Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 15/12/2006 23:45:59

Title: Is luminosity relative?
Post by: thebrain13 on 15/12/2006 23:45:59

Does luminosity vary based on your relative motion from it? Effectively, a dopplar effect for luminosity, except for instead of light being redshifted when you move away from it, the lightsource is less luminous. It seems that this would have to be the case, but Ive never heard it referenced, nor did I get any info about it on google.

Title: Re: Is luminosity relative?
Post by: daveshorts on 16/12/2006 11:40:40

It depends on the type of redshifting. If it is because an object is just moving fast, in one direction the brightness will be higher because the light will be blue shifted and in the other it will be red shifted, which may or may not cancel out. If it is red-shifted due to the expansion of the universe the light released will all be redshifted, and therefore loose energy and appear less luminous.

Looking on google there appears to be a quantity called 'comoving luminosity', which I think is the luminosity of an object if you were moving at the same speed as you.

Title: Re: Is luminosity relative?
Post by: Soul Surfer on 18/12/2006 09:39:04

The energy you recieve from a particular light quantum depends on its frequency so if it is red shifted you receive less energy from each quantum.  If it is blue shifted you receive more

Title: Re: Is luminosity relative?
Post by: thebrain13 on 18/12/2006 20:06:20

I think what I was getting at, was if you were moving away from the sun you would experience the flow of time on the sun as being reduced. Heres how, According to relativity of simultaneity, if you get farther away from an object and have a velocity relative to it in the opposite direction, all events would appear as happening slower. The reason being that just by being farther or closer to an object changes your perception of that objects time.

So if we were observing the sun, with a relative velocity in the opposite direction from it, it would be aging slower and therefore it could not emit the same amount of photons for us to receive. Therefore an object with a relative velocity from us would appear less luminous even after the red shift is accounted for.

Ive never heard this argument before. Did I error somewhere?

Title: Is luminosity relative?
Post by: thebrain13 on 09/01/2007 06:52:54

Any responses?

Title: Is luminosity relative?
Post by: lightarrow on 09/01/2007 16:18:50

Quote from: thebrain13 on 18/12/2006 20:06:20

I think what I was getting at, was if you were moving away from the sun you would experience the flow of time on the sun as being reduced. Heres how, According to relativity of simultaneity, if you get farther away from an object and have a velocity relative to it in the opposite direction, all events would appear as happening slower. The reason being that just by being farther or closer to an object changes your perception of that objects time.
So if we were observing the sun, with a relative velocity in the opposite direction from it, it would be aging slower and therefore it could not emit the same amount of photons for us to receive. Therefore an object with a relative velocity from us would appear less luminous even after the red shift is accounted for.
Ive never heard this argument before. Did I error somewhere?

If photons were particles spatially localized from the sun to the earth, I would agree with you, but I sincerely don't know if it's possible to localize them (QM says no, but QM could be slightly different in the future, who knows).

If the same reasoning is made with flashes of light, instead of photons, it's certainly true: the (average) frequency of the flash AND the number of flashes arriving per unit time would be less.

Title: Is luminosity relative?
Post by: Soul Surfer on 09/01/2007 17:18:40

Yes it would be dimmer however Unless you are moving with a velocity significant to the velocity of light the effect is minuscule but the frequency changes are much more detectable.  This effect is observable astronomically though.

Highly energetic sources that are rotating (like black holes and Neutron stars) can only get rid of energy from their poles i.e. along the axis of rotation and so they would tend to produce relatavistic jets of gas in in two directions opposite to each other but frequently only the jet pointing towards us is seen because its energy is blue shifted and enhanced at one time it was thought that the jets may be seriously asymmetrical but careful observations can detect the much fainter red shifted jet that is pointed away from us.