Is there a size, beyond which a system cannot be considered at once?

[Strange]

Yet you figure we can figure a way to apply our observations and data to equations that can accurately portray the thermodynamic state of a system millions of lys in diameter?

 

Yes.

 

How?

 

How can we possibly answer that? We (well, Mordred and others) could give you the full mathematical treatment. But you wouldn't understand that and would ask for simple explanation. You have had the simple explanation and either don't get it and/or reject it.

 

I don't think anyone can invent a model that would satisfy your particular intuition.

 

At what cosmic date are we freezing said system, inorder to consider it, at once?

 

There is no "frozen" description. We have a model of the evolving universe. You can look at conditions at particular times (e.g. temperature, pressure) and work out what would be happening in terms of plasma, particle interactions, etc.

[tar]

Strange,

 

Got sidetracked, looking up carbon dating, to set up a mind experiment to help illustrate my position, but got side tracked again, so I don't have a good setup, but my idea was to set up a theoretical situation where some radio active element would not have a way to be created until lets say the third generation of stars, and lets say this element would only be created for a few moments during a certain stage of a third generation star, and would have a half life so quick as that any appreciable quantities of the stuff would degrade to some other element in a few years.

 

Now the thought experiment is to think about how much of the stuff is currently extant in the Milky Way. Lets say a certain frequency signature would identify that some photons we are receiving now were emitted by the stuff. We know the stuff we currently see with this signature has already degraded, so we can not count that. And the signature of any that currently is extant will not be seen to be counted until later, depending on how far away it is, by which time, when we see the signature, THAT stuff will be degraded, so we can't count that either. So when determining the amount of this substance that is currently extant in the galaxy we can either incorrectly count stuff we see, or just guess at what we will be able to see later. The only way to get a proper count, is to observe the galaxy for 150 thousand years, record each sighting and the distance to the sighting and determine when and where an instance of the stuff was known to exist. Then one could pick that time 150 thousand years prior as a time to freeze, and figure how much of the stuff was extant at that moment.

 

While this could be done, the moment in time you froze the place at, inorder to do the count, was, at the point of determination, a moment that had occurred 150 thousand years ago, so the count would be of little value, in answering the question "how many atoms of this rare stuff are currently extant in the Milky Way?"

 

This difficulty is primarily because the system is too big to be considered at once.

 

Regards, TAR

[Strange]

In that example, you could take into account the distance that each measurement was made (and therefore how long ago), the half life and the formation mechanism and then extrapolate to calculate the average amount present. I wouldn't be surprised if there are elements where exactly that is done as many isotopes have half lives that are very short relative to the lifetime of stars, or even very short compared to the frequency of their production methods.

 

This seems to be yet another ill-defined "problem" you have invented that is simply based on a poor understanding of the methods and mathematics used.

[Mordred]

To add to Strange's comment. When we use our telescopes we also do a spectrum analysis. This spectrum allows us to identify elements and compounds.

One of my fav articles on element absorbtion lines etc is physics of the intergalactic medium

 

http://arxiv.org/abs/0711.3358

You may also want to look at the

 

 

Hertzprung-Russell diagram.

 

 

http://www.atlasoftheuniverse.com/hr.html

 

as well as the cosmic distance ladder

http://www.google.ca/url?sa=t&source=web&cd=11&ved=0CEYQFjAK&url=http%3A%2F%2Fterrytao.files.wordpress.com%2F2010%2F10%2Fcosmic-distance-ladder.pdf&rct=j&q=cosmic%20distance%20ladder&ei=X5KzVOXlFYKZyATCy4KABA&usg=AFQjCNGlvPOAHYvvgMKDzr4f8t3KAzw5Mg&sig2=ddvTSQZykycf5uRqs9dB6g

 

This is a handy slide show the later pages has some spectography examples

Some areas to look at on spectrography and elements is to look at the Lyman forest and the Rydberg formula

 

This link will get you started.

http://en.m.wikipedia.org/wiki/Hydrogen_spectral_series

[tar]

Mordred,

 

Thanks for the links. Nice stuff. Still working on them.

 

Seems like we have a great deal "figured out".

 

Interesting though that so much of it is implied, indirect, "figuring" and imagining. And so little of it is actual observation of current arrangements of the universe.

 

What what we see "means" is true, is an interesting question. And one at the basis of my concerns in this thread.

 

The closest star to us is 2 or three lys away. There is only one instance of the thing, yet we alternately see it in our sky at night, and imagine it currently shining in the cosmic present.

 

Which instance of the thing, do you figure is true? And which instance of the thing is included in your thermodynamic state calculation of the Milky Way?

 

Regards, TAR

[tar]

Let's say for instance that that closest star was spinning one time every 5 years and has undergone some event that was sending out a powerful stream of energetic particles in only one direction, (or perhaps also the diametricallly opposed direction). Let us also suppose that for two weeks every year or two we are in the path of this stream of particles, and then a year or two later in the path of the stream projecting out the opposite side of the star.

 

At any paricular moment, which way would you figure the stream of energetic particles was facing? Would you go by the experience on Earth, or would you go by the figuring of the timing and distance of the thing, and what orientation the star has in the cosmic present?

 

 

Regards, TAR

Actually that is a fantastic thought experiment, because from the cosmic now the projecting streams of particles, from both sides of the star, would have to descibe spiral arms, from the point of view of the cosmic now.

 

Interesting that that is how we visualize galaxies, both when we see one from its axia of rotation, and when we figure how ours looks from the cosmic now.