Quote:
Originally Posted by alpal
OK - when you've worked out how to explain a universe expanding into itself then please let us all know.
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It really depends what you mean by explain.
I think there are three broad things you might mean.
1. We have a set of equations that describe the process pretty well. Essentially the idea is that the metric, which tells you how far apart things appear to be, is changing, so that things seem to be getting further apart without moving. The metric is internal to the manifold of space-time and requires no 'background space' to be meaningful. This is all 'just' maths (sometimes referred to as the 'shut up and calculate' view of physics), but I think it counts as a real form of understanding/explanation. The description is good, and we can calculate and observe all kinds of interesting consequences.
2. We can interpret the equations as meaning that the universe is expanding, though not into anything, and that the universe is 'curved' or 'warped'. This is fairly removed from our day-to-day experience, but it seems a fairly reasonable conclusion from where I stand. You can see light curving - this is the warpedness. Blowing up a balloon while standing on the surface is a reasonable illustration of the expansion with fewer dimensions. As to what the universe is 'in', I don't see the need for anything 'outside', particularly since no one has ever glimpsed it. If you can't visualise 4-D spacetime, or a non-embedded manifold, then fair enough - they are difficult abstract concepts. But generations of maths and physics students have successfully absorbed the concepts to a reasonable degree, self included. I find this personally to be a good 'explanation' that I 'understand'.
3. Finally you can ask what is 'really' going on. Here you are outside physics, and indeed science. The generally accepted reality is that you can do maths and experiments and just leave it at that (point 1 above), or you can come up with a useful way of visualising or interpreting the maths and experiements (point 2 above), and that's it. There is no way I've ever heard of to meaningfully understand physics, other than to do maths and experiments and try to sum it up in a useful interpretation. If you insist that 'really' is what you need, then I'm not sure anyone can help, and I'd suggest your bar may be set a bit too high. Or maybe you are doing philosophy, which is not a field I really know a lot about.