There's something in the darkness...
So I've been seeing more and more stories about an elusive thing known as 'Dark Matter'. If you've watched the latest Star Trek
film, you might immediately draw parallels with the 'Red Matter' idea they use in it. The difference is, Dark Matter is not science fiction. There is an increasing amount of evidence that this stuff really exists and that it is absolutely fundamental to life as we know it. Given its apparent importance, I think I should probably explain what the big deal is, namely;
What the heck is it. Well, the best answer I can give is that it is matter that we can't see directly. When we look at the sky, we can see stars, moons and planets because they all either emit or reflect light. Dark Matter, however, is stuff that doesn't reflect or emit light. So all the talk of exotic, hypothetical particles is somewhat misguided - there is no reason why it can't just be forms of matter that we already know, but in places that we can't see. Alternatively, it may indeed be a new kind of matter - stuff that doesn't interact with light at all, and only shows itself by its gravitational influence. So that it could be all around us, all of the time...but we'll never be able to touch/see/feel it. Somewhat creepy, I feel.
It is of great importance to understand it. All evidence (that I outline later) suggests that around 80% of the whole universe is made up of the stuff! That's four-fifths of the universe that we cannot see or, as yet, understand. Being able to comprehend what's going on, then, will advance our knowledge significantly. Indeed, there is a chance of creating a light-speed spacecraft powered by the stuff. Are there aliens out there that have already figured this out? Are they currently zooming around the universe at light-speed with Dark Matter space transport? Maybe.
So then, where did this newfangled idea come from? Well, somewhat surprisingly, the evidence for Dark Matter started appearing in 1934, by the Swiss astronomer Fritz Zwicky. Unfortunately, Zwicky was a rather horrible man, had few friends and was considered by many as a bit of a loon. As a result, his ideas were seen in a bad light, thought ridiculous and ultimately ignored. Later on, astronomers and cosmologists alike started seeing more evidence for Dark Matter and it soon built up. Just some of the evidence includes;
There are many other, more technical, evidence that shows an apparent need for Dark Matter.
So that's that then. Dark Matter must exist. Not so - there are a few competing theories. The most basic of which says that we don't truly understand the nature of gravity, so all calculations to do with it are fundamentally flawed. It is a long held belief that General Relativity
(the accepted theory of gravity, I will use the acronym GR) is not a complete theory, so this counter-argument is perfectly acceptable. For example, GR does not explain what happens inside a Black Hole either.
Another idea is that Dark Matter could actually be neutrinos, particles physicists have known about for a long time. We already know that these do not interact strongly with regular matter (but can be detected, if you push hard enough) - but there is no adequate evidence that there are enough neutrinos in the universe to account for all the apparent Dark Matter. There are astronomers that are working on this, though, so it's a relatively popular idea.
There you have it then - Dark Matter in a blog. Still not really clear on it? Join the club. It seems every time we hope to tie all of Physics up in a little bow, something like this pops out of nature that we just don't know how to deal with. In all likelihood, we could fully understand what Dark Matter is in the years to come, feel good about ourselves, only to be slapped in the face by Mother Nature once again. But this is the reason I truly do love Physics - it's complete chaos.
film, you might immediately draw parallels with the 'Red Matter' idea they use in it. The difference is, Dark Matter is not science fiction. There is an increasing amount of evidence that this stuff really exists and that it is absolutely fundamental to life as we know it. Given its apparent importance, I think I should probably explain what the big deal is, namely;- What on Earth even is it?
- Why is it of such importance to understand it?
- Where does the idea come from and what is the evidence for it?
What the heck is it. Well, the best answer I can give is that it is matter that we can't see directly. When we look at the sky, we can see stars, moons and planets because they all either emit or reflect light. Dark Matter, however, is stuff that doesn't reflect or emit light. So all the talk of exotic, hypothetical particles is somewhat misguided - there is no reason why it can't just be forms of matter that we already know, but in places that we can't see. Alternatively, it may indeed be a new kind of matter - stuff that doesn't interact with light at all, and only shows itself by its gravitational influence. So that it could be all around us, all of the time...but we'll never be able to touch/see/feel it. Somewhat creepy, I feel.
It is of great importance to understand it. All evidence (that I outline later) suggests that around 80% of the whole universe is made up of the stuff! That's four-fifths of the universe that we cannot see or, as yet, understand. Being able to comprehend what's going on, then, will advance our knowledge significantly. Indeed, there is a chance of creating a light-speed spacecraft powered by the stuff. Are there aliens out there that have already figured this out? Are they currently zooming around the universe at light-speed with Dark Matter space transport? Maybe.
So then, where did this newfangled idea come from? Well, somewhat surprisingly, the evidence for Dark Matter started appearing in 1934, by the Swiss astronomer Fritz Zwicky. Unfortunately, Zwicky was a rather horrible man, had few friends and was considered by many as a bit of a loon. As a result, his ideas were seen in a bad light, thought ridiculous and ultimately ignored. Later on, astronomers and cosmologists alike started seeing more evidence for Dark Matter and it soon built up. Just some of the evidence includes;
- Galactic Rotation Curves. Stars in a galaxy orbit a central point (the galactic centre) and, with our understanding of gravity, things that are further away from the centre will orbit it slower. This is akin to the theory of space flight - as soon as you push enough to escape the gravitational pull of the Earth by going far enough away from its centre, you no longer fall as fast back to Earth as you do when you, say, jump up. However, what we see is that the stars that are further away from the galactic centre do not orbit it slower. In fact, all the stars in the galaxy orbit at pretty much the same speed. This is very confusing and implies that there is more mass than we can see, causing the stars to speed up - Dark Matter.
Galaxy rotation curve - the data points as you go right along the horizontal axis should fall downwards along the vertical according to our theory of gravity. However, it does not. The strange steep slope at the beginning need not concern you, this is something astronomers know about and can explain.
- Gravitational Lensing. This is a technique that uses Einstein's General Relativity, which basically says that light is bent by things with a large mass (in fact, it is bent by all things with mass, but only noticeable with large masses). The amount it is bent is proportional to the mass of whatever is bending it. Astronomers look at light coming from far away, see it being bent by a galaxy and from that can work out the mass of that galaxy. They can then compare this value with calculations of mass of the visible matter in the galaxy. However, what these calculations show, again, is that there is more mass there than we can physically see - Dark Matter.
Gravitational lensing
There are many other, more technical, evidence that shows an apparent need for Dark Matter.
So that's that then. Dark Matter must exist. Not so - there are a few competing theories. The most basic of which says that we don't truly understand the nature of gravity, so all calculations to do with it are fundamentally flawed. It is a long held belief that General Relativity
(the accepted theory of gravity, I will use the acronym GR) is not a complete theory, so this counter-argument is perfectly acceptable. For example, GR does not explain what happens inside a Black Hole either.Another idea is that Dark Matter could actually be neutrinos, particles physicists have known about for a long time. We already know that these do not interact strongly with regular matter (but can be detected, if you push hard enough) - but there is no adequate evidence that there are enough neutrinos in the universe to account for all the apparent Dark Matter. There are astronomers that are working on this, though, so it's a relatively popular idea.
There you have it then - Dark Matter in a blog. Still not really clear on it? Join the club. It seems every time we hope to tie all of Physics up in a little bow, something like this pops out of nature that we just don't know how to deal with. In all likelihood, we could fully understand what Dark Matter is in the years to come, feel good about ourselves, only to be slapped in the face by Mother Nature once again. But this is the reason I truly do love Physics - it's complete chaos.
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