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Things that puzzle me

  1. Feb 9, 2009 #1
    I have practically zero physics background. I understand a lot, but have zero knowledge of any higher physics math. Anyway, in pondering what things ive read related to physics, Im left with a few questions that completely puzzle me.

    First C... the speed of light is considered a constant, but is it relative to the emitter? Or is it a fixed absolute velocity which theoretically cannot be exceeded? Being relative to the emitter would make more sense, but leaves me with a lot more questions.

    Next, pressure. If you have a hypothetical container partitioned in the middle - one side has an absolute vacuum, and the other a single atom/diatom of a gas - will there be a pressure differential? And if so, it would seem that either the single atom is moving at relativistic speeds(?) to create such a vibration as to create pressure, or does it become more like a wave function in such a vacuum?

    Third - absolute velocity. If relativity states that as objects approach the speed of light, they become more massive, a) what happens to an object at an absolute speed of NIL, and b) if the said change in mass were observable, would one not be able to (theoretically) resolve the absolute speed of the earth by observing the nature of the very atoms/matter it is made of? and further, direction of velocity be resolved by moving an object at a different speed and observing the differences in the matter/atoms?
  2. jcsd
  3. Feb 9, 2009 #2


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    Hi drak, welcome to PF. Yes, there is a pressure differential. The atom/molecule occasionally strikes the container wall; the average force per unit area from these collisions constitutes the pressure. The side with the vacuum has a pressure of (essentially) zero. (Photons exist inside this vacuum, but let's ignore them.)

    The atom could have a speed anywhere from zero up to relativistic speeds; there are no constraints. The lower the speed, the less the pressure. Strictly, we don't want to describe the atom or molecule's movement in the container as a "vibration," since there is no restoring force. If the gas consists of a molecule such as H2 or NH4, though, some vibration will occur within the molecule.

    The atom/molecule has a wave function, but I don't believe we need to incorporate quantum effects to model the pressure suitably.
  4. Feb 9, 2009 #3
    A relative velocity compared to the emitter makes sense based on our natural intuition...two cars driving down the road in the same direction at slightly different speeds causes the rear driver to observe the other moving away from his car at a very slow speed. Einstein showed us that light does not work this way. If you travel close to the speed of light, you will "always" see light recede or approach you at exactly the speed of light. This fact leads to such strange behavior as "length contraction" and "time dilation".

    EDIT: This needs a little extra clarity. All motion is relative. Motion requires a minimum of two vantage points to be described. Light "always" travels at speed c from any (our) vantage point, whether we are in motion or not, and whether we are emitting the light or receiving it.

    Theoretically, nothing can travel faster than the speed of light. Anything that has mass can "never" move at the speed of light. This would require an infinite amount of force (and energy) to accelerate the object to speed "c". Only massless particles, like the photon, and the hypothesized particle that carries the gravitational force, gravitons, can move at the speed of light. Some theories suggest that some particles, like tachyons, can move faster than the speed of light, but this is very speculative and has never been proven.

    The discussion in the previous paragraphs assumes that the medium in which light moves is the vacuum. When light moves through materials, like glass, its velocity is no longer "c", but some fraction of it. In some mediums, it is possible for things to travel faster than light in that same medium. For example, see Čerenkov radiation: http://en.wikipedia.org/wiki/Cherenkov_radiation
    Last edited: Feb 9, 2009
  5. Feb 9, 2009 #4
    A bit more on absolute velocity, there is no such thing. Who is to say that the velocity of one star system is zero or another one is. If you try to measure relative to the "center of the universe" that may not be possible. The volume of universe may be expanding in a way analogous to the surface area of a balloon expanding as it is blown up. Any point on the surface of the expanding balloon is retreating from all other points but the center of expansion is not within the surface of the balloon. It has been suggested that our universe may be expanding in the same way, so even though every galaxy can be seen to be retreating from all of the others the center of expansion may not be in our physical universe.
  6. Feb 10, 2009 #5
    Respectfully, your statement that “absolute velocity” doesn’t exist is incorrect.

    “Absolute velocity” does exist, just as relative velocity exists. In fact, the constant ‘c’ associated with light-speed couldn’t exist as a constant in the universe if there was no such thing as “absolute velocity”. Light travels at an absolute velocity of 299,792,458 m/s through the vacuum of space therefore, absolute velocity does exist.
  7. Feb 10, 2009 #6
    I wasn't really happy with my answer and you hit on one point where I could have been more specific. I should have said perhaps no absolute velocity in the way the OP seemed to mean. The speed of light is the one true absolute velocity in the universe.
  8. Feb 10, 2009 #7
    So youre saying, if an object were to move through an empty universe, but theres no other object to reference it to, its not really moving............

    "If a tree falls in the forest and no one is there to hear it, did it make a sound?"

    This sounds pretty far fetched to me, like some sort of abstraction devised to solve the problem of absolute velocity.

    But if there is anymore reading on this subject I would love to see it, everything I can find on absolute velocity basically states we cannot resolve our own absolute velocity due to our limited frame of reference.
  9. Feb 11, 2009 #8
    Sure, it can be moving, but the problem is with describing "how" it is moving. The problem stems from the lack of an absolute reference frame; the abandonment of an absolute reference frame was important for the development of Einstein's theory of special relativity.
  10. Feb 11, 2009 #9


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    Right, Lightspeed is absolute, but "an absolute velocity of NIL" is impossible. So long as there are (or even can be) two or more objects in the universe, and they are (or could be) moving at different speeds, then neither is at rest relative to the other.

    This is also the response to the question about a single object alone in its own universe. Even if there is no other object, another reference frame can be imagined, with the object in question moving in that reference frame. Once the concept of an absolute reference frame has been eliminated, then no reference frame can be seen as prefferable to any other, even if the "other" is imaginary.
  11. Feb 11, 2009 #10
    Here's some interesting links. I learned many new things.

    See this thread, "Absolute Velocity of the Earth?":

    See this link (part way down the page) for various effects that influence the earth's velocity with respect to the CMB reference frame:
  12. Feb 11, 2009 #11


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    That single atom would be bouncing off the walls of it's side of the container. Over time, collisions between that atom and the walls would result in some average force / unit area (if the atom is bouncing randomly enough), and this would be the pressure.
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