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Tuesday
Oct192010

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: http://www.katemcalpine.com/

Saturday
Oct162010

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 http://arxiv.org/abs/physics/0608264
Friday
Oct152010

Physicists Gone Wild

General Relativity teaches us that the fabric of space and time is malleable; like a sheet of flexible rubber. When objects with mass and energy - like stars and planets - are around, the fabric stretches in such a way that the measure of distance around the objects gets distorted. General Relativity tells us that the gravitational force is simply a result of an illusion arising from such distortions of the fabric of spacetime. Depending on what astrophysical process is at work, the distortion realizes various beautiful patterns, dynamical and complex. One of the most interesting phenomena results from violent events, such as the collision of two black holes… The event creates a ripple in the curvature of spacetime, a gravitational wave that propagates out at the speed of light nudging everything in its way…

Gravitational waves are waves that stretch and squeeze distances across space in an oscillatory pattern - as they propagate through the universe. They go through most everything and travel immense distances - carrying within them information about violent events far far away. Unlike light, they can puncture through the primordial plasma of the universe; hence, detecting them would allow us to probe the first 100,000 years since the beginning of time, past the opaque plasma that obstructs our view (see my previous post on the cosmic microwave radiation)… detection of gravitational waves would allow us to verify the theory of General Relativity beyond anything achieved so far, directly confirming the existence of exotic objects such as black hole and cosmic strings. In short, these ripples of spacetime pack in them physical information about the universe and cosmology, information that is simply unavailable from any other source we have. We desperately need to "see" them!

In the 1970s, a first indirect evidence of gravitational waves was identified. Later the results were refined and confirmed. Two massive objects known as neutron stars can form a binary system - basically orbit around each other along a circle in an exceedingly dangerous dance. As they go around, they loose energy by emitting gravitational waves and spiral inward faster and faster for an eventual collision: imagine water flushing down the toilet… Astrophysicists were able to identify such binary systems and measure the period of the spiraling motion. Even though the gravitational waves could not be detected, the spin rate of the neutron stars was increasing precisely as predicted by General Relavity...

So, now that you're energized and romanticized about detecting gravitational waves, let's bring things down to reality… Think of a gravitational wave as a ripple on the surface of the ocean. As a water wave gets past you, you rise and fall with the passing disturbance. To detect gravitational waves, you need to measure the distance between two fixed points in space. Say you hold two fingers a few centimeters  apart and let a wave from some black  hole collision light-years away pass through them; you measure the distance between your fingers and you would see this distance stretch and squeeze as the wave passes through. Here's the catch: the effect from a typical gravitational wave is expected to change the distance between two fixed points by about the radius of a proton over a distance between the Sun and the Earth… How the heck you measure that??!! Well, unachievable goals have never stopped physicists from trying; and hence the title of this post. 

There are currently two huge gravitational wave detectors in operation. They are looking for ripples in the fabric of space as they pass by our neighborhood… they're called the LIGO detectors - Laser Interferometry Gravitational wave Observatory (I guess LIGWO does not sound right...). They are capable of measuring a change in length of about 1/1000 the radius of a proton over a distance 4 kilometers… the instrument is a marvel of technological precision and physics ingenuity… check out the accompanying really good video to learn more about LIGO.  

Thursday
Oct142010

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
Tuesday
Oct122010

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 http://arxiv.org/abs/0710.1129v1 arXiv: 0710.1129v1
Sunday
Oct102010

Back To The Future

Relativity tells us that Nature has a speed limit: no information can travel faster than the speed of light. That's quite fast, but not fast enough to travel the universe quickly enough. When you look at the Sun (please don't!), you see it as it was 8 minutes ago: it takes 8 minutes for light from the Sun to reach us. Stare at Saturn through a telescope: you are seeing it as it was 1 hour 30 minutes ago. The deeper you look into space, the further back in the past you are imaging... Our closest star, Proxima Centauri, is about 4 years in the past as we look at it today. We say it is 4 light-years away. Our nearest galaxy, Andromeda, is 2.5 million freaking years in the past! And the edge of the visible universe is 14 Billion years in the past... This means that when we look at the Cosmic Microwave Background radiation (see previous post), we are actually looking at the universe as it was starting out, in its infancy...

One of the strangest effects resulting from this time lag comes about when singular violent events occur far away: Supernovae or exploding stars... When a star exhausts its fuel and undergoes total (and I mean total) gravitational collapse, it typically undergoes a series of violent (and I mean violent) explosions... these are so nasty that near-by stars can be torn apart by the explosion... And we can see these events all the way from the safety of our Earth. They appear as a sudden brightening of an otherwise unspectacular star; and a subsequent dimming as the star starts to settle into a neutron star or, better yet, a black hole. The accompanying picture is that of Supernove SN2006X, before and after - much like pictures taken in hair transplant commercials. The star is 54 million years in our past. So, while the explosion was seen in 2006, it actually happened 54 million years ago... the star is there no more...

Saturday
Oct092010

The Information Paradox

What would happen to you if you were to jump into a black hole... we have three possible answers to this, two from General Relativity, the other from String Theory. In all cases, your fate would not be something to look forward to. But for many physicists, this would actually be a great way to go!

A black hole is a hole in the fabric of space and time, a collapsed star that sucks in everything around it through its immense gravitational pull. We now know there are billions of these extreme objects scattered throughout our universe... so, what would happen to you if you were sucked into a large black hole? Einstein's theory of General Relativity, which predicts the existence of black holes, suggests the following outcome: as you approach the surface of the black hole - called the horizon - you wouldn't necessarily notice anything special. But as soon as you cross the horizon, you suddenly will realize that you cannot communicate with your relatives outside the black hole, even if you wanted to… Ironically, this is perhaps the only time you would want to talk to your relatives! As soon as you cross the horizon, you cannot escape from the black hole ever again; nothing can, not even messages in a bottle. You will continue to fall towards its center as the forces of gravity become stronger and stronger, and pulls your internal organs apart... no one outside will hear your screams... 

Now add to this Quantum Mechanics. Steven Hawking showed in the 1970's that a black hole actually evaporates! When quantum effects are taken into account, one finds that a black hole has a faint glow that slowly dumps the stuff it's made of into the outside world as random radiation! And there arises the Information Paradox... So, you jumped into the black hole and disappeared forever; does then the black hole evaporate into random radiation? if so, where have you gone? the information that is you, where is it? is a black hole a sink of information? This is deeply troubling, for physics and for psychology. We believe information cannot be lost, only change its form! The Information Paradox preoccupied theoretical physicists for decades... We now think we have a good answer to this puzzle from String Theory... stay tuned for another post that will help save your sanity... 

The accompanying lengthy video (5 parts) gives you a history of Hawking and the Information Paradox he proposed.