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Sunday, 6 December 2015

Ask An Astronomer Anything At All About Astronomy (XI)

For some reason last week's post got a lot of attention, so this became a busy week. Find the complete list on the Q&A page.

1) Were there larger stars in the distant past ?
Yes.

2) Were there larger planets in the distant past ?
No.

3) Have we detected aliens ?
No.

4) Why shouldn't we assume mysterious signals are aliens ?
Because we don't know what alien signals would look like.

5) Why are we assuming aliens would communicate using technologies we can detect ?
The clue is in the question.

6) How come I can see Polaris all year round ?
Geometry.

7) Is the expansion of the Universe really like a balloon ?
Ish.

8) Do gravitational waves cause winds in space ?
No.

9) Will the collision of the Milky Way and Andromeda cause dangerous gravitational waves ?
No.

10) Was Praxis, the Klingon moon, destroyed by a gravitational wave ?
No.

11) Why do different planets move at different speeds ?
A wizard did it.

4 comments:


  1. Step 2. I asked a question if a man made black hole could be created in space, to make it grow large you could place it near a star, or maybe inside a moon for it to consume matter quickly to not disappear, and or evaporate.
    But hawking radiation is a problem with this, and someone said on a answers forum "You'd have to precisely aim the mass of a mountain range at it in 10^-20 seconds (say), to keep it from evaporating, and anything that approached it would be exposed to super high temperatures blasting away from it".
    Also another person also said on the answers forum "The smallest microscopic black hole that you can theoretically make is has a mass of 1 Planck Mass, and a diameter of 1 Planck Length. Unfortunately, this black hole will also evaporate in about 1 Planck Time, so you won't even be able to feed anything into it at the speed of light before it disappears".
    So how to get around this problem, if the microscopic black hole instantly has matter to feed on if you created the microscopic black hole in either a moon, or a planet, to get larger in mass, would this fix the problem.
    But would the radiation push the moon matter away from the microscopic black hole, so would the black hole not be able to grow big this way.


    Thank you for your help, anything helps.


    ReplyDelete

  2. Step 2. I asked a question if a man made black hole could be created in space, to make it grow large you could place it near a star, or maybe inside a moon for it to consume matter quickly to not disappear, and or evaporate.
    But hawking radiation is a problem with this, and someone said on a answers forum "You'd have to precisely aim the mass of a mountain range at it in 10^-20 seconds (say), to keep it from evaporating, and anything that approached it would be exposed to super high temperatures blasting away from it".
    Also another person also said on the answers forum "The smallest microscopic black hole that you can theoretically make is has a mass of 1 Planck Mass, and a diameter of 1 Planck Length. Unfortunately, this black hole will also evaporate in about 1 Planck Time, so you won't even be able to feed anything into it at the speed of light before it disappears".
    So how to get around this problem, if the microscopic black hole instantly has matter to feed on if you created the microscopic black hole in either a moon, or a planet, to get larger in mass, would this fix the problem.
    But would the radiation push the moon matter away from the microscopic black hole, so would the black hole not be able to grow big this way.


    Thank you for your help, anything helps.


    ReplyDelete
    Replies
    1. I would say the fact that microscopic black holes don't grow any larger is something to be thankful for, rather than a problem to be avoided. :)

      I suspect there is a threshold for the black hole's mass below which Hawking radiation is so strong that it becomes literally impossible for the hole to grow any more. At the atomic scale, radiation pressure is going to push any infalling matter away, delaying it so that the hole completely evaporates. At that size (I can't give you a numerical estimate) there is no hope - the initial mass of the hole simply has to be larger.

      Your best bet for creating a black hole which stands the greatest chance of growing would be to start inside a neutron star where the density is fantastically high. That will give you the maximum rate of mass accretion into the hole, so you can start with the smallest hole possible. You can't do this on Earth though, because without the tremendous mass of the neutron star to pressurise it, neutron-degenerate matter is extremely unstable and explodes.

      Basically, man-made macroscopic black holes are (for the foreseeable future) not a thing.

      Delete
  3. Just continue composing this kind of post.
    Polarlichter

    ReplyDelete