Posts By Date
Trending Science Stories

Entries in quantum (32)


Whither Supersymmetry?

All particles that physicists have so far discovered - the building blocks of matter and energy in the universe - carry a quantum mechanical property called "spin". The nomenclature is purposefully chosen to suggest that this property is akin to imagining tiny particles spinning around. However, this is not really a genuine picture since particles in quantum mechanics are fuzzes of probability - instead of tiny spinning billiard balls. Yet, this spin property of particles does behave very much like the good old notion of spin that we know and love - except for one important peculiarity: quantum mechanical particle spin is quantized… This means that, when measured, its value comes out as a discrete multiple of a universal number: spin 1, spin 2, spin 3, etcetera times the universal number. But it's more interesting than that. We also find particles with spins that are half-integer multiples of the same universal value: spin 1/2, spin 3/2, spin 5/2, etcetera. And nothing else. So, spin values arrange themselves on discrete levels - like the steps of a ladder - and come in two categories: integer spins and half-integer spins. 

These two classes have very different properties. Integer spin particles - called bosons - can be compressed together without resistance when there are no forces acting between them. Eventually, at high densities, they form an intriguing new form of matter called a Bose condensate. Superconductivity and superfluidity involve cooling things down to a point where such Bose condensates emerge - along with very peculiar macroscopic properties (see posts on superconductivity and superfluidity). Half-integer spin particles - called fermions - resist such compression: no two fermions can occupy the exact same state (this is known as the Pauli exclusion principle). The accompanying video shows a simulation of two fermions in a box. Quantum mechanically, the particles are simply probability lumps: red for high probability, blue for low. As the fermions are pushed together, they repel quantum mechanically; that is, there is no actual force law acting between these two particles! The repulsion is statistical in Nature. Notice in particular the interesting fringing pattern upon collision: remember these are supposed to be particles… that's quantum mechanics craziness for you (see post on visualizing quantum mechanics). 

All visible matter in the universe happens to be fermonic: for example, electrons, proton, and neutrons are all spin 1/2 particles. All forces of Nature are transmitted through the mediation of force particles: and all such force particles are for some reason bosonic. For example, the electromagnetic force is transmitted through a spin 1 particle known as the photon. These two classes of lego pieces of the subatomic world then have very different roles and character. A few decades ago, some theoretical physicists started contemplating the possibility that fermions and bosons may in fact be related; that there is a deep symmetry in the natural laws that connects these what otherwise are very different types of particles. For a lack of a better term, we call it supersymmetry. There is no experimental evidence for this symmetry yet. However, interesting things happen when such a symmetry is hypothetically considered. For one, one realizes that it is possible to talk about all the known forces of Nature as a single force law (see post on Grand Unification)… very elegant and perhaps very true. Another consequence has to do with understanding gravity within the crazy quantum world: string theory, the only theoretical framework that purports to combine gravitational and quantum physics - requires Supersymmetry to be logically self-consistent. While supersymmetry does not require string theory, string theory does need supersymmetry.

An immediate consequence of the existence of supersymmetry in Nature is the necessity of a plethora of additional building block particles of fundamental physics yet to be discovered. These particles are needed so as to be paired with the ones we already know in a manner that makes the catalogue of fundamental particles more symmetric - supersymmetric. The new particle physics accelerator in operation in Switzerland, the Large Hadron Collider, may be able to see some of these additional particles - and hence confirm the existence of Supersymmetry in Nature (see post 1 and post 2 on the LHC). The discovery of Supersymmetry in the next few years would undoubtedly be the greatest scientific discovery of the new century. But then, the century is rather young.


A Cure For Insomnia

According to quantum mechanics, everything is possible - no matter how crazy! You can walk through a solid wall... A friend of yours from across the continent can suddenly materialize next to you at a very inappropriate moment... How about jumping from Los Angeles to the moon? Sure, why not.... Everything is allowed - all Nature cares about is probability. This is the real world, not some bad science fiction movie... We know quantum mechanics is correct from an uncountable number of experiments; even from the fact we're around: atoms making us up would collapse instantly without quantum mechanics...

So, how come the last time you slammed your head on a wall, it didn't materialize on the other side? Well, it's all about probabilities. Quantum mechanics tells us that Nature itself - at a fundamental level - is not deterministic: nothing is certain, everything can happen, and the only thing that mother Nature keeps track of is the likelihood of something happening or not happening. When an event is highly unlikely or highly likely, we effectively see it in a deterministic light - like we're used to: it simply doesn't happen or happens in a predictive reproducible manner. In reality however, there is always a chance to get surprised or shocked...

So, how can you estimate the probability of an event from quantum mechanics? Here's a crude but correct method that you can use to impress people in a bar. First, identify the relevant mass, length, and time for the hypothetical event. Multiply the mass with the length times length; divide by the time. Take this resulting number and divide by ten to the power minus 34 (that's roughly Planck's constant): make sure you use kilograms for mass, meter for length, and seconds for time! You can easily convert units using google: for example, you could type "convert feet to meter". Now, take the number you got, multiply it by minus one, and raise 2.7 (that's "e" for the geeks amongst us) to the power of this number: this is an estimate of the probability of the event! That's it, you now know some quantum mechanics! 

Let's give it a try. Say I am considering slamming my head on a granite wall; it has been a particularly long morning, didn't get much sleep last night, and need to wake up somehow. I would like to know the probability that my head will go through the wall. That would certainly make the day more interesting.  So, I need a mass, a length, and a time. My head is rather substantial in weight: maybe 20 kilograms? The wall is about 1/3 of a meter in thickness. With the speed I am planning to slam my head on the wall, the time will be around one second. So, let's put things together with a calculator. 20 times 1/3 times 1/3 divided by 1 second. That's about 2. Divide by Planck's constant, that's 1E-34 on a good calculator. We get roughly 2E+34. Multiply by minus one. That's -2E+34. Final step: raise 2.7 to the power of -2E+34. That's tiny tiny tiny; your calculator will probably just say zero or just blow up... This means I would need to hit my head on the wall many many (many) times to expect to have it go through the wall once... I better start right away.

How about the probability for an electron to just appear out of thin air? Crazy, but how likely is that? You can look up the mass of an electron on google: type "mass of electron": you get 1E-30 kilograms. Say the freaky electron moves a distance of an atom's diameter - a reasonable thing to expect in the world of an electron? that's 1E-10 meters. Say it moves around a percent of the speed of light: electrons can move fast. The speed of light is around 1E+8 meters per second. A percent of that is 1E+6 meters per second. So, the time it needs to travel a distance of 1E-10 meters is 1E-10 divided by 1E+6; that's 1E-16 seconds. So, put these numbers together on a calculator: you'll get a probability of 2.7 to the power of minus one: or 0.37, that's 37% chance for an electron to appear out of thin air! Not bad at all. You now see why the microscopic world is so crazy, and our macroscopic world is so much more predictable...

And here's a simple calculator you can use to play around with this idea:

Length in meters Time in seconds Mass in kilograms Calculated probability

Grand Unification


Grand Unification has been the holy grail of physics for about a century. It is the belief that all forces of Nature are in reality different manifestations of the same force; that there is a deep unifying simplicity underlying the natural laws that we would be able to see if we could only unlock the key principles at work. There is good reason to believe that this belief is not an unrealistic one. 

By the mid 1800's, physicists had achieved a decent understanding of three forces prevalent in the world around them: gravity, electricity, and magnetism. Gravity was the earliest to be discovered and the most familiar one. Magnets had also been studied extensively by that time and were known to be sources of some mysterious non-gravitational force. And electricity had just been discovered through a series of experiments. Static electricity - responsible for the shock you get when you grab a door knob after petting a particular fluffy cat - had been identified even centuries before. 

In the late 1800's, a dramatic development occurred in theoretical physics: a physicist by the name of James Maxwell demonstrated on paper that the electric and magnetic forces are really the same force, the "electromagnetic force". They can simply be related by changing your perspective: if you just move around with respect to an electric force, you will see a magnetic force as well… The significance of this development was two-fold: it was the first time that we realized that Nature can fool us by appearing more complex than it actually is; and it was the first time that an entirely theoretical and conceptual process lead us to new physics. These two novelties were to become permanent themes in physics from then on.

In the early 1900's, two more forces of Nature were to be discovered: the "weak force" and the "strong force". Both ruled the world at very small distances - where quantum mechanics takes over. Their discovery had to be preceded with some understanding of the crazy quantum world first. The weak force is associated with radioactivity, while the strong force is responsible for nuclear power. And by the mid-1900's, theoretical physicists realized that all four of the known forces - gravity, electromagnetism, the weak force, and the strong force - are related to a series of profound symmetries within the natural laws, the so-called gauge symmetries (see previous post for more). But the four forces still looked very different. Can the success of uniting the electric and magnetic forces of the mid-1800's be replicated once again?

In the 1970's, another dramatic development demonstrated that this was indeed the case. A couple of theoretical physicists managed to show that the electromagnetic and weak forces are actually the same force law in disguise, the "electroweak force": one down, three to go. Their proposal involved the prediction of a new particle, the Higgs particle (see post on the God particle). This particle is yet to be discovered (however see post on the LHC), but the circumstantial evidence for the correctness of the electroweak theory has been so overwhelming that the authors of the work were quickly awarded the Nobel prize. 

So, we're down to three forces: gravity, the electroweak force, and the strong force. In recent years, we have learned that it is indeed very possible to unite the electroweak and strong forces as well - we call these frameworks GUTs (Grand Unified Theories). However, this program has many directions, as well as its share of problems. Only with more experimental data can it get pinned down definitively. But conceptually, there should not be any serious obstacles preventing us from uniting the electroweak and strong forces; unlike the case of their sister force…gravity.

So, that leaves the oldest force law, the gravitational force, the orphan of the story. Unfortunately, this last step of unifying all the forces of Nature is a major one: it involves resolving serious inconsistencies between gravity and quantum mechanics. To date, the best known candidate theory we have to address this issue comes in the form of String Theory (see previous post for a bit about this subject; more to come in due time…). It is however the case that testing this benchmark experimentally may come either centuries into our future or tomorrow… It feels like we are in striking distance of Grand Unification, yet the last hurdle is indeed a humongous one.

Accompanying this post are two video excerpts, parts of a longer documentary that explores this narrative. It's a total of 25 minutes of video for both, but the presentation content is well done and includes interviews with some of the most interesting theoretical physicists of our time - including Steven Weinberg, a co-author of the electroweak unification work.


The God particle...

Particle physics - also called high-energy physics - is the study of the fundamental constituents of matter and energy. The primary experimental tool is the particle accelerator: this is usually a multi-billion dollar state-of-the-art instrument that throws particles at each other at near the speed of light (see my previous post on the Large Hadron Collider). A typical experiment involves terabytes of data and hundreds of physicists. While the physics involved is now pretty well understood, the sociology of dealing with hundreds of ego-driven physicists still remains an interesting subject.

During the golden age of particle physics (1960-1980), the catalogue of all the forces of Nature and the building blocks of matter were charted and tabulated successfully. The result is a very elegant picture, with lots of symmetry and patterns. It is still elaborate enough however that discussing in too much detail would feel to a non-expert  like studying botany - not that there's anything wrong with botany. I'll summarize very briefly. All known matter is built from one of two types of particles: leptons (i.e. an electron), and quarks (i.e. a proton is made of quarks) - the naming scheme used in particle physics can get quite interesting. And all forces of Nature are traced to another category of particles called gauge bosons - yes, that's boson as in Bozo the Clown or the great Indian physicist Satyendra Bose: when two particles interact at a distance through a force, microscopically the process involves one of them spitting out the appropriate gauge boson and the other catching it. The back-reaction of throwing and catching the intermediary gauge boson results in a force on the two interacting particles. So, it's a zoo of particles in this microscopic quantum world, with leptons and quarks exchanging gauge bosons all the time; and thus the world goes around. We understand some aspects of this physics at a level of precision that is both gratifying and disturbing: in one famous measurable quantity, the theory predicts a number with 15 digit precision; the measurement agrees with it to all 15 digits… 

All is not rosy however with our understanding of this strange world of quantum particles. The highly successful theoretical framework that we currently have - called the Standard Model - requires the input of about two dozen parameters from experiment: numbers that we need to measure and depend on to be able to use the theory for predictions. For example, we need to measure and input into the equations of the Standard Model the mass and charge of the electron. This is somewhat of an ugly situation: the fewer "free" parameters a theory has, the better it is, the more fundamental. There's however another perhaps more important issue. The Standard Model predicts the existence of a particularly important particle - the Higgs particle. In the catalogue of the building blocks of matter that the Standard Model provides, all constituents have now been experimentally discovered - except for the Higgs… And the Higgs - popularly called the "God particle" - is a very very important missing link.

Let's add a bit of cosmology to the mix with a religious overtone. In the beginning, the universe was empty and void; and darkness was upon the face of the deep (i.e. dark energy). Then came about some matter from the Standard Model (see "graceful exit" in my previous post). However, temperatures were hot, very hot - too hot to have the Higgs around in large amounts. And all the other particles had no mass, they were massless much like the particle of light, the photon. Then the temperatures cooled down, and the Higgs particle condensed and filled the universe - like water vapor condensing at night on an open field. And there was much rejoicing. There was rejoicing because the Higgs particle interacts with the rest of the matter in the universe in a very interesting way: all other particles collide with the condensate of the Higgs that pervades space and, through this mechanism, acquire mass and substance… Hence, we get the proton, electron, then atoms, then us. So, the Standard Model requires a Higgs particle to make sense of the world we see around us. But we are yet to isolate one and identify it conclusively…

The Large Hadron Collider (LHC) - currently in operation in Switzerland - is hunting for the illusive Higgs. If we don't find it within the next few years, you will see particle physicists screaming and running around naked in the streets. If we do find it, but nothing else, the Standard Model would be complete for now - and depression will ensue on many physicists (including myself). Finally, if we find it in addition to a whole slew of new particles - perhaps candidates for dark matter - there will, once again, be much rejoicing. One last thing: there is a remote possibility that  the Higgs condensate IS the dark energy (see previous post on dark energy)… 

The accompanying video is a rap by the talented science journalist Kate McAlpine (known as alpinekat…) about the LHC. The words of the song actually have instructive physics content; it's worth to listen to carefully. Don't know however what to say about the dancing physicists… at least they're fully clothed. Check it out, and check out Kate McAlpine's webpage with more physics songs at:


On Beauty, History, and Her Story


In the early 1600's, two little known European scholars were on the verge of changing the course of human history forever - not through war or politics, but through scholarship. 

Tycho Brahe, a rich Danish nobleman, was obsessed with observing the night sky. He built the most advanced telescope of his time and started recording the positions of heavenly bodies meticulously. He collected remarkably detailed tables of numbers describing the positions of planets; but they were just that - numbers with no physical meaning. He struggled with making sense of his data - as well as with a chronic weakness for alcohol, loose women, and partying… Brahe was the first experimental physicist of history - in the modern sense of this term.  

Concurrently, a poor German scholar, Johannes Kepler, was incessantly trying to understand the rules by which the heavenly objects moved in the sky. But Kepler lacked data, numbers to look through, patterns to discover. He had already become famous for his mathematical skills, but he was dirt poor - with a mother in jail accused of witchcraft… Kepler was the first theoretical physicist of history...

Eventually fate brought Brahe and Kepler together. And after one night of heavy drinking, Brahe dropped dead and Kepler basically stole his data… he pondered over the long tables of numbers - positions of planets wandering the night sky. And from these numbers, Kepler's genius unraveled complex repeating patterns… he formulated his discoveries through three simple laws. And Physics had just been given birth to. As is typical of theoretical physicists, after this work Kepler got obsessed with some ill-conceived mathematical ideas; he eventually died as a war refugee… About a hundred years later, Isaac Newton was to finally see the big picture in an amazing work known as the Principia. Newton wrote: "If I have seen further it is by standing on the shoulders of giants" - referring to Brahe and Kepler.

Since then, Physics has been about observing Nature, measuring it, pondering over the measurements looking for patterns. And patterns are about symmetry. Think of a perfect sunflower, with a set of identical petals. If you rotate it around its stem by one petal, it does not change; it looks the same. We say the sunflower has a rotational symmetry. Since the beginning, physicists understood that symmetry was important for understanding the laws of Nature. After all, it's all about finding repeating patterns in measurements since Brahe's and Kepler's time. And when there's pattern, there's a symmetry: the elegant pattern of circling petals in the sunflower is a reflection of its rotational symmetry. But it was only in the 1900's when we finally understood the depth and importance of symmetry in Nature.

In 1918, a mathematician and physicist by the name of Emmy Noether was to change the way we think about Physics forever. While struggling to overcome discrimination against women, Noether focused on the role of symmetry in the natural laws. She managed to publish a seminal paper - while working from home: no institution would give her a job despite her fame simply because she wore a skirt… In this work, Noether stated and proved a remarkable theorem now named after her: every symmetry in Nature is associated with a quantity that remains constant in time. It is difficult to overstate the depth and beauty of this statement. Every observable that physicists measure and analyze arises from the fact that, in certain situations, the corresponding quantity can remain constant - and hence may be interesting. Noether was saying that Physics amounts to cataloguing the symmetries of Nature, and was providing a concrete prescription on how to proceed.

A table-top Physics experiment is performed at 2pm and leads to some measurements and results. It is then repeated at any later time, say 3pm; and it is found that the results have not changed. This implies that the laws of Physics governing the experiment are unchanged under a time shift or "time translation". There is then a symmetry at work in this setup - much like the case of rotating a sunflower without changing how it looked. Noether's theorem states that there must a quantity in this experiment that does not change in time, that remains constant. And the theorem identifies this quantity: we call it energy… Energy is constant by virtue of time translational invariance!  The reason we talk about the concept of energy at all is simply traced back to a symmetry. Even when energy is not conserved because of a lack of the required symmetry in a situation, we learn to still measure it to explore the new physics responsible for its non-conversation.

If an experiment is performed in my office (not that that'll ever happen); and then repeated in an office nearby with no changes in the results, we say that the laws of Physics at work are unchanged under "space translation". This is then again a symmetry. According to Noether's theorem, there is a constant quantity that we can track and study: we call it momentum… Momentum conservation is a result of space translational invariance!  Even when momentum is not conserved because of a lack of symmetry in a certain situation, we learn to still look at it to explore the new physics responsible for the non-conversation phenomenon: force!

Furthermore, every force of Nature discovered to date is now known to arise from a symmetry principle… The associated Noether constant is the charge of the force: for example, the electric charge for the electric force, mass for gravity. Hence, you quickly realize that Noether's theorem is very fundamental to the way we think in Physics and beyond. Noether's simple statement literally reorganizes the way we view Physics and the world; and provides a systematic tool for identifying patterns in Nature. Very few things in life get this beautiful and elegant.  

A simple reference article: Rubens de Melo Marinho Jr (2006). Noether's theorem in classical mechanics revisited

Frying Brains…  


This post is intended to take you all the way to the edge of sanity; so dim the lights, put on some good new age music, and brace yourself… String Theory starts with the premise that the building blocks of matter and energy are not necessarily particles - that is point-like packets of energy. The theory proposes that of the three pillars of modern physics - gravitation, quantum mechanics, and relativity - the first is really not formulated properly, but the last two are right on target. The implication is that this is the cause of the difficulty of putting gravity and quantum mechanics together: basically, blame it on gravity! The theory also proposes that all physical observables should be computable from scratch; there are no magic numbers floating around in Nature, we should be able to understand every bit of observation. The principles I just listed, while frugal and rather general, are extremely powerful. I can argue that this is all that is  needed to develop the entire field of String Theory. The idea is that anything logically consistent within this framework is fair game and is to be allowed…

After a couple of decades of hard work by a group of several hundred overworked string theorists, we now have a remarkably detailed picture of what this String Theory thing is - but the full picture is still incomplete. We are able to show that quantum mechanics can be married successfully with the gravitational force. And many outstanding puzzles of theoretical physics get very interesting resolutions, from strange black hole physics to particle physics botany. But the full narrative is still being written with research in progress, and we cannot tell yet whether this theory - in its current incarnation - is to survive the ultimate tribunal: experiments and measurements. However, given the successes of the theory, it is now highly likely that some of the new revolutionary ideas the theory has introduced into discussion are to survive within the ultimate future framework of fundamental physics.

As a result of all this, String Theory requires that the world has ten space dimensions… see my previous post about compactification to see an alternative mechanism - to the one I will discuss in this post - through which this setup can still lead to the observation of only three space dimensions. Since the building blocks of the theory are not necessarily point-like, one finds that these can be in the form of tiny strings; or even in the shape of membranes… in the full 10 dimensions, you can even have a three-brane or brane for short… that is an object like a membrane but extended in three dimensions instead of two. You won't be able to visualize this (I hope), but you can view a cartoon depiction of a two dimensional membrane: it is now a good time to play the first video attached to this post to make things a bit less abstract…

And here comes the big punchline: in the context of this theory, our universe can be a three-brane… we are living in the fabric of the brane that is flopping around in a higher dimensional space. Imagine the first video of this post with a population of insects living on the membrane that is flopping around. That is us; except its a brane extended in three space dimensions instead of two, and hence we perceive the world in 3D! We are confined and welded to the three brane. In fact, we are made of the stuff of the three brane: the ripples on the brane represent nothing but the matter in our universe, including ourselves! We can show from string theory that the way these ripples behave, scatter off each other, and evolve, is indeed in tune with all the stuff around us: electrons, atoms, all the forces of Nature that we have measured… This is simply shocking; that a picture of a brane flopping around in a higher dimensional space appears from the perspective of things living on the brane as the universe we actually see today… But it gets more interesting…

If we are a three brane flying and rippling through some higher dimensional space, there may very well be other branes floating around nearby: other universes. Check out my post on the Multiverse picture for more about this topic. Let's get all the way to the edge now. Imagine a gas of universes: instead of molecules making up this gas, it's branes all over! Each brane is a universe with miserable beings living in it. As is typical in a gas, constituents of the gas will frequently collide. So, imagine another brane, our evil twins, on a collision course with our brane, our universe. Time to play the second video clip attached to this post. What would we then see from the perspective of our universe during this collision process? String Theory tells us that we would observe a violent exponential expansion of our universe… well, that's what we actually observe today (see post on inflation)… the endpoint of the collision in the second video corresponds to what I referred to as "graceful exit" in the previous post; the collision itself: the Inflationary Epoch.

A reference article: James M. Cline (2007). Braneworld Cosmology PoSstringsLHC:011,2006 arXiv: 0704.2198v1

Fish in a Pond

Things are so because things couldn't have been any different… In recent years, cosmological observations have painted a remarkably detailed picture of the history of our universe. The bad or good news (depending on your perspective) is that some of the conclusions are astounding: results suggest an extremely delicate balance all around us, so delicate that if things were to be a tiny bit different at the beginning of time, perhaps we would not be around to ask any questions…

There are several such "coincidence" and "fine tuning" issues. In the beginning, the universe was filled with dark energy (see previous post) and underwent a dramatic explosive expansion. This expansion was exponential - physicists call it the Inflationary Epoch - where the fabric of space stretched faster than the speed of light. As the universe expanded, some normal matter was generated during a period cryptically called a "graceful exit"… The expansion was so violent, that it was highly highly (and I mean highly) sensitive to the initial condition of the dark energy pervading the universe. If things were a little different, this crucial epoch of expansion of the universe may not have been realized. And this inflationary epoch is crucially needed to explain why our universe is around… Here's another perverse coincidence. As the universe continued to expand, and is now known to undergo an accelerated expansion, space stretches away from us faster than light can catch up with it… so, our horizon - farthest extent we can see into the universe - is shrinking fast… As it happens, we live around the right period that allows us to just be able to see the whole universe… Several hundred million years later, the edge of the universe would have receded away from our visual horizon… On cosmological timescales, this coincidence is quite shocking and highly unlikely.

In the context of String Theory, these issues get addressed rather explicitly. String Theory is described by a set of equations whose solution is presumably our universe. The problem is that we sort of have a situation of an embarrassment of riches: the equations admit many many solutions - amongst them potential candidates for our universe - but these realizations are often very disparate in their conclusions on how the world should look like. Too many of these solutions do look like the world we live in - but the devil is in the details. And we don't know all the solutions… You may then say that String Theory allows many universes as possibilities: call it the Multiverse picture. How can we predict anything in such a situation! Which universe are we in? Are there other ones around? Where the heck are they? Why are we in this one? There comes the anthropic principle, an old idea that has received new life in the context of these modern questions. The idea is simple: of all possible universes, only a few select ones have the right conditions to have our kind of miserable life evolve in it… we are in this universe because if things were different, we wouldn't have been around… This does have a flavor of a mentality from the Middle Ages, doesn't it? it is absurd to ask some fundamental questions because there is no answer to them beyond: "it is so, because we need it to be so to be able to ask the question"...

Here's my revised version of an argument that goes back to the Cosmologist Linde in support of the anthropic principle. Imagine a species of sophisticated fish living in a pond whose temperature is 15C. After living their lives as high quality Sushi for a while, these fish develop intelligence and become sentient. Some of the fish adopt unglamorous careers of hard work with little benefits as physicists, and start measuring the temperature of the water. Fish with even lower self esteem become theoretical physicists, and start asking: "Why is the temperature so?". Can we derive some equations that predict the temperature? You are standing outside the pond looking into it and wandering: "These must be the stupidest fish in the world: the temperature is so because if it wasn't, this species of fish wouldn't survive and be around to ask the question"…

Personally, this scenario is very troubling to me. Does this mean there are some fundamental questions in physics we can never unravel beyond a lame anthropic argument? is this the end of fundamental physics then? I don't think so. There are some recent suggestions that one may be able to get a statistical handle on such questions: we may not be able to predict which universe of the many possible ones we live in, but perhaps we can say which ones are the most likely without considering a biological factor… And that may be good enough, whether we like it or not… after all, if the fish are to evolve, they need to start looking outside the pond...

The accompanying video is slightly on the edge… but still interesting enough to lead you to ponder over some of the implications of this subject that straddles physics and philosophy.

A reference article: Leonard Susskind (2007). The Census Taker's Hat arXiv: 0710.1129v1