Gravity, Light, and the Earth's Orbit: A Newbie's Questions

In summary, the conversation discusses the concept of radiation pressure and its potential impact on the Earth's orbit. It is noted that while photons do have momentum and contribute to the radiation pressure, it is a very small amount compared to the gravitational pull of the Sun. The conversation also briefly touches on the potential impact of radiation pressure from other sources in the universe.
  • #1
pete5383
85
0
Hey everyone. I'm kind of new to the whole physics world, but I've got a question I've been wondering about.

So the Sun attracts the Earth because of gravity (or bends space time that the Earth is traveling through), and my question is this: Since photons have momentum, and momentum must be conserved when the photons strike the Earth, how much collectively are the photons pushing the Earth away from the Sun? Obviously it must be very little, but does it cause even a noticable error when calculating the orbit of the earth? That is, if the sun did not emit light, would it appear as though it's gravitational pull has gotten stronger than when it -was- emitting light? Maybe a dumb question, but something I was pondering...thank you!
 
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  • #2
In principle you're right. Don't even need to talk about photons for that: classically, already, electromagnetic waves carry momentum as well as energy (btw, the photon momentum comes from that correspondence).
The transferred momentum on an object due to absorption or reflection of light is called radiation pressure.

However, as you suggest, it must be very small as compared to the gravitational pull. Didn't do the calculation, though.
 
  • #3
Agreed, someone should do the calculation just for fun
 
  • #4
Haha...yeah, sooooooomeone should do it...hm...so basically, the photons are pushing us away, but we just leave it out of mind because it's a pretty small amount?

After a long enough time, would we move slightly away, or isn't it even that much of an effect?
 
  • #5
Ok, I did this very quickly, so check if I didn't make any error...

Look at http://scienceworld.wolfram.com/physics/RadiationPressure.html

The radiation pressure (force per unit of surface) equals the energy flux divided by the lightspeed.

If we take it that the Earth receives about 1000 W per square meter of radiation from the sun, then that converts to 1000 W / (3.10^8 m/s) = 3.3 10^(-6) Newton per square meter, or Pascal (~ 33 10^(-9) mbar, which is a relatively good lab vacuum).

If we take the Earth radius to be 6500 km, then the cross section of the Earth is 1.3 10^14 m^2, so this results ini a total force on the Earth of:

4.4 10^8 N

(which is about the weight of a 40,000 ton mass, say, a big ship)

The Earth mass is about 6 10^24 kg,
so this results in an acceleration of about 10^(-16) m/s^2
 
  • #6
vanesch said:
Ok, I did this very quickly, so check if I didn't make any error...

Look at http://scienceworld.wolfram.com/physics/RadiationPressure.html

The radiation pressure (force per unit of surface) equals the energy flux divided by the lightspeed.

If we take it that the Earth receives about 1000 W per square meter of radiation from the sun, then that converts to 1000 W / (3.10^8 m/s) = 3.3 10^(-6) Newton per square meter, or Pascal (~ 33 10^(-9) mbar, which is a relatively good lab vacuum).

If we take the Earth radius to be 6500 km, then the cross section of the Earth is 1.3 10^14 m^2, so this results ini a total force on the Earth of:

4.4 10^8 N

(which is about the weight of a 40,000 ton mass, say, a big ship)

The Earth mass is about 6 10^24 kg,
so this results in an acceleration of about 10^(-16) m/s^2
This is interesting to me. Can you calculate the radiation pressure on the Earth from all other directions. I'm interested in knowing if the pressures at all other points on the earth, not directly in line with the sun, are greater or lesser than the radiation pressure from the sun.

Seems like even though all those stars are so far away, the sheer amount of radiation pressure would be much greater, considering every object in line with the Earth (and this is a very high percent of the observable universe) exerts some pressure.

Edit: I don't actually mean every point just say the same surface area exactally opposite from the sun.
 
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  • #7
GOD__AM said:
This is interesting to me. Can you calculate the radiation pressure on the Earth from all other directions. I'm interested in knowing if the pressures at all other points on the earth, not directly in line with the sun, are greater or lesser than the radiation pressure from the sun.

Seems like even though all those stars are so far away, the sheer amount of radiation pressure would be much greater, considering every object in line with the Earth (and this is a very high percent of the observable universe) exerts some pressure.

Given that the radiation pressure is the energy flux divided by the lightspeed, and that by far the main source of energy flux comes from the sun, and not from distant stars, by far the main contribution must come from the sun, no ?

The pressure is proportional to the energy flux...
 
  • #8
vanesch said:
Given that the radiation pressure is the energy flux divided by the lightspeed, and that by far the main source of energy flux comes from the sun, and not from distant stars, by far the main contribution must come from the sun, no ?


I really have no idea, that's why I asked :P I thank you for the reply though.
 
  • #9
Thanks vanesch! :smile: I'm going to have to read up a little bit on radiation pressure now. Thanks again for the calculations too
 
  • #10
vanesch said:
Given that the radiation pressure is the energy flux divided by the lightspeed, and that by far the main source of energy flux comes from the sun, and not from distant stars, by far the main contribution must come from the sun, no ?

The pressure is proportional to the energy flux...

The radiation pressure is related to the amount of radiation received, so the comparison here is valid. In fact, you could get a rough idea of the difference between the radiation pressure from the Sun and that from the stars by comparing the brightness of the light we receive from the Sun to that of the stars.

It would also be good for us to keep in mind that the stars are distributed almost evevnly throughout the universe (even though we only see them from the Earth's dark side), so they provide about the same pressure from all directions.
 

1. What is gravity and how does it work?

Gravity is a force that exists between two objects with mass. It is the force that pulls objects towards each other. The strength of gravity depends on the mass of the objects and the distance between them. This force is what keeps planets in orbit around the sun and objects on Earth from floating off into space.

2. How does light travel and what is its speed?

Light travels in a straight line and at a constant speed of approximately 186,000 miles per second. It is a form of energy that can move through a vacuum. When light is emitted from a source, such as the sun, it travels in all directions until it reaches an object or is absorbed.

3. What causes the Earth's orbit around the sun?

The Earth's orbit around the sun is caused by gravity. The sun's massive gravitational pull keeps the Earth in a constant elliptical orbit around it. The Earth's orbit is also affected by the gravitational pull of other planets and objects in our solar system.

4. What is the shape of the Earth's orbit?

The shape of the Earth's orbit is an ellipse, which is a slightly flattened circle. This means that the Earth's distance from the sun varies throughout its orbit. The point of closest approach to the sun is called perihelion, while the point of farthest distance is called aphelion.

5. How does the Earth's orbit affect our seasons?

The Earth's tilt on its axis and its elliptical orbit around the sun are what cause the change in seasons. When the Earth is tilted towards the sun, it receives more direct sunlight and experiences summer in that hemisphere. When the Earth is tilted away from the sun, it receives less direct sunlight and experiences winter in that hemisphere. The changing distance from the sun also affects the intensity of the seasons.

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