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Astronomy, Cosmology, and physics of old

  1. Oct 6, 2003 #1

    Nereid

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    How do we know that the physics we can do in labs here on Earth today is the same physics in galaxies a long time ago and far, far away?

    Take a collection of your favourite theories - the gravity of Newton, the electromagnetism of Maxwell, the quantum world of Schrödinger and Heisenberg, Einstein's relativity, Fermi's "little one" (neutrino) and the bizzare zoo of sub-atomic particles - how do you know they work on the Moon? On Proxima Centauri (the nearest star - other than the Sun - that we know of)? In the Andromeda galaxy (M31)? How do you know they worked before Chicxulub? before the trilobites? before the solar system formed?

    I would like to look at some of these questions from the astronomical perspective - how do astronomical observations support the idea that physics hasn't changed over the last ~10 billion years, and that it works just the same in distant quasars as it does here on Earth. Of course, there are plenty of other experiments and observations that show the universality of physics - the ancient nuclear reactor at Oklo for example - and maybe someone else will start a thread in Geology on that.
     
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  3. Oct 6, 2003 #2

    marcus

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    my friend, who is a trilobite, assures me that the Standard Model was working quite well to explain most particle physics 500 million years ago, but the sea urchins were complaining that it was not mathematically rigorous

    seriously, this is a very beautiful idea for a thread
     
  4. Oct 6, 2003 #3

    marcus

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    Nereid I will say the obvious and hope other people fill in the unobvious details. We have seen a quasar with redshift 6.4. Which means weve seen the spectral lines of hydrogen, I guess (I'm not sure about this) stretched out in wavelength by a factor of 7.4.

    that light is probably 12 billion years old, it goes way back

    If the mix of colors from a hydrogen atom was the same 12 billion years ago as it is today, then that says an awful lot of physics was the same back then

    I will try to verify some things and edit this, I know it is pretty vague, but thats my initial take on it
     
  5. Oct 7, 2003 #4

    selfAdjoint

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    Re: Re: Astronomy, Cosmology, and physics of old

    Almost literally ROTFLOL! Terrific!
     
  6. Oct 7, 2003 #5

    marcus

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    Re: Re: Re: Astronomy, Cosmology, and physics of old

    I guess you must know my friend the trilobite.
    glad to be on the same board with ya selfAdjoint and
    pleased you enjoyed that

    you probably know a lot of avenues for infering laws of
    physics have been staying pretty much the same
    and would be good to mention some

    I guess the gist of it is that a lot of presentday observations are
    actually of past events and also of distant events and
    we have a pattern of assumptions that keeps on working

    maybe there is no qualitative difference attached to something
    being distant or from distant past, all our observations are in the present and the all involve this web of assumptions
    including the laws and the fundamental constants
    and this web of assumptions continues to be confirmed
    with relatively minor adjustments now and then

    Let me hand the mike to you, to wrap it up. Nereid asked so we
    need something conclusive to finish up on
     
  7. Oct 8, 2003 #6

    Ivan Seeking

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    Re: Re: Re: Re: Astronomy, Cosmology, and physics of old

    I have read arguments for the evidence that G, C and possibly some or all other constants have changed over time. I also understand that were these values not exactly the values that they are, all sorts of horrible things would happen.

    If the former assertion that the constants have changed were true, it would seem that the physics of old would have been very different indeed. Has the evidence for the drift in these "constant" values been discredited?
     
  8. Oct 8, 2003 #7

    Nereid

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    The Moon and the Sun

    Establishing that the physics observed in earthly labs is the same as that in the distant universe is mostly a series of small steps. In fact, the determination of distances is called 'the distance ladder'. It's not unlike how surveying used to be done - before satellites, GPS, and even ariel photography - with interlocking grids of control points and much attention paid to consistency and systematic error.

    Stars play a key part in linking the physics of the lab to distance quasars, and so no surprise that the Sun has been intensively studied.

    But first, a warm-up: Newton couldn't take his prism and thermometer to the Moon, but we can. Some simple physics experiments have been done on the Moon, but high energy accelerators and micro-kelvin cold labs haven't been sent up to test the Standard Model and observe the behaviour of BECs there. Yet how many research proposals are there for such tests? If we haven't observed the decay modes of the K mesons on the Moon, how do we know they're the same as on the Earth?

    The retroreflectors left on the Moon by the Apollo astronauts have enabled the distance between the Earth and Moon to be measured with great accuracy. The data fit the models of the Earth-Moon system very well. So at least gravity and the propogation of light seem to be the same.
     
  9. Oct 8, 2003 #8

    Nereid

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    Re: Re: Re: Re: Re: Astronomy, Cosmology, and physics of old

    In another PF thread (in Theory Development I think) there has been some discussion of observations that might indicate a change in alpha (the fine structure constant) over cosmological time. While it's still an open question - the observations aren't consistent and convincing enough yet - my money is on it being constant. I don't recall seeing any serious reports of changes in other constants (plenty of data showing they are, in fact, constant).

    Do you have any links you could share with us?
     
  10. Oct 8, 2003 #9

    Ivan Seeking

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    Re: Re: Re: Re: Re: Re: Astronomy, Cosmology, and physics of old

    This was discussed quite a bit some years ago. If no one chimes in I will try to dig up any information that I can find; this was not fringe but it may have been short lived. I don't recall hearing this issue addressed directly in recent years.
     
  11. Oct 8, 2003 #10

    Ivan Seeking

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    Oh what the heck. A quick google [the search engine, not the number] yields some immediate results on your fine structure argument. I don't see any immediate references to what I have read in the past...unless this is related somehow...

    http://news.bbc.co.uk/1/hi/sci/tech/1991223.stm

    http://www.lns.cornell.edu/spr/2003-03/msg0049268.html

    http://arxiv.org/ftp/physics/papers/0101/0101013.pdf

    http://www.don-lindsay-archive.org/creation/constant_evidence.html


    http://www.science.unsw.edu.au/news/news_detail2.asp?id=63
     
  12. Oct 8, 2003 #11

    Nereid

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    Thanks Ivan; the don lindsay one is worth a read; main conclusion is worth repeating:
    The rest refer only to alpha (the fine structure constant), and all observations of time variability originate from the work of Webb et al. There's independent work, done by Bahcall, which finds no variability.

    In another PF thread this same topic arose; rather than repeat the discussion here, please click on the link (the relevant items are about half-way down the page):
    https://www.physicsforums.com/showthread.php?s=&postid=74925#post74925
     
  13. Oct 8, 2003 #12

    Phobos

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    A good intro link...
    Also, as mentioned, we have been to the Moon a few times and conducted a few tests...the results of which were consistent with our Earthly understanding. We've also sent many-a-spaceprobe throughout the solar system and the only unexplained physical oddity is the slight deacceleration of the Ulysses spacecrafts at the outer reaches of the solar system (anyone know if this oddity was noted for the Voyagers?).

    My initial thoughts re: Newton's gravity & relativity is that, if they differed in the past or differed in other parts of the observable universe, then we would be able to see some indication of it when doing deep space astronomy (e.g., viewing distant galaxies, etc.). The universe would have unfolded differently. But this is just off the top of my head and I have not delved into the research on this.

    So, until we go to Galaxy X (etc.) and conduct in-situ tests, we're going to have to make-do with our available observations which seem to say that the laws of physics are the same everywhere/everywhen. Of course, we still have a lot to learn about the laws of physics (e.g., dark energy is a biggie).
     
  14. Oct 8, 2003 #13

    Nereid

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    Pioneer, Ulysses, Voyager

    It was the two Pioneer craft which are heading out on an epic journey, 10 and 11. The Voyagers don't show the effect, partly because modelling for all other possible contributing factors is extraordinarily difficult to impossible (e.g. changes in the spacecrafts' orientations). AFAIK Ulysses doesn't show the effect. BTW, the analyses done as part of the recent Cassini test of GR included tests for the Pioneer anomaly; no signal in the data, but couldn't be ruled out either.

    For the rest, be glad if you contribute as our virtual experiment continues.
     
  15. Oct 10, 2003 #14
    HI, new here! I'm so glad I found this site.
    This is a question I've Had for a long time. As far as I can tell, there isn't much difference between a variable c and the redshift of distant galaxies. If other constants change in such a way that the fine structure IS constant, then is it still possible to differentiate between variable c and cosmic expansion? How would anyone know the difference? What would be the impact on theories such as Inflation?

    Sorry if this sounds a little naive. This the first time I've dared to ask this to anyone who undestands physics.
    I wonder if I should have started a new thread?
     
    Last edited: Oct 11, 2003
  16. Oct 11, 2003 #15

    marcus

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    it seems to me there are lots of ways to go about answering, so you may get a bunch of different explanations why variable c and expansion differ in effects that could be observed

    one thought is, a quasar was observed with redshift 6.4 which means that the wavelengths were stretched out by a factor of 7.4 (you have to add one to get the factor)
    So, how could a changing speed of light make wavelengths 7 times longer and frequencies 7 times slower?

    another thought---c gets into lots of formulas for how things work like eee-equals-emceesquare----if c changed a lot then stars would make different amounts of energy by fusion and act different and also atoms, like the H atom, would make different colors of light. A TINY change in c could leave things still recognizable but a big change would have non-linear weird effects in many facets of life----so c is not just tied to redshift

    another thought----expansion (or contraction) of universe was predicted by a theory constructed to explain gravity, before the redshift was noticed.
    expansion was not invented by somebody to explain the redshift---so if you find an alternative explanation of the redshift you still have a model of how gravity works which predicts that the universe is either expanding or contracting
    so if you make an alternative explanation for redshift then you are stuck with having to invent a new model of gravity (a new version of General Relativity) in which it is possible for space not to expand but just stay the way it is----inventing such a theory might be hard as it would have to explain the bending of light as it passes the sun and all the other things measured by experiements done to check GR

    IMHO the simplest is probably to stick with GR and view redshift as one of several bits of evidence that it is right

    it is a good question and right on topic with Nereid's thread about whether and how basic physical law and constants have changed over the billions of years----different people could answer in a wide variety of ways to this question. Welcome to board!
     
  17. Oct 11, 2003 #16
    Thank you very much, Marcus. The more I read the other threads, the more simple and naive the idea seemed. But I ran out of my own steam and needed help. It just seems odd, weird even, that with every passing moment there is more & more nothing between everything.
    Could It be the universe is just filling up with all those pesky negative energy electrons? HA HA.. just kidding! Thanks Much!
     
  18. Oct 13, 2003 #17

    Nereid

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    The Sun

    Stars are perhaps the most important bridge between the physics we observe on the Earth, and the physics of the farthest reaches of the universe, so it's worth spending some time on them.

    The Sun is the nearest star. Stating the obvious, it generates an awful lot of energy. What powers it? Simplistically, the conversion of H to He (with C, N, and O as intermediaries) in its core. Since we haven't been there to see first hand, who do we know? All that we can see is the photosphere - the 'surface' of the Sun - and the transparent regions above it, such as the corona. What can we say about all the in-between bits?

    Grab big handfuls of physics textbooks - gas laws, gravity, nuclear physics, plasma physics, etc - put them into a mixer, pour into a computer, and let it crunch on ... in other words, build a model. As better results come from the lab, feed them into the model; as more accurate observations of the Sun are made, use them to refine the model; as computers get more powerful, include more effects and make the elements modelled smaller; and so on.

    While there are still plenty of mysteries - how, in detail, does the Sun's magnetic field and the sunspot cycle work? how, exactly, does the corona end up with a temperature in the millions, while the photosphere (from where the corona's energy comes) has a temperature only in the thousands? - the main details are well understood.

    One of the more recent advances is helioseismology, which is like using earthquakes on the Earth to tease out the details of the Earth's interior, only on the Sun. It's a fascinating story; maybe someone will start a thread on it? Among other things, helioseismology can show sunspots on the side of the Sun facing away from the Earth, as if the Sun were transparent. It also helped refine the elemental abundance gradients within the Sun, right down to near the core.

    It might be argued that with a very detailed computer model and lots and lots of inputs, it would be surprising if the results weren't close to what's observed; that the room for tweaking is so great that the model doesn't really say anything about physics inside the Sun.

    This is not the case. There are actually very few free parameters in a solar model - the mass, composition, age, angular momentum, ... and that's about it - the rest is just physics from the lab. Much more remarkable is just how many GB or TB of observational data can be explained with so little in the way of inputs.

    However, there was a heart of darkness for several decades - there were some critical observations which the solar models could not explain; as the observations got better, the anomaly got worse. What was the mystery? How was it resolved?
     
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