String Theory (continued)

Let’s continue with our discussion of string theory from last week, which says that all matter can be broken down to a fundamental particle, called a string. Atom are made up of protons, neutrons, and electrons. Electrons aren’t made of anything smaller. Protons and neutrons can be broken down further into quarks, which also can’t be broken down any further. For this reason, quarks and electrons are referred to as fundamental particles. Even though quarks and electrons behave very differently, string theory says that they are actually very similar, and that the differences between them are due to these strings vibrating at different frequencies, similar to how a violin makes different sounds.

   The most interesting thing about string theory is that it requires extra dimensions of space. These curled-up dimensions are too small to see or experience. There are a great many ways of curling up these extra dimensions. String theory require seven extra curled up dimensions, so when added to our three familiar dimensions of space plus time we get a total of 11 dimensions.

   You can think of multi-dimensional space as a garden hose. If the hose is viewed from far away, it appears to have only one dimension—length. Now think of a ball small enough to enter the hose. Such a ball would move more or less in one dimension. However, viewed close-up, one discovers that the hose contains a second dimension—its circumference. For an ant crawling inside, it would move in two dimensions. This extra dimension is only visible when up-close, or if the ball we use is small enough compared to the hose.

The CMB temperature fluctuations from WMAP data seen over the full 

sky. The average temperature is 2.725 degrees Kelvin, and the colors 

represent small temperature fluctuations. Red areas are warmer and 

blue areas are colder by about 0.0002 degrees.

   It might be possible to detect gravitons indirectly. Remember, gravitons are the hypothesized fundamental particles that carry the force of gravity. Experiments are in progress to detect gravitational waves and although these experiments cannot detect individual gravitons, they could provide information about gravitons. Gravitational waves were hypothesized by Einstein whenever two massive bodies are in tight orbit with one another, such as the binary white dwarf stars discusses previously in this column.

   At the Large Hadron Collider (LHC), scientists will soon be ready to run at energies high enough to allow for the testing of extra dimensions as predicted by string theory. The thinking is that if they can smash particles together at high enough energies, part of the collision debris might escape into an extra dimension. By calculating the total energy before and after such a collision, they can see if less energy is present after the collision. If so, it could mean that energy is flowing to an extra dimension.

   The Cosmic Microwave Background (CMB) radiation might also contain clues about strings. It is interesting to think that in the first moments after the big bang, all the matter of the Universe was contained in an extremely hot, dense stew of fundamental string particles. If so, we might be able to look for an imprint in the background radiation as the universe inflated.

   NASA’s Wilkinson Microwave Anisotrophy Probe (WMAP) has been gathering information about the nature of the Universe for nine years, finishing in September 2010, and has provided us with some fascinating information, mapping the CMB radiation to produce a map of the microwave sky, revealing what the Universe went through during the first trillionth of a second (inflation). The Planck space observatory, a project underway by the European Space Agency, will improve on this by a factor of three and is scheduled to deliver final results near the end of 2012.

String Theory

One of the toughest problems in physics is unifying Einstein’s theory of general relativity, which describes gravity on a large scale, with quantum mechanics, which describes the other three fundamental forces acting on the atomic scale. Finding this “theory of everything” has stirred great debate in the scientific community. Devising experiments to test it has been unsuccessful so far, but that may soon change.

   After Einstein finished his work on general relativity, he spent the next three decades of his life trying to expand his theory of gravitation to include electromagnetism (the strong and weak nuclear forces had not yet been discovered). This search left Einstein isolated from the main body of physics which was consumed by the emerging field of quantum mechanics, and which Einstein never fully embraced. In the early 1940s, Einstein wrote “I have become a lonely old chap who is mainly known because he doesn’t wear socks and is exhibited as a curiosity on special occasions.” In 1950, he described his unified field theory and although his work was ahead of its time, his efforts were ultimately unsuccessful. Einstein’s dream of unifying the fundamental forces of nature would have to wait for physics and mathematics to catch up.

The worldsheet on the left shows a string splitting apart and 

the worldsheet on the right shows two strings joining together. 

The arrows indicate the direction of travel through spacetime.

   Since then, theoretical physicists have taken up the challenge. The most successful theory thus far is string theory. At its essence, string theory is an attempt at describing gravity at the quantum level. In string theory, all fundamental particles are not point like, but instead made out of tiny, one-dimensional, vibrating loops of string—like incredibly small rubber bands. These strings use gravitons to exchange the force of gravity back and forth. A graviton is a hypothetical particle that is itself another little loop of string. You start with a loop of string and that string splits in half, creating a second string. If you have many strings doing this at the same time, every once in a while the split strings will get mixed up with one another and exchange back and forth, and that is the origin of gravity in string theory. This splitting and joining can be viewed diagrammatically as a worldsheet. A worldsheet is the name coined by Leonard Susskind for a two-dimensional surface which describes the embedding of a string in spacetime.

   Leonard Susskind is widely regarded as the father of string theory. In 1969, he and other physicists began to explore the possibility that particles were made up of strings. What they discovered, to their amazement, was that these particles behaved as if they had gravitational forces between them. Split off little pieces, exchange them, and they create forces very similar to gravity.

   Initially, string theory was intended to describe protons and neutrons. But the strings that physicists describe now are a billion billion times smaller than a proton. Much smaller than we can see with any kind of microscope that exists and smaller than anything that can be detected by the Large Hadron Collider. Because of this, graviton detection is impossible. Even if we could build a detector the size of Jupiter, we would only be able to observe one graviton every 10 years at best and it would be impossible to distinguish it from a neutrino. And a neutrino shield would be so massive that it would collapse into a black hole!

   Next week we will finish up our discussion of string theory by talking about why it requires extra spacial dimensions and what experiments are underway to test string theory.

White Dwarfs

Three illustrations showing white dwarfs 

orbiting each other and then colliding.

Our Sun is said to be a main sequence star, meaning that it fuses hydrogen to form helium. What happens when it exhausts its supply of hydrogen fuel? Once hydrogen fusion stops, there is not enough pressure on the star's core to create a fusion reaction with helium. Because there is no longer any outward push from fusion, the star begins to collapse from the crush of gravity. This collapse create more and more pressure in the core until helium fusion begins, while some of the remaining hydrogen burns outside of the core. The products of helium fusion is carbon and oxygen. The star expands and becomes a red giant. Its life as a red giant is brief on an astronomical scale—only a few million years. It will then eject its outer layers as it reaches the end of its burn cycle, leaving behind a collapsed core known as a white dwarf surrounded by a planetary nebula. 
  A white dwarf is the final evolutionary state of about 97% of all the stars contained in the Milky Way. White dwarfs are very dense—they pack a mass comparable to that of our Sun into a volume similar to the size of our Earth. A white dwarf is composed mostly of the fusion products of helium: carbon and oxygen. It is also surrounded by a thin layer of helium, and sometimes hydrogen. Fusion quickly stops so the white dwarf has no source of energy and will cool over many billions of years, eventually to the point where it no longer shines at all.
  Astronomers have recently found two white dwarfs stars orbiting one another once every 39 minutes. Binary white dwarfs are exceedingly rare. Out of the 100 billion stars in our galaxy, only a few dozen have been found. Located about 7,800 light-years away in the constellation Cetus, these two stars orbit each other at a distance less than that from the Earth to the Moon.
  The fate of these stars is already determined. Because they orbit one another so closely, they produces gravitational waves, ripple-like distortions in spacetime that were predicted by Albert Einstein in his theory of general relativity. The gravitational waves carry away orbital energy, which has put the stars in an ever-tightening spiral. In about 40 million years, they will crash into one another and merge to form a new star. If their combined mass were more they would collide and turn supernova, but since they aren't massive enough, they will be reborn and begin to shine once again.

The Mystery of Antimatter

Despite the apparent variety of materials in the world around us, we know that everything is made of one thing: matter. But when the universe was born about 14 billion years ago in the Big Bang, two types of matter were created: ordinary matter, which we and everything around us are made, and antimatter, a kind of opposite version of matter. The existence of antimatter was first predicted by the British physicist Paul Dirac in 1928. We know antimatter exists because we can find traces of it in cosmic rays and because scientists have made small amounts of it in particle accelerators. The universe is vast, but it seems to be made almost entirely of matter. So where did all the antimatter go?

Experimental area at CERN’s Antiproton Decelerator The actual antiproton decelerator ring is behind the concrete shielding seen on the left.

   When matter and antimatter come into contact, they annihilate each other, creating vast amounts of energy in the process. A reaction between on gram of matter and one gram of antimatter would release the same energy as exploding 43,000 tons of dynamite. We think that equal amounts of matter and antimatter were created in the Big Bang, but something happened in the first few minutes after the Big Bang to shift the balance in favor if matter so that when matter and antimatter destroyed each other, some matter was left over. This was the matter that went on to form the universe that we know today. Physicists don’t understand why there was an imbalance between matter and antimatter. In fact, this is one of the biggest mysteries in science today. What we do know is that antimatter is a mirror-like reflection of matter, but not quite. Antimatter is said to violate CP-symmetry (CP stands for charge and parity), meaning that there is not total cancellation between particles and their antiparticles. Understanding this process will help us understand why we live in a universe made of matter and not antimatter. But to really understand antimatter we need an experiment that can recreate the conditions just after the Big Bang. 

   Scientist at the European Organization for Nuclear Research (CERN) are attempting to do just that with several ongoing antimatter experiments. Antimatter is difficult to create but even more difficult to keep without it reacting. CERN’s Antiproton Decelerator holds the antiparticles before injecting them into one of several ongoing experiments. They can hold antiparticles for up to 1,000 seconds—a lifetime in the realm of particle physics. These experiments all have one thing in common: they want to pinpoint the differences in antimatter when compared to matter. It is their hope that someday this mystery of the universe will be solved.