A blog about physics from a well caffeinated grad student.
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?
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?
Here’s an ammusing little happening in the world of physics from Nature News.
You’ve probably heard about the mysterious Dark Matter. It’s a hypothesized form of matter that is much more abundant than the “regular” matter we have come to know and love; protons, neutrons, electrons, etc. We’re pretty sure it’s out there, we’re just not sure what it is! There have been many theories as to what this matter is and how it behaves, but there hasn’t been any detection, direct or indirect, of dark matter… except perhaps until now.
A satellite mission called PAMELA has recently collected some data that seems to give indirect support for a specific model of dark matter. The PAMELA team recently gave a preliminary presentation of their data and invited physicists to attend, of course. It seems that tempting physicists with this data is like tempting a pack of starving wolves with a T-bone. Some of them took it upon themselves to bring their digital cameras (with consent of the PAMELA team) to this preliminary display. This data has not yet been officially released into the public domain, however, these starving physicists took it upon themselves to use the data they had photographed and released papers which pointed out how the data indirectly supported their theories!
I’m not sure what to think of this. I think many people in the field aren’t sure what to make of this either. It does seem a bit cheeky… but is it wrong? We’ll just have to wait and see…
Just goes to show you how desperate particle physicists are getting for dark matter data.
haha.
There have been a lot of cool things I’ve found on the web recently (mainly videos). I thought I should share.
First of all, there’s this video of a water balloon hitting the ground at high speed in slow motion. It doesn’t break, but there are awesome shockwaves.
Next, here’s a video of an experiment to demonstrate that traffic jams happen even without a bottleneck. You can see the compression wave traveling down the road. And also, here’s one of a java applet that simulates the same thing!
Here’s a link to a video of a mini-experiment showing the basic principles behind a particle accelerator. It uses a n electric bowl and a ping pong ball!
Next, a game of Asteroids that takes special relativity into account! Finally!
Now a more directly educational link; here’s Sean Carroll on Bloggingheads talking about cosmology. He explains what exactly “big bang” means, and what Inflation is. He also talks about the validity of new theories of the early universe, the multiverse theory and cuts through most of the crap that the media tends to churn out.
Finally, here’s a video of a pool filled with a non-newtonian fluid (corn starch in water). Just watch. It’s awesome.
No, not the economic kind of inflation… I’m talking about cosmic inflation. You might have heard of it before. I’ll give you the gist of it. There are many problems with the big bang theory. When physicists talk about the big bang, they aren’t really referring to the “bang” part (strangely enough). What they actually mean is the part 10-44 seconds after the “bang”. The big bang theory starts with some specially fine tuned initial conditions for the content and expansion of the universe and just lets it evolve from there to get our universe today. It doesn’t, however, explain what gave rise to those initial conditions. Inflation tries to do just that.
So what is inflation? Inflation is a rapid (exponential) expansion of our universe that we suspect happened just after the creation of our universe. This is, of course, unimaginably faster than the expansion rate of the universe right now. It might seem like a very strange thing for a universe to do, but if we accept the idea, we find that it solves several problems with the classical big bang model. Inflation describes how the universe could become initially so flat and homogeneous but still have small fluctuations that evolved into the galaxies we see today.
One of the strangest qualities about our universe is that it is almost perfectly flat. When I say flat, I don’t mean: like a piece of paper. I’m talking about a generalized idea of flatness. If one measures the curvature of the universe to be positive, then a triangle in this universe will have interior angles adding up to more than 180 degrees. If the curvature of the universe is negative, triangles will have angles adding up to less than 180 degrees. Our universe has a curvature of almost zero, so triangles have angles summing to almost exactly 180 degrees.
Why is a flat universe so strange? It’s because a flat universe is unstable. By that I mean that a positively curved universe will tend to collapse in on itself and a negatively curved universe will tend to expand so quickly that galaxies won’t have time to form. The best I can do at a simple explanation as to why inflation explains this apparent “coincidence” is that it has to do with how the contents of the universe (radiation, matter, dark energy), the curvature, and the expansion rate all relate to each other (the Friedman equations). An exponentially expanding universe will force itself to become more and more flat.
What could cause this inflation, you ask? Well, there are many theories of inflation that attempt to naturally account for an inflationary period, but what it mainly boils down to is the, so called, Dark Energy (aka: Cosmological Constant). That’s right, the same thing that’s confusing astrophysicists and cosmologists about the universe’s current accelerated expansion. Dark Energy, put simply, creates a repulsive gravitational force and results in an accelerated expansion of spacetime. The explanation as to why we are not “inflating” today, is that the Dark Energy today has less of a repulsive gravitational effect than it did in the early universe. Physicists are still trying to (cleanly) explain this transition.
I could keep rambling about how neat inflation is, but I think it’s best for everyone that I stop here.
The Hubble site has created a very snazzy new (flash) page that tries to explain dark energy and the expansion of the universe to the layman. It is definitely worth checking out, and has lots of pretty animations.
The beginning is definitely my favorite part. It has the atmosphere of an epic theatrical trailer, hinting on doomsday scenarios. Though, viewer be warned, even the Hubble site is not without fault. Sean Carroll has voiced his beefs with the accuracy of their explanations in the wonderful blog Cosmic Variance. That’s not to say they are all wrong, they have just chosen some poor wording on occasion.
I recently attended a lecture given by Lawrence Krauss which he called “Our Miserable Future”. It was a very interesting talk, giving an overview of the long term fate of Life, the Universe, and even Cosmology.
Consider this: The universe is expanding, and the farther away a galaxy is from us, the faster it will appear to travel away from us. This means that beyond a certain distance, galaxies will “appear” to travel faster than the speed of light. I should not say “appear”, really, because if a galaxy seems to travel faster than the speed of light away from us due to the expansion of the universe, then the light emitted from that galaxy will never be able to reach us. So, in fact, past a certain distance, we won’t be able to see any galaxies at all.
Now realize this: In 1998, astrophysicists discovered that the universe is not only expanding, it is an accelerated expansion. This will mean that the galaxies we see today, will eventually “speed up” and appear to travel faster than the speed of light, so we won’t be able to see them. Eventually, (well beyond the death of our sun, so don’t hold your breath) we won’t be able to see any galaxies in the universe. A very lonely thought.
What’s worse, the only way we know about universal expansion, is because we can measure other galaxies and see them traveling away from us. So well into the future, astrophysicists will have no proof that their universe is expanding.
Which kind of makes you wonder, eventually one of the amazing observations in physics won’t be observable. How many other amazing observations have we already missed by not evolving earlier after the big bang to discover them?
So, you’ve heard about the expansion of the universe. If you haven’t, well…surprise! The universe is expanding. But what does this mean? Well, in a previous post, I explained a bit about how space and time are not rigid, they can be bent. You might like to think of the universe as being made of rubber (I do…sort of). So what physicists really mean when they say the universe is expanding is that the rubbery spacetime in which we live, is being stretched.
Some people may confuse the idea of an expanding universe with the idea that all of the galaxies are moving away from each other. This is not a great way of thinking about it. What’s better is to think of this rubber sheet and draw some dots on it that represent galaxies. Now, if you start stretching the rubber sheet it will look like the dots are all moving away from each other, but actually, with respect to the rubber, they aren’t moving at all (very zen).
You may have heard that some galaxies appear to move away from us at speeds greater than the speed of light. Being the knowledgeable person that you are, you worry about Einstein’s special relativity being violated, since nothing can travel faster than the speed of light. Actually, there is no problem. Since expansion does not equal motion, the apparent speeds of these galaxies are not due to movement, but to the expansion of spacetime, so Special Relativity is not violated.
This may tie your brain in a knot at first, but I’ll say it anyway; there is no center of the universe. Remember that the universe is like a piece of rubber. If we were riding on one of those dots drawn on the surface while the rubber was being stretched, what would we see. Well, we would actually look around and see all of the dots moving away from us. This is because since the universe is being stretched the same amount in all directions, the distances between all dots are increasing at the same rate. It doesn’t matter which dot (galaxy) we choose to ride on, it will still look like all of the other dots (galaxies) are moving away from us. So since what we see doesn’t depend on from where we are looking, there can’t be any center.
Now consider this: if the very space in which we exist is expanding, then why aren’t the atoms in our body expanding away from each other? Why aren’t we just falling apart? Well, on very small distance scales, this effect is tiny… very tiny. The atoms in our body hold on to each other by electromagnetic forces. On larger scales (like a galaxy), gravity is the dominant force holding things together. That’s why galaxies don’t disintegrate because of spacetime expansion, gravity holds the stars together.
Neat things to read…
