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Terminology that is inconsistent with basic physic definitions

  1. Apr 7, 2004 #1
    When reading advanced physics articles, I frequently find terminology that is inconsistent with basic physic definitions. For example: "pure energy". I have no idea what that expression means. I have read much about space-time, the arrow or time, etc, but no definition of "time". Where might I find a source of physics terminology and definitions, as used today?
  2. jcsd
  3. Apr 8, 2004 #2
    Henry Jones,

    As I was reading your post I happened to have sitting next to the "McGraw-Hill Dictionary of Scientific and Technical Terms". Maybe something like that is what you're looking for.

    But if you don't mind sorting through some irrelevant junk, it's hard to beat a Google Search for info on just about anything.

    By the way, I looked up both your terms in that MG Tech Dictionary.

    Nothing for "pure energy" although that term is usually used the context of high energy or cosmological physics. It comes from the matter-energy equality that Einstein discovered a hundred years ago. Most things in the universe today are mixtures of matter and energy. But it's thought by physicists that at the instant the universe was formed, there was no matter. It was just pure-energy. Within a small fraction of second the energy began turning into matter.

    Time is a tricky one! So is its counterpart-space, or position. Physicist sort of wave their hands on these, and say "we know what we mean by time and space, and if there's ever any disagreement we'll settle it with some math!" The MG Tech Dictionary did have a definition for time, something like, "The dimension of the physical universe which at a given location in space, specifies the ordered sequence of events." Not bad.

    By the way, in Einstein's first paper on his theory of relativity, which was fundamentally about time and our perception of it, did he resort to some fancy arcane definion to base his theory on? No. He talked about what it means to say "the train will arrive at the station at 7 pm". We all know what he means! :wink:
  4. Apr 9, 2004 #3


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    Actually time isn't normally defined any more than "x" is defined. Time is one of the coordinates one used to specify an event. It's value depends on the coordinate system chosen. Normally, "time interval", the time span between two events is of much more interest (in the same way that the distance between to points is of more interest than just the coordinates of a point in some arbitrary coordinate system) and that is what is defined.
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