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Kepler's laws and the Doppler Effect

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alai
932678.  Wed Aug 15, 2012 7:39 pm Reply with quote

'yorz wrote:
I knew about Kepler and about the Doppler effect in sound. What was new to me is that light can be subjected to it as well.

Pretty important in astronomy and physical cosmology, given the whole "wait, everything is red-shifted... Big Bang! Inflation, anyone? Big Rip, perhaps?" sequence of reasoning.

 
Posital
932703.  Thu Aug 16, 2012 1:49 am Reply with quote

Although there are concerns that Doppler doesn't account for everything red-shifty astronomically...

 
alai
932852.  Thu Aug 16, 2012 12:12 pm Reply with quote

That I did not know. Tell us more! Gravity being "bold" again?

 
gruff5
932898.  Thu Aug 16, 2012 2:59 pm Reply with quote

I wonder if the Big Bang red-shift of galactic light is, strictly speaking, a Doppler effect?

The galaxies en masse aren't proceeding away from us through space, but it is space itself that is stretching/expanding and the light waves are stretched along with it.

That seems subtlety different.

 
'yorz
932899.  Thu Aug 16, 2012 3:04 pm Reply with quote

Good lord! Finally! I began to thunk I was the only one who found this interesting. Almost an entire month! (well, -ish)

 
tetsabb
932903.  Thu Aug 16, 2012 3:12 pm Reply with quote

'yorz wrote:
Almost an entire month! (well, -ish)

Relatively speaking, only

 
'yorz
932905.  Thu Aug 16, 2012 3:14 pm Reply with quote

Quite inflated.

 
GL1
938028.  Sun Sep 09, 2012 11:23 pm Reply with quote

'yorz wrote:


I knew about Kepler and about the Doppler effect in sound. What was new to me is that light can be subjected to it as well.
Thought it was QI.


Hello, first post from me so I have no idea of the local netiquette, please forgive any transgressions.

Kepler is most famous for his laws of planetary motion, which were one of the giant's shoulders that Newton was standing on when he came up with his laws of universal motion.

There are some interesting consequences of these laws, one of which is that the solar system, when considered as a whole, revolves around its own centre of mass. This centre of mass is sometimes inside the Sun, sometimes outside the Sun. Which means when I was born the Earth was rotating about the Sun ( or rather a point inside the Sun), but when my brother was born, the Earth was not rotating around the Sun (it was rotating about a point outside the Sun). So to answer to the question "Does the earth rotate about the Sun?", you must first establish when.

More info on wikipedia as always http://en.wikipedia.org/wiki/Barycentric_coordinates_%28astronomy%29
Kepler worked for Tycho Brahe, who was very interesting, and already discussed here
http://www.qi.com/talk/viewtopic.php?p=450802&highlight=tycho+brahe#450802

 
Jenny
938146.  Mon Sep 10, 2012 4:18 pm Reply with quote

Hi GL1 and welcome to the QI forums :-)

I love this QIness of that post.

 
gruff5
938289.  Tue Sep 11, 2012 11:16 am Reply with quote

hi GL1, we share name initials (presuming those are your initials?)

Yes, that was most QI, actually VI, I would say.

 
GL1
938342.  Tue Sep 11, 2012 2:31 pm Reply with quote

Thank you.

Looking at the original subject. I don't quite understand what the poster was pointing out is linked between Doppler shift in light and Kepler.

Kepler inspired the name of the space telescope. This uses brightness changes to identify Earth like planets near the habitable zone.
Often Doppler spectroscopy is used to check the redshift (light doppler effect as was pointed our earlier) after that to confirm the findings, but I don't think Kepler actually does that bit.

Interesting stuff about the kepler search for planets is that lots have been found, enough for the scientists to say there are at least 1.6 planets per star in the galaxy. Another is in the scientists engaged in the search for habitable planets have various criteria for the habitability. One is "Habitable Zone Distance", which is the distance a planet is from the center of the postulated Habitable Zone of its star. If applied to the solar system, Mars scores higher than Earth. Another measure is "Habitable Zone Composition", which is the measure of how suitable the chemicals on the planet appear for life. If applied this to the solar system both Mars and Venus score higher than Earth!

As gruff5(my initial twin) pointed out, an interesting thing about the "redshift" is the other non-doppler contributor to it. If the light is coming from far enough away, which also means far enough ago since it takes time to get here, then the universe has expanded in that time too, including the part of the universe that the wavelength of the light spanned, meaning it has been shifted red by the very expansion of the universe. Both relative speed and expansion impact redshift.
In the case of the cosmic microwave background radiation , arguably the oldest light there is, the wavelength is now 1,100 times longer than when it was emitted. The remnants of the doppler aspect of it can be measured too, which shows that the solar system seems to be moving relative to the glimpse of the early universe that the cosmic microwave background radiation is, at about 370km/s.

GL1

 
gruff5
938485.  Wed Sep 12, 2012 7:44 am Reply with quote

GL1 wrote:
...
In the case of the cosmic microwave background radiation , arguably the oldest light there is, the wavelength is now 1,100 times longer than when it was emitted. The remnants of the doppler aspect of it can be measured too, which shows that the solar system seems to be moving relative to the glimpse of the early universe that the cosmic microwave background radiation is, at about 370km/s.

GL1

Yes, and we (the solar system) are moving towards the CMB that lies in the direction of the constellation Leo - which is my birth 'sign'.

Cosmic, man!!

Can we think of the CMB as a kind of marker for 'absolute space'?

 
GL1
938520.  Wed Sep 12, 2012 11:03 am Reply with quote

Quote:
Can we think of the CMB as a kind of marker for 'absolute space'?

The Cosmic Microwave Background Radiation allows a velocity to be measured, but its not really absolute, it is a relative velocity between us and the part of the early universe that emitted the particular part of the radiation we are witnessing.

An absolute point cannot be derived either since, the CMBR literally came from everywhere. It was created by the universe when the universe's density had decreased to the point became opaque to light. That means when light could be emitted and no longer immediately collide with a particle capable of absorbing it. The whole universe shone like a light-bulb. (It was around the same temperature as a light-bulb filament). Having not been absorbed, it is this light that reaches us today as the CMBR.

The CMBR was formed at 380,000 years after the big bang event. There is another background radiation too, neutrinos being really really hard to absorb found the universe to be opaque, a lot earlier than light. A mere 2 seconds after the big bang they found the universe opaque and we can detect these too. Being so hard to detect we don't have as much data as we do on the CMBR

 
gruff5
941698.  Wed Sep 26, 2012 6:45 am Reply with quote

my dear alter-ego, GL :-) are we not mixing up 'opaque' with 'transparent' there?

I would instead say that when the temperature of the expanding universe had dropped sufficiently to allow the ionised plasma of nucleons and electrons to form neutral atoms (of mainly H and He) was when the universe became transparent to light (electromagnetic radiation) and the CMB we observe today was emitted.

The CMB, unsurprisingly, is in the same inertial frame as the 'Hubble Flow', which is the inertial frame you get when you average out the local velocities of all the galaxies. This universal inertial frame (there can be no other, in this universe) provides a logical frame against which all other velocities can be measured. Call velocity through it relative or absolute - perhaps we shall one day see evidence that it does mark out an absolute space?

 
'yorz
941778.  Wed Sep 26, 2012 3:18 pm Reply with quote

GL1 wrote:
Looking at the original subject. I don't quite understand what the poster was pointing out is linked between Doppler shift in light and Kepler.

The poster is a tad baffled as well - I am sure there was a link, but am struggling to cough it up now. Will keep trying.

 

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