces on each plate should be zero.

 

[Bomb#20]

The thing to ask isn't whether it'll stop spinning, but rather, what force is acting on it to stop spinning?

PERPLEXITY DAY...

Conservation of Angular Momentum​

Despite the energy loss through thermal radiation, the ball's angular momentum must be conserved in the absence of external torques. ...

The thermal radiation applies an external torque. Photons leaving a point on the spinning object's surface in the direction of the spin are blue-shifted; photons leaving in the direction opposite the spin are red-shifted; the momentum of a photon is proportional to its frequency.

I don't think that works--either the object cools and reaches 0K,

Nothing ever reaches 0K; it just gets closer and closer. All the effects we're talking about here are stronger for fast-spinning objects than slow-spinning ones, so none of them should be able to literally stop anything from spinning. I think for purposes of this thread we need to take "stop spinning" to mean "angular momentum has a half-life".

or it's heated by incoming energy that should be just as distorted as the outgoing energy.

As far as I can see, that doesn't cancel out the effect, but adds to it. Both incoming and outgoing photons push harder if they're slowing down the rotating body than if they're spinning it up. Am I wrong?

 

[Bomb#20]

Put two metal pates in space as heat radiators next to each other at the same temperature. The outer surfaces will radiate into space cooling the pates. The two faces of the pates facing each other will radiate and absorb equally from each other for a net energy loss of zero.

The net reaction forces on each plate should be zero.

Why would that make the net reaction forces zero? It sounds like the photon back-and-forth would tend to push the plates apart*, at least as long as the plates remain hotter than the cosmic microwave background.

(* The thermal repulsive force is probably dwarfed by their gravitational attraction.)

 

[Elixir]

Put two metal pates in space as heat radiators next to each other at the same temperature. The outer surfaces will radiate into space cooling the pates. The two faces of the pates facing each other will radiate and absorb equally from each other for a net energy loss of zero.

The net reaction forces on each plate should be zero.

Why would that make the net reaction forces zero? It sounds like the photon back-and-forth would tend to push the plates apart*, at least as long as the plates remain hotter than the cosmic microwave background.

(* The thermal repulsive force is probably dwarfed by their gravitational attraction.)

Zackly what I was thinking, but IANAP.

 

[unapologetic]

Will a spinning celestial object, such as Earth, ever stop rotating?

What about the Moon?
Has it truly stopped rotating?
Is it gravitationally locked to always face Earth?
Or is it coincidence that it's rotation exactly equals it's orbit around Earth?
Will that ever change?

 

[Elixir]

Tidal Locking

Tidal locking occurs when an astronomical body's rotation period matches its orbital period around a partner, resulting in the same side always facing the partner. This happens due to gravitational interactions that create tidal bulges on the body. The gravitational pull on these bulges exerts a torque, gradually synchronizing the body's rotation with its orbit[1][3][5].

The Moon is tidally locked to Earth because its rotational period matches its orbital period, causing us to see only one side. This synchronization results from energy dissipation over millions of years due to tidal friction[1][3][4].

 

[Shadowy Man]

Will a spinning celestial object, such as Earth, ever stop rotating?

What about the Moon?
Has it truly stopped rotating?

It is rotating.

Is it gravitationally locked to always face Earth?
Or is it coincidence that it's rotation exactly equals it's orbit around Earth?

It’s not a coincidence. Tidal forces cause then rotational period to equal the orbital period.

Will that ever change?

The rates may change but barring a massive collision it is likely to remain tidally locked.

 

[Loren Pechtel]

The thing to ask isn't whether it'll stop spinning, but rather, what force is acting on it to stop spinning?

PERPLEXITY DAY...

Conservation of Angular Momentum​

Despite the energy loss through thermal radiation, the ball's angular momentum must be conserved in the absence of external torques. ...

The thermal radiation applies an external torque. Photons leaving a point on the spinning object's surface in the direction of the spin are blue-shifted; photons leaving in the direction opposite the spin are red-shifted; the momentum of a photon is proportional to its frequency.

I don't think that works--either the object cools and reaches 0K,

Nothing ever reaches 0K; it just gets closer and closer. All the effects we're talking about here are stronger for fast-spinning objects than slow-spinning ones, so none of them should be able to literally stop anything from spinning. I think for purposes of this thread we need to take "stop spinning" to mean "angular momentum has a half-life".

Yeah, it won't ever reach it but the photons will be getting longer and longer and thus the effect smaller and smaller.

I agree with the idea of a half life. Every situation is going to be different but my gut says that for any given situation it will follow a half life pattern.

or it's heated by incoming energy that should be just as distorted as the outgoing energy.

As far as I can see, that doesn't cancel out the effect, but adds to it. Both incoming and outgoing photons push harder if they're slowing down the rotating body than if they're spinning it up. Am I wrong?

I think you're right.

 

[Loren Pechtel]

Will a spinning celestial object, such as Earth, ever stop rotating?

What about the Moon?
Has it truly stopped rotating?
Is it gravitationally locked to always face Earth?
Or is it coincidence that it's rotation exactly equals it's orbit around Earth?
Will that ever change?

The moon most certainly is rotating. It is tidally locked but that doesn't mean no rotation, that means 1 day = 1 year (but remember that's relative to it's primary (Earth), not to the sun.) And the moon does not always present one face to Earth. It's orbit is not circular so it wobbles back and forth. Were it a perfect sphere we would be able to see 59% of it over time--in practice there are spots near the poles we will never see from Earth even though we would see the same spot on a perfect sphere.

Once a tidal lock is established only a major outside disruption could end it. That will happen eventually, though. Any system not in a bidirectional tidal lock will slowly move towards lock--Earth isn't locked, thus the Earth is slowing down, transferring momentum to the moon. Eventually the gravitational binding of the moon to the Earth will cease to be the dominant force and it will wander off to orbit the sun instead. At the limit of the Hill Sphere the binding energy is zero and it will just wander off, in the real world this will never happen because as objects approach this limit they become more and more vulnerable to external forces and something is going to pull it away. Now, whether this will actually happen or not is a question I don't believe the stellar physics guys have a definitive answer for. As the sun's hydrogen runs low it's going to get big and red. There is little doubt that Earth's current orbit will be within the outer envelope and any planet here would be brought down by drag. But the sun is also blowing off mass in the stellar wind and as it gets bigger and redder this wind will considerably intensify. As the sun loses mass Earth's orbit will grow larger. Venus will burn. Mars will live on. Earth is within the uncertainty of their estimates of how stars evolve.

 

[Bomb#20]

Once a tidal lock is established only a major outside disruption could end it. That will happen eventually, though. Any system not in a bidirectional tidal lock will slowly move towards lock--Earth isn't locked, thus the Earth is slowing down, transferring momentum to the moon. Eventually the gravitational binding of the moon to the Earth will cease to be the dominant force and it will wander off to orbit the sun instead. At the limit of the Hill Sphere the binding energy is zero and it will just wander off, in the real world this will never happen because as objects approach this limit they become more and more vulnerable to external forces and something is going to pull it away.

We're not going to lose the moon. The earth's Hill sphere is about three times bigger than the orbit the moon will have when it stops moving away, when the earth becomes tidally locked. The lion's share of the earth's original rotational angular momentum has already been transferred to the moon, so it hasn't got all that much further out to go.

 

[DrZoidberg]

The idea of "perpetual motion" as an exceptional and unattainable condition is medieval, and relies on the notion that motion requires an energy input to be sustained.

That's a local truth. If you live at the bottom of a gravity well, with an atmosphere, and you are so completely under the influence of those things at all times that you don't even notice them, then such "laws of nature" as 'perpetual motion without an external force is impossible', or 'nature abhors a vacuum', are easy to demonstrate.

When you realise that the vast majority of everything IS vacuum, your perspective needs to change, or your ideas will become obsolete.

A spinning object in isolation will spin forever.

A planet is not entirely isolated, of course. Tidal effects from the Sun, Moon, and other planets and even distant stars and galaxies, all act to modify the rotation of the Earth. But the distances are large, gravity is weak, and the influence is inversely proportional to the square of the distance, so it will take a LONG time for any effects to become significant.

The tides slow the rotation of the Earth. Tne Moon is the largest tidal influence, because although it's small, it's very, very close by (in cosmic terms), at only half a million miles. The Sun is also a noticable influence; it's far away, but it's very large. No other object has an effect we can easily measure, or can detect without highly specialist equipment, over short timescales (and by 'short' I mean 'less than the time that human record keeping has existed').

So yes, in a sense, the spinning of an isolated neutron star is "perpetual motion", in that the motion will continue indefinitely.

But in the sense that the complete phrase "perpetual motion" implies an unknown and supernatural influence to counter the supposed natural tendency for all motion to stop, which is how the phrase is most commonly used in philosophy, it is an erroneous concept, based on the medieval mistake of ascribing local, (earthbound) conditions and observations to the universe as a whole.

Medieval observers saw the Moon and the Sun revolving around the world, and couldn't understand why they didn't slow down like everything else. Worse still, the planets occasionally did slow down, but then speeded up again! Trying to explain this led to all kinds of weird and arbitrary hypotheses (google "epicycles" if you want to know more about the knots these guys tied themselves in, trying to explain what they could see).

Three simple (but not at all obvious) ideas did away with the philosophical need for supernatural "perpetual motion"; The hypothesis that the universe is mostly vacuum; The hypothesis that the Earth is itself in motion, and not the fixed centre of the universe; And the hypothesis that an object in motion will continue in motion unless acted upon by a force.

As all moving objects near the Earth's surface are acted upon by forces applied by the air or water through which they move, and as we cannot feel the motion of the Earth, these three ideas were very hard to accept, and very difficult to demonstrate. But we now know them all to be the case. And, impressively, the likes of Copernicus, Galileo, Newton, and Kepler were able to demonstrate them before we had even achieved flight, much less spaceflight. Now we can actually go and see for ourselves that space is mostly vacuum - but we knew it before we went, just because of a handful of really impressive thinkers who were able to interpret the very mixed signals we were getting from our observations, which were the result of our abnormal situation: On a planet with an atmosphere, in orbit around a star, and orbited by a large and nearby Moon.

You can say the same thing way simpler.

Without gravity acting upon the Earth, then slowing down, would be doing work. Yes, if left alone the Earth will be perpetually spinning. All in accordance with the laws of physics.

 

[Loren Pechtel]

Once a tidal lock is established only a major outside disruption could end it. That will happen eventually, though. Any system not in a bidirectional tidal lock will slowly move towards lock--Earth isn't locked, thus the Earth is slowing down, transferring momentum to the moon. Eventually the gravitational binding of the moon to the Earth will cease to be the dominant force and it will wander off to orbit the sun instead. At the limit of the Hill Sphere the binding energy is zero and it will just wander off, in the real world this will never happen because as objects approach this limit they become more and more vulnerable to external forces and something is going to pull it away.

We're not going to lose the moon. The earth's Hill sphere is about three times bigger than the orbit the moon will have when it stops moving away, when the earth becomes tidally locked. The lion's share of the earth's original rotational angular momentum has already been transferred to the moon, so it hasn't got all that much further out to go.

The Hill Sphere isn't a hard limit. Beware the werewolves, Earth currently has two moons. The second one will soon be lost, though.

(I hope this link behaves--works for me, but it doesn't like it when the board tries to preview it.)

Radware Bot Manager Captcha

Also, remember that there are two systems involved: Sun-Earth and Earth-Moon. Remember, everything locks eventually. Since the Earth-Sun lock is a year then the Earth-Moon lock is also a year--but a year is well outside our Hill Sphere. Given sufficient time the moon is lost.

 

[Shadowy Man]

Once a tidal lock is established only a major outside disruption could end it. That will happen eventually, though. Any system not in a bidirectional tidal lock will slowly move towards lock--Earth isn't locked, thus the Earth is slowing down, transferring momentum to the moon. Eventually the gravitational binding of the moon to the Earth will cease to be the dominant force and it will wander off to orbit the sun instead. At the limit of the Hill Sphere the binding energy is zero and it will just wander off, in the real world this will never happen because as objects approach this limit they become more and more vulnerable to external forces and something is going to pull it away.

We're not going to lose the moon. The earth's Hill sphere is about three times bigger than the orbit the moon will have when it stops moving away, when the earth becomes tidally locked. The lion's share of the earth's original rotational angular momentum has already been transferred to the moon, so it hasn't got all that much further out to go.

What will the orbital radius be for the moon when it stops moving away? How would you calculate that?