What does When the Universe was one minute old mean?

[bapowell]

Good question, John. I'm not immediately sure what the answer is. I would suggest that once B reaches the event horizon, his clock is equally redshifted relative to A's clock. Perhaps they both fall in at the same time according to B, while A thinks that A falls in first. There's no paradox here, since simultaneity is observer dependent.

 

[John Bleau]

Thanks bapowell,

I consider the question still open, though it has strayed somewhat from the original post (though it's still related in a manner I was hoping to clarify). My own conception of the event horizon is similar to the speed of light in SR - we can accelerate as much as we please, the speed of light remains unattainable. Likewise for "approaching" the EH. But possibly this is a topic for another thread after which we could return to this one. Or maybe it's already been broached - I don't know, I'm new here.

Anyway, thanks, I learned quite a bit.

At any rate, I have to get back to work. I'll pop by tomorrow or next week.

 

[DevilsAvocado]

There's another much more sophisticated version of this.

Okay, that explains it.

 

[DevilsAvocado]

... My own conception of the event horizon is similar to the speed of light in SR - we can accelerate as much as we please, the speed of light remains unattainable. Likewise for "approaching" the EH.

I think the answer to your EH question is here:

http://en.wikipedia.org/wiki/Event_horizon#Interacting_with_an_event_horizon"
...
For the case of the horizon around a black hole, observers stationary with respect to a distant object will all agree on where the horizon is. While this seems to allow an observer lowered towards the hole on a rope to contact the horizon, in practice this cannot be done. If the observer is lowered very slowly, then, in the observer's frame of reference, the horizon appears to be very far away, and ever more rope needs to be paid out to reach the horizon. If the observer is quickly lowered by another observer, then indeed the first observer, and some of the rope can touch and even cross the (second observer's) event horizon. If the rope is pulled taut to fish the first observer back out, then the forces along the rope increase without bound as they approach the event horizon, and at some point the rope must break. Furthermore, the break must occur not at the event horizon, but at a point where the second observer can observe it.

Attempting to stick a rigid rod through the hole's horizon cannot be done: if the rod is lowered extremely slowly, then it is always too short to touch the event horizon, as the coordinate frames near the tip of the rod are extremely compressed. From the point of view of an observer at the end of the rod, the event horizon remains hopelessly out of reach. If the rod is lowered quickly, then the same problems as with the rope are encountered: the rod must break and the broken-off pieces inevitably fall in.

These peculiarities only occur because of the supposition that the observers be stationary with respect to some other distant observer. Observers who fall into the hole are moving with respect to the distant observer, and so perceive the horizon as being in a different location, seeming to recede in front of them so that they never contact it. Increasing tidal forces (and eventual impact with the hole's gravitational singularity) are the only locally noticeable effects. While this seems to allow an in-falling observer to relay information from objects outside their perceived horizon but inside the distant observer's perceived horizon, in practice the horizon recedes by an amount small enough that by the time the in-falling observer receives any signal from farther into the hole, they've already crossed what the distant observer perceived to be the horizon, and this reception event (and any retransmission) can't be seen by the distant observer.

Edit: http://en.wikipedia.org/wiki/Apparent_horizon" is observer-dependent.

 

[John Bleau]

Thanks for your reply, DA.

This suggests to me that if our universe is emerging from a white hole or falling into a black hole, then there is no reaching the singularity. It would mean that, in fact, we have been emerging from the Big Bang indefinitely.

I suspect that "when the Universe was one minute old" is a projection of our local geometry to the singularity. On the rubber sheet model, this would be akin to our extending a straight line and not having it curve along the rubber sheet.