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The anthropic principle suggests that perhaps the universe is the way it is because if it were any other way, we would not be here to ask the question. However, this principle can't explain why the universe continues to exist once that question has been asked. One would expect that if this was the simplest possible universe that allowed this question to be asked, then it would end pretty soon afterwards.

Perhaps there is an extension to the anthropic principle that says something like "this universe is the simplest possible universe in which some particular task can be achieved". Perhaps that task is understanding the universe (as in the Douglas Adams principle "if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable" and it's corollary "this has already happened - several times"). Or maybe it's something else, as the universe does still seem to be here.

Here's an interesting little analogy I thought of the other day. In the theory of Speciai Relativity (SR) the result of any experiment is the same if the entire experiment is moving at constant speed in a constant direction, so there is really no way to tell if you are stopped or moving at a constant speed, so the concept of "absolute rest" is not really meaningful from a point of view of science (you can only be "not moving" with respect to something, like the planet Earth for example).

Somebody who believes in God is kind of like somebody who believes that they are at absolute rest. There is no way to prove or disprove that claim, scientifically speaking, but if it makes them happy and it isn't doing anybody any harm then there's nothing wrong in believing that.

The trouble comes, of course, when somebody else believes just as strongly in a completely different god, or when they are moving at 100 miles per hour towards the other believer and claiming that it is they, not the other guy, who is actually in a state of absolute rest. If the two believers are firm in their beliefs they may choose to disagree amicably as they pass each other, or if they are fundamentalists they may choose to blow the heads off the heretics and unbelievers. This is where religion (and unprovable beliefs in general) stop being harmless and start to become a real problem for society.

Perhaps that is the real answer to the "science verses religion" debate. Science deals in relative things, religion deals in absolutes. Science is perfectly compatible with any given religion but two different religions are fundamentally incompatible with each other. Hence there are plenty of Christian scientists and plenty of Islamic scientists but no Christian Islamists.

My philosophy is that one should believe what one likes, but should let others believe what they like as well and should not try to blow the heads off those who disagree with you. Don't try to push your absolutes on someone else who may have their own absolutes already. We can all agree as long we only speak in relative terms ("this guy is at rest relative to the Earth") rather than in absolute terms ("this guy is at absolute rest). Relatives are universal and absolutes are personal. Science is universal and religion is personal.

This post is about something that almost certainly doesn't exist and what it would be like if it did.

The things in question are Cosmic strings. These are not to be confused with the superstrings of string theory (which are generally sub-atomically tiny). Cosmic strings are very big things and you would know if you had one in your back yard.

If you have a bunch of objects (be they atoms or grains of sand or planets or black holes) and you put them in a row far out in deep space, gravity will eventually pull them all together. But suppose you had an infinite number of such objects and you were somehow able to put them into an infinitely long row. There is no way for gravity to pull them together because there is no center of gravity to pull them into - each object is pulled up by all the objects above it and equally down by all the objects below it.

Even so, tiny irregularities in the spacing of the objects in such a string would tend to cause them to condense out into increasingly large (and widely spaced) lumps like droplets of water on a dew-laden spider's web. But what if the string is perfectly homogeneous? Then there is no way for such irregularities to creep in and the structure is completely stable. This is something like what a cosmic string would be like.

It was thought (before a better explanation came along) that such strings could have formed very early on in the development of the universe (as "topological defects"). Such strings would be very dense - about the mass of Earth for each mile in length, but thinner than a proton. So they would be sort of like black holes stretched out along an infinite line instead of confined to a particular point. But despite all this mass, they would not cause a gravitational force. Instead, their effect on space-time would a bizarre and subtle one. Suppose you took a circular walk around a cosmic string, facing it all the time. When you got back to where you started you'd think you would have turned through 360 degrees, but due to its weird effect on the surrounding geometry, you would actually have found that you would have turned through slighty less than that. Circles only have about 350 degrees instead of the usual 360 when they encircle a cosmic string.

Cosmic strings could also be used as a time machine. If you were to have two parallel cosmic strings passing close to each other at very high speed and you move around and between them in just the right way you could theoretically go back in time. Unfortunately (or perhaps fortunately), this isn't very practical for going back to last thursday week and warning your earlier self that it would probably be better for all concerned if he/she/you didn't get out of bed that day (one of the most common reasons for wanting to travel through time).

Following on from yesterday's post...

Suppose the hypothesis that the universe is infinite is true, and that any event with a finite probability will happen an infinite number of times. Now, given a set of observations about the universe (such as every observation you have ever made), what is the probability that those observations were made in the first stage of the universe (i.e. the one we have generally assumed that we were in) and what is the probability that those observations were made in one of the macroscopic quantum fluctuations in the time after the heat death of the universe?

The set of observations is finite, so the amount of matter required for them to be made is finite. So quantum fluctuations will cause these observations an infinite number of times for each sufficiently large volume of space. In the first stage, those observations can happen at most once for the same volume of space. So given the set of observations, it's practically certain that they are post-heat-death.

But if the universe is post-heat-death, why can we see so much? A much smaller universe would have allowed for almost as many observations (including the existance of the entire human race) but be much more likely. Perhaps the human race (and hence somebody to observe the universe) cannot exist without all those very distant galaxies.

No-one knows exactly how the universe will end. There are several possibilities. One is that everything will fall into black holes, which will then fall into each other and coalesce until every particle in the universe is in one place just as they are thought to have been at the beginning.

A second possibility is that the expansion of the universe will accelerate faster and faster until all the fundamental particles are ripped apart from each other at speeds exceeding the speed of light so that they can never affect each other again and every little particle effectively ends up alone and impotent in its own universe, allowing nothing more complicated to exist.

A third possibility is that the universe is completely balanced between those two extremes, Goldilocks-style, and will continue to expand but at a decreasing rate so that it would theoretically stop expanding at all, but infinitely far in the future. In this case life within the universe can theoretically go on as normal for a very long time. However, eventually all the stars run out of fuel. The smaller ones evolve into white dwarves, then cool to brown dwarves and black dwarves. The larger ones evolve into black holes and neutron stars. Eventually all the protons in the black dwarves decay into positrons and gamma rays and the black holes evaporate via Hawking radiation until only the neutron stars are left. Much later, the neutron stars quantum tunnel into black holes which then themselves evaporate relativity rapidly. At this point the universe is just a homogeneous sea of electrons, positrons and photons. The electrons and positrons will eventually annihilate leaving only photons. From then on, nothing really changes.

But here's the weird part. We have a universe, empty apart from some weak radio waves, for an infinite period of time. Now, quantum-mechanically it is possible for empty space to just create a small piece of matter and a small piece of antimatter spontaneously, from nothing. In fact, this is happening all the time but normally these annihilate again in a very short time. It's very unlikely, but sometimes these particles will stay around a little bit longer. In some cases they may even be around for long enough to be joined by other fluctuation-generated particles. Very rarely you'll get a whole bunch of such particles together at once. Even more rarely still there will be enough of these particles to form an entire planet or solar system or stellar cluster or galaxy or galactic cluster or even a pile of matter the size of the currently observable universe. These things are all incredibly unlikely but given an infinite amount of time even the most unlikely things will eventually happen so long as they are possible.

So eventually, every sequence of events that has ever been played out will play out again just by random chance. And every possible sequence of events involving a finite amount of matter (including your life, and mine, and all conceivable variations thereupon) will play out just by random chance, an infinite number of times.

When I explained this to my friends, they said "wait a minute, so you believe that (given the universe is flat, the third possibility), at some point in the future the following sequence of events will happen:

1. a perfect replica of the Earth as we know it will spontaneously form from nothing
2. all of the salt dissolved in all of the water in all of the oceans of this replica world will spontaneously leap out of the oceans, hundreds of feet into the air
3. this salt will then spontaneously form itself into a giant peanut orbiting the planet
4. the peanut will spontaneously turn into a small green shining baby and
5. all of this will happen an infinite number of times?

I had to confess that while phrased like that it did seem rather ridiculous, that it what the theory predicted. Some of my friends are rather strange people.

Continuing on yesterday's theme of light being bent by gravity, what happens when gravity is really strong, such as around a block hole? At 1.5 times the Schwarzchild radius (the "point of no return") of a black hole, light is bent so much that it actually orbits around and around the black hole! If you were at that distance from a black hole and looked out at a tangential angle, the surface below you would look perfectly flat and you would see the back of your own head all along the horizon!

But if light going around in circles is weird, time going around in circles is even weirder. According to General Relativity, deep within the bowels of certain rotating black holes it may be possible to move around in a circle and end up not only where you started but also when you started. Gravity is so strong that it bends time into loops and events can occur which are their own cause and their own effect. A time machine. All this is hidden behind the event horizon of the black hole so we could never actually observe this time travel going on but it's pretty weird nonetheless.

This is one of the most mind-boggling things I learnt about while studying physics at university. That and one of the last lectures I ever attended, which was given by Sir Martin Rees and was on the subject of the fate of the universe in the extremely long term. I'll write about that tomorrow.