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")