Black Holes (Physics)

Member Name: Midrange

Product:	Physics
Date:	05/04/09 (15 review reads)
Rating:

Advantages: Black Holes - mysterious, invisible, fearsome and fascinating

Disadvantages: I haven't gone into any deep science here - message me for more information

The Supermassive Black Hole

As you may have heard in the news recently, our galaxy contains a Supermassive Black Hole or, as astronmers like to call them, a SMBH. As a physics teacher, one of the most frequent questions I am asked about this discovery is: "are we all going to get sucked into it!?" The quick answer is, reassuringly, "No". The probing mind will them ask, "why not?" and it is this question that I will answer here.

What is a Black Hole?
First of all, it's worth explaining what one of these beasts actually is. I apologise to you as the reader if I am moving too slowly. If you get impatient with my overly cautious style, please leap ahead.
A black hole is defined as "a region in space in which the gravitational field is so strong that nothing can escape once it has passed the event horizon". The key points to draw from this are
1) It's a region in space - that is, it takes up a certain amount of space, it is a "thing". I
2) Nothing can escape once it gets too close. Nothing can escape, not even light.
3) If you are OUTSIDE the event horizon, you are safe. If you are INSIDE the event horizon, you can never get out.

The event horizon is like an imaginary "safety barrier" around the black hole. Any intrepid explorer, particle of dust, or beam of light that crosses over the event horizon will be unable to move fast enough to escape the clutches of gravity. The heavier the black hole, the larger the event horizon is. More mass = more weight = larger event horizon. The SMBH in the centre of the Milky Way has a mass of roughly 1,000,000 times our Sun. The event horizon, therefore, stretches far out into space... but here's the key...

Why Don't We (on Earth) Get Sucked In?
The Earth, and more specifically the solar system, are billions of miles away from the centre of the Galaxy. The event horizon of the SMBH doesn't even get close to us. Indeed, the event horizon only stretches out a couple of hundred miles.
The Earth won't get sucked into the SMBH for the same reason the Earth doesn't get sucked into the Sun. As the Earth is in orbit, it is always moving with some speed to escape the gravitational pull of the Sun. It's a bit like rolling a penny into one of those conical money boxes - the penny rolls around and around and doesn't fall in instantly. The difference with the Earth-Sun system is that the Earth doesn't spiral inwards.


The Strangest Objects in Space
So now we know that we're safe from the SMBH - and any distant Black Hole for that matter - let's finish by looking at some of their unusual properties...
- Black Holes are infinitely dense. This is not to say that are infinitely heavy. Mathematically, BHs have been shown to contain a singularity at their centre - a point in space where ALL of their mass is concentrated. Density is defined as mass per unit volume. If the volume is zero, then the density is infinite. The singularity is not very well understood - what happens to space and time at a point of infinite density?
- Time travel. If you were able to sit right on the event horizon (neither escaping or falling in) you would be able to look out into the Universe and witness the whole of time rushing past your eyes. It would be like watching a movie in fast-forward. If you were able to survive at the event horizon (which you wouldn't, see below) you could use this bizarre effect as a form of time travel. You could step forwards from the event horizon and essentially step forwards in time.
- Spaghettification. As an object approaches a black hole it will experience different forces of gravity along it's length. The front of the object will be pulled more by gravity than the rear - simply because the front is closer to the Black Hole. This will cause the object to be drawn out along it's length - like a long string of spaghetti. Any human being undergoing this painful experience would be unlikely to a) enjoy it, b) survive. The same would be true for space ships or probes.
- Fundamental Particle? Black Holes have only mass, spin and (arguably) charge. This is similar to the fundamental particles of nature - protons, neutrons, electrons etc. Could it be that Black Holes are just an extremely massive type of fundamental particle?
- All across the Universe we find black holes. It is believed that one is found at the centre of every galaxy - perhaps they are crucial to galaxy formation - a central hub for all other stars to orbit around?
- Evaporation is the only way a Black Hole can be "killed", so to speak. Steven Hawking developed the theory of how Black Holes could gradually dissolve away over time. The larger the black hole, the longer it will take to evaporate - just like a puddle of water in the sun.
- Micro Black Holes - undoubtedly you'll have heard of the Large Hadron Collider at CERN. One of the risks of turning on the LHC was the possible production of a microblack hole. The key feature about microblack holes is their size - because they are so small they are liable to evaporate (that's the technical term) very, very rapidly. For this reason they were deemed to pose no threat, and the LHC project went on.

Summary: Non-technical article about the fascinating objects known as "Black Holes"