The David Boyle / Dr. Dave Goldberg letters

David Boyle:

My admittedly vague understanding of Hawking's model of the Big Bang seems to me to suppose that there was a very definite "beginning" of the universe in a linear sense. First A happened, then B, then C, but the track was already laid down before the train started forward.

My admittedly vague understanding of Einstein's theory of relativity seems to me to suppose that linearity is a subset of universal being, rather than the reverse. A, B, and C are all there enfolded in the superdetermined structure of the multiverse, and the order that you pass through them in depends entirely on your approach vector.

I imagine an orange rolling across the ground, apparently in a straight line, without any real sense that it's actually just interfacing parts of its surface with the surface of a much larger sphere, the big picture of its motion revealing the truth of its straight line to be a petty illusion. Is that where this arrow of time nonsense comes from?

David Goldberg:

I'm afraid Hawking has very little to do with it. The Big Bang picture predates him by a good margin. But you're basically right. The timeline of the universe seems somehow inconsistent with the notion that "time is relative" to the motion of the observer. Sort of. First, how you're right. When cosmologists talk about the big bang, we need to be clear about whose clocks we're using: whether we imagine people floating around at "average" points in the universe, or whether they are uniformly spaced. This is a "gauge choice" and affects the details of things like the growth of structure. They do _not_ affect_ the order in which events occurred. Relativity describes two events as "timelike separated" when no matter how you observe them, "A" always precedes "B" (or vice-versa). If there really is an ambiguity between the two, they are "spacelike separated." If there are two events that happen in the same place as one another but occur at different times, those events are timelike separated to everyone, and it makes sense to talk about the timeline (as we do in our book, and everyone else does in theres). That's exactly how it worked in the Big Bang.

Make sense?

David Boyle:

Right, I guess Hawking strikes me as someone so caught up in the prettiness of his mathematics that he ignores the fact that a lot of what he says makes no sense. So, I like to pick on him (I can't really do math that well). Er, so I was being a bit of a smart-ass, I actually know who Ed Hubble was, sort of.

You asked me if what you said made sense. After thinking about it for a couple of days, yeah it does, but I sort of get the feeling someone is playing a joke on us. Honestly, very little about physics makes sense to me for long before my head starts to hurt and I need to go shoot the dog with the garden hose.

So, some follow-up questions that have been bugging me since I was 12...

What would happen if you got in the millenium falcon, pointed it , say, north, and flew off towards the edge of the universe as fast as possible, then looked in the rear-view mirror? There'd be an event horizon described by the limit of the expanding universe, right? Does that mean we live in someone's singularity? The idea that we're all in a black hole appeals to me because of a> the fractal geometry of nature seems like a good indication that it is, in fact, turtles all the way down (it would make sense to me if our universe contained nearly infinite variant subsets of itself) and b> you could get some kind of escherian top-down conservation of mass if the matter we were funneling into black holes was being replaced by being funneled in from somewhere else.

This apparent expansion of the universe really feels like it must be related to the sense we have of time moving "forward".

My Dad thinks the red-shifted light isn't indicative of an expanding universe at all, it's just what happens to light when it gets old and goes really far. Can you explain to me why he's full of shit?

Regards!

David Goldberg:

Sorry for the delayed response. As you might imagine, I got swamped with entanglement followups after my last column.

Anyway, if by "as fast as possible" you mean "at sublight speeds," then the short answer is that the horizon for the millenium falcon would be the same as the horizon for whatever star system it was passing. Event horizon and horizon, in this case, can be used interchangably.

If, on the other hand, you mean "at superlight speed", then I'm afraid the question is ill-posed. We just don't have an answer.

Dave Goldberg, Ph.D.
Associate Professor, Drexel University Department of Physics
"A User's Guide to the Universe: Surviving the
Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty"
http://www.usersguidetotheuniverse.com
io9.com's "Ask a Physicist"
askaphysicist@io9.com