How is molecular hydrogen detected?

[Chronos]

I agree, twofish. Most of the genius ideas posted on this site seem plagued by a shocking ignorance of math and fundamental physics.

 

[Bobbywhy]

twofish-quant, thank you for your comments in post #70 above. "Crazy ideas" musn't be ignored without thorough consideration of their source. Since I am not a professional scientist your viewpoints are enlightening...one of the great benefits of PF! I certainly learned a lot by following the exchange between you and JDoolin!

 

[twofish-quant]

"Crazy ideas" musn't be ignored without thorough consideration of their source.

It's quite complicated...

The reason I mention Roger Penrose is that when he is talking about neuroscience he really is a crank. However, curiously the things that make him a crank in one field makes him totally brilliant in another. If you hear him talk about neuroscience, you can tell it's the same person that is doing work in quasi-crystals. However, he got lucky that in one area, the data seemed to end working in his direction, and in another it hasn't.

The thing about "crazy ideas" is that it's a matter of time allocation. OK, someone mentions a nutty idea. Now what? What exactly is it that you want me to do with it?

One problem with cosmology is that there are realms in which it's not clear what is "crazy" and what itself. Once you get into the inflationary era, then it's not clear what's nutty and what isn't. However, one thing that I don't think that the general public doesn't quite appreciate is that most of cosmology happens in "non-crazy" areas. Popular works in cosmology play up the "weird physics" and often miss the point that most of cosmology takes place in situations where the physics isn't weird at all.

For example, once you get past the very, very, very early universe, you are just talking about "gas and gravity." Some of the big mysteries involve things that are unlikely to involve any super-weird physics. Galaxy formation, and early star formation for example.

 

[JDoolin]

That was before I realized that Special Relativity and cosmological General Relativity were in direct conflict, and the FLRW metric assumes as one of its premise that the Lorentz Transformation equations are invalid at large distances.

Yes. This is something that people are aware of, and I remember my cosmology professor mentioning it in an early lecture. It's not an assumption so much as a consequence. Once you start with the premise that the universe is isotropic and homogenous, then at large distances things are going to be flying away from each other at > c, and Lorenz transformation will break down.

Essentially, if you try to do cosmology using special relativity, you end up getting a big inconsistent mess. So you either do things with Newtonian gravitation or else you do it with full general relativity.

The reason that Einstein is considered a genius, is that once you figure out that the speed of light is finite, it's not easy to come up with a theory of gravity that's consistent. The "obvious" ways of adding relativity to gravity don't work.

There are people that believe that if Einstein hadn't come up with the key ideas, that we'd still be struggling trying to figure out how to make gravity work with relativity.

Here is my problem in arguing with you guys. The continually retreating premise. If I say that FLRW metric is based on the premise that Lorentz Transformation is invalid at large distances, you claim that that is not a premise. The premise is homogeneity and isotropy.

But when Milne showed you that isotropy is possible without homogeneity, so long as you account for special relativity, then you change your story. Now, that is impossible because it doesn't fit with the FLRW metric.

But yes, a lot of Gravitation, Relativity, and World Structure is devoted to explaining that argument. Explaining how and why Einstein and Eddington are wrong--that isotropy is indeed possible without homogeneity. And the premise on which the FLRW metric is based, is fundamentally flawed.

If you would acknowledge that point, then you might begin to focus on my real argument instead of straw-men.

 

[JDoolin]

It's quite complicated...

The reason I mention Roger Penrose is that when he is talking about neuroscience he really is a crank. However, curiously the things that make him a crank in one field makes him totally brilliant in another. If you hear him talk about neuroscience, you can tell it's the same person that is doing work in quasi-crystals. However, he got lucky that in one area, the data seemed to end working in his direction, and in another it hasn't.

The thing about "crazy ideas" is that it's a matter of time allocation. OK, someone mentions a nutty idea. Now what? What exactly is it that you want me to do with it?

One problem with cosmology is that there are realms in which it's not clear what is "crazy" and what itself. Once you get into the inflationary era, then it's not clear what's nutty and what isn't. However, one thing that I don't think that the general public doesn't quite appreciate is that most of cosmology happens in "non-crazy" areas. Popular works in cosmology play up the "weird physics" and often miss the point that most of cosmology takes place in situations where the physics isn't weird at all.

For example, once you get past the very, very, very early universe, you are just talking about "gas and gravity." Some of the big mysteries involve things that are unlikely to involve any super-weird physics. Galaxy formation, and early star formation for example.

I feel that Penrose made a tremendous error in claiming that Lorentz Contraction is invisible. I've actually not read Penrose account of it, but Terrell's description is online somewhere, and I posted my arguments against it on my website.

On the other hand, I have very much enjoyed reading "The Emperor's New Mind."

All I know of Penrose's ideas of neuroscience are what I read in that book, so I may be unaware of exactly how nutty he is, but I didn't see anything in "The Emperor's New Mind" that was particularly nutty or controversial. In fact, as I recall, I was excited to find someone who essentially agreed with me. I think the main point that Penrose was making was that people have opinions, and computers don't. A computer is able to compute, but it is utterly unable to make a judgment of whether that information is interesting or the effort was worthwhile. Where do those opinions come from? It is doubtful that our opinions and emotions are going to be successfully emulated with a computer in their current form. I don't know what exactly your issue with Penrose is, but given your prediliction for building straw-men, I wonder whether you are arguing with Penrose, or a straw-man version of Penrose?

I don't think that Penrose was wrong, but that Penrose was aware of something that perhaps you're not. That human-beings are opinion-generating engines. That's our function, to figure out what we value, and pursue it. Whether you agree with Penrose, or not, that that function derives from quantum mechanics, it actually relates to the argument that you were making earlier.

Your argument was that if I believe something that the consensus does not believe then it is a waste of time to spend ten years of my life on it. That is your opinion. My opinion is that those ten years were not at all wasted. While it has been painful, humiliating, and humbling at times, I got a Masters degree in physics, and a Masters degree in mathematics during that time. I became gainfully employed as an adjunct professor. I have learned a lot about physics, mathematics, logic, and emotions during that time. I've learned over the years that my opinions are not something I'm stuck with, but nobody can change them for me, either. I have to take personal responsibility for my opinions; and Penrose, pointing out that my opinions may be somehow quantum mechanical in nature--might not be terribly useful in neuroscience, but it has been helpful in my own personal psychology.

As an example, just six months ago, I realized that I was a victim of an opinion that I had--I completely lacked faith in other people. I had seen the evidence that they were continually disappointing me, never listening to me, and I had long accepted that they were going to continue to do so, and I might as well just accept it. But I decided that I was going to quantum-mechanically change that opinion, and believe that people are NOT going to disappoint me. It changes how I interact with them, and other people around me are starting to do things to impress me.

You could do it now, if you would just acknowledge my point, that Milne's model really is isotropic and nonhomogeneous. And then if someone went out and actually read "Relativity, Gravitation, and World Structure" and went out and edited the Wikipedia article so that it wasn't full of lies and nonsense about Milne's model being a zero mass version of the FLRW metric, but is, in fact, an isotropic, but nonhomogeneous distribution--an exploding sphere of matter--that would also really impress me. It would be like a "Quantum change" or something.

 

[JDoolin]

I followed Halton Arp for years, thinking intrinsic redshift was a characteristic of quasars and that quasars were "connected" to low-redshift galaxies. I even wanted to believe the Tifft quantizatiion of redshifts. After many years of trying I gave up and got on with my learning process. My lesson learned: do not let wishful thinking and emotion control my science.

Oh Jonathan Doolin, I watched and listened carefully to -1, -3, -4, and -5 above. -2, as you say, is not functional right now. Too bad, though, as the first time through those four I was confused. So, I watched and listened to all four a second time. Now I am a trained listener and a professional public speaker (Toastmasters International=28 years) but, unfortunately, I never could figure out what message you were trying to communicate in each of those four modules. Perhaps your written script could be revised to be more coherent and to clearly address the point you want to make. The point is I couldn't find the point. Do the titles "spoonfedrelativity" intimate that even babies should understand your explanations?

Since you seem to have some great passion for Milne's Cosmology, could you simply write up a clear, professional paper with all your work and ideas, and then send it to some publisher that would then submit your work to peer review. Would that not either prove or disprove your approach?

Cheers, Bobbywhy

Let's also acknowledge another point. If I'm right, it could be embarrassing and humiliating to a LOT of people. Pretty much everybody in the field of modern cosmology. Over the years, I realized that even if I'm right, it's not really that great news. In fact, if I'm right, cosmoloogy will become in some ways a lot more boring. No more searching for Dark Matter. No more great mysteries, about the fundamentals. You'd just be looking for details. Details, details, details. If I'm right, a good part of the sense of wonder about the universe will be gone.

I'm basically the idiot in the back of the room, when the teacher is doing the super-complicated equation and the only part of it that I understand is that 3+5=8 not 7. While everyone else is following along, figuring out all of the neat principles that are derived from assuming isotropy AND homogeneity, I just am too stubborn to go along with it.

When people finally notice that the idiot in the back of the room is actually right, that the teacher made a typo at the beginning, it really doesn't mean the idiot is brilliant. It just means that the idiot was right.

I really don't have a paper to write, and twofishquant is correct. I'm not a research scientist, and I certainly don't have anything *new* to say, based on new data. I don't read that much. And I don't know any of the unwritten rules of submitting an article. I just get stuck at the point where it stops making sense, and I can't read any further.

I could happily prepare a series of lectures, if I had someone who wanted to listen, but I really can't prepare a paper to send to a group of people who really really REALLY don't want to hear what I have to say.

 

[JDoolin]

http://arxiv.org/abs/1110.3054

And it was published in A&A

http://adsabs.harvard.edu/abs/2012A&A...537A..78B

One bit of constructive criticism is that you *really* need to learn how to do basic library research. Once you have that one paper, you'll find a cluster of people that are working on Milne-Dirac universes.

I got to that paper, by going to http://arxiv.org/ and typing in "Milne cosmology".

It's a very good example of how to present a "nutty theory" that passed peer review. The fact that you can get the right baryon-acoustic oscillations with Milne-Dirac is a non-obvious and interesting result. Also, even if it's wrong, it's interesting. The situation is that in order to have a Milne universe, you have to have no gravity. You can get no gravity by either having an empty universe or by having a universe that is 50% antimatter in which the antimatter counteracts the gravity of the matter.

Also, you have to read papers critically. The supernova data they cite is old and noisy, and more recent data move things away from the Milne baseline.



Again, figuring this stuff out is why you have to learn to do library research but...

That's what AEGIS is for. If we do the experiment and it turns out that anti-matter has gravitational repulsion, then people get free trips to Stockholm.

There is the "magic wand" or "tooth fairy rule" of cosmology papers. In any paper, you are allowed one free wave of the magic wand. The problem with the Milne-Dirac papers is that they wave the magic wand twice. Once assuming that antimatter is repulsive and the other that assumes that somehow there are vast quantities of hidden antimatter in the universe.

If AEGIS shows something unexpected, then you now have only one magic wand wave.

Thank you very much for the link. I have to admit I am confused about the Milne-Dirac model. But perhaps there is some way I could contact those researchers, with my questions. Typically of these articles, I find myself illiterate; unable to get past the first assertion; Equation (1), you see, simply looks wrong to me. It doesn't make any mathematical sense.

http://screencast.com/t/9WYFYl8F

 

[twofish-quant]

Here is my problem in arguing with you guys. The continually retreating premise. If I say that FLRW metric is based on the premise that Lorentz Transformation is invalid at large distances, you claim that that is not a premise. The premise is homogeneity and isotropy.

It's a premise because homogenity+isotropy+GR= FLRW.

But when Milne showed you that isotropy is possible without homogeneity, so long as you account for special relativity, then you change your story.

Who is this "you"? There are lots of people on this thread, and we don't all think the same.

I don't have any reason to dispute that isotropy is possible without homogenity, and that if you assume isotropy without homogenity, that you can get a metric that is different than FLRW. This is a well-defined mathematical question, and I have no reason to conclude that Milne got this wrong.

So what?

Now, that is impossible because it doesn't fit with the FLRW metric.

No. It's *possible*. It's just not what we observe. FLRW metric works because it's consistent with observations and Milne isn't.

Explaining how and why Einstein and Eddington are wrong--that isotropy is indeed possible without homogeneity. And the premise on which the FLRW metric is based, is fundamentally flawed.

I really don't see why this is relevant except as a historical footnote. Einstein and Eddington were wrong about a lot of things, so there's no reason why they should be right about this argument, but since I'm more interested in cosmology than science history, I don't see why this matters.

We *observe* that the universe is homogenous and isotropic, we then take these observations and create models that are consistent with these observations which gets us FLRW. It may indeed be *possible* for the universe to be non-isotropic. It's would also be *possible* for the Earth to have two moons or for me to be drinking diet Pepsi instead of Coke right now.

Lot's of things are *possible*. That's why you have to go out and see what is actually there.

If you would acknowledge that point, then you might begin to focus on my real argument instead of straw-men.

Ultimately, the only arguments that are going to convince me are "I took these measurements, and they fit the Milne model better than FLRW." Right now we are talking theory. If you assume X, Y, and Z, you must conclude A. If A isn't the case, then you look at where you messed up with X, Y, and Z.

As a *theoretical* point, the Milne model is inconsistent with any gravity model that approximates Newtonian physics. Since we *observe* that Newtonian physics works in most situations (and we can define the situations in which it doesn't), this poses a big problem.

The problem is that you are doing philosophy rather than physics. You are treating isotropy and homogenity as if they were mathematical axioms when they aren't.

With physics the only arguments that matter are those that are grounded in empirical observation. You can convince me that if we assume X, we get the Milne model. That's not hard. The problem is getting to observation.

We *observe* that the universe is homogenous and isotropy at large scales. That gets you to FLRW. If we observe something different, then we toss FLRW.

 

[twofish-quant]

Your argument was that if I believe something that the consensus does not believe then it is a waste of time to spend ten years of my life on it.

I'm not telling you how to live your life. You can live life the way that you want.

But for me, I want to do productive stuff. This means working on several things, so that if I "strike out" on one, I can get somewhere with another. It's not a matter of agreeing with consensus. It's a matter of agreeing with God. If God says that the universe does or does not work in a certain way, then no amount of effort is going to change that.

Also it's important to be *original*. I spent eight years trying to get supernova to explode with convection and came to the conclusion that it couldn't be done. This is not a waste because it was *original* in 1998. Now if someone does exactly the same thing that I did, it would be a waste of time, because it wouldn't be *new*.

The other fun stuff is to work on something for which there is no consensus. There is no consensus model on how supernova work, or for how black hole jets work, or for how galaxies formed.

My opinion is that those ten years were not at all wasted.

Again, this is a matter of personal philosophy, so if you don't agree with me then fine, but what knowledge does humanity have now that it didn't have ten years ago as a result of your efforts?

I like advancing the frontiers of knowledge. Sometimes it's something silly and trivial. You code a program one way, and you find that it doesn't work. That's "new knowledge". I can point to stuff that I know now that I didn't know a week ago, and that adds to humanities knowledge of the universe.

So when someone does something and at the end of it, humanity learns nothing, I think that's a waste. Again this is a personal view.

You could do it now, if you would just acknowledge my point, that Milne's model really is isotropic and nonhomogeneous.

I haven't worked through the math, so I don't know for sure, but I have no reason to doubt this. However, if that's true then you have a problem since the universe appears to be statistically homogenous. Also, if you are arguing that this was Milne's position, then I have no real reason to question that.

Milne's model being a zero mass version of the FLRW metric, but is, in fact, an isotropic, but nonhomogeneous distribution--an exploding sphere of matter--that would also really impress me.

Definitions shift. When cosmologists talk about the Milne model, they are referring an a zero mass version of FLRW. Now this may not be what Milne himself had in mind, but this is an issue of scientific history, which I'm not that interested in.

If you accept any sort of Newtonian-like universe, then Milne model=zero gravity.

 

[twofish-quant]

Let's also acknowledge another point. If I'm right, it could be embarrassing and humiliating to a LOT of people.

I don't think so. Guess what?

Scientists *love* it when the universe throws people a surprise. If it turns out that the universe isn't decelerating you are talking about enough papers and grant to last the next 20 years.

In fact, if I'm right, cosmoloogy will become in some ways a lot more boring.

You *kidding*? If Milne is right, that means that most of what we thought we known about gravity gets tossed in the trash. Milne in a non-empty universe is inconsistent with GR, which means that every thing involving gravity has to be rewritten.

And then there are the big open questions now. How did galaxies form? We toss out all of the old equations and rewrite everything. How does this effect black holes? Heck, do black holes even exist.

No more searching for Dark Matter.

Not true. If it turns out there is no cosmological dark matter, then we *still* have to look for galactic scale dark matter.

No more great mysteries, about the fundamentals. You'd just be looking for details. Details, details, details. If I'm right, a good part of the sense of wonder about the universe will be gone.

The wonder is all in the details.

While everyone else is following along, figuring out all of the neat principles that are derived from assuming isotropy AND homogeneity, I just am too stubborn to go along with it.

Look outside. If you can point to something out there and say, look this isn't homogenous, then people will listen. Also isotropic and non-homogenous universes have this weird fine tuning problem, which is if the universe is non-homogenous then why did God decide to put is right in the center of the non-homogenity.

If you have a non-homogenous universe, and you end up near the center of that universe, then you need to figure out how you ended up there. If it turns out that we are in the center of an inhomogenity to one part in 10^-3, then you can say it's random. If you look at the universe and it turns out that you are at coordinate 0.000000000000000 then you really have something weird happening.

Anytime you solve one mystery, you have a million new ones. If we establish that we live in the Milne universe, that's only the start of the mystery.

I could happily prepare a series of lectures, if I had someone who wanted to listen, but I really can't prepare a paper to send to a group of people who really really REALLY don't want to hear what I have to say.

It's a tough crowd, but part of being a physicist means learning to "enjoy the fight." It's like entering a boxing ring. If I enter the ring against a heavyweight champion, and he smiles at me, that's not "fun." I want them to try to knock me out. The first time you go into the ring, you'll probably get knocked out in ten seconds. Second time, maybe you can last for a minute. Maybe after a few years, you might actually win a round.

The most important part of getting a Ph.D. is the defense. That's when you present your results to your dissertation committee and they try to tear you to shreds. It's intellectual cage fighting, and they'll only hand you a sheet of paper when you can show that you can defend yourself in the arena.

 

[twofish-quant]

Typically of these articles, I find myself illiterate; unable to get past the first assertion; Equation (1), you see, simply looks wrong to me. It doesn't make any mathematical sense.

The article assumes that the reader knows general relativity, and equation (1) is the GR metric which corresponds to the Milne model of the universe.

If you look hard, I'm pretty sure that there is something out there that presents a simple introduction to GR.

Also, if you want to start thinking about homogenity

https://telescoper.wordpress.com/2011/06/22/cosmic-clumpiness-conundra/

Again, the assumptions of homogenity and isotropy are not mathematical axioms but rather working assumptions that seem to be accurate.

 

[JDoolin]

I haven't worked through the math, so I don't know for sure, but I have no reason to doubt this. However, if that's true then you have a problem since the universe appears to be statistically homogenous. Also, if you are arguing that this was Milne's position, then I have no real reason to question that.

Thank you. But actually, I encourage you to question that! Don't take my word for it; go and actually read "Relativity, Gravitation, and World Structure," if you can find the time. At least skim the chapter where he discusses his differences of opinion with Eddington, and the use of a-priori reasoning. Look at the big diagram at the front of the book. See if what I'm saying is true. I'd like to understand why people argue with me over this. The Milne model seems to be misrepresented everywhere I look, so unless you are looking at the same book I am, you actually DO have reason to question that I am telling you the truth.

 

[JDoolin]

The article assumes that the reader knows general relativity, and equation (1) is the GR metric which corresponds to the Milne model of the universe.

If you look hard, I'm pretty sure that there is something out there that presents a simple introduction to GR.

Also, if you want to start thinking about homogenity

https://telescoper.wordpress.com/2011/06/22/cosmic-clumpiness-conundra/

Again, the assumptions of homogenity and isotropy are not mathematical axioms but rather working assumptions that seem to be accurate.

The type of homogeneity assumed in the FLRW metric is an infinite homogeneous distribution of matter. Milne's model does not claim there should (or should not be) some kind of "cosmic clumpiness" on the small scale. Rather, it predicts increase in the density towards infinity at the extreme periphery of any observer's view.

There's two competing effects at the extreme periphery. You have higher density, but younger universe. The younger universe means we should have less galaxies. The higher density means we should have more galaxies. I would think we should expect to find, in this high density, low age region, a large number of supernovae, but very few fully formed galaxies. But at those distances, 10, 20, 30 billion light-years, we possibly can't see anything as dim as a galaxy anyway.

This is why I'm so suspicious when you say that Milne's model is being tested with the data. You need to be able to point at something that tells me clearly that there is no increase in density at the extreme periphery of the visible universe, and I have strong doubts that we have observations that are that good.

(This paragraph has been edited with Chronos' correction. Thanks, Chronos.) If I were to take a wild guess about what to expect from Milne's model, at the extreme periphery of the universe, I would expect there to be more Type II supernovae toward the edge of the universe, and fewer Type I supernovae. Based on my reading of an introductory astronomy book, it sounds like Type I supernovae come from white dwarfs collecting enough matter to reach critical mass for carbon-detonation, while Type II supernovae are the truly giant stars going supernova. Since the most distant regions are extremely young, the Type II supernovae would dominate, and the older Type I supernovae would be relatively less common.

Also from Milne's Model, there should be a directional difference in the distribution of galaxies. I would expect an asymmetry in the distribution of galaxies that fairly precisely matched the asymmetry in the dipole anisotropy of the CMBR.

Also from Milne's Model, if I am correct that there are secondary accelerations, there should be a local region where Hubble's constant is large and low variance, indicating projection from a more recent event, and a more distant region where Hubble's constant is small, and high variance, indicating an older part of the universe; a projection from a more ancient event.

Now, if none of these expectations are happening, then perhaps we can reject Milne's model based on comparing hypotheses to data. But as long as Milne's model is being rejected because it is a zero-mass model, then we're not rejecting Milne's model. We're rejecting a straw-man.

 

[Chronos]

I believe you had a dyslexic moment there, James. Type I supernova are classified as white dwarf detonations, Type II supernova are the ones resulting from core collapse of massive stars.

 

[JDoolin]

I believe you had a dyslexic moment there, James. Type I supernova are classified as white dwarf detonations, Type II supernova are the ones resulting from core collapse of massive stars.

I corrected it above. Thanks.

 

[JDoolin]

Now I am a trained listener and a professional public speaker (Toastmasters International=28 years) but, unfortunately, I never could figure out what message you were trying to communicate in each of those four modules. Perhaps your written script could be revised to be more coherent and to clearly address the point you want to make. The point is I couldn't find the point.

I have made three more modules. Hopefully you will find these more coherent.

http://screencast.com/t/npv2XLleRl7o. In this module, I discuss why is it that I am multiplying 1 mile per hour times the age of the universe.

http://screencast.com/t/Rie6CCJR8: In this module I discuss the reasons why a kinematic model of the universe has been rejected. From what I have seen, they are all based on straw-man arguments posing as ruductio-ad-absurdam.

If I'm right, http://screencast.com/t/d5rmfSFIr9: In this module, I discuss the particular reasonong which was addressed earlier in posts 45, 47, and 48, 53, 56, 54, in this thread. For your convenience, I am re-posting all the relevant parts of that argument below. You will notice that post 56 and 54 have been reversed, chronologically, in the tradition of the dialectic--in post 56 I had posted a hypothesis, that distant forces were observer dependent, and then later realized that twofishquant had already posted the contradiction to this hypothesis--that indeed the distant forces could, with bookkeeping, be found to be the same.

Do you have some other reasoning, perhaps based on an application of Gauss's Law? I'm asking that because I'm pretty sure that I've seen such an argument made by none other than Einstein himself. However, I don't remember where I saw it; some book I've long since returned to the library (in frustration).

http://www.ast.cam.ac.uk/~pettini/Physical%20Cosmology/lecture02.pdf

Ah, yes, thank you for that article. This was exactly the argument that I was thinking of. I've seen this argument in books, but I had never found it online. I was calling it "Gauss Law" but it is "Birkhoff's Theorem."

While I am essentially in agreement with Birkhoff's theorem, the article you reference is making a major error in its application, (and if I am not mistaken, Einstein made this same mistake, and was perhaps its originator.) If you are calculating the forces on particles A, B, C, and D, it is completely inappropriate for you to draw a circle around an arbitrary observer O, and then treat all of the mass in that circle as though it were a point mass at point O.

It would make much more sense to account for the masses near the objects A, B, C, D, respectively, to calculate the forces that are acting upon them.

(The other major error in the article is equation 2.3... Failure to apply time dilation and the relativity of simultaneity.)

Here's another way of thinking of it...

I have point A. You can argue that all of the forces are balanced at point A, so it doesn't accelerate.

I have point B. You can argue that all of the forces are balanced at point B, so it doesn't accelerate.

AHA! You say, the universe must then be non-accelerating!

But that doesn't work. If I start with point A as my origin, and then look at point B, I find that there is a force at point B pulling it toward point A. But wait, I just showed that the forces are balanced if I take point B as the origin? What gives?

The issue here is that the forces at point B when viewed from point A are *different* from the forces at point B when viewed from point B, because when you change coordinate systems then the forces change. But how can that be? Don't forces stay the same when you change between inertial coordinate systems?

Yes, but from point A's point of view, point B is not an inertial coordinate system, it's accelerating, and because it's accelerating, when you switch between point A and point B, the forces change. From point B's point of view, it's an inertial coordinate system, and A is accelerating. So when you switch between A and B, you have to change the forces to take into account the fact that the coordinate systems are non-inertial.

From A's point of view, there is a force on B pulling B toward A, and there is zero forces at A. Now when you switch to B's point of view, you are a non-inertial reference frame from A's point of view. To make it inertial, you have to subtract the forces that are acting on it. That causes the forces at B from B's point of view to go to zero, and then causes the A to go from zero force to the opposite of what was the force that A sees acting on B.

So if you take any point as the origin, you will see a force of zero for that point, but you will see non-zero forces for points other than the origin.

Now then you see how the universe works. We don't have any infinite clouds, but we have clouds that are "practically" infinite. You take something like the interstellar medium with a one light year cube, and then take a piece that is much, much smaller, and see how you calculate gravity.

Thank you for giving further explanation here.

I still think that ignoring the relativity of simultaneity is a flaw, but I also realize now that I was misunderstanding Einstein's argument.

He was literally saying that the force on a distant particle is an observer dependent quantity, while time is an observer independent quantity. That seems amazing to me, and I will have to think about it further.

And if you do your bookkeeping right, you come up with the same answer. The important thing is to keep track of what reference frame you are in so that you can account for non-inertial effects correctly.

I meant to ask, precisely what bookkeeping is done, so that you all get the same non-zero answer? I can agree that with correct bookkeeping, you should get the same answer, but by my calculation that answer should be zero. Because I don't believe that "correct bookkeeping" is represented by drawing a circle extending out to the radius of the object and stopping (as is done on page 2-3 here: http://www.ast.cam.ac.uk/~pettini/Physical%20Cosmology/lecture02.pdf ) . I think the circle needs to be drawn to encompass a large volume around the object. At least, make the circle large enough to encompass the masses that are in the object's immediate vicinity.

With this sort of bookkeeping, all observers would, indeed, agree that the force on the object would be the same. But the "same value" that everyone would come up with, would be zero.

 

[twofish-quant]

Thank you. But actually, I encourage you to question that! Don't take my word for it; go and actually read "Relativity, Gravitation, and World Structure," if you can find the time.

It's not so much a matter of reading books but rather doing the math.

Also, it *is* known that you can have homogenity rather than isotropy and isotropy without homogenity. Again, it's not a matter of reading books, but just thinking about the situation.

I'd like to understand why people argue with me over this.

I don't think that people are arguing with you over *this*. I think people are arguing with you over something else.

Mathematically, you can have an isotropic universe without homogenity, so what? Big deal.

The Milne model seems to be misrepresented everywhere I look

I don't think it's misrepresentation rather than "definition creep." When cosmologists talk about the Milne model they are talking about a situation in which galaxies move according to the velocities of Milne. Unless you are willing to throw away GR, the only way this can happen is if you have an empty universe.

It's not so much an effort to misrepresent Milne, but rather salvaging something.

 

[twofish-quant]

The type of homogeneity assumed in the FLRW metric is an infinite homogeneous distribution of matter.

Right. But for FLRW to work you just need things to be bigger than the cosmic horizon. If things cut off at 100 billion light years, you get the same results.

But at those distances, 10, 20, 30 billion light-years, we possibly can't see anything as dim as a galaxy anyway.

Yes we can. This is a very simple calculation. Take the absolute brightness of a galaxy. Take the detection limit of our current telescopes. It's a very easy calculation to show that the drop off in galaxies that we see is *real* and not an observational effect.

It's not even close.

You need to be able to point at something that tells me clearly that there is no increase in density at the extreme periphery of the visible universe, and I have strong doubts that we have observations that are that good.

Again, you need to be able to do library research. I've been more patient than most people at "spoon feeding" you data (since this may be of interest to someone else), but at some point you have to be able to do your own library research. (Start with google and early galaxies and dark ages).

And yes, our observations *are* that good. Again, detection limits is something that you can do research on your own. It's a very simple calculation that you can do, and I'll leave it to you as an exercise to figure out how to do it.

It's not even close.

Also from Milne's Model, there should be a directional difference in the distribution of galaxies. I would expect an asymmetry in the distribution of galaxies that fairly precisely matched the asymmetry in the dipole anisotropy of the CMBR.

We don't see this.

Now, if none of these expectations are happening, then perhaps we can reject Milne's model based on comparing hypotheses to data. But as long as Milne's model is being rejected because it is a zero-mass model, then we're not rejecting Milne's model. We're rejecting a straw-man.

No we aren't. For Milne to be consistent with GR you have to have zero mass. Therefore to accept Milne in the absence of matter, then you have to reject GR. Since we have good observational tests for GR, this is something that people aren't going to do lightly.

 

[twofish-quant]

I meant to ask, precisely what bookkeeping is done, so that you all get the same non-zero answer?

Once you have matter that is in an accelerating reference frame then the acceleration becomes a "fictious force".

I think the circle needs to be drawn to encompass a large volume around the object. At least, make the circle large enough to encompass the masses that are in the object's immediate vicinity.

If you have an closed sphere, then the masses that are outside the circle between the origin and the point are canceled out by the mass on the other size of the hollow region.

With this sort of bookkeeping, all observers would, indeed, agree that the force on the object would be the same. But the "same value" that everyone would come up with, would be zero.

Yup, but if you are in a non-interial reference frame then zero force would = acceleration.

 

[twofish-quant]

The younger universe means we should have less galaxies.

Once you get past z=8 or so, we see *NO* galaxies and *NO* stars.

 

[twofish-quant]

I think that you still don't "get it."

On the northwest corner of 1st Avenue and 14th Street in New York City, there is fast food place that sells hot dogs. Now someone argues that there is a French restaurant there.

You cannot by any purely mathematical or philosophical argument refute that position.

It is perfectly mathematically and philosophically possible for there to be a French restaurant at the corner of 1st ave and 14th street. There is no logical contradiction for there to be a French restaurant at the NW corner of 1st and 14th. If you ask me to prove through logical arguments that there isn't a French restaurant there, I can't.

But there isn't. You can go to that location, and see that it's a hot dog joint. If you can't get a plane ticket to NYC, you can go onto google maps, and see that there isn't one there.

Same goes with cosmology. I cannot by pure mathematics or logic show that Milne is wrong. I can just look a the sky and show that he is wrong about how the universe is set up, and most of those measurements were taken decades after Milne was around.

Also, the point of theory is to tell the observers what to look for. You are asserting (incorrectly) that we can't see distant galaxies because our telescopes aren't good enough. Now even if that were true, then the question should be "how good do our telescopes have to be?"

One of the points that I'm trying to make here is that cosmology is not philosophy. It's grounded in observations in much the same way that oceanography is.

 

[Chronos]

My apologies for referring to you as James, Jonathan. I perceive your error as one of philosophy, not science - e.g., I agree with twofish. Cosmology is a conjecture founded on observation. While I agree cosmology is still largely a matter of conjecture, it is a conjecture based on observational evidence. Once you leave the realm of observational evidence you enter the realm of metaphysics.

 

[SpiffyKavu]

Golly, this thread has gone off topic. I was going to say that the Fermi telescope has good things to say about molecular hydrogen (not always well traced by CO), but I have a feeling a new topic would be better...

 

[twofish-quant]

While I agree cosmology is still largely a matter of conjecture, it is a conjecture based on observational evidence. Once you leave the realm of observational evidence you enter the realm of metaphysics.

And this is a serious, serious philosophical problem when you deal with things like multiverses, pre-event zero, and the anthropic principle stuff.

However, once you get past the very, very early universe, you don't have to worry about these issues.

One problem with the way that cosmology is presented to the general public is that there is so much focus on the "this is *WEIRD* and *SPOOKY* stuff" that people aren't aware that most of cosmology isn't different from oceanography or planetary science, and observing the big bang isn't any different from observing the moon. We know the moon is there because we can see it. We know the big bang happened because we see that too. In some ways, we know more about the formation of the universe than we do about the formation of the moon.

 

[JDoolin]

My apologies for referring to you as James, Jonathan. I perceive your error as one of philosophy, not science - e.g., I agree with twofish. Cosmology is a conjecture founded on observation. While I agree cosmology is still largely a matter of conjecture, it is a conjecture based on observational evidence. Once you leave the realm of observational evidence you enter the realm of metaphysics.

There IS an error of philosophy here, but I don't think it is mine.

The problem is that you are doing philosophy rather than physics. You are treating isotropy and homogenity as if they were mathematical axioms when they aren't.

If homogeneity is NOT a mathematical axiom, then it should not be possible to make a mathematical argument with it as your premise.

Once you start with the premise that the universe is isotropic and homogenous, then at large distances things are going to be flying away from each other at > c, and Lorenz transformation will break down.

The issue here is that your cosmology professor actually IS using the idea of an infinite homogeneous universe as an axiom. He then uses that axiom to draw logical consequences. He then uses those logical consequences to throw out the idea of Special Relativity applied at large scales.

 

[George Jones]

I have closed this thread. There have been several pages of violations of Physics Forums Rules, which I explicitly posted earlier in this thread (post #26).