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Please correct me if I am wrong, but I believe that for every force, there is an equal and opposite force.
My question is: What is the equal and opposite force to radiation pressure (i.e. the force that the sun's electromagnetic radiation exerts on the Earth's magnetosphere)?
Thank you.
Established Member
It's actually for every action there is an equal and opposite reaction.
I don't know a great deal about radiation, but I would imagine it's like anything else. If you set a 20kg weight on a table, the table pushes back. That's why it doesn't collapse.
Order of Kilopi
primummobile. Yep, and the sun does push at us a tiny bit, SEE:http://en.wikipedia.org/wiki/Radiation_pressureOriginally Posted by primummobile
Established Member
Thanks for the link. The article is a little over my head, so I would appreciate clarifications, regarding my questions below...
I noticed this sentence in the Wiki article:
"Radiation pressure is about 10−5 Pa at Earth's distance from the Sun[9] and decreases by the square of the distance from the Sun."
Follow up questions:
Does the Sun push us away from it a tiny bit, or towards it a little bit? or both, b/c "every action has an equal and opposite reaction"? Does it depend if the radiation is reflected or absorbed?
Is there any kind of tie-in with gravity? That is, does something that absorbs (or reflects?) EM radiation move very slightly towards the source emitting the radiation, and is it a function of the inverse-square of the distance from the source?
Thanks again.
Established Member
Whatever outward pressure the radiation from the sun applies to the Earth is easily overcome by the gravitational attraction between the Earth and Sun.
I'm not sure what you're asking in the second question.
Established Member
Okay, so the radiation pressure pushes the Earth away from the Sun.
I guess I am wondering if there is an equal and opposite reactionary force to the radiation pressure that pulls the Earth towards the Sun.
Is radiation pressure the opposite force of gravity? I noticed they both have inverse square functions.
Sorry for the stupid questions.
Thanks.
Established Member
It doesn't pull the Earth toward the Sun, the Earth just pushes back against the pressure.
Think of putting your hand on a brick wall. When you press on the wall, the wall presses back on your hand. You don't think about it that way, but it's really the electromagnetic force in the wall pushing back against your hand, and that's what makes the wall solid. The wall isn't being 'pulled' toward your hand... it is pushing back.
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But what I am trying to understand better is
is there any relationship between radiation pressure and gravity?
i.e. the Sun's radiation pressure pushes on the Earth, and the Earth pushes back towards the Sun.
Is the Earth pushing back towards the Sun a form of gravity, if not gravity itself?
I am pretty sure there is NO relationship between radiation pressure and gravity, but I would like to be sure.
Thanks.
Established Member
You are right. There is no relationship between the two. Any type of mass deforms the fabric of spacetime, and that deformation manifests as gravity.
You can see a representation of it in the images here:
http://en.wikipedia.org/wiki/Gravity#General_relativity
But keep in mind that the 2-Dimensional sheet used to represent spacetime is only for clarity. Mass is actually warping space time in all 3 dimensions. It's only drawn that way to make it easier to visualize.
Order of Kilopi
You are correct, the radiation pressure and the gravity from the Sun are two separate things, and they are not related.
Other folks have touch on something, but it would be good to make it explicit: when object A applies a force to object B, the reaction force is not on object B. The reaction force is applied by object B on object A. So when a horse pulls a cart, the reaction force is a force on the horse, not another force on the cart.
Conserve energy. Commute with the Hamiltonian.
Order of Kilopi
The equal and opposite force to radiation pressure is the force that stops or reflects the photons.
If I push on a wall, the wall pushes back on me. Earth pulls me down due to its gravity, but I pull it up toward me with the same force. Modeling radiation pressure the same way, as you want to do, photons apply a force on an object and the object applies that same amount of force to the photons in the opposite direction. If the radiation pressure pushes a planet away, then the planet pushes the photons away in the other direction.
Established Member
interesting.
sorry for the gory example, but it's all I could think of...
the bullet that hit JFK caused JFK's head to push back towards the bullet, correct?
So in the same way, couldn't a light particle from the Sun hitting the Earth cause the receiver of the light (the Earth) to push back towards the EM radiation's source (the Sun) just a tiny bit?
In reality, the Earth would be pushing back against each photon individually. But with a constant flow of radiation, the phenomena could be observed as gravity.
In other words, the observed phenomena of gravity (of the Sun) would then just be the cumulative effect of the Earth pushing back on the photons hitting it (which came from the Sun).
This would then explain why planets orbit the Sun at the Sun's equator...b/c EM radiation is emitted more at the Sun's equator than at the poles (due to centrifugal force/oblateness caused by rotation)
Order of Kilopi
As you lean against a wall, can the wall push on you and make you move toward it? If you fire a paintball at a can, can the paintball hit the can and push the can back toward you?
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I am having trouble with this. I thought:
-the wall does push back on you.
-the can does push back towards the paintball.
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This is completely wrong. The two phenomena are different. Just because they are both forces acting on the Earth does not mean they are the same thing. Planets orbit in a disk because of the angular momentum of the original cloud they were formed from and friction. Nothing to do with radiation pressure from the Sun.
Vulcan Administrator
Thread moved to ATM at OP's request.
Everything I need to know I learned through Googling.
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If we look at Uranus, it's moons orbit at the planet's rotational equator (vertical to the plane of the solar system).
How would the "angular momentum of the original cloud" theory account for this?
It would appear to be more elegant if they orbit at the rotational equator, b/c that is where Uranus emits the strongest EM radiation (with the radiation pressure hitting the moons on this vertical plane).
Order of Kilopi
Some do. Others don't. It has a number of irregular satellites.
It doesn't. It's suspected that Uranus was struck by a large protoplanet early on, which would account for the inclination of its rotation. Some of the moons could have been formed in the same event. Also, equatorial orbits are the most stable for inner moons - either they'll be in equatorial orbits, be perturbed into equatorial orbits, or they'll be lost.How would the "angular momentum of the original cloud" theory account for this?
Sorry, that makes no sense. Uranus is cold, and radiation pressure wouldn't work that way anyway. Gravity is the key issue here.It would appear to be more elegant if they orbit at the rotational equator, b/c that is where Uranus emits the strongest EM radiation (with the radiation pressure hitting the moons on this vertical plane).
I say there is an invisible elf in my backyard. How do you prove that I am wrong?
The Leif Ericson Cruiser
Order of Kilopi
The can is pushed away from the paintball, not pulled toward it. Likewise, the paintball is pushed away from the can.
With this simple, collisional model of radiation pressure, there is no way that Earth can be pulled toward the sun due to these collisions.
Order of Kilopi
This doesn't work at all. When the Earth "pushes back" on the photons, that affects the photons, not the Earth. Again, if you have a force causing an object to move, the reaction force does not cause that same object to move. The reaction force is a force on a different object.
Moreover, if gravity were a side effect of radiation pressure, then we'd feel little attraction to the Earth (from which we receive relatively little radiation) and a large amount of attraction to a bright lamp (which emits a large amount of radiation).
The wall does push back on you. That affects you, not the wall.
Yes, the can pushes back on the paintball. That affects the paintball: the force causes the paintball to stop moving. The only force on the can is the force from the paintball, which causes the can to move away from the paintball.
This is a pretty common misconception (well, not the gravity thing, just the whole action-reaction issue). The important thing to remember is that only the forces acting on an object affect its motion. When you apply a force to an object, there is a reaction force, but that reaction force is not on that same object, and so has no effect on the object's motion. Rather, that reaction force is applied to whatever was the source of the original force, and may change the source's motion. When you push something away from you, you feel a force pushing you away from that object; the reaction force doesn't affect the object in any way.
Conserve energy. Commute with the Hamiltonian.
Established Member
Let's see if I have this right:
Using the paintball example, the equal reaction to the acceleration of the paintball is the backward force upon the person firing the paintpall gun with a tiny percentage of it affecting the Earth. When the paintball hits the can, momentum is transferred to the can overcoming it's inertia and the effect of Earth's gravity and moving it in the same direction the paintball is moving.
The same thing is happening if the paintball hits a wall except the energy imparted to the wall is more spread out and the more solid the wall is, the more momentum that is transferred to the Earth's rotation and movement in it's orbit around the Sun.
Once the paintball has left the gun, it's actions have no effect on the gun. In the same way, radiation from the Sun will no longer directly affect the Sun, however the energy imparted to the Earth from the radiation will affect the orbital dynamics between the Sun and Earth.
Is that pretty much correct?
Established Member
Here is the force balance for radiation pressure:
At the sun:
The sun emits radiation in all directions nearly equally. This cancels, causing no net acceleration for the Sun.
Among the Photons:
This radiation is composed of photons, which travel in a straight line. The photon density will decrease proportional to r^(-2), similar to the gravitational flux.
At an object:
An opaque reflective object will absorb all photons indecent to its surface. This will produce an inverse square force proportional to the surface area of the object away from the Sun, unlike classical gravity which is an inverse square force proportional to mass towards the Sun.
Putting numbers to this, a black bearing the density of Earth but with a radius of roughly 2.2 mm will feel a radiation pressure equal to the gravitational force, and will move in a straight line. Smaller objects will be pushed away from the Sun, as the radiation pressure is stronger than gravity.
If the objects are perfectly reflective the radiation pressure is doubled and a radius of 4.4 mm is needed. Osmium is rather black, but is four times denser that Earth. Thus an Osmium speck would only need to be 0.5 mm for the radiation pressure to outweigh gravity.
Since the radiation pressure is proportional to the area of the object (r^2) and gravity is proportional to the mass of an object (r^3), smaller objects will feel more radiation relative to their gravitational force. If you are trying to argue that gravity is a result of radiation pressure, you might want to read about LeSage's theory of gravity.
http://en.wikipedia.org/wiki/Le_Sage...of_gravitation
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While you're there, OP, read the entire Wiki article on Gravity. Gravity has nothing to do with radiation pressure.
I'm not a hardnosed mainstreamer; I just like the observations, theories, predictions, and results to match.
"Mainstream isn’t a faith system. It is a verified body of work that must be taken into account if you wish to add to that body of work, or if you want to change the conclusions of that body of work." - korjik
Established Member
Thanks guys.
Order of Kilopi
The opposite action to the light from the Sun which is pushing the Earth away a little bit, is that the sun was also pushed back a little bit when it radiated the light.
But since the sun is radiating light in all directions at the same time, the pushes on the sun from the light it emits cancels out.
Radiation pressure has nothing to do with gravity even though they do both follow an inverse square law.. Most things that act at a distance follow an inverse square law in how much they influence each other. One way to think about it is that the total influence has to out over a sphere centered at the sun, the area of this sphere is the square of the distance so the ratio of any fixed size area to the whole of the sphere is some constant factor divided by the square of the distance.
It's not a stupid question, it's an ignorant question, and there's no shame in ignorance when you're trying to learn.
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