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Movement through time?

  1. Apr 5, 2004 #1
    Our earth spins on its axis at 1600 km/hr, and then travels around the sun at some higher speed (i don't remember what) and then our galaxy spins on an axis at a higher speed, and it in turn revolves around some Great Attractor at the speed of c.
    So IOW are we traveling at a net speed of c? If so, are we in some sort of free fall effect? :confused:
  2. jcsd
  3. Apr 5, 2004 #2


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    No, we are not travelling at c.

    I should point out that, in general, you cannot define a velocity (e.g. "I'm travelling at 0.2c") without providing a point of reference (e.g. "relative to the Earth").

    The only speed in the universe that you can specify without needing to provide a point of reference is the speed of light. Why? Because the speed of light is measured the same by all observers; this is the central postulate of relativity theory. Light, travelling at c, appears to travel at c for all observers, no matter how they're moving.

    However, no massive body can travel at c, so you cannot specify the Earth's velocity without measuring it with respect to something else in the unvierse. The fact that the Earth appears to moving at a different speed to someone in a space shuttle versus someone on Mars versus someone in the Andromeda Galaxy means the Earth cannot be travelling at c.

    - Warren
  4. Apr 6, 2004 #3
    The question is always "velocity with respect to what" In the case of the earth, the sun, the milky way etc - there is a composite motion wrt to something. Ether types have adopted the CBR is a universal reference frame since we can measure a net Doppler shift with respect thereto.

    There is also the notion that all frames are in reality traveling at c - that is when you consider an event with a spatiotemporal beginning and ending and add the two vectorially (temporal distance squared combined with the spatial distance squared) - you will get the spacetime velocity (the interval squared) which is constant in all non accelerating reference frames
  5. Apr 7, 2004 #4
    I may be wrong, but Einstein thought of acceleration and gravity two of the same thing. When you feel acceleration there is no difference between that and gravity.
    So his idea was that we all must be accelerating because we feel gravity. the only way to be completley still is to actually be in a constant free fall that perfectly counter-acts the effects of gravity.
  6. Apr 7, 2004 #5
    what does this acronym indicate? "with reespect to"?

    What is that? Something like a wormhole that allows time travel?

    You are correct.

    Good point.
    Last edited: Apr 7, 2004
  7. Apr 7, 2004 #6
    Mattrixman - Gravity and Inertia are somewhat inseparable - Einstein's principle of equivalence is usually quoted for authority on this subject - but the correspondence may be even deeper - Einstein opined that we notice an inertial reaction when we accelerate a mass wrt (with respect to) the universe (i.e., space) but he also said that the same force would be generated if the universe as a whole were accelerated with respect to the mass - a relativity principle for acceleration. Feynman once commented that gravity may be nothing but an inertial reaction because we do not have a Newtonian reference frame - he didn't pursue it very far - but if you consider that the universe (Hubble Sphere) is expanding at the velocity of light at a uniform radial rate, then the volume is actually accelerating. When you make a volume to surface transformation via the divergence theorem (Gausses' theorem) you arrive at an isoptropic acceleration that is about c^2/R or roughly 10^-10 meters/sec^2 .. of the same order of magnitude as the gravitational constant. As a mater of fact, the dimensions of G are (vol/sec^2 )per kgm. The fact that gravity and inertia act equally upon the same mass constant is not fortuitous coincidence - it is a consequence of the fact that gravity is in reality a form of inertia
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