# An object above absolute zero radiates energy

1. Aug 25, 2013

### eightsquare

An object above absolute zero radiates energy. This implies that object on earth too radiate energy(infrared?) My question is, if the earth were not moving, we would be much slower, so would we radiate in some other wavelength of the spectrum? We are essentially travelling really fast because of the earth's movement.

2. Aug 25, 2013

### davenn

I couldnt see it affecting the amount IR radiation coming from our bodies

Dave

3. Aug 25, 2013

### eightsquare

How so? We would be slower.

4. Aug 25, 2013

### davenn

why would we be slower ?
if you walk a kilometre your body is still going to expend the same amount of
energy making your legs move regardless of whether the earth is stationary or moving

Dave

5. Aug 25, 2013

### HallsofIvy

Staff Emeritus
You seem to think that the speed of an object affects the amount of radiation it admits. Where did you get that idea?

6. Aug 25, 2013

### phinds

Slower relative to WHAT? Motion is relative. The radiation of the Earth has nothing to do with the speed that it is traveling with respect to other bodies. That speed, by the way, varies from zero to .999999% of c with all values in between. Why would picking one of those frames of reference have anything to do with how much the Earth radiates?

7. Aug 25, 2013

### eightsquare

Well greater speed means more kinetic energy and higher kinetic energy means higher temperature and higher temperature means more radiation(that's the way I learnt it in school). Which part is wrong?

8. Aug 25, 2013

### davenn

its wrong in the way you are relating things ( or not relating things)
as phinds said ... motion is relative

did you read my post post #4
does that not make sense to you ?

Dave

9. Aug 25, 2013

### Staff: Mentor

If the earth would not orbit the sun, observers at other stars would see a slightly different spectrum and this spectrum would not depend on the position of earth in its orbit. The difference would be extremely small, however - we orbit sun with a velocity of ~30km/s, or 1/10000 the speed of light. Observers at other stars could (at most) see a shift of just 2/10000 between "earth moves away from us" and "earth moves towards us".

The total emitted light in the frame of earth is independent on the motion of earth relative to anything else.
The total emitted light as seen in other frames might depend on those other frames.

No it does not. Temperature is related to unordered kinetic (and other) energy. The motion of the whole earth is not unordered.

10. Aug 25, 2013

### eightsquare

Nice to get a civil explanation. So this basically means the more the atoms are banged about the more they radiate? Why is this?

11. Aug 25, 2013

### davenn

because the collisions cause an increase in temperature
Bang a piece of metal plate with a hammer ... you will find that both the head of the hammer and the metal plate get warm

Dave

12. Aug 25, 2013

### eightsquare

I know that. I'm asking WHY this increase in temperature causes more radiation.

13. Aug 25, 2013

### Staff: Mentor

More unordered motion -> more (and higher-energetic) collisions.

14. Aug 25, 2013

### davenn

trying to think of an example if something that gets hotter the faster it moves ....

take a jet fighter ... it does ... its metal skin warms up. and altho the fighter travelling at say, mach 1 ( the speed of sound) has lots of kinetic energy, its not that that is warming the metal skin up.
Its the friction of that metal against the air ( air resistance). if you flew that fighter in a vacuum (with no air)
then the skin wouldnt warm up .... no friction generated heat

cheers
Dave

15. Aug 25, 2013