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

[tar]

When considering the total energy or the total mass, or the total entropy, or the total momentum of a system, what happens when the far reaches of the system are separated by a significant distance, where light or gravity cannot get from one end of the system to the other in less than a moment?

 

Let's say a moment is between 1 1/2 and 2 1/2 seconds, and is somehow related to how we sense and remember and think in terms of existence and cause and effect and such.

 

A system, larger than half a million miles in diameter, would not be understandable as happening "at the same time". Any equation, like the application of the gas law, would be suspect, unless "when" the far reaches were being considered, was well defined.

 

So is there a size limit for considering a system within one equation?

Edited December 20, 2014 by tar

[Ophiolite]

Surely that is wholly contingent upon what the equation is and how accurately you want the answer.

[studiot]

Good morning tar.

 

Your question is a bit woolly, but a good one.

 

No there is no general size limit.

 

But thast does not mean we do not have to take it into account.

As Ophiolite says it depends in part on the equation

But also in part on the size and nature of the system.

We often have to consider the spread of the activity through the system.

 

Many good practical examples come from earth sciences since the Earth is very large relative to individual particles.

 

Earthquakes start locally and eventually affect all parts.

Oceanographers have a term called ocean mixing or flushing times.

It is possible for one one of a bar of rock to be molten,but the other end to be liquid, at the same time because of heat transference rates.

Children know this phenomenon in relation to ice lollys.

 

I was once involved in modelling what happens to pressure in gas pipeline networks thousands of miles remote from a breach.

 

In all cases the equations themselves do not depend upon scale, but some parts of the network remain unaffected for considerable periods.

 

There is even a branch of mathematics dealing with size independence, to whit fractals.

Edited December 20, 2014 by studiot

[swansont]

Another example is sending a spacecraft to the moon, or Mars, or to land on a comet. If you can account for how the system will evolve in time, you can model it properly.

 

At the other end of that you also have steady-state conditions, where the delays don't matter.

[tar]

Studiot,

 

So in particular I am concerned with the rotation of a Galaxy, in regards to the proposal of the need of dark matter to be "present" to explain the rotational speed. How can one figure such a large item as a galaxy as one thing, happening at once, according to a simple gas law?

 

Regards, TAR

[swansont]

Studiot,

 

So in particular I am concerned with the rotation of a Galaxy, in regards to the proposal of the need of dark matter to be "present" to explain the rotational speed. How can one figure such a large item as a galaxy as one thing, happening at once, according to a simple gas law?

 

Regards, TAR

 

It's gravity, not a gas law. It's also an example of something close to steady-state; if a galaxy's radius is 50,000 LY, then it's only sensitive to fluctuations below that time. But the mass transients on that time scale aren't going to be that big. Galaxies are billions of years old.

[Strange]

So in particular I am concerned with the rotation of a Galaxy, in regards to the proposal of the need of dark matter to be "present" to explain the rotational speed. How can one figure such a large item as a galaxy as one thing, happening at once, according to a simple gas law?

 

The issue here is orbital velocity. You can consider each star, grain of dust or hydrogen atom independently. The only other thing that matters is the total mass inside the orbit.

[tar]

swansont,

 

I don't know how mass transients are figured. It seems likely that a mass would "pull" from the direction where it was when we see it.

Which is not the direction the thing is in now, as the light and gravity took some time to get here.

 

So the distance between here and the observed item would matter, in determining its current position, if that is what one was trying to accomplish. Or is its apparent position the only one we care about?

 

When considering something the size of a galaxy, if you considered apparent position only, your positioning of the close stuff would be more correct than your positioning of the far away stuff.

 

My concern, is that the size is discounted and somehow ignored or considered unimportant, as if the age simplified the matter, where to me, the age increases the complexity.

 

For instance, how do we think about a question such as "how many supernova happened in the galaxy in 2014?"

 

If we saw one last week, did it happen in 2014 or did it happen 5 10 or 50 thousand years ago?

If one is happening now 35 thousand lys from here, does that count as happening in 2014 or do we have to wait 35 thousand years to see it, to count it as happening in 2014?

 

So are mass transients figured as effective mass, or actual mass? And what perspective makes any sense to take, that could see the whole galaxy at once?

 

Regards, TAR

 

Strange,

 

The "total mass" inside the orbit, is a good example of my question. The orbit the Sun takes around the center of the Galaxy is very large and the stuff within it, very substantial. How do we figure all that at once, when it takes tens of millions of years to complete an orbit, and 50 to 150 thousand years to even see where the other end is at?

 

Regards, TAR

[Strange]

 

The "total mass" inside the orbit, is a good example of my question. The orbit the Sun takes around the center of the Galaxy is very large and the stuff within it, very substantial. How do we figure all that at once, when it takes tens of millions of years to complete an orbit, and 50 to 150 thousand years to even see where the other end is at?

 

The mass is calculated from the number and types of stars, measurements of amounts of gas present in between, and so on.

 

Orbital velocities measured either using Doppler shifts or, in some cases, direct measurements of movement over time.

[swansont]

A supernova doesn't change the mass all that much — most of the star's mass remains nearby, so it all contributes to the rotation of the outer stars of a galaxy.

[tar]

Swansont,

 

I was not suggesting the super novas changed the mass of the galaxy, I was suggesting that its hard to know when to count a thing as one instance of a thing.

 

Perhaps you have a point, that it would not matter that much to the rest of galaxy whether a supernova's mass was spread out over a relatively small section of the galaxy, nearby where the mass stood as an intact star. But perhaps that difference that the spreading out of the mass would make, would increase over time. For instance, after 1billion years subsequent to a super nova, that star's mass is highly distributed in dust and has accumulated to form perhaps another star, and given of light, which is energy and therefore mass, that has had a billion years to travel at the speed of light, away from where the star stood, a billion years ago. Such affects would be 1000 times less in only 1 million years and 10 thousand times lest in the mere time it takes light to cross a galaxy, but there would still be wide ranging effects. Effects that ranged as wide as the electomagnetic waves and particles and dust could reach, in the time frame in question.

 

Let's say for instance incredible amounts of cosmic rays are issued forth from a supernova, and you are located 1 light year away. In a year you get boiled, but in two years...you are safe because the cosmic rays have already passed your location.

There are other solar systems with planets that might be in the path of the cosmic rays eminating from that super nova, but your location is no longer in its sights.

 

Now, when figuring the mass of the galaxy, by the methods suggested above, by Strange, are you figuring the mass equivalent of the cosmic rays, and their position and timing? Would it not matter if you took the "pictiure" now or last year, in terms of whether that mass was in the direction the star use to be in or in the other direction?

 

Does it not matter at which instant you are "freezing" the galaxy in order to take your census of the place, in terms of its mass and the postions and concentrations of that mass?

 

Regards, TAR

 

Regards, TAR

[Mordred]

Here is the thing tar. We consider the entire observable universe in a singular system. Beyond that there us no data. Google the FLRW metric for a model of the observable universe

So The only scale limit is confined according to data and measurements. Any scale can be modelled with sufficient data.

[swansont]

The amount of mass lost in radiation is small compared to the mass of a star. Our sun converts about 4 million metric tons of mass to energy every second. If it did so at that rate for 1 billion years, that would be about .0068% of its current mass. Even doing so for the estimated 10 BY lifetime of such a star, that's a tiny fraction of its mass.

 

http://solar-center.stanford.edu/FAQ/Qshrink.html

http://curious.astro.cornell.edu/question.php?number=563

[tar]

Mordred,

 

I am questioning the statement "beyond that, there is no data".

 

We have been looking at the stars and documenting what we see for what, maybe 5 thousand years?

 

Do we have ANY current data from beyond a sphere 5 thousand lys in radius?

 

We see stuff happening currently that is happening at vast distances, further away than even the far reaches of our emmense galaxy. Its the timing difference between what is happening at this point in the Milky Way, and at some point on the other side of the place, that I am questioning here, in asking whether there is a size limit to what you can reasonable consider one system, in total, all taken "at once" in a calculation, or inference.

 

Such is my consternation when told that scientists have figured that the universe is not only expanding, but is currently accelerating its expansion. Really? You can take the status of the entire system as one thing and make a statement like that, based on the information gleened about the place over the last 5thousand years? And all this based on doppler shifts that show a thing is mostly moving toward us or away from us, or moving left and right or up and down against the background of really distant items? Those really distant items could have turned around and headed back toward us a billion years ago. We would not have any data about that turn around for billions of years.

 

SwansonT,

 

" If it did so at that rate for 1 billion years, that would be about .0068% of its current mass." So If you took 147 Suns, together they would convert a sun's worth of mass into energy and send it outward every billion years. Supernovas seem to actually beam large amounts of mass away into energy, in the person of a beam of cosmic rays heading out in some particular direction. Black holes seem to spew off a high energy beam as well. I don't know the numbers for the natural ways that energy is converted back to mass, nor "currently" what percentage of the universe is mass, and what is energy, but its an interesting thought experiment to perform, in reference to my current question of how large a system you can consider at once.

 

Let's say for instance, that there is a certain cycle that occurs where through various mechanisms energy is converted to mass and mass to energy, and just for the sake of argument the cycle has an average period of 100 billion years. That is, that after 100 billion years, any selected peice of mass, was likely once energy instead, and any selected quantum of energy was once a particle with mass. Under this thought experiment when you see a distant quasar, the image is made up photons which carry with them energy which was once mass. Concurrently the area where the quasar once stood, has not only undergone some large amount of stellar evolution, and continual shining and concurrent loss of mass, but has been subjected to the photons of energy coming in from all directions, from all the other items in the universe, for 13.8 billion years. Which items of energy and mass are you going to count, in reference to that quasar, when doing your census of the system which is the observable universe, when the items of mass and energy related to that quasar are spread out across the entire observable universe? At least in a spherelike volume with a radius the distance between us and the quasar's location.

 

It would be like having in your hand, your own skull as a 7 year old. You can't take the place at once.

 

Regards, TAR

[swansont]

SwansonT,

 

" If it did so at that rate for 1 billion years, that would be about .0068% of its current mass." So If you took 147 Suns, together they would convert a sun's worth of mass into energy and send it outward every billion years.

No, that's .0068%. If you took 147 suns, they'd convert 1% of a sun's mass into energy every billion years. You need 14700 suns to convert 1 sun's worth of mass every billion years. IOW, for a star like our sun this is a slow process, and assuming steady-state for the mass is a pretty good assumption.

 

Supernovas seem to actually beam large amounts of mass away into energy, in the person of a beam of cosmic rays heading out in some particular direction. Black holes seem to spew off a high energy beam as well. I don't know the numbers for the natural ways that energy is converted back to mass, nor "currently" what percentage of the universe is mass, and what is energy, but its an interesting thought experiment to perform, in reference to my current question of how large a system you can consider at once.

Since these systems represent a huge amount of energy, then could emit a large amount and still have it be a tiny fraction of the total. The numbers are important.

Such is my consternation when told that scientists have figured that the universe is not only expanding, but is currently accelerating its expansion. Really? You can take the status of the entire system as one thing and make a statement like that, based on the information gleened about the place over the last 5thousand years? And all this based on doppler shifts that show a thing is mostly moving toward us or away from us, or moving left and right or up and down against the background of really distant items? Those really distant items could have turned around and headed back toward us a billion years ago. We would not have any data about that turn around for billions of years.

Argument from incredulity isn't much of an objection. You could look at the evidence for expansion, for instance

http://en.wikipedia.org/wiki/Accelerating_universe#Evidence_for_acceleration

 

Our observation isn't simply a matter of making one observation one day and another some time after that. We can leverage the fact that the universe is expanding and look at what we would expect without acceleration and what we see.

 

You don't need to observe something along its entire trajectory to know something about its speed. If a ball is 100 m away 1 sec after it was thrown, you know it was going a minimum of 100 m/s at some point along its trajectory. Knowing the speed when you observe it gives other info — if it's moving faster than 100 m/s now, you can infer it's speeding up. Slower than 100 m/s, and you can infer it has slowed down.

[Mordred]

We also don't rely on just observation of expansion in terms of distance measurements alone. Part of the evidence for expansion Is the thermodynamic properties we see today and at the time of the CMB. The universe is 2.7 Kelvin today at the time of the CMB it was 3000 Kelvin. This drop in average temperature is only possible with an increase in volume. See ideal gas laws (cosmology)

http://en.m.wikipedia.org/wiki/Equation_of_state_(cosmology)

[Strange]

Now, when figuring the mass of the galaxy, by the methods suggested above, by Strange, are you figuring the mass equivalent of the cosmic rays, and their position and timing? Would it not matter if you took the "pictiure" now or last year, in terms of whether that mass was in the direction the star use to be in or in the other direction?

It wouldn't make any difference. The mass and energy of those cosmic rays would be orders of magnitude less than the errors in the overall figure.

Do we have ANY current data from beyond a sphere 5 thousand lys in radius?

 

We have direct observational data going back to 380,000 years after the big bang. We have indirect evidence and extrapolations based on well understood physics, going back nearly 380,000 years before that.

 

Its the timing difference between what is happening at this point in the Milky Way, and at some point on the other side of the place, that I am questioning here, in asking whether there is a size limit to what you can reasonable consider one system, in total, all taken "at once" in a calculation, or inference.

 

I was going to give the apparent homogeneity of the universe as an example in response to your initial question. The reason for the inflation hypothesis in the big bang model is to explain this uniformity because it seems to have required the entire (observable) universe to have been small enough to have been a single uniform entity. (There seems to be another thread on this "horizon" problem.)

 

Those really distant items could have turned around and headed back toward us a billion years ago. We would not have any data about that turn around for billions of years.

 

True. But they could also have turned into chocolate elephants and started dancing a jig. With no evidence to support either suggestion nor any mechanism by which it could happen, there seems little reason to consider it.

 

I don't know the numbers for the natural ways that energy is converted back to mass

 

e = mc2.

 

But it doesn't make any difference. Apart from the fact that the amounts of energy you are talking about are tiny, they are also still in the galaxy so it makes no difference to the total.

 

Let's say for instance, that there is a certain cycle that occurs where through various mechanisms energy is converted to mass and mass to energy, and just for the sake of argument the cycle has an average period of 100 billion years. That is, that after 100 billion years, any selected peice of mass, was likely once energy instead, and any selected quantum of energy was once a particle with mass.

 

A) there is no evidence for this cycle so why would we consider it?

B) It wouldn't make any difference to the total mass-energy (and therefore gravitational effect) of the galaxy.

 

You just seem to be making up random stuff in the hope it might show current science to be wrong. But the actual evidence is more important.

[tar]

SwansonT, Mordred, Strange,

 

I appreciate that the science and accepted measurements of Z and proposed volume and density calculations point to an expanding universe. My intent is not to try to prove current science wrong, but to question if the inferences make sense.

Taking a measurement and fitting it to a mathematical model is appropriate. Establishing a mathematical model thusly and seeing that measurements fit the model is good confirmation that the model is workable.

 

However, the well understood science is a somewhat similar body of knowledge that we had just a short time ago, when everybody thought the universe was slowing down its expansion. My conondrum is figuring out which pieces of information are derived from assumptions and data that was properly vetted for time and distance considerations, and uncomfortably, whether or not the time and distance assumptions made were based on knowledge and measurements taken before or after certain insights and constant adjustments were made.

 

It is not unusual to find a cyclical nature to the reasoning of time and distance, where an assumption provides the distance and time figures that are plugged into the equation from which assumptions about distance and time are inferred.

 

Like Z.

 

And now, the areas of the universe that are just now visible to us as CMB, which at the time of photon launch were 380thousand years old, are now 13.8 billion years old. Those areas are thusly two different sizes in terms of volume, the way we see them, and the way they are. Yet Strange submits that there is no universal now, as if it is not required that the universe actually be, at this current moment, larger than it appears.

 

So, with the differences in models between current physicists, and the changes in models over the years as new inferences are made and new observations made to fit into the models, and with the "difference" in models derived from what we see, as opposed to what we know to have been the case, and what we know must currrently be the case, it is hard to visualize the model that a particular scientist is using when making a particular statement or writing a particular formula that encompasses an entire system that is larger than a moment in radius.

 

Regards, TAR

[Strange]

However, the well understood science is a somewhat similar body of knowledge that we had just a short time ago, when everybody thought the universe was slowing down its expansion.

 

This has changed because there is new evidence.

 

My conondrum is figuring out which pieces of information are derived from assumptions and data that was properly vetted for time and distance considerations, and uncomfortably, whether or not the time and distance assumptions made were based on knowledge and measurements taken before or after certain insights and constant adjustments were made.

 

 

You would probably need to do a degree in cosmology, or at least study the appropriate material, much of which is available online, in order to be able to answer that for yourself.

 

So, with the differences in models between current physicists, and the changes in models over the years as new inferences are made and new observations made to fit into the models

 

Actually, the models are changed to fit the data. The other way round would be pseudoscience.

[tar]

SwansonT,

 

Sorry about my large mistake, missing the percentage, but energy released by suns does not stay in the galaxy for long. Maybe a hundred thousand years, but after that it is outside the galaxy. Otherwise we could not see other galaxies. So mass/energy calculations should include a time component, as some energy has left, is leaving and will leave, according to whether you are considering energy on this side of the galaxy or on the other.

 

Regards, TAR

[Strange]

Do the math. You will get a number very close to zero. It will certainly be many orders of magnitude smaller than the errors in estimating the mass of the galaxy.

 

(Once again, we are back to you saying "I think this is significant" when a back-of-the-envelope calculation shows that it is obviously not. You need to work some of these things out so that your "gut feel" becomes more relaistic.)

Edited December 29, 2014 by Strange

[tar]

Strange,

 

I have seen many a term dropped as not significant that I thought was significant. Even in this discussion it is often claimed that the effect I am talking about is too tiny to make a difference. When I know the difference between a item we see, and the same item after the time it takes light to get here from it, is significant. Items that are 1 billion lys away have had 1 billion years to evolve and move. You cannot talk of two instances of a thing that can only be one instance of thing. Doing so would allow for a significantly incorrect system equation.

Regards, TAR

Edited December 29, 2014 by tar

[Strange]

As I say, work it out for yourself, and then show us that the energy lost by electromagnetic radiation is significant (i.e. larger than the errors).

 

There is no point having the "but it seems significant to me" argument all over again. If you can't be bothered to work it out, then you have NO RIGHT to claim it is significant.

[tar]

Strange,

 

But here, the how, of how when is to figure the thing, is the area where the significance resides.

 

Consider the amount of energy a star puts off. It is quite significant. Significant enough to cause a chemical reaction at the back of your insignificantly tiny eyeball. This energy that that visible star put off has gone off in all directions, has been going off in all directions and continues to go off in all directions. Enough photons, enough energy hits a small thing like an eyeball, at a distance of a few ten thousands of lys, to cause us to see the star. How may eyeball size areas are there in this area of the galaxy that the energy of that star is currently hitting? Some photon from that star is currently hitting an eyeball size area a billion lys from here. And that occurence can be said to be occuring in every direction, at every distance that there is, from that star. Locations in and outside the galaxy are currently receiving photons from that star. In addition that hypothetical area in question that is currently receiving a photon from that star, also received one yesterday and a year ago and a billion years ago from that star and will recieve one next year and the year after. This same thing can be said for every eyeball size area in the entire universe, or at least within the same number of lys distance as years old that the star in question is.

 

Seems a significant amount of energy to me. With or without numbers attached.

 

Regards, TAR

[Strange]

Seems a significant amount of energy to me. With or without numbers attached.

 

Without numbers attached, you are still relying on the same, tired old argument from incredulity: "it is amazing I can see light from distant stars, therefore it must be important". You seem to be using "significant" to mean having a noticeable effect; i.e. you can see it.

 

It is still an insignificant proportion of the mass of the galaxy (which was the subject of discussion) and playing silly word games won't change that.

 

If you want to claim that the energy released by stars is significant in this sense (i.e. has some relevance to the subject you are discussing) then please demonstrate it. It just requires a little basic arithmetic. (I am not going to do it for you because I don't think you will learn anything from that.)

 

If you are too lazy to support your own argument then I don't see why anyone else should take it seriously, either

Edited December 29, 2014 by Strange