Speed of the Sun: Facts & Measurements

[John Richard]

That isn't really the problem. The problem is that some kids grow up watching it and their opinions are formed. Later some of them become fascinated with physics and want to learn. The danger is that when they start, they might try and build the science around the model and that is backwards. Starting with iffy assumptions can start you off in the wrong direction. You need to learn the facts before you make assumptions. Starting off with assumptions and looking for facts that support the assumptions isn't the best way to learn.

Another thing is that often people getting into physics start by trying to conceptualize things that may not even be true. Trying to wrap your head around things that are impossible to wrap your head around isn't very productive. You do all this reading and thinking and in the end your still just left with a blown up balloon with dots or a sheet and a bowling ball. The fact is that these analogies are misconceptions themselves if you take them seriously.

If your misconceptions made you interested enough to learn then they did you a favour. If you had no misconceptions what would you need to learn? Analogies, by there very nature, are not that which they are seeking to explain. This is really a debate for the philosophy section, but reading your post I would say you have unearthed the fact that you may have had, or may still have the wrong ideas. Congratulations, you've just justified the process of learning.

 

[John Richard]

wow. i didn't know that about the CMB radiation and, although i had not previously believed in an aether (and i still don't), i am intrigued that the reference frame that is coincident with the CMB is not any more important than any other. it surely seems to have a unique and universal claim.



yeah, you guys, can you explain why the the unred-shifted CMB would seem to be moving relative to the center of our galaxy without a notion that our galaxy is moving relative to some universal frame of reference that can me measured by anyone else in the universe?

O good, back to the question. I may be wrong but I think this has to do with the nature of the observational isotropy of the speed of light. The frequency of light can only be affected by its source. Once propagated any observer will observe the finger print of the velocity of the light source. The observers motion does not affect this.

So effectively we can calculate the speed of the light source with reference to the universal speed limit which is C. Then we can cross reference with other observed motions to arrive at a topology of interrelated motions.

Happy to be shot if I'm wrong!

John

 

[W3pcq]

If your misconceptions made you interested enough to learn then they did you a favour. If you had no misconceptions what would you need to learn? Analogies, by there very nature, are not that which they are seeking to explain. This is really a debate for the philosophy section, but reading your post I would say you have unearthed the fact that you may have had, or may still have the wrong ideas. Congratulations, you've just justified the process of learning.

That is a good point, but I think that my argument still stands. That goes back to what I said earlier about how conclusions must be stretched to gain public interest.

I can't speak for anyone but myself, but I feel that the misconceptions in the media have been destructive to my interest in learning. I agree that some, and I would have no idea about percentages, go into physics trying to understand misconceptions on tv. Me, I have a natural curiosity about how things work and what is out there. The misconceptions in the media have only made me fustrated. The misconceptions in the media also may lead one to believe that all the mysteries have been solved, so what is the point.

I don't think that anyone has the right ideas when the idea is a speculation. If you would claim that the big bang model is the exact right idea, I would disagree. Maybe best educated guess, but nothing more.

 

[marcus]

that is right, but it is not 600 km/second
If you are talking about the sun's motion relative to CMB, the speed is about 370 km/second.

yeah, you guys, can you explain why the the unred-shifted CMB would seem to be moving relative to the center of our galaxy without a notion that our galaxy is moving relative to some universal frame of reference that can me measured by anyone else in the universe?

our galaxy IS moving relative to a universal frame that could be determined by anyone else. there is an absolute standard of rest which Hubble already knew about in the 1930s.
one can define what it means to be at rest with respect to the Hubble flow

the Hubble law v = HD is not exactly right because the solar system is moving 370 km/s in the direction of Leo. that means recession velocities of galaxies in that direction are not quite as high as they should be according to v = HD
and recession speeds of galaxies in the opposite direction from Leo are just slightly higher than they should be. Redshift data has always needed to be corrected for the Earth and sun's motion relative to the Hubble flow.

then when CmB data started coming in it just CONFIRMED this and allowed the solar system velocity to be measured more precisely, both speed and direction coordinates.

this doesn't have to do with AETHER as i understand it. Maybe you call it aether but I don't. It is just the uniform expansion (the Hubble flow). you can be at rest with respect to it or not. if you are moving relative to it then it will not be uniform or isotropic.

 

[marcus]

... The frequency of light can only be affected by its source. Once propagated any observer will observe the finger print of the velocity of the light source. The observers motion does not affect this.
...

What you say seems to be at odds with what is generally taught in college physics. Something's odd, either factually or in your use of terminology.

As ordinarily understood, BOTH the observer's motion and the expansion of distances affect the frequency of the light.

I'll illustrate with an example. In the case of thermal radiation, temperature gives a good handle on frequency because they are proportional. You know that the CMB was emitted from partly ionized gas at a temp of about 3000 kelvin and its temperature now is about 2.76 kelvin

Distances expanded by a factor of about 1100 during the time the light was traveling. The redshift is NOT A DOPPLER EFFECT and was caused by the stretching out of distances.

The gas that emitted the CMB light (which at the time was part visible part infrared, like from a 100 watt bulb) was about 40 million LY from our matter when it emitted the light.

It was receding from us at a speed of about 60c (sixty times the speed of light).

Today, that same matter, whose light is just now reaching us as CMB, is about 45 billion LY from us, and receding at a speed of 3.3c (slightly over three times the speed of light).

The redshift, which has reduced the frequency by a factor of 1100, is entirely attributable to the fact that while the light was traveling on its way to us cosmological distances increased 1100-fold.

the only doppler effect involved is a few MILLIkelvin caused by the Earth and sun motion of 370 km/second in the Leo direction.

You may have noticed that 1100 is the approx. ratio between the two distances I mentioned, the 40 million LY (distance then) and 45 billion LY (distance now)

Ooops, bedtime. don't have time now to discuss more. Anyway after some light is emitted, its frequency can be changed a lot by things that happen subsequently and also by the observer's motion (which in this case has only a small millikelvin effect but can have a larger one)

 

[W3pcq]

Is the doppler effect of EM radiation compatible with C according to special relativity?

 

[Janus]

Is the doppler effect of EM radiation compatible with C according to special relativity?

The standard Doppler effect equation has to be modified to a relativistic form, but the basic concept of the Doppler effect remains the same.

 

[marcus]

Is the doppler effect of EM radiation compatible with C according to special relativity?

The standard Doppler effect equation has to be modified to a relativistic form, but the basic concept of the Doppler effect remains the same.

To expand on what Janus said, the Doppler equation in Special Relativity (which only works for speeds less than c) says that the frequency ratio is
\sqrt{\frac{c-v}{c+v}}

Here v is the speed of the emitter relative to the receiver, and positive v denotes increasing distance. You can see that if v = 0.5c then the frequency ratio is
\sqrt{\frac{0.5}{1.5}} = 0.58
That means that the frequency of the received light is only 58 percent of the frequency of what was sent.

If you think in terms of wavelengths, they get longer and the formula is just the reciprocal. In astronomy the wavelength ratio can be written 1+z, so you might see this equation for a Doppler shift
1+z = \sqrt{\frac{c+v}{c-v}}

the point that needs emphasis IMO is that these Special Relativity formulas do not work for calculating the cosmological redshift.
In the case of redshift, the recession speeds are often greater than c, and the formulas break down in that case.
So in a beginning astronomy class you get taught a different formula for the redshift which does not depend on velocity.
1+z = \frac{a(t_r)}{a(t_e)}
Here t_e is the time the light was emitted and t_r is the time it was received. The a(t) is the scale factor at time t. The ratio is the factor by which the universe expanded during the time the light was in transit.

 

[John Richard]

What you say seems to be at odds with what is generally taught in college physics. Something's odd, either factually or in your use of terminology.

It was receding from us at a speed of about 60c (sixty times the speed of light).

Today, that same matter, whose light is just now reaching us as CMB, is about 45 billion LY from us, and receding at a speed of 3.3c (slightly over three times the speed of light).

Marcus, thank you for your reply. I think I was just factually wrong and I appologize to rbj.

Sometimes, stating what you think you know, gets a better answer than any question you may have asked. That is certainly true for me in this case.

You speak of speeds in excess of C and of CMB just reaching us and this makes it clear to me that I have a big gap in how I'm interpreting the big bang model. I understand that the CMB wave length will stretch as a result of the expansion. But, I am under the impression that we were inside the big bang, which means that the CMB will have to have been around us from the start.

I am also under the impression that nothing travels faster than light and that all points of observation will measure light at the same speed. I had assumed therefore, that the CMB that is around us is phase shifted from origin by a measure of light years equal to the time since the big bang. That the expansion has effected only the frequency.

I do not think it is fair of me to press on your time just to teach me. I would be very grateful if you could suggest some reading that might clear up my misconceptions.

Once again, thank you for taking time to help me.

John

 

[marcus]

... which means that the CMB will have to have been around us from the start.
...

Yes that is right! Caveat, we are talking about the mainstream picture here. there is a lot of evidence that it is right as far as it goes.

To get used to visualizing standard cosmology it can help enormously to play around with a cosmology calculatory. Several people here at PF have made their own----Jorrie, hellfire and I don't know who else. there are some threads where they discuss features.

but most of us use online java calculators by Ned Wright or Siobhan Morgan.
Go here
http://hubblesite.org/newscenter/archive/releases/2008/08/full/
http://scienceblogs.com/catdynamics/2008/02/liveblogging_the_high_redshift_1.php

It talks about a young galaxy at redshift z = 7.6 (if confirmed) observed as it was 12.8 billion years ago during the first wave of star formation
this is conference news, it hasn't been published yet AFAIK

So suppose you want to know the recession speed of a galaxy at z = 7.6
First try Ned Wright's
http://www.astro.ucla.edu/~wright/CosmoCalc.html
just plug in 7.6 for z, over on the left

what I get from Wright's is that the light travel time was 12.97 billion years which is a little off from the 12.8 they say in the press release but small discrepancies don't matter here. and I get that the age of the universe when the light was emitted was 0.7 billion years. Is that what you get?

the important thing to notice is over on the left Ned Wright puts in the default values of H=71, and Omega_matter = 0.27 and Lambda = 0.73. they are accepted parameter values, widely used. So when you go on to Morgan's calculator you should take them along with you and plug them in too. Morgan wants her students to work more and consciously put them in.

So then go here
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html
and over on the left, put in 0.27 for matter, 0.73 for Lambda, and 71 for Hubble.
Then type 7.6 into the z box and see what you get. You should get some recession speeds!

Cosmology is largely non verbal. It is applying a mathematical model to the geometry of the universe, and fitting it to many different kinds of data.
That mathematical model is built into Wright's and Morgan's calculators.
When you play with those calculators you are playing with the best model of the universe we have today.

Truly enormous amounts of data of many different kinds are used to challenge the model. it must fit to the data. that is how the numbers 0.27, 0.73 and 71 are determined. The more data, the more accurate these estimates get, and the better the fit.

What do you get for the speeds? I get about 3.3c and 2.3c. 3.3c is the recession speed of the galaxy THEN when it emitted the light (some 12.8 billion years ago) and 2.3c is the recession speed NOW as the light is arriving to us and coming down the telescope.

The distance now should be greater than the distance then by a factor of 1+z which is 8.6.

How about checking to see if that is about right? 1+z is the factor by which the light wavelengths are stretched, so it must be the factor by which distances expanded in the universe while the light was in transit to us. If the ratio doesn't work out right, please tell me!
==========================
I gathered a bunch of useful links here:
https://www.physicsforums.com/showthread.php?p=1610331#post1610331

 

[AstroLiv]

I think I previously posted this comment wrongly:

Hi Marcus,

Do you have any idea where can I find a good multimedia, or other interaction to well understand the Solar Dopplergrams? I have difficulty answering the following question:

At about what speed is the right edge of the sun moving away (receding) from us at the equator?

Thank you in advance for your help.

Liv.