Date: July 27, 2011
Title: Ripped Apart by a Black Hole
Podcaster: Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
Description: Black Holes. The very term brings up images of extreme astrophysics, of science fictional weirdness, of scary objects out there just waiting to swallow you up. Are you afraid of being dropped into a black hole? You should be; it would be bad. What’s more, you shouldn’t only be afraid of being dropped past the event horizon of a black hole. You should be afraid of getting ripped apart on the way in!
Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.
Sponsor: This episode of “365 Days of Astronomy” is sponsored by — NO ONE. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
RIPPED APART BY A BLACK HOLE
Hello, and thank you for listening to 365 Days of Astronomy! I am Rob Knop, Professor of Physical Science at Quest University Canada.
Black Holes. The very term brings up images of extreme astrophysics, of science fictional weirdness, of scary objects out there just waiting to swallow you up. Black holes are left behind by the explosions of really big stars, and supermassive black holes are found at the cores of galaxies. There is no doubt that black holes are among the most glamorous of astronomical objects– which is pretty impressive for an object that emits no light. Dropping something into a black hole is the quintessential way to get rid of it; once it’s gone past the event horizon, it can never get out.
Are you afraid of being dropped into a black hole? You should be; it would be bad. Of course, chances of it happening are a whole lot less than chances of you dying peacefully of old age in your bed, but still. What’s more, you shouldn’t only be afraid of being dropped past the event horizon of a black hole. You should be afraid of getting ripped apart on the way in!
How do black holes rip apart things that are falling into them? To understand that, we’re going to have to talk about the ocean. If you’ve been to the beach by the ocean, you know about tides. At “high tide”, the ocean is higher than it is at “low tide”. If you leave your shoes on the sand near the edge of the ocean at low tide, they may get washed away by the water when the tide comes in and the ocean rises. Why is it that the ocean tides come and go? What makes it rise and fall in its regular pattern, high tide coming approximately twice a day?
Tidal forces, as physicists call them, surprisingly have nothing to do with the ocean. The ocean is just responding to tidal forces. Tidal forces have everything to do with gravity. Tidal forces are just when the strength of gravity is different at different distances from some object. In the case of the tidal forces that cause the ocean tides on Earth, the object that’s the most important is the Moon, although there’s also a contribution from the Sun. The Earth, as you know, is a ball, and the Moon orbits the Earth. At any moment, there will be a side of the Earth that is closer to the Moon than is the center of the Earth. The opposite side of the Earth is farther from the Moon than is the center of the Earth.
Gravity is stronger the closer you are to something. On the surface of the Earth, we experience the strength of gravity as being the same everywhere. However, that’s because we never get very far from the surface of the Earth. The radius of the Earth is about 6000 kilometers. If you climb up a building so that you’re 100 meters above the ground, that hardly makes any difference in comparison to 6000 kilometers. However, when you consider the entire diameter of the Earth, which is about 13,000 kilometers, the difference in distance from one side to the other side does start to be important.
The Moon stays in orbit around Earth because it is held in orbit by the Earth’s gravity. Just as the Earth pulls on the Moon due to its gravity, the Moon pulls back on the Earth. The gravity of the Moon on the side of the Earth that happens to be closer to the Moon is stronger than the gravity on the far side. This is a tidal force! It’s a difference in the strength of gravity at two different positions. Because gravity is stronger on the parts of the Earth closer to the Moon, those parts would tend to get stretched inwards. Likewise, because gravity is weaker on the parts of the Earth farther from the Moon, those parts would tend to drift outwards. The net result is that the tidal force of the Moon acting on Earth tries to stretch the Earth out.
However, the Earth doesn’t really stretch all that well. It’s being held together by its own gravity. What’s more, most of the stuff of the Earth is solid, and doesn’t flow around very well. However, bits that do flow will do so. We notice that most with the water in the oceans that make up much of the surface of the Earth. While the whole Earth hardly gets stretched at all, the flowing parts– the water– get stretched more. On the side of the Earth facing the Moon, you get a buildup of water– that’s the stuff that’s been pulled in closer to the Moon. You also get a buildup of water on the side of the Earth opposite from the Moon– that’s the stuff that’s tried to drift away as a result of weaker gravity. If we’re at a point on Earth where there’s a pile-up of water, we would call it high tide. If we’re at a point where the water has moved away, we would call it low tide. As the Earth rotates once every day, we will experience two high tides; one when the moon is overhead, and one when the moon is directly beneath our feet, overhead on the opposite side of the Earth.
Have you ever noticed that your weight is different at midnight with a full moon overhead than it is at noon on the same day? Of course you haven’t! What this tells you is that the gravitational effect of the Moon is much smaller than the gravity of the Earth if you’re on the surface of the Earth. Nonetheless, it is this gravitational effect, acting over the whole of the Earth, that drives the ocean tides. So, if you’ve ever been caught in a tide at the ocean, you have indirectly felt the effect of the Moon’s gravity even though you’re on the surface of the Earth. But, you will never feel it directly.
In order for tidal forces to be strong enough so that you can feel them directly, you need to get very close to a very massive object. That tends to be pretty difficult. You don’t feel the tidal forces of the Earth; you don’t feel the Earth’s gravity trying to stretch you out and rip you apart. Could you get closer to the Earth than the surface? Not really. You could bore a tremendously deep hole and get closer to the center of the Earth, but then some of the Earth would be above you, and no longer pulling you all towards the center of the Earth. How about a more massive object? Surely the tidal forces at the surface of the Sun are greater than they are at the surface of the Earth? Actually, they aren’t! While gravity itself is nearly 30 times stronger, it turns out that because the Sun is so much bigger than the Earth, the stretching effect due to tidal forces at the surface of the Sun is even less than the stretching effect at the surface of the Earth… which already you can’t feel.
So how do you get close enough to something massive enough in order to create appreciable direct tidal forces? You need something very massive, and very small. The smallest that you can make something of a given mass is, you guessed it, a black hole. A black hole the mass of the Sun would have an event horizon at a radius of only three kilometers. You’d never make it there, though. As you get close enough to something that massive, the rate at which gravity changes with distance becomes huge. If you were falling feet first towards a solar mass black hole, the strength of gravity on your feet would be so much stronger than the strength of gravity on your head that you’d be torn apart long before you were able to fall into the event horizon.
Now, you might think that the bigger the black hole, the worse this stretching effect might be. However, it turns out that this is not the case. In fact, the bigger the black hole, the weaker the tidal forces get at the event horizon. So, if you were falling into a black hole that was several thousand times the mass of the Sun, you wouldn’t get ripped apart at all; that is a black hole that you could make it through the event horizon without being turned into paste first. If you want to go jumping into a black hole, don’t find one of the stellar mass black holes that are undoubtedly scattered throughout the disk of our Galaxy. Rather, go to the center of our Galaxy, and jump into the 4 million solar mass black hole that you can find there.
All of this is great as a thought experiment, but does it ever actually happen? Well, yes; we’ve already talked about the ocean tides. If you’re more than 20 or so years old, you may remember comet Shoemaker-Levy 9, the comet that crashed into Jupiter back in 1994. However, it wasn’t one humongous crash, but a whole bunch of big crashes spaced out over the course of a week. The reason was that on a previous pass by Jupiter, the comet– which was only weakly held together in the first place– got torn apart into a bunch of fragments by the tidal forces of Jupiter.
It turns out, however, that we have in fact observed an event that we believe was something being torn apart by the tidal forces of an actual black hole. Back in March of 2011, the Swift satellite observed an X-ray flare that was originally believed to be a gamma ray burst. However, as the same object emitted a series of additional flares, it became clear that this object was something different. Over the course of the next couple of months, Swift observed a series of X-ray flares, declining in brightness and lasting longer each time they happened.
What is probably going on here is that in a galaxy a few billion light years away, a star was orbiting a black hole in an elliptical orbit. As that orbit decayed, the closest pass of the star by the black hole became too close, and the black hole started to rip the star apart. Each time the star made a close pass, more of the material was ripped off of the star. The gas swirled around the black hole, and some of it was then accelerated out through a jet at the pole of the black hole, generating the flares that we saw. As time went by, the star passed closer and closer, getting more and more torn apart, until it was completely ripped up. After that, the gas would swirl around and eventually fall into the black hole. Yes, indeed, it seems that we were seeing the death screams of a star being torn apart by and eaten by a black hole.
Cool.
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.