This passage is excerpted from an article by Pankaj Joshi that originally appeared in “Scientific America” magazine (© 2009 by Pankaj Joshi).
- Modern science has introduced the
- world to plenty of strange ideas,
- but surely one of the strangest is
- the fate of a massive star that has
- reached the end of its life. Having
- exhausted the fuel that sustained
- it for millions of years, the star is
- no longer able to hold itself up
- under its own weight, and it starts
- collapsing catastrophically.
- Modest stars like the sun also
- collapse, but they stabilize again at
- a smaller size. Whereas if a star is
- massive enough, its gravity
- overwhelms all the forces that
- might halt the collapse. From a
- size of millions of kilometers
- across, the star crumples to a
- pinprick smaller than the dot
- on an “i.”
- Most physicists and astronomers
- think the result is a black hole,
- a body with such intense gravity
- that nothing can escape from its
- immediate vicinity. A black hole
- has two parts. At its core is a
- singularity, the infinitesimal point
- into which all the matter of the
- star gets crushed. Surrounding the
- singularity is the region of space
- from which escape is impossible,
- the perimeter of which is called
- the event horizon. Once something
- enters the event horizon, it loses
- all hope of exiting. Whatever light
- the falling body gives off is
- trapped, too, so an outside
- observer never sees it again.
- It ultimately crashes into
- the singularity.
- But is this picture really true?
- The known laws of physics are
- clear that a singularity forms, but
- they are hazy about the event
- horizon. Most physicists operate
- under the assumption that a
- horizon must indeed form, if only
- because the horizon is very
- appealing as a scientific fig
- leaf. Physicists have yet to figure
- out what exactly happens at a
- singularity: matter is crushed, but
- what becomes of it then? The event
- horizon, by hiding the singularity,
- isolates this gap in our knowledge.
- All kinds of processes unknown to
- science may occur at the
- singularity, yet they have no effect
- on the outside world. Astronomers
- plotting the orbits of planets and
- stars can safely ignore the
- uncertainties introduced by
- singularities and apply the
- standard laws of physics with
- confidence. Whatever happens in
- a black hole stays in a black hole.
- Yet a growing body of research
- calls this working assumption
- into question. Researchers have
- found a wide variety of stellar
- collapse scenarios in which an
- event horizon does not in fact
- form, so that the singularity
- remains exposed to our view.
- Physicists call it a naked
- singularity. Matter and radiation
- can both fall in and come out.
- Whereas visiting the singularity
- inside a black hole would be a
- one-way trip, you could in
- principle come as close as you like
- to a naked singularity and return
- to tell the tale.
- If naked singularities exist, the
- implications would be enormous
- and would touch on nearly every
- aspect of astrophysics and
- fundamental physics. The lack of
- horizons could mean that
- mysterious processes occurring
- near the singularities would
- impinge on the outside world.
- Naked singularities might
- account for unexplained high-
- energy phenomena that
- astronomers have seen, and they
- might offer a laboratory to explore
- the fabric of spacetime on its
- finest scales.
- Event horizons were supposed to
- have been the easy part about
- black holes. Singularities are
- clearly mysterious. They are places
- where the strength of gravity
- becomes infinite and the known
- laws of physics break down.
- According to physicists’ current
- understanding of gravity,
- encapsulated in Einstein’s general
- theory of relativity, singularities
- inevitably arise during the collapse
- of a giant star. General relativity
- does not account for the quantum
- effects that become important for
- microscopic objects, and those
- effects presumably intervene to
- prevent the strength of gravity
- from becoming truly infinite. But
- physicists are still struggling to
- develop the quantum theory of
- gravity they need to explain
- singularities.
- By comparison, what happens to
- the region of spacetime around
- the singularity seems as though it
- should be rather straightforward.
- Stellar event horizons are many
- kilometers in size, far larger than
- the typical scale of quantum effects.
- Assuming that no new forces of
- nature intervene, horizons should
- be governed purely by general
- relativity, a theory that is based on
- well-understood principles and has
- passed 90 years of observational
- tests.