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The first time I realized that grown-ups were not infallible was when I was trying to find out whether beams of light can bend.

Someone (I don't remember exactly who - either a parent or teacher I expect) told me that light always travels in straight lines. Someone else (another parent or teacher) told me that in fact gravitational fields could cause light bend. They couldn't both be right, so one of them had to have been lying to me. This was a very confusing thing for a young child to understand (not the gravity thing, the grown-ups lying thing) and I vaguely remember being rather upset about it.

I got over it, but it wasn't until I was learning about gravity at university that I found out that they were both right, sort of.

Gravity does affect light - that much has been determined experimentally. I have seen Hubble Space Telescope pictures where a single galaxy seems to appear in multiple places (or is even "smeared out" into a circle called an Einstein ring) due to the light being bent by another galaxy.

But gravity works by causing time and space to be curved in the vicinity of massive objects. What does it even mean for a line to be straight if the space it lies in is curved? It's a similar situation to trying to find straight lines on the surface of the Earth. There aren't any very long ones because the surface of the Earth is itself curved (cue comment about Christian fundamentalists). However, we can find the shortest distance between two points. On the surface of the Earth we can do that by finding a "great circle" (i.e. a circle who center whose radius is the radius of the Earth and whose center is at the center of the Earth) that connects the two points, and following it. That's why the shortest route from London to Seattle goes way up North into the Arctic circle (try it with a globe and a piece of string if you don't believe me).

Similarly, we can find the shortest path between two events in spacetime (such as a photon leaving a distant galaxy and that same photon entering your eye or telescope), even if that spacetime is curved by gravity. Some of these shortest paths turn out to be exactly the paths that beams of light follow. So light does actually follow straight lines with the right definition of "straight", even when it is being bent!

What is even weirder is when you look at shortest paths through spacetime when the thing moving through spacetime is not moving at the speed of light. In empty space, these are exactly the straight lines you would expect. But in a gravitational field, they are curves. In fact, if the gravitational field is large enough in spacetime these curves are parabolas - i.e. the sort of curves you get by throwing some object and watching the arc it makes. Now these are obviously not straight paths in space, but it turns out that they are "straight" paths in spacetime. So now when watching a baseball game you can confuse your friends by saying "my, look at that spacetime geodesic" when someone hits a fly ball.

When you sit on a swing and a friend twists the swing up and then lets go, you spin around as the swing untwists. If you are spinning fast enough, you'll notice that your extremities seem to get "pulled away" from the axis of rotation. They don't really, of course, it just seems that way because the force you need to exert on them to keep them moving in a circle is towards you just like the force you'd need to exert to counter an outword pulling force.

According to the theory of General Relativity, if you were standing still and all the matter in the universe were rotating around you (with a speed proportional to distance from you), the motions of all this matter would exert the same forces on you that you feel by spinning around in a non-spinning universe. In other words, there is a sort of symmetry between spinning around in a non-spinning universe and staying still in a spinning universe. This line of thinking suggests that in a universe which was completely empty apart from you (if that were possible) you would be able to spin around without experiencing these centrifugal effects.

But the only reason we feel these effects is because you need to exert a force to accelerate things (like limbs) - inertia. This doesn't seem to have anything to do with the rest of the universe. Or does it? Perhaps inertia only happens because of gravitational interactions with the rest of the universe as a whole, and that whenever we experience the relationship between force and acceleration we can infer the existence of the rest of the universe from that. Perhaps there is something in the mystical idea that "all things are connected" after all.

One of the most beautiful and general principles in physics was discovered by Emmy Noether. Hers is a fascinating story by itself, but I am a physicist not a biographer, so here comes some science-y stuff. Noether's theorem says that for every symmetry in a system, there is an associated quantity that is conserved. To see what that means in practice, it is useful to look at some examples.

Space is symmetrical in that (in the absence of matter like electrons, protons and galaxies), one piece of space looks very much like another piece of space. If I do an experiment in one part of space, then slide it over to another part of space and perform the experiment again, the result will be the same. This symmetry leads to conservation of momentum. If the second piece of space is different from the first piece of space (for example because it has a planet in it) momentum will not be conserved as a particle moves from one piece of space into the second (it will hit the planet and its momentum will change).

Time is also symmetrical in that if I do an experiment at one time and then do it again in the same place at a later time, I'll get the same result. This symmetry leads to the conservation of energy.

Another symmetry that space has is rotational symmetry. If I do an experiment with the apparatus pointing one way, then reorient the apparatus and do the experiment again pointing in a different direction, you'll get the same result. This symmetry leads to the conservation of angular momentum. Near the surface of the earth there is a rotational asymmetry due to gravity (there is a "special" direction - down). This assymmetry causes a pendulum to change angular momentum as it swings backwards and forwards (if you do it in a symmetrical place, such as far away from any sources of gravity, it will go around and around in a circle - it will have constant angular momentum).

Most of the time, our universe acts the same as it would if it was "flipped" the way a mirror-image reflection is flipped. This "mirror image" symmetry leads to the conservation of a property called the "parity" of a fundamental particle. The mirror image version of a particle has the opposite parity. However, it seems that there are some occasions when parity isn't conserved - in these respects our universe acts differently to a hypothetical "mirror universe", identical to ours in every respect except left and right being swapped. The apparent symmetry turned out to be an asymmetry.

I was recently trying to convince someone that that Special Relativity (SR) was more correct (in the situations where it is applicable) than classical (Newtonian) physics. One argument I used is that irrespective of experimental evidence for SR, SR is actually a simpler theory than classical physics, when you write each theory down in their simplest forms.

In this form, physics looks very different from classical "high school" physics. A lot of concepts which are classically very different turn out to be the same thing in relativity. Space turns out to be the same thing as time. Different angles (as in rotation) turn out to be the same thing as motion at different (constant) velocities. Electric fields turn out to be the same thing as magnetic fields.

This theory has a "parameter", a value which isn't predicted by the theory and must be determined experimentally and plugged into the theory to make it complete. This parameter is called "c" and is usually known by its physical meaning "the speed of light in a vacuum".

Now, one could conceivably get classical physics in this same form by plugging in a "c" value of infinity instead of 299,792,458 metres per second. Doing this causes time and space (and angles and velocities, and electricity and magnetism) to separate out like an emulsion of oil and vinegar left to stand for a while. Only the finite value of c causes these concepts to mix (and the smaller the value of c, the more they mix and the more pronounced relativistic effects become).

"Great," you might say, "so Einstein might have been wrong all along and all this weird time dilation/length contraction/mass equals energy stuff could all be bunk." The trouble with that, though, is that with c=infinity, the model corresponds less well to observed experimental results - the time dilation effects that have been measured are not predicted, and the speed of light is predicted to be infinity.

But the place where this model diverges most drastically from reality is magnetism. The c=infinity theory predicts that there should be no magnetism at all. Trying to add magnetism back in to a non-relativistic theory causes all sorts of complexities and irregularities. In fact, it was trying to remove these irregularities that brought about relativity in the first place. Really, the simplest way to have a consistent theory of magnetism is relativity with all the non-intuitive concepts that entails.

I think what I like most about this image isn't the high technology involved in combining images of the same structure from 4 different telescopes, nor even the deep insights into the physics of galactic collisions that it provides. What I like most about the picture is all the pretty colours.

More specifically, I'm fascinated by what it means that this picture is so colourful. What it does mean is that there within this galaxy there are regions of space where qualitively different things are going on. There's got to be at least 7 or 8 different colours in that picture. Each of these colours represents a region of space with a particular distribution of dust and gas and stars of various different sizes. All the different colours mean that there are lots of different such distributions. In some places, space is mostly full of dust and gas. In others, there are stars which are mostly very old. In others, there are stars which mostly very young. Why different areas of space have such different characteristics is a mystery to me - that's why I find the different colours so fascinating.

Here's another fascinating picture. This isn't a painting, an artist's impression or a computer graphic, it's a photograph (albeit a very high-tech one) taken by the Hubble Space Telescope. If you had a sufficiently powerful telescope and pointed it in the right direction, that's exactly what you'd see. It's really hard to get a sense of the incredible scale of this galaxy from the picture, but it's about 50,000 light years across. That means even the finest details you can see in the dust rings surrounding the galaxy are each hundreds of light years across. This thing is just unimaginably big, yet the size of these fine details compared to the size of the whole thing makes it look like something on a much more familiar scale. One would expect something that big to be quite smooth but that isn't the case.

It seems that the universe has a great deal of structure at all scales, like a fractal.

There's a fascinating article on The Edge's World Question Center where they ask a big pile of distinguished thinkers about what they believe, but cannot prove. Curiously, right before I came across this article, I was thinking myself about things that I believe but don't have any evidence for - things that I believe simply because believing them makes me happier than not believing them. In the sense that there are some of these things, I suppose I am religious, though not in the traditional "organized religion" sense (I prefer to choose my own beliefs rather than following a pre-packaged set). For your derision I will outline these beliefs here.

I believe that I have free will, that is that there is an irreducible "something" in the universe that is "me" and that is not bound by deterministic laws of physics (i.e. that if it was simulated perfectly on a computer, no matter how much the simulation might act like me it would still not be me). I belive that quantum mechanics may leave room for this sort of dualism.

I believe that we will eventually figure out how to construct a theory of physics that includes both quantum mechanics and gravity as special cases.

I believe that true love is real - that there is more to this feeling than just a chemical reaction in the brain that has evolved to give children a better chance of survival into adulthood by providing them with stable families. (Call me a romantic.)

I also believe that I will live forever. Not in the Christian sense of living forever in heaven - I believe I will live forever in this universe. This is kind of an unusual one, so I feel I should justify it a bit (though I do not pretend for a moment that this constitutes any sort of proof). Suppose for a moment that Everett's many worlds formulation of quantum mechanics is the correct one - i.e. that whenever any fundamental particle could go either one way or another way, the entire universe is effectively "splits" into two universes identical in every respect except that this one particle goes one way in one universe and the other way in the other universe.

Now, suppose one of these choices lead inevitably to my death, while the other choice allowed me to remain alive. From my point of view, the irreducible "me" continues to exist only in one of the universes, so that is the universe that I experience. I cannot experience my own death because (by definition) I am no longer there to experience it once it is complete.

This seems to have worked pretty well so far - there have surely been lots of quantum mechanical events which, if they had turned out in a way differently than the way they did, would have lead to my death. However, that in itself is not very strong evidence since the probability of my death so far has probably been fairly low, quantum-mechanically speaking (if we discount the improbability of my conception in the first place, which I do because I wasn't alive then). Lots of people have survived to my age in my universe, and none of them quantum-mechanically needed to in my universe in the same way that I quantum-mechanically need to in my universe. However, when one day I become the oldest living human being and continue to live for much longer than anyone else has ever done, this line of thought will be more convincing.

That leads to a rather depressing-sounding scenario - I will get to watch everybody that I love die. The only way I get to die is if the universe inevitably gets to a state where no life at all is possible - if it collapses in a "Big Crunch" or expands at an ever accelerating rate leading eventually to all atoms being ripped apart. Which are also rather depressing scenarios in themselves. However, I find these possibilities significantly less depressing than the possibility of dying, so I don't worry about it too much. I'm excited to see what happens!

Believing this behooves me to support certain causes - those which will ensure I continue to be comfortable in the very long term. I want our planet to continue to be a nice place to live. I want the rest of the human race to not become extinct, so that I always have somebody around to talk to. I also want medical technology to continue to improve so that, no matter how many parts of my body start to fall of, I'll always be able to get them stuck back on or replaced so that I can continue to have a good quality of life. With this belief system these worthy causes are also in my personal interest.

Although I can't die I can still suffer a great deal of pain, so this immortality does not excuse me from having to take care not to get into a car accident, and if you say "so you think you're immortal? Prove it by shooting yourself in the head with this gun" I will refuse since the most likely outcome of accepting would be having to live with terrible brain damage.

Despite the depressing sides, this idea holds a certain comfort for me. Not having any limit on my lifespan frees me from thinking "oh I must do this before I die, and this, and this" and getting frustrated that I probably wouldn't be able to achieve them all. There are still lots of things I want to do, but there's no rush as I have plenty of time. It's kind of the opposite of the "live every day as if it was your last" philosophy.

Note that nothing in this theory mentions me by name, so anyone else can apply this theory to themselves just as I can. From your point of view, you will also experience living until the end of the universe, and experience everyone else dying. There is no contradiction here because of the many worlds interpretation of quantum mechanics - you continue to live forever in your universes, I continue to live forever in mine. You die in my universes, and I die in yours. Of course, you might not believe that this is really how things work, but if things do work like this you don't have to believe it for it to happen. Of course, this theory isn't falsifiable - if this theory turns out to be wrong you won't be able to tell me "I told you so", and if it turns out to be right I won't be able to say "I told you so" either (at least to anyone who has been born already).

While checking the links in this post, I discovered that apparently I am not the first person to have thought of this.

Suppose that some time in the future, humankind figures out a way to make possible time travel into the past. Suppose, furthermore, that it turns out that the universe is holonomic - that is, there is only one past and one future and we can't change it (so we can't create an "alternative 1985" like in Back To The Future II). Effectively, the entire history of the universe would be predestined. So the going back in time would also be predestined. Anything that the time-travellers do while on their trip to the past would have already have happened and would always have happened. So we know that the time travellers cannot kill their past selves (or they would not be in there in the future to make the trip). In fact, anything that they do in the past must guarantee that the time travellers are alive and sufficiently healthy in the future to be able to make the trip back (which would not necessarily be the case if the time travellers were not there). So effectively, the time travellers would be their own guardian angels, "protecting" their past selves (deliberately or not) simply by the virtue of existing in the past.

Now, suppose that you are a future human in possession of a time machine. You don't worry about "responsible time travel" since you can't break anything or rewrite history. It's only natural for you to wonder just how far back in time you can go. With sufficient technological advancement, you might be able to go right back to the moment at the very beginning of the universe. And, just maybe by going back to that point in time you actually initiate the creation of the universe - you become God.

Sometimes I wonder if the universe is like that. But I suspect it is not - I find it very difficult to believe that free will is just an illusion.