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Is zero velocity measurable?

  1. Jan 11, 2010 #1
    Relative to our sun, it's possible to calculate the speed at which the Earth is moving through space in it's orbit. So it's possible for an astronaut to position himself in a fixed position just slightly outside the Earths orbital path and observe the Earth passing by at the velocity the mathmatics say it should be traveling. This speed obviously changes depending on what point Earth is in it's eliptical orbit around the sun since it speeds up and slows down during each pass around the sun.

    Our sun being part of the milkyway is "swirling" around with all the other stars in our galaxy. So relative to the black hole that is theorized to be in the center of our galaxy, it's possible for an astronaut to position himself at a fixed point relative to the center of our galaxy where he could observe the sun passing by as it orbits this black hole.

    Since the universe is expanding and galaxies are moving away from each other, it should also be possible for an astronaut to position himself at a fixed point in space where he could observe the milkyway galaxy passing by. However, in my two examples above, this "fixed" point is always relative to another existing object.

    So I'm questioning the possiblity to measure TRUE zero velocity. If an astronaut is floating in a "fixed" position in space, is there any mathmatics that can prove he is indeed staying in one spot? What would be the determining factor of true stillness in space if there is no known point that can be used as a reference?
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  3. Jan 11, 2010 #2


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    There is no such "fixed" position in space. Position is always relative to something. Einstein's SR proved that we don't need to have this "aether" that everything is relative to.
  4. Jan 11, 2010 #3
    If the Big Bang is true, then there is an originating point that everything is expanding from. When the age of the universe was calculated, the mathmatics of galaxies moving away from this point was reversed. So if a point has been calculated that was the origin of the Big Bang, then wouldn't that be the reference point to determine a fixed point in space?

    What has me questioning the measureability of absolute stillness in space relative to the origin of the Big Bang is the fact that everything in the universe is expanding... including space itself as well as time.

    If space and time is expanding, even with the center of the univers as a relative point, can a fixed point be mathmatically explained?
  5. Jan 11, 2010 #4


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    The big bang didn't occur at any one point in space, it WAS space...expanding!
  6. Jan 11, 2010 #5


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    Precisely. The scientific theory we call (through misguided historical accident!) The Big Bang, does not imply that the Universe began from a single point. What we do know with a high degree of certainty is that the early universe was very hot, dense and very uniform in density from place to place. There was not a empty region outside of some initial fireball that flung the universe out into the void. Every part of the early universe was in the same hot, dense state. When we say the 'universe is expanding' it means if you take any given region, then everything within that is getting further apart from everything else.

    This may sound strange, and raises the obvious question of whether the Universe must be infinitely big. The short answer is that we don't know. What we do know is that due to the finite speed of light, there is a finite region of the Universe that we can see, given the finite age of the Universe. Within this region, the Universe is homogenous (roughly the same everywhere) and certainly the early universe (what we call the Big Bang) was very homogenous within this entire observable region.

    Now, as Matterwave said, all motion is relative. There is no such thing as absolute motion and indeed this is a very important concept underpinning relativity. That being said, in practice when doing cosmology it is very convenient to define what we call 'co-moving' co-ordinates. In these co-ordinates, anything which is simply moving with the general expansion of the Universe is said to be 'at rest' and we define velocities from that rest point. It turns out to be a very usefull way of doing things. It also has a physical basis. The 'afterglow' of the Big Bang, the Cosmic Microwave Background Radiation (CMB) fills the whole universe (and gives us a lot of the information that we know about the Big Bang). For objects at rest with respect to the general expansion, the CMB will look the same in all directions. For anyone moving with respect to it, one side will be a bit redshifted and the other a bit blueshifted due to the Doppler effect.

    Measuring our own velocity with respect to the CMB was an important early step on the path to making precision measurements of the CMB.
  7. Jan 11, 2010 #6
    When you impart velocity to an object it gains kinetic energy. Now forgive me if relativity has some say in this, but couldn't you find out the absolute velocity of an object by measuring its energy?

    If we know how much energy it takes a hydrogen atom to exist (thus at zero velocity), then by measuring the energy of the particle we could work backwards to find how fast it really is travelling, regardless of local space.
  8. Jan 11, 2010 #7


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    Kinetic energy is just as relative as velocity. So when you measure an objects kinetic energy you are measuring it with respect to you. If you measure the kinetic energy of a ball sitting next to you in a car driving down the road, you will measure it as 0. Someone sttting at thr road side will measure it as some non-zero value.
  9. Jan 11, 2010 #8
    Doesn't work since it turns out that energy is as relative as velocity.

    Doesn't work. If you are travelling along with the hydrogen atom, you end up with a different energy than if you travel at a different speed with the hydrogen atom.
  10. Jan 11, 2010 #9


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    Kinetic energy is irrelevant. It only matter between you and the object you collide with.
  11. Jan 12, 2010 #10
    So if you destroyed the atom with an anti-hydrogen atom you would only get as much energy back as its mass + local velocity, rather than mass + absolute velocity?

    If so where does the rest of the energy go?
  12. Jan 12, 2010 #11
    Not local velocity, but relative velocity between the two.
  13. Jan 12, 2010 #12


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    What do you mean by "local velocity"? Relative velocity? And, again, there is no such thing as "absolute velocity". And there is no such thing as the "rest of the energy". As everyone here has told you repeatedly, energy is relative to the frame of reference- the amount of "energy" in a moving object depends upon the frame in which it is measured.
  14. Jan 12, 2010 #13
    Yes as espen has put it in better words, local velocity meaning relative velocity.

    Energy being relativistic, is there then anything in the universe that is absolute?
  15. Jan 12, 2010 #14


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    Yes, there are absolutes (a more technical phrase might be 'invariant quantity'). The important revolution that came with GR was the understanding that we need to consider a 4 dimensional 'space-time' when doing calculations, rather than 3 spatial dimensions that could be de-coupled from time.

    This means that any 3 dimensional quantity will not be absolute, or in other words, different observers will disagree about the size of something. The normal velocity is a three dimensional quantity; how fast is my spatial location changing with time? Since different observers disagree about distances and times, they will disagree about speeds.

    On the other hand there are 4 dimensional invariant quantities that everyone does agree on. There is for instance, the length of the '4 velocity' vector, defined as (dt/d\tau, dx/d\tau, dy/d\tau, dz/\dtau) where \tau is the time experienced by the observer and t is the time as the observer sees for the object being observed. This might be a bit confusing, so let me give an example.

    Say the observer and the object have no relative velocity. In the case dx/d\tau etc are all zero and dt/d\tau = 1; the times of the two things agree as they are at rest with respect to each other. If the object started moving with respect to the observer, then we would have for instance dx/d\tau > 0. However, we also know that objects with relative motion have a time dilation between them, such that dt/d\tau < 1. If you worked through the full details you'd see that this means the total length of the 4 velocity is preserved in the sense that someone at rest with respect to the object and someone moving with respect to it would calculate this thing to have the same length.

    There is also the 4 momentum which is also an invariant. You can also have the 4 acceleration etc.
  16. Apr 23, 2010 #15
    Can't you be 'fixed' and 'relative' at the same time to something/anything at any give moment in time? Just because everything in the Universe is moving doesn't mean there isn't a fixed co-ordinate. There may be nothing in it or there may be something in it for an instant. It would, in fact, be a three dimensional, physical co-ordinate, with an additional co-ordinate of time. That would be your 'space-time' co-ordinate.
  17. Apr 24, 2010 #16
    Sure, we can define a coordinate system as one where some reference object is moving at such-and-such a nonzero velocity. Is that what you mean? It's no different in principle from defining a coordinate system in which a reference object has zero velocity. Some coordinate systems are more convenient than others. But deciding between them is just that: a matter of convenience. There are infinitely many possible coordinate systems we could pick. As far as the laws of physics go, the choice of which velocity to call zero is as arbitrary as the choice of which direction to call up.
  18. Apr 24, 2010 #17
    I hope I don't throw of your question in a direction you're not interested in;
    I apologize if so:

    I just wanted to comment on the (possibly side-) issue of differentiating
    between signal and noise when the values you are trying to measure are very
    small: how do you tell a very small value appart from plain noise.?
  19. Apr 26, 2010 #18
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