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?

McGill physics students: Been there, done that, made the T-shirt.

The physics undergraduate students at my old university, McGill (based in Montreal), have exercised their creative talents and come up with a T-shirt design to unite them all. It portrays the great difficulty that every McGill undergrad goes through on their way to completing a B.Sc. in physics. It, of course, does this all with style, sophistication, and whit.

Here it is for your pleasure:

If you don’t get the joke here, you’re probably not familiar with quantum mechanics. That’s okay. Let me try to explain it a bit. The Greek letters (psi) around the frowny face form the expectation value of … the frowny face (pain). What that is loosely suggesting is that if you asked (measured) a statistically significant number of McGill physics students, the average emotion you would measure would be pain.

Another of their potential designs uses the idea of spherical cows (metaphor for approximations):

I especially love the insight that you can set the tail to zero here. It really simplifies things.

Can you fool a photon?

The world of the small is very spooky
… at least it is to me. Recently a few researchers showed just how spooky it is by actually preforming Wheeler’s thought experiment. Wheeler’s thought experiment is really a test of the wave-particle duality of matter. I’ll give you the inside scoop.

Some Background About The Quantum World

First you need to know about particle interference patterns if you don’t already. It turns out that if you fire a beam of subatomic particles (like photons) at a wall with two slits and then look at the pattern the particles form on a screen behind this double slitted wall, you won’t just get two splotches of particles that look like the two slits. What you actually get is an interference pattern, just like that of a wave (like light, or water). Ok… weird. You might think, “Ok, well the particles must be interfering with each other, let’s just try firing one particle at a time“. To make things weirder, if you fire one particle at a time through this double slit, you get the same pattern! Each individual particle is interfering with itself! Now… to make things weirder still, if you set up a detector to try to measure which slit each individual particle passes through, you suddenly get the regular, dull pattern of two splotches of particles! It’s like the particle has a mind of its own and knows when you are looking and when you aren’t (this is not the case, of course). It will only interfere with itself when you’re not looking… as if being like a wave is taboo or something.

Wheeler’s Experiment

The next thing a physicist would try to do after realizing that (s)he had been outsmarted by quantum mechanics is to, well… fight back. Wheeler said to himself, “Let’s try to trick the particles into thinking we’re not looking when we actually are“. You can try to do this by only choosing to “look” or not (IE: measure which slit the particle passed through) after the particle passes through the double slit wall. This type of measurement is called Delayed Choice. This decision would have to be utterly random to work properly and physicists have figured out clever ways of doing this effectively.

So what happens? Do we trick the particles?

Turns out: NO. The particles have outsmarted us again. They behave exactly as they normally do depending on whether we’re looking or not, even if they don’t “know” we’re looking. They interfere when we’re not looking and just pass straight through the slits when we are. Spooooky! This is in complete agreement with the quantum mechanical (wave-particle duality) description of matter and it actually rules out many other theories.

This world just keeps getting weirder and weirder, doesn’t it? When you think about it though, what says that subatomic particles should behave like tennis balls, or rocks? It’s a completely different realm of reality. We find it spooky because we’ve adapted to the large scale laws of physics, and expect that when you throw a tennis ball through a hole in the wall, it will come straight out the other end no matter whether your eyes are open or closed. I’ll let you continue this philosophical tangent on your own.

Quantum Entanglement

Entanglement seems to be a hot topic in popular physics. So I thought I’d take a stab at explaining it in a layman friendly manner. Before I do that, though, I’m going to have to give you a brief intro into the nature of quantum mechanics.

-A Crash Course in Quantum-

Okay. Firstly, quantum mechanics tends to deal with very very very small things, like electrons. So from now on, I’ll talk about electrons like they are the only focus of quantum mechanics, however, all subatomic phenomenon must be handled with quantum mechanics. Electrons have a property called spin, for now, you can imagine that it is like the electron… well… spinning. If I say that an electron is “spin up”, then it means if you look at it from the top, you will see it spinning counterclockwise. “Spin down” means it is spinning in the other direction if you are looking at it from the top. I’ll also say things like “spin right” and “spin left”, which mean similar things to spin up and down except looking at the electron from a different direction.

So, how does spin work? Well, let’s say I have a beam of electrons and I want to know which electrons are spin up, and which are spin down. I’d send all these electrons through a machine and some would come out spin up, some spin down. If I took only the spin up ones, and sent them through another machine that asks the same question (are you up or down?) it would tell me that all of them are up. No surprises here. What if I instead took all of those spin up electrons and asked them, are you right or are you left? The machine would tell me that half of them would be right, and the other half would be left. Strange.

Now something even stranger. If I took the left electrons I’d just filtered out, and asked them, “are you up or are you down?”, maybe you’d think that since I was using only up electrons before, that they would all be up. No! Again half of them would be up, and half would be down! It’s like they forgot that they were spin up to begin with!

An important bit of insight is that I didn’t say which of these electrons in the beam were up, down, left or right. I only said what a certain percentage of these electrons would be. In fact, in quantum mechanics, one can NEVER predict whether a specific electron will be up or down, only a probability that it will be up or down. And it’s not that we don’t have enough information. The very nature of subatomic particles is that of probability until you make a measurement.

So what does this have to do with entanglement? When we entangle two electrons we process them in such a way that their spins must cancel out. So, if one is up, the other must be down. If one is right, the other must be left. This is called conservation of angular momentum, but the name is not important, just the idea. But remember, as I said before, you can never know whether one specific electron is up, down, left or right… only the probability that it will be. Here’s the weird part. If two electrons are entangled and one is sent hundreds of miles away from the other, conservation of angular momentum must still be true and their spins must cancel out. If we took one, and asked it, “are you up or down?”, we might find that it happens to be up at the moment. At that very instant we know with absolute certainty that the other must be down! And we didn’t have to measure the other to figure this out.

Now, some people may be quite skeptical at this apparently amazing phenomenon and might reason this way: “If it’s all probability, isn’t it like flipping a coin and discovering that the heads side is pointing up and then being amazed that the tails side of the coin is pointed downwards? Isn’t this obvious?”. Simply put: no. The coin has two distinct sides that will always be heads and tails. Electrons, before they are asked to reveal their spin direction, don’t have a specific spin. By mathematical and physical requirement, their spins must be considered as only a set of probabilities for being up, down, left or right.

Einstein didn’t like this idea at all. It looks like once the first electron’s spin is measured, that information about the first electron’s spin is transfered instantaneously to the second one, even if they are lightyears away! This would violate Einstein’s principal of relativity that says nothing (no information) can travel faster than light. This is known as the EPR paradox (stands for Einstein, Podolsky, and Rosen). One resolution to this “paradox” is that no information is actually transfered. We can’t control whether the first electron is up or down, left or right. Since we can’t control the first electron’s spin, we can’t transfer any information.

So hopefully, I have explained this well enough to make you sufficiently confused about nature and reality. A great physicist, Richard Feynman liked to say, “If you think you understand quantum mechanics, you don’t understand quantum mechanics!“ I’m glad to say, that with my education in physics, I am becoming progressively more and more confused about the strangeness of the very very very small.