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A&C reference library

by Phobos
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marcus
#127
Jan27-12, 01:48 AM
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Yenchin came up with a possibly useful iconic picture of the Einstein field equation (basic GR equation)
http://www.zamandayolculuk.com/cetin...ity_worlds.jpg
marcus
#128
Jan28-12, 10:02 PM
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Emmy Noether's original paper, in English translation:

http://arxiv.org/abs/physics/0503066
http://arxiv.org/pdf/physics/0503066.pdf

Section 6 on page 12 discusses the fact that conservation of energy does not hold generally in the curved spacetime of GR---something pointed out by David Hilbert.
The paper first appeared in the Nachrichten der Königliche Gesellschaft der Wissenschaften zu Göttingen (1918)
marcus
#129
Feb29-12, 03:58 PM
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Standard Solar Model, thanks to Phyzguy:
http://www.ap.stmarys.ca/~guenther/e...n/ssm1998.html

Gives percentages of H, He, and heavier elements. Derived by a computer model of fusion burning in core starting with pre-star gascloud abundances. Provides basis for estimating the remaining lifetime of the sun.

=============

The estimated total entropy as of today of the universe within our cosmic event horizon might be of interest to someone.
http://arxiv.org/abs/0909.3983 This was published in Astrophysical Journal in 2010.
One of the co-authors is Charley Lineweaver.
Here's a conference presentation writeup based on it, some nice color visuals.
http://www.mso.anu.edu.au/~charley/p...ganParisv2.pdf
marcus
#130
May10-12, 10:44 PM
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When I type this into Google and press space, or equal sign, Google calculates the present critical energy density of the universe.
3c^2(71 km/s per Mpc)^2/(8pi*G)

that is because it can interpret "71 km/s per Mpc" which is the present value of the Hubble rate H
and because the formula for the critical energy density is
ρcrit=3c2H2/8πG

It gives the answer in PASCALS but a pascal is the same as a joule per cubic meter. the same unit works for both pressure and energy density.
So essentially it tells you the density in question is 0.85 nanojoule per cubic meter.
It's gratifying how much the Google calculator recognizes and is able to calculate.
It knows things like "mass of earth" "mass of electron" "radius of sun".
So you can put a term like "radius of earth" into a formula you want it to calculate, like type in
2pi*radius of earth
and you get a bit over 40,000 kilometers.
I guess you could say that the Google calculator is a library resource
marcus
#131
Jul22-12, 07:10 PM
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A nice interactive graphic illustrating the various size scales of the universe. I'm not sure if we posted it before. Indications are it's new this year, at least in this version, so probably not.
http://htwins.net/scale2/
New PF member Wakabaloola gave us the link.
marcus
#132
Aug11-12, 04:05 PM
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A nice pedagogical thing. Pairs of masses orbiting each other send out gravity waves--yes we know this. It was the basis of the 1993 Nobel prize to Hulse and Taylor who observed a binary pulsar. But how about an example close to home?

By orbiting the sun, the Earth radiates 196 Watts of power in gravity waves. the frequency of the wave is presumably one cycle per year, very low frequency

How do you know? Wikipedia has the formula for the radiated power. Type the following into google search window, which functions as a scientific calculator:
32/5*G^4/c^5*(mass of earth*mass of sun)^2(mass of earth+mass of sun)/(1 AU)^5

It knows what G is and what c is, and what the masses of earth and sun are. So you don't have to look any of that up. When you type that in, or simply paste it in, if you want, and press space or equal sign, it will say 196 watts.

thanks to Mfb for this idea.
marcus
#133
Sep7-12, 11:13 AM
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A new kind of cosmological calculator. Tabular output instead of oneshot. It reveals patterns: maxima, crossovers, relationships between columns in the table... So it is somewhat better for learning than a one-shot. Perhaps quite a bit better, I don't now yet. We need a place to keep "user manual" type information---what you can do with it. At least temporarily until there is a page somewhere online you can link to. For now here is the most user-manualish information I've seen:

Quote Quote by Jorrie View Post
The 'single-step problem' has been solved in CosmoLean_A17 and the 'few issues' with the flexible rounding of column data are gone as well, or so I hope. Please try it out and report any anomalies.

The most important differences are:
  1. The info-popups have been mostly reworded and include comments as received in PMs.
  2. The stretch range inputs arranged to be more consistent with the output table, from highest to lowest stretch.
  3. Some 'logic' built into the input processor so that 'one-shot' outputs are intuitively achieved by either making s_step zero, which gives output for s_upper only; or by making s_upper and s_lower equal to each other, irrespective of s_step.
  4. The number of decimals (rounding) of column data are adjustable individually. Becomes active on clicking Calculate and will remain so until changed again, reset clicked, or the page is refreshed.
  5. Overall accuracy has been improved by resolving some coding issues. It now seems to work accurately up to s =10 000.
  6. Some input validation and protection against crash of program are included. More to be considered.
  7. On the drawing board: "into the future" (s < 1).
marcus
#134
Sep30-12, 06:41 PM
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This by Ruth Durrer What do we really know about dark energy?
might be good to have handy as a reference.
http://arxiv.org/abs/1103.5331
It explains what is really being measured
azzkika
#135
Jan17-13, 07:24 AM
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Quote Quote by marcus View Post
This by Ruth Durrer What do we really know about dark energy?
might be good to have handy as a reference.
http://arxiv.org/abs/1103.5331
It explains what is really being measured
Sorry to be cynical, your posts are most informative to an amateur like me so many many thanks for the work you and others put in on this site, but doesn't this link just state that the observations don't fit the theory so we add in X to balance the equation and X is called dark energy. Essentially the universe doesn't behave as expected is my understanding. Maybe no dark energy is required just a better understanding of the universe.
marcus
#136
Jan17-13, 03:20 PM
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Quote Quote by azzkika View Post
Sorry to be cynical, your posts are most informative to an amateur like me so many many thanks for the work you and others put in on this site, but doesn't this link just state that the observations don't fit the theory so we add in X to balance the equation and X is called dark energy. Essentially the universe doesn't behave as expected is my understanding. Maybe no dark energy is required just a better understanding of the universe.
It's important to be skeptical. To have deep reserves of skepticism about manmade theories.
This could translate into a cynical attitude when one hears scientists who should know better gushing about "dark energy" to get media attention, or sell books, or get funding. But those are INDIVIDUALS.
I would not be cynical about the current cosmic models, they may actually be more solid and justified than you think and if they turn out wrong, well, that is not dishonorable! Theories are meant to be tested and eventually improved or discarded. I would rather be skeptical about the theories---no disrespect.

You might like this recorded interview with Carlo Rovelli:
http://edge.org/conversation/a-philosophy-of-physics

Rovelli is one of today's prominent physicists and is skeptical both of "dark energy" and of "string theory". But derogatory only of the HYPE. He has what I think is quite a reasonable attitude about the cosmological constant Lambda, which people CALL "dark energy" but there is no evidence that it is any kind of energy. It behaves, as far as we can tell, like a simple slight constant curvature. As such it has a natural place in the Einstein equation. For 90 years or so people knew this constant curvature naturally appeared in the GR equation but most people assumed the constant was zero.

The GR equation has room in it for TWO geometric constants, Newton's G and Lambda. Lambda is a curvature that for a long time we thought it was zero, and it happens not to be. That's all.
To see that viewpoint explained, google "rovelli prejudice". You get a paper by Bianchi and Rovelli called "Why all these prejudices against a constant".
Or go here: http://arxiv.org/abs/1002.3966 and click on PDF.

Since this is a bibliography thread, it is better to make a separate thread to discuss questions like this. If you want, start one. This thread is mainly for useful links to research papers, videos and other online resources.
julcab12
#137
Jan20-13, 08:08 AM
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Quote Quote by marcus View Post
http://edge.org/conversation/a-philosophy-of-physics

Since this is a bibliography thread, it is better to make a separate thread to discuss questions like this. If you want, start one. This thread is mainly for useful links to research papers, videos and other online resources.
(Disable lurker Mode). . Awesome article and thanks for sharing Marcus. I really like his approach and mentality towards science and "how it should be". Really change my view on what is "ugly" or how it's not needed.

"I think science is not about data; it's not about the empirical content, about our vision of the world. It's about overcoming our own ideas, and about going beyond common sense continuously. Science is a continuous challenge of common sense, and the core of science is not certainty, it's continuous uncertainty. I would even say the joy of taking what we think, being aware that in everything we think, there are probably still an enormous amount of prejudices and mistakes, and try to learn to look a little bit larger, knowing that there is always a larger point of view that we'll expect in the future."
marcus
#138
Jan21-13, 01:02 AM
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I'm glad you liked it!
I happened to see your response--was it to the article by Bianchi and Rovelli "Why all these prejudices against a constant?" or to the piece in Edge?--because I came here to post this page from Ned Wright's cosmology site:
http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html
He thinks light travel time should not be used in press releases as a measure of distance.
And he gives an argument in support, and proposes making more use of the redshift number.
Proper distance would be another good alternative---what you would measure by any ordinary means (e.g. radar) if you could somehow stop expansion long enough to measure.
marcus
#139
Feb13-13, 01:47 PM
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Mordred found a recent paper which gives a detailed treatment of the extent of the Habitable Zone under various conditions---different kinds of planetary atmospheres.
http://arxiv.org/abs/1301.6674
marcus
#140
Apr10-13, 11:08 AM
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Introductory cosmology video lectures on Pirsa.
The lecturer is a young guy named Matt Johnson. I watched Lecture 1 and got a good impression of him. He seems fast, alert to questions, and well-organized. It is blackboard rather than slides, which is normal for these Pirsa introductory lecture series. That doesn't slow him down because he writes quickly and legibly. Click on enlarge, to fill the screen.

These series normally run to something on the order of ten lectures. The first three have already been given and are online. Their URLs are:
http://pirsa.org/13040062/
http://pirsa.org/13040063/
http://pirsa.org/13040064/
marcus
#141
Jul3-14, 06:29 PM
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Good 43 minute video of probing interviews with Abhay Ashtekar and Ivan Agullo.
https://www.youtube.com/watch?v=IFcQuEw0oY8
Skydive Phil made it.
http://www.physicsforums.com/search....archid=4190288


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