| From: Zardoz ® | 04/11/2001
19:36:28 |
| Subject: re: COSMOLOGY FAQ | post id:
484070 |
SPACEWhat is space.? From: James R (Avatar) We can label any point in space with three co-ordinates, measured from some arbitrary reference point. For example, I can say a certain point in the centre of my lounge room is three metres north, two metres east and two metres up from a fixed point in the corner of the room. We can label any point in time using one co-ordinate, measured from an arbitrary fixed time. For example, I can say this is the year 2001, as measured from an arbitrary year zero. Combine the two and we can label any event in spacetime. For example, I can say a molecule of oxygen passed through the spatial point I mentioned above at 12.20 pm, 17 April 2001. I can say I was born in a certain place at a certain time. The big bang happened at a certain place and a certain time. That's all spacetime is, really - a set of coordinates. Imagine you have your spacetime coordinate grid set up. You can label events with particular cordinates. Now you can start to measure the differences between events in terms of space and time intervals. For example, I can say event A occurred two metres to the west of Event B. Event C occurred 2 minutes after Event D. It turns out that the measured intervals are NOT constant, but depend on the speed of the observer of the events and the local gravity. This is how Einstein's relativity differs from the Newtonian picture. According to Newton, all intervals would be constant, always. How does Einstein account for the measured differences in space and time intervals? He specifies that one thing which is constant is the speed of light for any observer. In addition, the forms of the laws of physics must appear to be the same in certain special reference frames. These postulates lead to the idea that the spacetime "gridlines" are stretched and distorted by gravity and motion. Thus, the distances between gridlines can vary. This also applies to "gridlines" of time. |
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