[ja1s2d3f4 (OP)]

To quote Schroeder, and in reference to Einstein's theory: "Waves of sunlight reaching Earth are stretched longer by 2.12 parts in a million relative to similar light waves generated on Earth."

I am not debating that truth. I was just wondering if somebody could give me, or guide me to, some information that could explain how we know that those waves are actually stretched? In other words; how do we know the Sun didn't just give off light with that frequency?

[NakedScientist]

Thanks for the question - we've put it to Prof. Brian Fulton who was the guest on tonight's show about astrophysics. Once I hear from him I'll post it back here.

TNS

[NakedScientist]

Answer from Professor Brian Fulton, Dept. of Physics, York

There are two ways in which light arriving at the earth could be stretched to longer wavelengths ("red-shifted").

The first is if the object emitting the light was moving away
from the earth.  This is the doppler effect, and a common analogy is with the way the "pitch" of a police siren changes as the car comes towards you and then away from you.  The re-shift of light arriving at the earth from distant objects is very common and reflects the fact that because of the general expansion of the universe, distant objects are moving away from us.  Now, in terms of the sun, the earth doesn't have any significant motion towards/away from the sun (not quite true as the orbit is not quite circular - but a tiny effect).  

Hence I don't think that could be the origin of a parts per million effect.

The second is if the light is being emitted from an object which has
a high gravitational field.  If you think of throwing a stone in the
air, it loses energy because of the gravitational attraction of the earth, and eventually falls back to the ground.  So light trying to get away from the earth loses energy.  In the case of light the effect isn't big enough to stop it (well, actually, in a Black Hole it is!) but the loss of energy shows up as a lengthening of the wavelength. I was surprised that the effect for the sun was as big as parts per million, but I've just calculated it and it is indeed true -the figure your visitor quotes is exact."