But doesn't the block "start" with the big bang - I mean the block doesn't extend "backwards" eternally.
There is no start. independently from the Big Bang. That is the point.
To see that, we need a little analogy, which, despite being a bit superficial, it is useful to convey the notion.
Consider the surface of the earth (a sphere, more or less). Let’s ideally replace one dimension of space (the north-south direction) with a dimension of time. In this model, if you (one dimensional being extended in the east-west spacial direction, in this model) seat watching TV anywhere on the surface, you will feel like being dragged in time towards the south pole as the clock ticks.
As time passes, the east-west space circumference seems to increase until it reaches its maximum extension at the equator. All objects around you seem to move farther away from you (perceived as universe expansion). Then it seems to shrink again as time passes and you are approaching the south pole.
This is a very simplified model of a 2-dimensional closed universe that gives observers in it the illusion to start with zero spacial extension and to end the same way. Something like a Big Bang and a Big Crunch.
But the whole Universe as a whole is an immutable sphere with a 2-dimensional spacetime that does not begin, nor ends, nor moves at all. All events that punctuate it, including you watching TV at the space and time location that corresponds to, say, 53-Madison New York, are also eternal and all existing at once. The event of your birth is still there (farther north from NY) and the event of your death is also already there (much farther south from NY
).
So, this 2-dimensional continuum does not need to come from somewhere. Despite having observes on its surface that think that Universe started at the north pole (Big Bang )and will end at the south pole (Big Crunch), there is no start and no end for the existence of the whole sphere.
A good introduction is here ->
http://archive.is/0pC8J
Ciao
- viole