Black Holes for dummies

July 25, 2006 2:34 PM

Can someone please explain to me in very basic terms the modern understanding of black holes and worm holes as a conceivable means of space/time travel?

I have been reading up on Kip Thorne's theories, and I think they are fascinating. However I'm sure that I'm not really grasping it all due to the advanced language and terms-- I have no science background.

Can someone give me a 'for dummies' version of how Kip Thorne (and others who share his view) think the black hole/worm hole phenomena can be harnessed for space/time travel? I'm getting a bit buried in all the 'naked singularities' and 'membrane paradigms.'

I have Wiki'd and googled Kip Thorne's work, but I'm hoping for something reasonably straightforward I can read or watch and feel I have a solid understanding of the concepts at play here. Thanks!

posted by np312 to science & nature (10 comments total)

I can't speak to the time travel issue, but I might be able to help out on the space travel.

Grab a piece of normal copy paper. Roll it up. This represents the fabric of space/time. Think of a worm hole as a sort of tunnel that bridges between the top of the roll and the bottom (through the center). If you entered the worm hole, you can travel to the other side more easily than if you went all the way around.
posted by lunalaguna at 2:53 PM on July 25, 2006

The wikipedia page on wormholes is pretty good (about as simple as possible, and covers the major problems). Briefly, the way that general relativity works, is you can specify any shape for space-time, and the equations of general relativity tell you where all the mass is. So people can say "suppose there is a wormhole" and the equations come back and tell them where to put the mass to make it.

Fortunately, there are serious problems: stable (i.e. non-lethal) wormholes typically require matter with negative mass-energy. This stuff has never been observed, and from what I know, would cause all sorts of weird problems if it actually existed. The most serious problem would be the existence of stable, traversable wormholes, which can be used to create time machines. And of course, once you have time machines, there's no stopping the endless streams of bad movies...

Technically this isn't impossible though, so you if you really want to kill your grandparents before you are born, there's still hope.
posted by Humanzee at 3:17 PM on July 25, 2006

Fortunately, there are serious problems: stable (i.e. non-lethal) wormholes typically require matter with negative mass-energy. This stuff has never been observed, and from what I know, would cause all sorts of weird problems if it actually existed.

The other thing is that if you're willing to allow negative mass-energy, you can build yourself an honest-to-Bob warp drive instead. I think these so-called "warp drive" spacetimes are worse-behaved than the wormholes spacetimes — in the sense that they tend to violate more of the energy conditions that physicists like their matter to obey. All the same, it goes to show what kind of weird & wonderful things you can conjure up in GR if you're willing to discard the constraints of normalcy. Guess that's why I went into the subject...
posted by Johnny Assay at 4:47 PM on July 25, 2006

> Fortunately, there are serious problems

Well, you don't hear *that* often...
posted by baylink at 8:22 PM on July 25, 2006

Sometimes even respectable physicists venture into Gene Ray territory. It seems to me that wormholes are the 21st century equivalent of those pictures on medieval maps labelled "here be Dragons".

You will never see a working wormhole in your life. Nothing to see here. Move along.
posted by flabdablet at 8:46 PM on July 25, 2006

And of course, once you have time machines, there's no stopping the endless streams of bad movies...


Oh yes there is.

Kill M. Night's parents... before he's even born! Mwewahahahahahahah!!!!
posted by incessant at 11:54 PM on July 25, 2006

"Contact" by Carl Sagan is a fantastic read and spends some time describing/theorizing this stuff.
posted by hermitosis at 6:47 AM on July 26, 2006

my personal favorite hypothetical time travel macguffin is the Tipler Cylinder. take a long cylinder of neutronium, spin it up to the something approaching the speed of light, and the math says that you can plot a course around it that will take you backwards in time. no wormholes or negative mass-energy required, although you'd want to stay far away from the ends of the cylinder.
posted by the painkiller at 8:05 AM on July 26, 2006

From a 100-level science overview course I took last semester:

Student: "Can people travel through wormholes?"

Teacher: "Sort of."

Student: "Can you elaborate?"

Teacher: "Given current technology, people can travel through wormholes the same way they can travel through a tree shredder."
posted by Orb2069 at 1:51 PM on July 26, 2006 [1 favorite]

It's worth bearing in mind that the painkiller's suggestion has problems of its own. Namely that the cylinder needs to be essentially ten neutron stars strung in a line, and three alone will quite promptly collapse into a black hole.

So anyway - wormholes. Wormholes don't exactly follow the analogies described above, as far as I know, in that they represent a shortcut even when the 'long way round' is set in flat space. This is from memories of Thorne's book, which I read a long time ago, before I was an astrophysicist.

Time machines in general then... the thing about them is that generally speaking they're regions where the gravity twists time so much that you can follow a path that doesn't just end up a bit earlier than you might otherwise expect (as most gravitational fields do - they dilate time somewhat) but they do it so much that your starting point is before your finishing point. The key point about this to take home is probably that these time machines are locations in space and time, and not machines that move through it. This means you can only move around in the region that the time machine is present in, as the rest of the time it's not there for you to use. This is handy as it gets round the obvious lack of time travellers here today. They don't have a time machine here today to use.

Imagine that day to day you're continually walking uphill, upwards being forwards in time. A gravitational field might make this 'hill' less steep, so you go up less quickly as you walk around. A time machine might let you walk downhill from some person's point of view, whilst as far as you're concerned you think you're still walking uphill, going forwards. Sort of. Analogies will only get you so far. Anyway, once you run out of downhill you'll have to go back uphill (when you get to the point when the time machine came into existence). Normally general relativity will stop you doing this downhill thing, but there are possible situations in which this might not apply. These have been mentioned above (wormholes, Tipler cylinders). We've not seen them anywhere in the universe though, and Hollywood scriptwriters have explained as well as anyone why this can be a bad thing to allow.

When it comes to killing grandparents and all that, I think the jury is out (as it is when it comes to whether these things can exist at all). We could argue about that for some time and probably not get very far, except to say that I'm not aware of any of the proposed solutions (e.g. parallel universes, not being allowed to kill your grandparents) have been ruled out. Not surprising really given our lack of time machines.
posted by edd at 3:53 PM on July 26, 2006