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Entries in gravity (41)

Monday
Nov082010

How To Mutate A Physicist

The deep universe is littered with violent astrophysical events: exploding stars (supernovae), colliding galaxies, merging black holes tearing the fabric of space-time, crazy fast spinning pulsars, movies by Quentin Tarantino… these events can create powerful shockwaves that kick intergalactic or stellar gas particles to immense speeds and energies. These particles - often simply protons (i.e. Hydrogen atom nuclei), but sometimes also X-rays and atoms like Carbon - attain unimaginably large energies. They are all around us, so energetic that they pass through our bodies and much of the Earth's rock layers. We call them collectively and cryptically "cosmic rays".

Cosmic rays - unlike the neutrinos of my previous post - do occasionally interact with organic matter (i.e. biological cells) and play an important role in the evolution of life on Earth: they can help mutate DNA when they penetrate a cell, and mutations are a key ingredient in the process of evolution. What is quite astounding about cosmic rays is their extremely high energies. Physicists often measure energies in "electron Volts" - some useful unit of energy related to the more familiar ones such as Joules or Calories. To put things in perspective, the largest particle accelerator built by the pitiful human race generates particles with energies around several trillions of electron Volts. Cosmic ray particles have been detected with energies around a million times more… but not much more than that. Lower energy cosmic rays - in the thousands of trillions of electron Volts range - can be detected directly through detectors raised to the edge of the atmosphere with balloons; higher energy ones are seen indirectly through showers of particles they generate when they collide with the Earth's atmosphere. Much is not known or understood about these mysterious visitors from outer space. But in the past few decades, we have started tracking them down actively and aggressively.

The first video features the late celebrated science narrator Carl Sagan and gives a great overview of the subject. The second video is part 1 of a 3 part series that goes into more details.

Sunday
Nov072010

WIMPs And MACHOs

Only 4% of the stuff in our universe is directly visible. Of the remaining 96%, 70% is a mysterious anti-gravitating substance called Dark Energy (see post 1 and post 2 for more). That leaves 26% of less crazy but still exotic stuff called Dark Matter.

Dark Matter cannot be seen, but its presence can be deduced from its gravitational pull on other visible stuff in the universe. It is now believed that each galaxy has a spherical halo of dark matter, typically larger than its size. Recently, using the Chandra X-Ray Observatory, a most dramatic evidence of galactic dark matter was observed - perhaps bringing us the closest we've been to actually seeing this exotic stuff. Two galaxies underwent a graceful collision, each dragging with it its dark matter halo (see post on galaxy collisions). See attached first video for a simulation. The red stuff in the video is visible matter/gas; the blue, dark matter. As the stars and gas in the galaxies skimmed past each other and slowed down, the dark matter whizzed by, past the colliding visible stuff: dark matter interacts weakly hence can travel further during a collision. This effect could be seen in the motion of the visible gas in the galaxies as the dark matter pulled on it and slowed the collision - like the effect of molasses or air drag! 

Currently, there are several candidates for what dark matter may be, from the least exotic to the most lunatic. On the least crazy side, dark matter may be made of MACHO's… yes, MACHO stands for MAssive Compact Halo Object: dead stars - rather small brown dwarves, black holes, neutron stars - littering the universe. We can't see them, but they pull on things gravitationally. Another more exotic category are WIMP's… WIMP stands for Weakly Interacting Massive Particles: subatomic particles that we have not yet discovered in our labs that (1) interact weakly with the rest of known particles - and are hence difficult to detect; and (2) that have potentially large mass, perhaps large enough that it is too energetically costly to create them in the lab. WIMPs can come in different flavors: lame neutrinos (see post on neutrinos), and/or particles needed to have supersymmetry in Nature (see post on supersymmetry), and/or particles that arise from having a universe that has more than three spatial dimensions (see post on higher dimensions). Beside MACHOs and WIMPs, there are other possibilities as well, the most interesting of which are axions - light particles that have an intimate relation to the nuclear force. In short, we have no clue what dark matter is at this point in time. But the thrill of the chase is the way physicists earn their paychecks after all…

The second video accompanying this post is a general discussion of what is known about dark matter so far.

Monday
Nov012010

When Galaxies Collide

We now know that our universe is filled with billions and billions of galaxies; each with billions and billions of stars in it - and often a supermassive black hole at its center devouring its stars... And when you have billions of galaxies, some will occasionally collide! As we image the universe, we see around us colliding galaxies at various stages of their collisions; we hence can reconstruct these majestic events - like a crime investigator reconstructing a crime scene. Through computer simulations, we can then model colliding galaxies and compare with observation. The first video shows a montage of a simulation superimposed with actual images from telescopes of colliding galaxies. The shear power and beauty of these cosmic dances should bring you to tears... 

Speaking of bringing you to tears, the second video is about our own galaxy's collision course with a neighboring galaxy, the Andromeda galaxy. Make sure you wrap up your life's to-do list as soon as possible...

Sunday
Oct312010

Renormalization

You are about to read about one of the most profound and beautiful principles in all of physics. It is simple, yet fundamentally important. It was formulated in the 1970's by physicist Kenneth Wilson. Wilson later on mentioned that he had thought about the problem for more than 10 years - during this period, he did not publish a single paper… the subject had consumed him. And in one year, he finally released several papers announcing his discovery and was immediately awarded the Nobel prize for the work…

Imagine you are to study a physical system that is rather complex, with many many constituents. For example, a gas in a box with numerous molecules bouncing around. You also happen to have several pairs of ACME magic eyeglasses that can make your vision immensely more powerful - each one progressively more powerful than the previous - allowing you to probe the gas in more detail. You first stare at the box of this uninteresting gas with your own limited vision and perhaps describe it with a handful of measurements: the strength by which the molecules are bouncing off each other, the mass of the molecules, and maybe a few others - all usually determined indirectly by measuring things like temperature, pressure, and density. You then write some equations that describe the system and use this new theory to make predictions. This is presumably a crude description because you could not see much details about the underlying dynamics. But maybe it's good enough?

Now, imagine you put on the progressively more powerful pairs of magic eyeglasses: you now can see more of the detailed interactions of the molecules and make new measurements accordingly. Does your previous theory of the gas change? There are two possibilities: your theory is "renormalizable", or it ain't. If the theory is renormalizable, you can then use your original equations to describe the more detailed physics you are now aware of; the only change you need to do is to tweak the handful of parameters that you originally had in your theory: the strength by which the molecules are bouncing off each other, the mass of the molecules, etcetera. The equations you wrote originally are otherwise unchanged. If you think about this for a moment, that is simply amazing! That means the details of the physical system are all tucked into a handful of parameters only… 

The other possibly is that your original theory is NOT renormalizable. This means that - as you probe more details of the system - you discover you need to modify your equations, perhaps add to them new physical parameters that you didn't need to previously to make good predictions. As you use more powerful eyeglasses, you may find that you always have to add new modifications to your theory ad infinitum - a plethora of new parameters depending on how accurate you want to describe the system. Still, for a desired level of accuracy, you can still write a bunch of equations and do physics. But this theory now is said to be "effective" - good enough to a certain precision but not beyond. This implies that you really do not understand the fundamental physics underlying the physical system; you just approximate things the best that you can. In contrast, a renormalizable theory can in principle be the ultimate most fundamental description of the physics at hand - an exact theory. 

Every physics theory in the world falls in one of these two categories (to be careful, it's actually three categories - with the addition of a possibility called superrenormalizability, but this is mostly inconsequential for our discussion). Interestingly enough, of the four known forces of Nature - electromagnetism, the weak force, the nuclear (strong) force, and gravitation - three happen to be renormalizable! All except the gravitational force… Thus, in reality, the accepted description of gravity - General Relativity - cannot be fundamental… From Wilson's amazing and general ideas, we know with certainty that we really do not fundamentally understand the oldest force law around us! We also know that our description of gravity will surely fail when we come across a pair of eyeglasses that can see down to 10 to the power minus 33 centimeters… And we know that - until then - we should be ok with using General Relativity, unless some other unforeseen mechanism (like extra dimensions… see previous post) kicks in.

The accompanying graph shows how the three parameters that quantify the strengths of the three renormalizable forces - electromagnetism, the weak force, and the nuclear force - change as we probe smaller distances (with more powerful "eyeglasses"). Higher energy on the horizontal axes corresponds to higher detail. Otherwise, the force laws do not change structural form! Adding supersymmetry to the mix (see previous post), one finds that, at some small distance of about 10 to the power minus 29 centimeters- all parameters unite in strength as shown… this is the notion of grand unification (see previous post for more) - that all three forces are different manifestations of the same force law!

 

Wednesday
Oct272010

When Stars Explode

When a star exhausts its Hydrogen fuel by burning it into Helium, it collapses under its own weight. For small stars like our Sun, the star eventually fades away into a rather unremarkable object known as a White Dwarf - neatly placed at the center of a spectacularly colorful cosmic painting made of remnant star dust (see post on planetary nebulae). 

If the original mass of the star is large enough however, this collapse ignites the Helium - through nuclear fusion - and burns it into heavier elements such as carbon and oxygen. This process continues until the core of the star transforms into a ball of iron - with lighter elements surrounding it in layers like the layers of an onion. No nuclear fusion can burn the iron core. At that point, gravitational collapse takes on a catastrophic character and a violent (and I really mean violent) explosion tears the star apart - along with its neighborhood… The explosion can be dramatically witnessed from Earth as intense light and copious X-ray emissions. This is called a Type II Supernova and typically involves the release of an amount of energy equivalent to the detonation of 100…twenty five more zeros nuclear warheads… Another type of equally dramatic explosion that we regularly witness in the deep cosmos is known as Type Ia Supernova; this involves two orbiting stars in a dangerous gravitational dance - think of it as salsa gone wrong… in both cases, the space around the star ends up covered with star debris - evidence of a violent event in its past.

Whatever remains of the star after a Supernova further collapses until all the particles making up its atoms get converted to a single type of particle known as a neutron. Neutrons are electrically neutral but resist tight packing due to quantum mechanical effects I will talk about in another post. The end result is known as a Neutron Star - a dense ball of neutrons. This is a very very peculiar object: typically 50-100 kilometers in size, but immensely dense - heavier than our Sun… It can spin at very high rates, emitting a sweeping beam of X-rays and other cosmic radiation from its poles - like a lighthouse beacon. We can see these beams from Earth and measure the spin rate; we call these objects Pulsars.  This is the end game for a star that was originally larger than our Sun but was still lighter than five times the solar mass. For even larger stars, the intense quantum mechanical pressure generated from packing neutrons is not enough to stop the collapse. The result is instead a black hole (see post on black holes). The largest Supernova recorded so far occurred in 2006; the mass of the original star was about 150 times that of our Sun…

The first accompanying video gives a brief overview of supernovae, and talks in particular about the Type Ia kind. The second video talks about neutron stars that arise from Type II supernovae. I also prepared a short slideshow of images of famous supernovae in a third video. The soundtrack is titled "Chinar Es" - roughly translates to "You are glorious" - an ancient Armenian tune composed around year 700 A.D. by Nerses Chnorhali. I titled some of the slides with the year of the star explosion.

 

Monday
Oct252010

The Secret Of Misery

Check out the funky video attached to this post. A water droplet appears to "undrop" itself from a twig. It's just a video run in reverse. Why is it that this process feels instinctively unnatural? Is it at all possible to setup the water mass in the lake in such a precise way that a drop will suddenly emerge and latch onto the twig as shown? Here's a surprise for you: the equations describing this water droplet cannot distinguish between forward and backward in time! there are absolutely no reasons from the perspective of the microscopic physics for this process not to happen in either directions: dropping is equivalent to "undroping"… What then creates this perceived asymmetry? An asymmetry suggesting that the water droplet dropped into the lake, instead of the reverse; an asymmetry that distinguishes past from future… Why does time appear to come with an arrow pointing into a future? Why do we age uncontrollably and helplessly, and perceive a distinction between past and present, and an uncertain future?

The field of Thermodynamics - often called Statistical Mechanics in our modern era - is one of the oldest disciplines of Physics. It is also the most robust - the one that changes least over time (no pun intended) through the discovery of new laws of Nature. Thermodynamics deals with statements about the macroscopic bulk behavior of physical systems that involve a very large number of constituents. The air in a room contains an unimaginably large number of molecules moving around erratically across a large expanse. We don't care about where a particular molecule is at a particular time. We care about averaged quantities that can be used to quantify the whole mass of the air. So, we talk about, for example, the temperature of the air instead: roughly speaking this is the average energy per molecule - basically a statistical assessment of the state of the air. 

There's another very important macroscopic quantity that is particularly useful for quantifying physical systems with a large number of constituents: entropy. Entropy is a measure of the level of disorder in a system; the more the disorder, the higher the entropy. If you take the air in a room and pack it in a small box, you've decreased its entropy: the molecules have less room to be disordered in and hence are relatively more ordered. The water droplet on the twig falls down into the lake and dissipates its molecules within the large mass of the lake's water: the droplet mixes with the lake's waters and increases its entropy by reaching out to a more disordered state. As we age and eventually drop dead, the molecules in our body - which are in a rather highly ordered state (specially in my case) - will disintegrate as we decompose and become fertilizer.. our entropy increases in this rather grim scenario as well.

One the laws of Thermodynamics proposes that the entropy of the universe cannot decrease; it must increase or, at best, stay the same. Hence, Nature is determined to increase disorder all around us. You can view this as due to the following: a more disordered state involves more underlying possibilities; so, it is statistically more likely to find yourself in such a state. The water droplet prefers to drop instead of "undroping" because that would increase its entropy - the number of possibilities for its molecules that can then roam freely the waters of the lake. We could in principle - with enough charisma - reverse the process by having the water from the lake jump up onto the twig. But that will require us to expand large amounts of energy as we carefully prepare the water in the lake with the proper initial conditions; and expanding energy will require burning calories;  and burning calories amounts to a net increase in the entropy of the universe since it involves dissembling the molecules of the food we consume… No matter what you do, the disorder in the universe will not decrease. 

Pretty interesting, isn't it? Complex systems - particularly the universe - will tend to increase their level of disorder, driven by statistical considerations. And that is precisely what creates the asymmetry between the past and the future! Did we just understand why time comes with an arrow pointing into a future? Not really. The universe apparently started in a highly ordered and hence unlikely state, creating this uncontrollable drive toward resettling itself into disorder; hence we perceive an arrow of time. We now understand, given the initial state of the universe billions of years ago, why time moves forward in one direction. That is certainly quite amazing. But why did then the universe start in such a highly unlikely state? If the most disordered state is the most likely one, why didn't things start off in such a maximally disordered state in the first place! There then would have been no need for this misery due to a constantly advancing time that buries our past and memories into dust… things would have been disordered dust from the outset… We owe our very existence to that initial highly unstable, unlikely, fine tuned, and ordered state of the universe 14 billions year ago; but we also can trace to it all the pain we suffer with the passing of time.

Sunday
Oct242010

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.