View Full Version : Getting Close to Compact Objects...

2002-Feb-20, 06:24 AM
By those, I mean neutron stars and black holes. This was inspired by Subrahmanyan Chandrasekhar's lament about the lack of good strong-field tests of general relativity.

X-ray binaries are poor candidates for such places, because their X-ray luminosities are often a few thousand times the total luminosity of the Sun, meaning that if one got to Saturn's distance (10 AU), one's spacecraft surface would be heated by the X-ray flux to ~ 1000 K unless one ran a *lot* of coolant through it and to some *big* radiators. And that's just the X-ray flux; many X-ray binaries have massive OB-supergiant companions that have luminosities typically 10^5 the Sun's -- one has to retreat a factor of 10 to keep from overheating (1000 K at 100 AU).

And at that distance, the supergiant would have an angular size of a few minutes, looking like a blazingly bright blue spark. Much of its luminosity would come off near 100 nm in the ultraviolet, which would cause a severe case of sunburn, if not skin cancer, unless it could be filtered out.

Strong-field effects would be observable over a few times the object's Schwarzschild Radius, which is 3 km for the Sun, making 4 km for a typical neutron star and 30 km for a 10-solar-mass black hole, in the range of several X-ray-binary objects. This amounts to a milliarcsecond at some safe distance like 100 AU.

Radio pulsars may be a better choice, but there are important features of their magnetospheres that I've been unable to find, such as particle densities and energies, and particle leak rates.

However, some clues are that the spin-down luminosity is usually somewhere around 10 solar luminosities, and that a radio pulsar's X-ray flux is typically 0.001 that. A radio pulsar's radio-wave emissions are apparently much weaker; it's been difficult for me to find precise figures, however.

But the X-ray luminosity of an isolated pulsar will be enough to make the closest safe distance become around 0.1 AU, meaning a strong-field-region angular size of a fraction of an arcsecond. This is a mild improvement over an X-ray-binary's value, but still not very big.

Dead pulsars and isolated stellar-mass black holes would be very difficult to find, but they could be approached to within a few thousand km, which is where tidal forces become about 1 Earth gravity per meter, which is a reasonable upper limit. An object sent from a great distance would do closest approach in only a few seconds, offering not much time for detailed observations. However, the strong-field region gets an angular size of a few minutes of arc, which is much easier to see.

But the easiest time would be with the 10^6 - 10^9 solar-mass black holes thought to inhabit the centers of many galaxies. One could approach the Schwarzschild Radius, the radius of no return, while getting tidal forces of 1 Earth gravity / kilometer or less, which should be safe for all but very large spacecraft; one also passes by that point with a timescale of about a minute -- plenty of time for observations.

But if one is not careful, one's spaceship will pass that point of no return -- and pass it while experiencing those relatively gentle tides. And one can spend the next 15 seconds to 5 hours, depending on the mass of the hole (the more massive, the longer the time) contemplating one's ultimate fate -- to be squeezed and stretched into oblivion in a fraction of a second as one approaches the center of the hole.

There is such a hole in the center of our Galaxy; if one could travel there, one could experience all those nice strong-field gravitational effects at first hand without getting either baked or crushed/stretched.

2002-Feb-21, 11:56 PM
And, if you created a gravity lens around your spaceship, you could technically fly right down into it. Question is, would you smash into something?

With a predictable level of uncertainty, if observations are right, a GRB will come our way at some point and give us a feel up close. Not exactly the same, but certainly a present from way out there.