Black holes for beginners part I

What are they?

I'll start with what they aren't.

What aren't they?

  • Cosmic vacuum cleaners – black holes do not suck material up. Sucking implies a difference in pressure. There is no air in space.
  • Tunnels to other universes – black holes lead no-where. What happens when you fall into a black hole will be discussed later.
  • Giant swirling purple things – not swirling in the usual sense, and definitely not purple. They are black, hence the name.

An introduction to gravity.

What goes up must come down, right? Wrong. There is something called the 'escape velocity'. What this is is that if I stood on Earth's surface and threw a ball up, it will never come down provided I throw it faster than Earth's escape velocity (about 11.2 kilometres per second, or 25 000 miles per hour). If I constructed some giant cannon to fire a ball faster than this upwards, it would carry on forever.

What does escape velocity depend on? It depends on the mass of the Earth. The bigger the Earth, the faster you have to go to escape the pull of gravity (and never come down). It also depends on the size of the Earth, though.

The smaller the Earth is, the closer you are to most of the stuff the Earth is made of. If you halve the size of the Earth, you're twice as close to the opposite side when you stand on it, right? You're also twice as close to the centre, where most of the matter that makes up the Earth is.

This means gravity acts more strongly on you (the closer you are to something, the stronger the pull of gravity). A smaller Earth (but one weighs the same i.e. has the same mass) would have a stronger pull of gravity and so a bigger escape velocity.

Where does a black hole fit into this?

Suppose I were to take the Earth and crush it so it becomes smaller and smaller (but still weighs the same). It would be very difficult, that's for sure. For a long time, not a lot would happen. Eventually, I would run into some more serious difficulty. All the atoms in the Earth would be essentially touching each other. I would have to force the electrons closer to the centre of the atom. Eventually, the electrons would be forced into the centre of the atom where they would combine with protons to form neutrons. You would end up with a mass of neutrons, so dense that a lump the size of a sugar cube would weigh 2 billion tons! The Earth would have become a neutron star.

Nothing in the universe can travel faster than light. Einstein showed us this. So how small would the Earth have to be for the escape velocity to be faster than light?

Part II


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  1. * SAM says:

    This stuff is so mind boggling. Last time Steve tried to explain it to me, I nearly killed myself.

    | Reply Posted 11 years, 9 months ago
  2. * xxxx says:

    it is really intresting

    | Reply Posted 10 years, 3 months ago
  3. * xxxx says:

    will i did understand more thigs than ever before about space and black holes

    | Reply Posted 10 years, 3 months ago
  4. * Kenneth Wade Wilson says:

    If there was a tiny blackhole within reaching distance of you and you were to reach out and touch it, sticking your finger into the event horizon, getting it back would NOT be a matter of escape velocity. The problem would be the fact that time flows slower near a mass than it does away from it. So, sticking your finger into the event horizon would cause it travel much slower in time than you would be. So, if you tried to pull it out, it would not come out as fast as you moved your arm and you would likely lose it. Indeed, your heart would not be able to pump blood through it once it was below the event horizon, so it would be lost and you’d have to tear it off by pulling your arm away. Of course, this is an oversimplification, for your body outSIDE the event horizon would be torn apart by the time rate differences between the parts closest to the black hole and the parts farthest. Your nipples would go one way, your shoulderblades the other, and what was left of you outside the black hole would fall into it almost as rapidly rapidly. A black hole is a charicature of a planet, with the same effects on you as an astronaught, but over much, much shorter distances. The amount of energy you would need to keep yourself from falling into it at reaching distance would be a saturn five rocket going full blast, just to give you a hover. Escape velocity wouldn’t enter into it. You could leave Earth at five feet per minute if you had enough fuel. Escape velocity only applies to objects that aren’t being pulled or pushed away and are therefore in “free fall” in a trajectory upward. Such an object must travel at escape velocity, or its orbit will decay and pull it back into the black hole.

    | Reply Posted 6 years, 3 months ago
  5. * Kenneth Wade Wilson says:

    In other words, escape velocity initially refers to groundspeed while in orbit, not to how fast you are moving upwards. Escape velocity actually starts out equal to groundspeed and goes down. As your groundspeed reaches escape velocity, you begin to move upwards away from the ground faster and faster until your movement away from the center of the Earth becomes equal to your escape velocity if you haven’t accellerated any more than that. During this, your groundspeed approaches zero plus or minus a fudge factor of how fast the planet is rotating. If Earth rotates at two thousand miles an hour, then your Earth ground speed while you are nearing Alpha Centauri at Earth’s escape velocity is two thousand miles an hour and your velocity away from earth is earth escape velocity. These relationships are never mentioned in explanations of event horizons around black holes, so there is likely a LOT of disinformation about black holes, even in textbooks. It aught to be explained the way I have just explained it. It isn’t. I was adopted and raised by a university scientist. He told me science is a marketplace of ideas, so let the buyer beware.

    | Reply Posted 6 years, 3 months ago

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