Krauss and an Overview of Cosmology

I’ll write a more interesting post soon, but for now, I present Lawrence Krauss. Here he gives, what I’ve decided to be, the best one hour overview of cosmology I’ve ever seen. (via richarddawkins.net)

What do you all think about his suggestion that getting a universe from nothing is natural?

Let’s get philosophical: what’s your existential preference?

Quantum mechanics is weird. It gets even weirder when you try to interpret what the theory is telling you about “reality”. In fact, I’m taking a course at the moment called: Interpretations of Quantum Mechanics. I’m hoping eventually I’ll get some blogable material out of it.

For now, I have a question for you all. It will be a purely subjective question (not like last time). I’ve blogged about the random nature of the quantum world before, and I’ve also given an account of an experiment that demonstrated the requirement (under certain assumptions) for an inherently random world, but the nature of reality is still a hotly debated topic in the world of physics. Some physicists reject the notion of a world that is fundamentally random and instead consider the possibility that we’re not seeing the whole picture. They come up with, so called, hidden variable theories that attempt to explain away the randomness by postulating some hidden property in the small world that we can’t directly measure. I’ve also recently come across a paper that hypothesises that the (random) quantum mechanical nature of the very very small could be an emergent phenomenon; that is to say (in pedestrian terms) we aren’t squinting hard enough to see all of the information about a quantum system and this lack of information results in seemingly random behaviour.

I wish I understood these things well enough to explain them here… but I don’t. Instead I’d like to know what your personal preference is for reality and why.

Which description of reality are you (secretly?) cheering for? Are you more comfortable with the completely non-random deterministic view of the world, or are you instead enjoying the idea of a world built on random behaviour? AND WHY?

Eternal life – Dyson vs. Krauss

I’ve been meaning to post this for a while, but kept putting it off because I anticipated it being a rather long post. Several months ago I attended a lecture given by Lawrence Krauss at the CUPC. He gave us an overview of a “debate” he had with Freeman Dyson about whether or not life could exist forever. Keep in mind, this is not an argument for the likeliness of eternal life, it’s just simply addressing the possibility of it. In physics, the questions about whether or not something is even remotely physically possible are, many times, the most fun! And the ideas Krauss shared with us that originated from his back-and-forth with Dyson were so fun and interesting that I thought I’d take a stab at reproducing an overview of it all here. Keep in mind, I will be glazing over all of the mathematics and so if you want a more in depth look at the derivations of these results you should probably check out the original papers (here is Dyson’s; here is Krauss’s). They are enjoyable to read if you have a physics background (and maybe even if you don’t). So here it goes. Dyson vs. Krauss. But before we begin this faceoff, we need to buckle down and tend to a question that is begging to be answered:

…what do we mean by “life”?

Firstly, I must mention that we are not talking about eternal life for a single being. This debate was focused on eternal life for, say, a civilization albeit one that may evolve. Secondly, living things come in many shapes and forms, some of which we may not yet be aware of. It seems unreasonable to make the assumption that all forms of life are like those on earth; carbon based, dependent on water to survive, etc. In any case, Dyson and Krauss are both physicists and so for the purposes of their debate they were more concerned with the physics of “life” than its biology. Let me put it like this: we are not really concerned with the biological processes that lead to the thought “I think therefore I am”, we are simply concerned with the existence of the thought itself to define “life”. In other words, by “life” we really mean consciousness, or more simply, computation. Consciousness seems to have a lot to do with the firing of neurons which go about processing information much like a computer (or perhaps a quantum computer). Whether or not consciousness is really akin to some kind of computer program is a whole new debate in itself (perhaps some neuroscientist readers can comment on this). Despite this, computation must at least have a lot to do with consciousness and so surely by investigating the eternal existence of computation we won’t be doing too badly.

So, what restricts us from running a computer program for all time? Well, the first barrier is: energy. Hopefully you are familiar with the fact that the universe is expanding. Not only is it expanding, it is expanding at an accelerated rate. It turns out that this puts a constraint on the amount of energy any civilization can harvest to keep them alive (computing). With a finite amount of energy available one might give up at this point and declare that life, which requires energy to sustain itself, can’t exist for an infinite amount of time. Dyson, however, was still optimistic. He realized that living things are less concerned with physical time and are more concerned with, what he calls, subjective time. Living things measure time by the number of thoughts they have, so if a civilization can have an infinite number of thoughts using only a finite amount of energy, one could say that they have achieved eternal life. This subjective time depends on the temperature at which the entity operates. So if we assume that the civilization has the ability to change its temperature at whim, at first glance it seems like the civilization can have an infinite number of thoughts (live for an infinite subjective time) if it keeps decreasing its temperature for all time (getting closer and closer to absolute zero, but never exactly zero). That strategy (again, at first glance) will allow an infinite number of thoughts using only a finite amount of energy.

So, is this strategy really possible? Well, in answering this question we come to the next roadblock: heat dissipation. Computation generates heat (there’s a reason your computer gets warm when you turn it on). Living things will also generate heat. Even if we ignore all of the heat generated from familiar biological functions and only focus on the heat generated from thinking, we still have a minimum rate for heat production of a living entity. This heat has to be radiated away at a rate greater or equal to the rate at which the heat is produced, or the entity will “die” (there’s a reason your computer’s CPU needs a fan). Dyson considered this and deduced that the best way to get rid of waste heat would be through electromagnetic radiation. However, going through the math he deduced that the rate of radiation of waste heat this way would depend on the temperature and the number of electrons of which the entity was made. And if the life form kept reducing its temperature in this way, there would eventually be a time when it could not radiate its heat fast enough with only a finite number of electrons. So, this couldn’t work. Did Dyson give up?

Nope.

Think about this: what if you really really wanted to go about running a computation on your laptop but your fan couldn’t cool it off quickly enough. What would you do? What Dyson would probably do, is run the computation for a while, put the computer into sleep mode, let it cool off, wake it up, continue the computation and then repeat this until the computation was done! That’s exactly what he suggested a civilization might try to do to live forever; namely periodically hibernate in order to get rid of the excess waste heat! The civilization could continually lower its temperature (decrease its metabolism) and periodically hibernate for longer and longer in order to have an infinite number of thoughts using a finite amount of energy.

A nice strategy… but this is where Krauss stepped in and poked a lot of holes in this argument. The first caveat comes from the necessity for some kind of alarm clock to wake up the civilization from its hibernation. Any alarm clock is inevitably going to be performing some kind of computation in order to calculate when it should “ring” and tell the life forms to wake up and smell the coffee. This alarm clock is subject to the same laws of physics as the life forms themselves and, as such, will eventually use up all energy reserves by the same arguments as above (since a hibernating alarm clock would defeat the purpose).

The second caveat comes from the fact that we are living in a universe which is expanding at an accelerated rate. It turns out that a universe with that property will be permeated by background thermal radiation (analogous to Hawking radiation) which means a lower cutoff for temperature. In short, in a universe undergoing accelerated expansion there is a minimum temperature, which means that Dyson’s strategy of continually reducing a civilization’s temperature won’t work.

Now, you may have heard a bit about quantum computers and be thinking: “… but quantum computation doesn’t necessarily require any energy. You can, in principal, do as many computations as you like without generating heat as long as you don’t measure the result”. If you did think of that, great! However, as Krauss pointed out, you’ll necessarily have to radiate heat if you want to do any erasing in order to prepare for a new computation. If you had an infinite amount of memory storage available you could ignore that point, but any civilization’s memory storage is limited by the number of particles it has access to, which is (as with the case of energy) limited in supply. Krauss sums up this point well.

Thus any civilization can have only a finite total memory available, and resetting registers is therefore essential for any organism interacting with its environment, or initiating new calculations. While an existence, even nirvana, might be possible without this, we do not believe it is sensible to define this as life.

So right now it looks as though life (as some form of computation), by its very nature, must end. Mortality is a necessity of life. I am actually fond of this wistful result. I find it gives life more meaning and makes it more precious… but that’s just me. What do you think?

A limit to our knowledge?

Over at backreaction, Bee posted some nice questions about the present and future of physics and askes whether there will be a limit to our knowledge. There are a hoard of responses to that post as well.

“Phenomenology” is the word of the day, and sometimes I can’t but wonder what if that fundamental theory – should it exist – indeed does not make any testable predictions. Just consider it for a moment: There is a fundamental theory, but it makes predictions only in ranges far outside what we can measure. With the focus on phenomenology, aren’t we then potentially discarding the path to go? It is not even that I believe it to be the case that a theory of quantum gravity would not have observable effects, but that possibility certainly exists (and who cares what I believe). So then what? What can we know? Can we know what we can know? What will happen to physics? Would the pursuit of such a theory still count as science?

For those who aren’t aware as to what she is referring, Bee is talking about a Theory of Everything (TOE) that physicists are spending much of their time searching for nowadays. A TOE would be a theory that simultaneously describes all particles and interactions between them via the four fundamental forces (Electromagnetism, Gravity, Strong Force, Weak Force). So far, gravity is the renegade force that is causing us problems. It just doesn’t like to play nicely with the others. When we try to combine it in the same way that was used to combine the other forces, we get gobbledygook results and infinities that can’t be swept under the rug. So far, String Theory has been the popular candidate for a TOE, but so far it hasn’t made any testable predictions, which is a major criticism of the theory.

I personally don’t think that there should be any need to worry about a TOEs predictions being permanently outside the range of what we can measure. Certainly there will be a limit to how big we can build a supercollider, but eventually, I think we will be able to test predictions simply by looking into the sky and studying the remnants of the Big Bang, which is surely the largest explosion we could possibly study. Let me explain. The deeper into space one looks, the farther back in time one looks also (since the light takes some time to get here). So looking deep into space we eventually see what is now called the Cosmic Microwave Background (CMB). This is the light from the stage in the universe’s evolution when the dense “soup” of particles quickly became transparent and allowed light to freely move through it, some of that light is now arriving at earth. The appearance of this CMB can give us information about how the universe formed. A Theory of Everything should be able to explain certain features of the CMB, and so we can indirectly check the validity of the theory. There may be certain predictions of any Theory of Everything that cannot be directly verified, however, I think many aspects of a TOE will have certain consequences for the evolution of the universe which can give indirect evidence for the validity of the theory. But, as always, I reserve the right to change my mind in the future…

Another interesting hypothetical Bee asked about was the following: Suppose there are two TOEs. Up to the limits of what we can measure, they both seem to be valid, but fundamentally they paint a very different picture of our universe. A silly example would be: Two theories, both unify the four forces and describe all particles and the CMB and all that. But one of them is based on the fact that the universe is made of chocolate, and the other is based on the fact that the universe is made of cheese. How would we know whether or not the universe is made of chocolate or cheese? I think that if both theories accurately described the observations we made of the world around us then, from a scientific standpoint, it really wouldn’t matter what was really going on. In this scenario, thinking that the universe is made of chocolate as opposed to cheese would not impact scientifically, it would only impact socially. It would surely be an interesting question, and I think peoples’ world views would differ greatly between the two ideas, but the science would not. For this reason I think it would be a time to leave the question up to philosophers to answer, at least until we found a discrepancy between our scientific observations and one of the theories. We could certainly assume that the universe is made of chocolate to help give us a decisive picture of the world in which we live. But the beauty of science is that at any point if something smells fishy, we can change our theory to better describe the world.

As for the question as to whether or not this is likely to happen, I don’t know. I don’t really think I have a good enough feel for the field yet to make a worthy guess. What I do know is that although it may sometimes appear otherwise, the world is full of clever people and (if we find a way to stop global warming) clever people will continue to be born. Science is historically a field known for its surprises. As (i think) Richard Feynman once said: Major scientific discoveries are very seldom followed by the cry of “Eurika!“. More often than not, they are followed by: “Huh. That’s odd…“.