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Can we measure absolute velocity?

  1. Feb 19, 2004 #1


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    Is there a method of determining exactly how fast the earth is moving through space? By this I mean the overall velocity with the galaxy/galactic group.
    I know we can measure changes in the parallax motions of the stars, but can we measure our absolute velocity without looking outside the earths sphere?

  2. jcsd
  3. Feb 19, 2004 #2


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    We can measure our speed with respect to the cosmological background radiation by adjusting for redshift/blueshift, but we can't really be sure thats an "absolute" reference frame. According to Einstein, such a concept is unnecessary.
  4. Feb 20, 2004 #3


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    No, we can not measure "absolute velocity" for the very good reason that there is no such thing as "absolute velocity"!
  5. Feb 20, 2004 #4


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    What about the universe expansion rate? Wouldn't its instantaneous velocity be absolute?
  6. Feb 20, 2004 #5


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    you suggest looking at the "instantaneous velocity" of expansion
    but the distant galaxies are all receding at different rates depending on how far away they are
    the more distant they are the faster they recede away from us.

    so this is not a good way to get an absolute standard of speed
    (recession speeds being all different)
    but we dont need another absolute speed standard besides the one we alreay have----we already got c.

    from your question I can filter out two questions actually
    A. is there an absolute standard of speed? (answer is Yes)
    B. is there a special distinguished frame of reference
    that can be used to measure speeds?
    an intrinsic idea of being "at rest" in the universe?

    cosmologists often use the expansion of the universe ("Hubble flow") to define an absolute rest frame

    this corresponds to the one russ_watters mentioned: the CMB frame

    the solar system is said to be moving 370 km/s in the direction of the constellation Leo with respect to the Microwave Background and that is the same as saying that it is moving 370 km/s in the direction of Leo with respect to the Hubble flow.

    I can get some links, the speed was measured by COBE satellite and before that by a Lawrence Lab U-2 airplane observatory.

    This is the 1996 report of the COBE project
    that determined our speed to be 370 km/s
    http://arxiv.org/astro-ph/9601151 [Broken]

    This shows a starmap with the hotspot in Leo made by the U-2 airplane:

    If you are at rest with respect to Background that means that there is no doppler hotspot in one part of the sky caused by your moving towards it and no doppler coldspot the other way caused by your moving away from it-----in all directions the Background looks the same wavelength mix, the same temperature.

    if you are at rest with respect to the Hubble flow that means in all directions you see the distant galaxies are receding at the same rates (which are greater the more distant they are)
    so the expansion around you looks uniform or symmetrical or as they say "isotropic". If there are people living in other galaxies then they also have access to this simple universal idea of being at rest

    and being at rest with respect to expansion process (Hubble flow) is the same as being at rest with respect to Backgound.

    it is a very useful reference frame for cosmologists so in their papers you will see velocities given sometimes relative to this frame

    I will try to find a link to illustrate.

    this says our local group of galaxies is moving 627 km/s
    with respect to Background (CMB)
    Last edited by a moderator: May 1, 2017
  7. Feb 22, 2004 #6


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    Thanks marcus. I didn't realize
  8. Feb 22, 2004 #7


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    lots of wonderful things in store then

    BTW did you look at this?

    it shows the hotspot in the cosmic microwave background
    against the star-map (to give landmarks)

    if you can locate Orion in the sky tonight then you
    can see the direction of our absolute motion

    face south
    around 8PM Orion should be overhead (on meridian)
    and to the left of Orion , more to the east, is the
    constellation Leo where the doppler hotspot is.

    later in the evening Orion should be moving to the right,
    more to the west, and Leo should rise some and be
    more overhead

    by midnight Leo should be on the meridian (the dividing
    centerline between east sky and west sky)

    I'm assuming you are in northern hemisph otherwise right and
    left are mixed up

    the cold spot in the microwave background
    (cold because we are fleeing from it and the freqs get
    doppler downshifted) is in constellation Aquarius

    wont be seeing that until summer or fall

    catch Leo while you can
  9. Feb 22, 2004 #8


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    Don, of course youre in N. hemisphere, Kansas I see!

    Well the big story in astronomy in the past 10 years is
    they finally measured the Hubble parameter
    Wendy Freedman's team did it with the Hubble Space Telescope

    all the other stuff you hear about (dark energy, accelerating expansion, and so on) mostly depends on inferences and
    knowing the Hubble parameter (the main gauge of expansion rate)
    is basic to all that inference.

    so when you hear people talking excitedly about this and that, the reason they can confidently make all those inferences is because
    H is now between tight error bounds

    In 1998 the "Hubble Key Project" team reported that it was
    71 plus or minus 3 or something like that
    between 68 and 74, if I remember, at some level of confidence.

    So you better begin by understanding what that 71 means

    In the clunky-sounding units astronomer are unfortunate enough to use, it means

    71 kilometers per second per Megaparsec

    this is something to get straight---a megaparsec is a tom-fool unit of distance which is about 3 million lightyears

    this says that (except for random movements and accidental noise like that)
    if you go out one Megaparsec you see stuff receding at 71 km/s

    and if you go out two Megaparsecs you see stuff receding at 142 km/s

    and if you go out three M's you see stuff receding at 213 km/s

    that is, on average, and discounting our own motion towards Leo.

    if you subtract off our own motion in the Leo direction of about 370 km/s

    then you get a picture that is fairly symmetric that all around us things are receding at speeds proportional to their distance by this proportion they call H.

    good luck understanding the universe, heh heh, nobody every has:wink:
  10. Feb 22, 2004 #9


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    Yes I did marcus. There's so much to learn. Mostly I've only been looking at the "small" of physics. But I do look at the sky in wonder.

    Orion is easily visible this time of the year. I'll look for Leo as you susgest. Cool stuf.

    I'll keep reading your links. Thank you.

  11. Feb 22, 2004 #10
    Can we construct a physical space similar to ours in which one may measure absolute velocities exactly (like we now measure the speed of light)? In an exponentially inflationary universe, does even acceleration appear absolute?
  12. Mar 2, 2004 #11


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    I got a message today from Ian

    "Have you any idea if this is the direction of motion of our galactic group or just the direction of the Sun."

    this is something that Warren (chroot) knows about in some detail
    and I hope he sees this and responds
    in addition to my response

    there are several absolute motions that astronomers have determined fairly accurately

    there is our galaxy's going something about 600 km/s in the direction of Crater (I am being imprecise)

    but it is a bit like a spinning "Frisbie" sailing along and turning
    at the same time
    so a point on the frisbie, like the sun,
    if the spin was carrying it backwards
    could be going not so fast

    and in fact the sun is on the side of the galaxy where the spin is carrying it backwards and it is going only 370 km/s

    and also in a slightly different direction (coordinates in Leo, not Crater) which happens because of the galaxy being tilted with respect to its direction of motion and the sun's orbit around Galactic center not being exactly planar and nagging trivia like that.

    the basic picture, neglecting minor details
    is that the sun's speed IN THE GALAXY is about 250 km/s in a direction that happens currently to be roughly BACKWARDS from
    where the galaxy is going.

    and the absolute sun speed is 370 km/s

    because it has that 250 km/s knocked off

    so if the sun wasnt going backwards it would be traveling
    620 km/s towards crater, along with the center of the galaxy and
    the average motion of the galaxy. (the 620 = 250 + 370 is
    only approximate)

    now about the LOCAL GROUP there was that article that Warren found where they give the speed and exact coordinates of the absolute motion of the local group

    And as I recall it was not very different from what Milkyway is doing on its own. Milkyway is very massive----I think even more than Andromeda which is the other biggie in our group.

    so whatever Milkyway is doing is roughly typical of the local group.
    I think the group is also going to a direction in Crater but maybe just not exactly the same numerical coordinates.

    the coordinates are impossible to remember

    as in daily life, there are unlimited ways of defining collectivities, your choir your church your town your county your territory your continent your planet

    so it is always an openended vague game you start playing when you say where are "we" going

    so there is also the Virgo Cluster
    with which our local group is tenuously connected and which
    I am told is heading towards a vague big patch of sky called
    "Hydra-Centaurus" where something with a hokey name of "the great attractor" lives

    well for a start I would not bother with all that because
    saying Crater is almost the same but more precise
    Hydra and Centaurus are huge constellations spread all over the southern sky map

    so it is very hand-wavy to say HydraCentaurus as a direction

    and it is very vague to talk about where the Virgo Cluster and its hangers-on are going because IMO it cannot be determined very well in the first place

    so in the infinite tower of clustering I tend to stop with the local group and not worry about larger groupings


    so suppose you went out tonight and wanted to see where our galaxy or (what is almost the same thing) our local group
    is going

    you should probably spot it by looking for a diamond-shape
    rhombus of stars called Corvus

    Corvus is in the southern sky and it is a close neighbor of Crater
    so since Crater is dim
    it is a good thing to look for this Corvus diamond of 4 stars with one in the middle

    Corvus is like an oldfashioned crossed-sticks-and-paper kite that kids used to fly.
    Last edited: Mar 2, 2004
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