There seems to be some confusion [among the students in my Physics and Society class] about spacetime, which is completely understandable because it’s difficult to visualize. I’m going to review some of the main ideas of it here, hopefully clearing up some confusion along the way.
The main quote many people gave in their summaries [of the lecture on Special Relativity] is that “space and time are no longer separate”. But what does this mean?
- It means that two observers having relative velocity near the speed of light have different perceptions of time (rate of time passing) and space (positions in space).
- Note that space, in this case, means the three directions that can be used to label a position. This is not to be confused with the idea of “outer space” or “empty space”, the emptiness between planets’ atmospheres and between nebulas and galaxies and all that.
- The emptiness of outer space just means that there isn’t anything at any of those “empty” locations. Even if you go past these regions really quickly, they will still be empty! However, the three perpendicular directions you use to describe the positions will be different from those of someone moving at a high velocity relative to your own.
- So each observer has their own framework of space and time, which means they each can describe an event by its position (in three dimensions, say, “left-right”, “forward-backward”, and “up-down”) and the time it seemed to occur (in one dimension, which we can call “earlier-later”) according to that observer’s clock.
- Since the two observers’ clocks tick at different rates, they age at different rates, and they disagree on whether two spatially-separated events are simultaneous in time or not. In particular, measuring the length of something involves simultaneous measurements of position, and if two observers disagree on what is simultaneous, they will also disagree on the length of the thing being measured.
This does NOT mean that space and time are the same thing: each observer can always tell the difference between space and time, within each one’s own inertial frame!
- Space and time are just two separate aspects of the same thing: that mysterious-sounding “spacetime”.
- Spacetime (i.e. careful attention to both space and time) is only needed when trying to compare situations from one inertial frame to another, because the two observers disagree on some aspects of space and time.
- What makes them both aspects of the same crazy “spacetime” concept is that the way time dilates (stretches), and the way length contracts (squishes) along the direction of motion, both depend ONLY on the relative speed between the two inertial frames.
So although the idea of spacetime seems weird compared to our normal intuitive way of looking at things, keep in mind that there is a bigger framework that allows the two parts to be mixed together. The only time that this bigger framework is needed is:
- when comparing points of view that are moving at some relative velocity,
- and only if it is some significant percent of the speed of light.