WMAP Results

Discussion in 'Physics & Math' started by chroot, Feb 25, 2003.

  1. zanket Human Valued Senior Member

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    3,777
    Ned Wright’s Cosmology FAQ says: “The Universe was not concentrated into a point at the time of the Big Bang. But the observable Universe was concentrated into a point.” There’s a nice graphic of that in the link.
     
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  3. zanket Human Valued Senior Member

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    This doesn’t resolve the paradox. Speed is distance traveled divided by the time of travel. Whether measured locally or from afar. All the observers in the paradox measure the same distance between A & B. You measure a different time of travel than A & B do. Hence, light does not appear to travel at the same speed, c, to all observers. I think you would need to show that one of my boldfaced statements is false.
     
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  5. GundamWing Registered Senior Member

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    Does no one see my last post?

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  7. zanket Human Valued Senior Member

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    Re: Your geydinkin experiment makes no sense

    You didn’t believe the stuff I posted in the other thread about gravitational time dilation? Here’s another link about it. Note within: “This [time dilation] is distinct from the time dilation from relative motion.”
     
  8. GundamWing Registered Senior Member

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    367
    Re: Re: Your geydinkin experiment makes no sense

    What you are then IMPLYING is that there is a differential in the gravitational field (i.e., the guy on the building is 'not' accelerating at the same rate as the guys on the ground). This again, means there IS relative motion!. You can't have people accelerating in two different ways, and have them be moving 'stationary' with respect to one another.

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    If there isn't a sufficient difference in their acceleration, then you wouldn't see any relativistic effects anyway. If you saw a sufficient difference, the building would probably become unstable and collapse.

    IF you are saying there is NO differential, then please go back a bit and read my last post again.
     
    Last edited: Feb 26, 2003
  9. chroot Crackpot killer Registered Senior Member

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    Great... my "real science" thread has been hijacked into another "relativity is false" bullshit thread. James, we need another forum for this bullshit.

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    I implicitly meant the observable universe.

    - Warren
     
  10. chroot Crackpot killer Registered Senior Member

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    Re: Re: Re: Your geydinkin experiment makes no sense

    Dude, gravitational time dilation exists. Time runs slower deeper in the gravitational well. Calm down. James is right -- it's just the failure of simultaneity, again.

    - Warren
     
  11. GundamWing Registered Senior Member

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    367
    Re: Re: Re: Re: Your geydinkin experiment makes no sense

    I know that warren -- my point is that even if gravitational time dilation was acting on them -- you wouldn't experience a '50% difference' in clock rates unless there is a sharp difference in the acceleration experienced between the two positions. You only need to discuss simultaneity assuming that you have this huge building that lives in this huge gradient (to get a 50% difference in clock rates... time dilation, on earth, for example, is 1 part in 10^9, according to your own link zanket).

    Anyway, from your last post I see now that you're trying to put this whole thing in a highly non-homogeneous gravitational field (which you failed to mention in the original problem), which then leads back to JamesR answer. :bugeye:
     
    Last edited: Feb 26, 2003
  12. zanket Human Valued Senior Member

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    3,777
    The guy at the top of the building measures the signal traverse one light-microsecond in two microseconds, or 0.5c. Are you saying such a measurement is invalid? “Failure of simultaneity” isn’t cutting it for me.

    Regarding my suggestion about dark energy, you said:

    The answer to this hypothesis is an uneqivocal no simply because gravity is far too weak.

    and I said:

    Can you elaborate? It seems to me gravity can be arbitrarily strong. For example, if the light from a distant galaxy skirted a black hole, the light could be accelerated to an arbitrarily large velocity. Again, you would measure the light’s arrival velocity at c, but the acceleration of the light relatively quickened the light’s source clock to an arbitrarily fast rate.

    Until the paradox is resolved, I’ll rephrase as:

    Can you elaborate? It seems to me gravity can be arbitrarily strong. For example, if the light from a distant galaxy slingshotted around a black hole, the light could be blueshifted to an arbitrarily large degree, quickening the light’s source clock to an arbitrarily fast rate compared to our clocks.

    Which means the space from the region represented by the light should be expanding faster locally from our perspective than Hubble’s constant predicts. Or so it seems to me.

    Does this imply that the value of the Hubble constant varies amongst observers with different clock rates?
     
  13. zanket Human Valued Senior Member

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    3,777
    Re: Your geydinkin experiment makes no sense

    The difference in clock rates was a given, and OK in principle.

    Then please respond to my rebuttal to James R.
     
  14. GundamWing Registered Senior Member

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    367
    Re: Re: Your geydinkin experiment makes no sense

    Your rebuttal related to the fact that speed is distance / time.

    See: http://phyun5.ucr.edu/~wudka/Physics7/Notes_www/node93.html

    It curves both space and time, you assumed it only curves time. :m:
     
  15. zanket Human Valued Senior Member

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    3,777
    Re: Your geydinkin experiment makes no sense

    That doesn’t resolve the paradox. Now if you added to that something like “If you had assumed it curves both, then I will see B’s watch stop at one microsecond as B expects, because (insert logical explanation here),” that might be a resolution. A comment not applied to the paradox isn’t.
     
  16. island Registered Member

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    zanket wrote:
    <i>"Now if you added to that something like “If you had assumed it curves both, then I will see B’s watch stop at one microsecond as B expects, because... "</i>

    ...length at the lower gravitational potential is shorter, and so the guy at the top of the building measures the signal traverse one light-microsecond in one microsecond.

    All of the effects follow naturally from curvature.
     
  17. zanket Human Valued Senior Member

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    3,777
    Now that’s a solid answer! But it doesn’t resolve the paradox, because gravitational length contraction applies only radially, not along the circumference (ground). So all the observers in the paradox measure the same distance between A & B. Kip Thorne puts it this way in Black Holes and Time Warps: “A [ruler], when oriented in a circumferential direction, measures precisely the lengths of true, flat spacetime. However, when oriented radially, it shrinks by an amount that is greater the nearer it is to the [black] hole, and therefore it reports radial lengths that are larger than the true ones.”

    You’ll find sites with the opposite information, like this one, but they are attempting to explain gravitational length contraction using a rotating wheel, which contracts the circumference rather than the radius (misleading!). Gravity causes no length contraction along the circumference of a mass, because gravity does not act in that direction.
     
    Last edited: Feb 26, 2003
  18. chroot Crackpot killer Registered Senior Member

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    2,350
    The problem with your electric-chair apparatus, zanket, is that

    the observer on the roof is not directly measuring the light going between observers A & B

    When a clock moves at a high relative velocity, it appears to slow down. This means its internal workings -- electromagnetics included -- appear to slow as well. Light bouncing inside the clock appears to run slow to the distant observer, as inferred by ticks or clicks or whatever. But this light bouncing around inside the clock never gets to the distant observer! If the clock were to send a pulse of light to the distant observer, the distant observer would measure its speed as c, as it goes through his apparatus.

    You can't directly measure the speed of light that doesn't go through your apparatus.

    In other words, the constancy of the speed of light is a local phenomenon -- something that you quite clearly should have known.

    - Warren
     
    Last edited: Feb 26, 2003
  19. wesmorris Nerd Overlord - we(s):1 of N Valued Senior Member

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    9,844
    That's extremely interesting, that's for the info. Hmm. So really there IS an absolute "rest" reference frame, even though it's always changing?

    Oh, and I'd swear I read in SCIAM a few months ago (maybe last summer) that it had been determined that the expansion of the universe is accelerating. Was that speculative? I thought it had been resolved... the big freeze wins.

    If it's true that the expansion is accelerating, would it effect the accuracy of the measurement regarding the age of the universe?
     
  20. chroot Crackpot killer Registered Senior Member

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    2,350
    Well, it's not "absolute." It's just the rest frame of the Hubble flow. As far as physics experiments are concerned, any other reference frame is just as good. The CMBR rest frame is unique, though, in that the universe gave it to us.
    It is largely accepted that the universe is accelerating, but a non-accelerating universe has not been completely ruled out by the data. There are still some allowed parts of parameter space that describe universes that do not accelerate -- though it is true that most of the evidence is leaning toward acceleration.
    Nope.

    - Warren
     
  21. synergy Registered Senior Member

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    Actually, the rest frame of the Hubble flow may be unique enough so that some physics experiments would show different results. Consider the philosophical idea (I believe developed by Ernst Mach or somebody else who knew Einstein) that gravity is not an attraction to a body but rather a shielding (by that body) of the repelling force of everything else in the universe (my own interpretation of his words). Let me know if this makes you laugh.
     
  22. chroot Crackpot killer Registered Senior Member

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    2,350
    Well, certainly -- the WMAP satellite is one such physics experiment.

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    - Warren
     
  23. synergy Registered Senior Member

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    143
    No, I see now I was wrong...
    If we are living in the 3D "skin" of a 4D "sphere" then no one place is central, so the "repelling force" would not, as I visualized, reduce anything at the "center" into a black hole, because there is no such "center".
     

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