Bridge building with Albert Einstein and Nathan Rosen. Or, why Stargate can’t happen…

The classical problem with falling into a black hole is a pretty simple one. You don’t come out. Ever. Also, as you go down, you’re turned into spaghetti.

Gravitational lens attributed to the presence of Dark Matter. NASA, public domain.
Gravitational lens attributed to the presence of Dark Matter. NASA, public domain.

Or do you? Albert Einstein’s General and Special Theories of Relativity – both of which have been absolutely proven through every single test ever – tell us nothing comes out of a black hole, because the velocity needed to escape one is greater than the speed of light.  But Stephen Hawking says it does – and has spent a lot of time and effort figuring out the mechanisms.

It’s to do with quantum physics. According to quantum physics, information cannot be destroyed. Although that word doesn’t quite mean to physicists what it does to everyday English speakers. In a quantum sense ‘information’ means ‘information about the universe’, which includes the existence (or not) of particles, how they are configured, how they are spinning and so on.

You can see the problem. Stuff vanishes into a black hole – drops through the event horizon and is lost to any possible detection by us. In a quantum sense, information about the universe has just gone forever. Except quantum mechanics says it can’t. A paradox!

Hawking solved that. Actually, he says, black holes ‘leak’, because of those same quantum principles. Specifically, a particle isn’t positioned in a defined place – it occupies a range of possible locations. If a particle is on the black hole’s event horizon (the point where escape velocity exceeds lightspeed), the uncertainty of location means it could be on either side. And a proportion of the time, it will be outside. Thus, it leaks.

The principle is known as ‘Hawking radiation’, and the smaller the black hole, the faster it leaks. Eventually they evaporate. It’s why any micro-sized black holes made from a few hundred particles in the LHC at CERN aren’t a danger – they’ll dissipate faster than they can grow. You’ll also note that no human observation is required for the quantum wave function to collapse and make all this happen. None. Nada. Zip.

Since then, Hawking has also resolved the problem of information vanishing with the particles whose location happens to resolve on the inside of the event horizon. More on that another time.

Recently, though, there’s been a suggestion from a team at the University of Lisbon under Diego Rubiera-Garcia that even Einstein’s determinist physics might generate a leaky black hole.

It’s a riff on an old idea. According to general relativity, a black hole is where space-time curvature becomes infinite. Normal physics totally break down. It’s known as a ‘singularity’, and it’s a point that is infinitely small. We’re shielded from it by the event horizon.

Back in 1935, the Einstein and his colleague at Princeton, Nathan Rosen found a way of reconciling general relativity with particle physics, concluding that it was theoretically possible for particles to ‘bridge’ spaces. Here’s the paper, ‘The particle problem in general relativity’, published in the Physical Review in July that year:

One place where such ‘bridges’ might occur was where space-time curvature was very high. The practical problem was the instability of these ‘Einstein-Rosen Bridges’. Sure, physics might break down in a singularity, and it might link to another point in space or time (space-time is just one thing – so by strict Einsteinian principles, a wormhole can go not just to any place, but to any when). But according to Einstein and Rosen, the wormhole might not exist for long enough to let a single photon through. And even if it did, because it was infinitely thin, all you’d get out is a stream of undifferentiated mass/energy.

So much for Stargate (an idea pinched, I might add, from Robert A. Heinlein’s Tunnel in the Sky). And yes, Interstellar bites the dust too (I’m sure the makers knew the issue, but hey, they flew into a Hollywood Black Hole…)

One or two physicists did try to make the theory work – check out this paper:

But now there’s a new idea. The postulate is that if the centre of an event horizon contains a ‘wormhole’, it won’t contain a singularity. Furthermore, the ‘wormhole’ has a finite scale. That means information could pass through.

That still doesn’t mean a Stargate scenario. The size of such a wormhole is still infinitesimally tiny and, while you might not be ‘spaghettified’ falling into it (reduced to an infinitely long and infinitely thin stream of mass/energy), you’d still end up as a fairly passable imitation of linguini.

The maths work out. Whether the reality follows is, of course, another matter. I kind of hope it does. And then I kind of hope we can figure out a machine to exploit it – a device that can travel anywhere in time and relative dimensions in space.

wright_weird-universe-cover-200-pxWill we? Well, on the plus side, the real world is a LOT stranger, when you get down to relativistic and quantum level, than anything imagined on TV. The down side is that it’s strange in ways that aren’t likely to meet our dreams – strange, indeed, in ways we can’t dream. But you never know…

For more on the weirdness of our universe, check out Explaining Our Weird Universe 1 – cool stuff about just how odd our universe actually is.

Copyright © Matthew Wright 2016


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