Expert: Jayendra Upadhye - 11/17/2004

Question
Hi there, do these statements make sense?  why/why not?

1) if the sun's surface became much hotter (while the Sun's size remained the same) the Sun would emit more ultra-violet light but less visible light than it currently emits
2) a black sheet of paper absorbs all the light that falls on it and emits no radiative energy at all
3) if a distant galaxy has a substantial redshift (as viewed from our galaxy), then anyone living in that galaxy would see a substantial redshift in a spectrum of the Milky Way Galaxy

Answer
Hi Jim,
Earlier Jonny asked me questions with SAME titles!
Anyway..
1 - true. because, the emission spectrum of the sun would start shifting towards the ultra-violet, ie yellow to white to ultra-violet. (examples white dwarf stars).

2 - True (but conditionally).
The paper MUST BE BLACK in all ranges of the spectrum for that to happen. OFTEN THAT DOES NOT HAPPEN, THINGS ARE BLACK IN THE VISIBLE RANGE, BUT EMIT AT OTHER FREQUENCIES!
Anyway, if the paper was truly black in all infra-visible-ultra ranges, or the complete electro magnetic spectrum, THEN "no radiative energy " part would be really true.
[in practical life this does not happen, and in all likelyhood, the paper DOES emit infra-red heat rays.

3 - Yes that is true because of the principle of reciprocity
which says, that for any two bodies, relative effect on one body appears same when viewed from another body.
that is why if two people are approaching each other, BOTH see each other as "coming nearer"! ie: relative velocity term applies equally to both of them.
However, this principle breaks down in terms of time, at relativistic frames of reference, where the person travelling close to the speed of light "appears to be aging slowly" to the person that is stationary, and the stationary guy "appears to be aging faster" to the faster guy. This happens due to time dilation at speeds close to that of light. i used the term "appears" because, to each of the guys, his own aging process appears normal enough, and the problem lies with the other guy!

however in case of red shift, this principle still holds good.

pls do rate the answer.
Jayen