Don't Try This At Home

Here we go again - General Relativity. This time a relatively simple question - in fact two - what exactly will you see as you watch your Asronaut companion as he falls into a black hole, and what will he see?

Suppose for argument's sake we start off at a reasonable distance, say maybe 40,000 km outside the event horizon of a minimum mass "natural" black hole (one formed from gravity) of 2.3 solar masses.

40,000 km is the circumference of the earth at the equator.

The rocket engines just counteract the pull of gravity, so that your craft stays stationary.

Your companion decides to make the ultimate sacrifice for the sake of science, and steps out of the airlock.

Immediately, gravity starts to pull him towards the black hole, and he drifts past the rear of the craft.

You train a telescope on him.

As he falls nearer and nearer to the event horizon, because of the rules of General Relativity, two things happen.

1. The image you see of him gets redder and redder and fainter and fainter.

2. He seems to fall more slowly the nearer he gets to the event horizon.

(NB the "event horizon" is that whuch if crossed, nothing can get back)

These are real events, not optical illusions.

The redshift is "gravitational" redshift, not "doppler" redshift, which is caused by objects moving rapidly away.

The first effect is due to the gravity not "slowing" the light down, but reducing it's frequency. Eventually you friend will completely fade away from view. He will still be there, you just can't see him, because gravity squeezes the energy from the light coming from him.

The second effect is more dramatic. He will appear to fall ever slower and ever slower. If he waved to you, and if you could see it, it would be like seeing a super slomo camera. The reason is that gravity actually slows down time. It does not appear to, it actually does.

So a seeming paradox is reached - your suicidal friend eventually becomes invisible and stops in time ( a bit like Alexander the Great below). If you could see him, he would appear frozen in time at the event horizon.

So what is happening?

To answer this, we must go to the second question - what does your suicidal friend see?

As he falls in at normal speed, he sees the actions of YOU getting forever faster, BUT he can still see you in normal light. The only difference is that his field of view is reduced. He will see the entire universe closing in on a porthole-like opening in a black void. The further he falls in, the smaller the porthole gets until when he crosses the event horizon, and he does, that porthole shrinks to nothing, just before the whole Universe stops dead. When his world is completely black, he has passed over the event horizon in what he perceives as normal time.

How can reconcile this seemingly paradoxical view?

The clue is in the name of the theory - "relativity". But who is "right"?

That's easy - since the suicidal friend is accelerating HE is the one who experiences the changes, BUT, as his body experiences them all at the same time, it all seems normal to him. To keep the balance of things, he perceives the same as you, timewise, but it is actually the suicidal astronaut whose time actually slows down.

What you see is more bizarre that what he sees.

From your point of view, he never gets there.

From his point of view, he gets there in a finite amount of time but you, and the rest of the universe, disappear from his sight, all squashed into an infinitely small "porthole" which winks out of existence.

All this is academic, of course, since, he would be "pummelled" to death way before any of this (see !" A New Meaning to Gymnastics")

Light does a Bender

Gravity bends spacetime (spacetime is 3 dimensions of space, one of time). In the diagram, the space is seen as two dimensions, and the earth bends it as if it were a rubber sheet.

If you have a black hole instead of the earth, the singularity inside the black hole actually punctures the sheet. A "singularity" is our name for classifying the unclassifiable. When matter is squeezed into nothingness, physics breaks down completely. The "black hole" is actually a name for the area inside of which nothing can escape. The terms "black hole" and "event horizon" mean the same thing, whereas "singularity" is the term for the thing in the middle of it all.

The straight lines in the diagram are called geodesics (light paths), and as is clear, they are bent.

So light, although composed of massless photons, is affected by gravity, because gravity is not really an attraction, but a geometric effect.

OPINION:

This is one reason why I believe gravity will never be united with the other three forces, which are gauge fields. Gravity is not a gauge field, but geometry.

Explanation of a Black Hole

There has been lots of bad press about the LHC concerning the possibility of producing "mini black holes that could swallow up the Earth".

Call this post the "anti-Newspaper" if you like.

Ok, so, Black Holes - what are they?

Keeping it simple, here we go.

Stars. Stars are simply large bundles of stuff (mainly hydrogen or protons) that are powered by nuclear reactions, the same kind as in the atom bomb. The reason they do not explode like an atom bomb is that there is so much stuff there, that gravity keeps it all in a ball and contains the millions of atom bombs going off every second.

It is a matter of balance between outward nuclear explosions, and the pressure inwards from the gravity.

That's for a normal star.

However they do not last forever, and there will come a point when there is no hydrogen left to explode. The star shrinks until gravity raises the inside temperature to be hot enough to explode the next element, Helium. When that is finished, it shrinks more, and burns even heavier elements.

Even this can't last forever, and there comes a point when it can no longer make any atom bombs at all.

It gives up at this point, and IMplodes, but the implosion goes right through the centre and what is seen is an EXplosion outwards. We call this a Nova.

What is left is a lot of stuff getting thrown out into space, leaving a small, dense stellar core the size of a small planet. This is called a white dwarf, and will cool into a red, then brown, then black dwarf. Gravity is stopped by outward atomic pressure.

That is for a normal star.

If the star is just a tad bigger to begin with, there is too much stuff in it to make a dwarf star, and the protons add electrons to themselves and all become neutrons, and the star shrinks to a smaller size. Then the IMplosion again, but what is left this time is a Neutron Star, exactly like an atomic nucleus, but the size of a city. Gravity is stopped by outward neutron pressure.

That is for a slightly larger than normal star.

If the star is bigger than about 2.3 times the size of our sun, nothing can stop the shrinking and gravity completely takes over.

To everyone outside, the star is crushed out of existence and becomes a Black Hole.

The size of these black holes depends on the original star, so at 2.3 times bigger than the sun, the black hole would be roughly the size of a football field.

Clearly, most stars are far bigger than the sun, which would mean that their black holes could be as big as a planet. Hovever, this would require a billion billion billion billion tonnes of mass to create.

All due to gravity.

The upshot is that a black hole is formed from compression, of any kind, but we only know about how gravity does it, However, it has been conjectured that during the Big Bang, pressures were so great that the possibility of black hole formation at that time is entirely feasible, bearing in mind that all the stuff of the universe was there at the time to create them.

The fears that the LHC can form mini-black holes are completely groundless because a) the pressure there is about a trillion trillion times less than needed, b) there is simply not enough stuff to make one even if the pressure was available, and c) in order to "swallow the earth" the created black hole would have to be at least as big as the earth.

Ever tried pushing a water melon through the eye of a needle?