Let's continue on with special relativity. My previous entry focused on the time aspect of special relativity, on how time elapses more slowly on an object moving through space at high speed. Today, I'd like to take up the space aspect and the joint concept of spacetime. Just to be clear, there are three things to be concerned about: space, time, and spacetime. As before, I will be drawing from Brian Greene's book, The Fabric of the Cosmos.
First, in discussing relative space, Greene gives the example of how two city planners could design different street grid systems for the same town, which would be equally valid. Within the limits of my keyboard, one might look like this:
_ _ _ _ _
_I_I_I_I_
_I_I_I_I_
_I_I_I_I_
_I_I_I_I_
_I_I_I_I_
and another might look like this:
_ _ _ _ _
_/_/_/_/_/
/_/_/_/_/_
_/_/_/_/_/
/_/_/_/_/_
More interesting yet is the combination of space and time into spacetime. For this, Greene invokes the idea of a flipbook, a little book of sequential photographs on soft cardboard pages stapled together. When you run your thumb over the pages from beginning to end, of course, the pages appear to show a movie.
A flipbook -- comprised collectively of pages that each depict what happens at "a region of space at one moment of time" (p. 53) -- thus represents spacetime. What makes this topic so interesting is that two observers to the same event do not have to have the same "still photo" images on their respective flipbook pages. Writes Greene about the introduction of special relativity:
Until 1905, it was thought that everyone experiences the passage of time identically, that everyone agrees on what events occur at a given moment of time, and hence, that everyone would concur on what belongs on a given page in the flip book of spacetime. But when Einstein realized that two observers in relative motion have clocks that tick off time differently, this all changed (p. 55).
Whereas one observer can think that certain things are going on at one point in time, "Another observer, moving relative to the first, will declare that the events on a single one of these pages do not all happen at the same time" (p. 55).
"Instead, observers moving relative to each other cut a block of spacetime up into pages, into time slices, in different but equally valid ways" (p. 56).
Another metaphor, similar to the flipbook, Greene uses for spacetime is a loaf of bread, which different observers can slice at different angles. At this point, Greene then introduces the "kicker" to the story: "not everything in relativity is relative" (p. 58). He continues:
Even if you and I were to imagine slicing up a loaf of bread in two different ways, there is still something that we would fully agree upon: the totality of the loaf itself. Although our slices would differ, if I were to imagine putting all of my slices together and you were to imagine doing the same for all of your slices, we would reconstitute the same loaf of bread. How could it be otherwise? We both imagined cutting up the same loaf (p. 58).
Absolute space does not exist. Absolute time does not exist. But according to special relativity, absolute spacetime does exist (p. 59).
I hope this information is clear to readers, at least "relatively" so.