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Comment Re:von Neumann probes (Score 2) 391

A real head-scratching conundrum about the universe is explaining why it's not already overrun with self-replicating robots.

Nah, that's easy : it actually *is* overrun, what else do you think all the dark matter is?
These robots are monoliths with ratios 1:4:5. Because they are black and full of stars, they are very hard to see against the cosmic background.

Comment Re:We may hear from Philae later (Score 1) 337

Of course, the comet will also start shooting off monster steam blasts, which could easily blast Philae off at escape velocity.

Only in Hollywood movies.

Meanwhile they asked this question to the ESA people (watch the press conference), and it turns out that (1) the expected mass emissions of the comet are not that large, so no monster blasts, and (2) the lander is still pretty massive (about a metric tonne) so it has a large inertia, and will be hard to move by the escaping gas. In summary, they didn't expect it to go anywhere soon (unless it got hit by a meteoroid).

Comment Re:95 years but (Score 1) 120

There's no such thing as a passive detector.

Sure there is. There is nothing special about a detector. If you can put a whole cat (=a bunch of atoms) in a superposition of quantum states, you can also include the detector (=a bunch of atoms) in that superposition.

It only works if the inside of the box (including the detector) is isolated from the rest of the universe. Then there is a superposition of 2 states: (1) the radioisotope didn't decay, the detector detected nothing, the cat is alive; and (2) the radioisotope decayed, the detector detected the emitted particle and released the neurotoxin, the cat died.

Once you break isolation (i.e. coherence), the rest of the universe entangles (at random) with one of these states, and the other state "collapses".

The big question is: what happens with the collapsed state? Was it absorbed in the other state? Did it entangle with a parallel universe? Did it become disconnected from reality?

Comment Re:Why it matters (Score 1) 293

Why would matter exiting a wormhole be more energetic? It would be less energetic.

In a blackhole matter enters, undergoes E=mc^2 and is re-emitted as conveniently detectable radiation. In a wormhole, matter potentially traverses the length and remains as matter - which means no gamma ray bursts, just whatever heat you pick up from jostling around with all the other matter that might be doing it.

The gamma ray burst energy doesn't come from matter to energy conversion. Recent studies found they consist of normal matter (atomic nuclei). This gets accelerated to near light speed by conservation of momentum in the accretion disc, and the frame dragging effects of the black hole geometry.

Light doesn't exit a black hole at all. Hawking radiation consists of twin pairs of photons that form just outside the event horizon (geometric energy to matter conversion). One falls into the black hole, the other escapes.

Matter falling into a black hole will gain a lot of kinetic energy from the gravitational potential alone, which counters light speed at the event horizon. And on the other side, if stuff can get out again, there can be no event horizon, so the potential well would be less steep. Classic worm holes even are theorized to have a repulsive gravity on the exit end, so matter would be accelerated even more.

Comment Re:Why it matters (Score 1) 293

Why should there be exits? What if they go to another universe?

I was talking about the classic kind of wormhole. Either it has a direction, and then there should be a 50/50 chance that any end is an exit, or it has no direction and both ends can act like an exit.

If they go to another universe, then I would expect other universe's wormholes to connect to ours too, in a similar ratio (otherwise our universe would be very special, and lose matter/energy).

Is there one nearby that we can observe with our extremely primitive and limited technology? Would we know it if we saw it?

Matter almost falling into a black hole, but escaping, is the source of some of the most energetic bursts of cosmic rays, and we can detect those from half a universe away. It would not be unreasonable to expect the matter/energy that comes out to be even more energetic and also have a much greater quantity. Again, only assuming stuff exits a wormhole.

Comment Re:Why it matters (Score 3, Interesting) 293

However, there are hypotheses that wormholes to be stabilized require using negative matter

If Sag A* is a wormhole, and required stabilizing, then it would have destabilized long long time ago, since it has been constantly gobbling up regular matter (albeit infrequently lately).

I doubt anything could pass through a wormhole, since that would probably break causality or the laws of thermodynamics. Also, we should have detected stuff coming out of the other side (maybe not of this one, but there should be "exits" all over the universe).

If wormholes exist, my guess is they will be more like a pair of entangled black holes. They would look like normal black holes, until you did a careful statistical analysis of Hawking radiation of both.

A rock store eventually closed down; they were taking too much for granite.