I’ve been reading The Fabric of the Cosmos and just watched the 3 hour Nova special based on his book The Elegant Universe. Brian Greene provides an entertaining walk through of the history of physics in comprehensible layman’s terms and the quest for the holy grail of physics: a single, unified theory that explains how everything works. Here’s the run down…
1665 – Newton proposes that the force that makes apples fall to Earth is the same force that makes the moon go ’round the earth. This revolutionized our thinking because it brought the Earth and the Heavens together in that the same forces govern them in the same ways.
1905 – Einstein suggests that the speed of light is the fastest speed anything in the universe can go. This flies in the face of Newton’s work in that Newton had assumed that gravitational effects are instantaneous between objects. For example, in Newton’s universe, there would be no delay between a shift in the gravitational force of the sun and Earth’s response to it. If nothing travels faster than light, and light from the sun takes 8 minutes to reach Earth, then if the sun suddenly disappeared, we shouldn’t feel it instantaneously, but 8 minutes later (at the soonest). So, if Newton’s theory was that bogus, then why do the planets rotate around the sun?
1916 – Einstein conceives of the universe as having 3 dimensions plus the dimension of time. (Think of our four dimensions this way: if I want to do really important stuff, like hang out with friends at the corner bar, I need to know 4 things: the 2 cross streets, what floor they are on, and what time to arrive.) In a universe of massless objects, these four dimensions work pretty much like a flat, undisturbed trampoline (which he called “space-time”). But put something heavy on that trampoline and it sags; the heavy object warps the playing field, making objects of less mass roll into the pit like a golf ball on a tricky putt-putt course. Plop something as heavy as the sun on the fabric of space-time and the earth on the edge of its dip, and the earth will get sucked in towards the center. This is the theory of General Relativity. (BTW, it turns out that Newton wasn’t a crackpot. His equations and theories hold up in the limited world of observable phenomenon. They only start to break down at extremes of cosmic proportion.)
Einstein didn’t stop there. About 50 years earlier, James Maxwell had discovered the mathematical justification for the reason that your compass goes bonkers when lightning strikes: electricity and magnetism are the same thing! Imagine how revolutionary and exciting it was to, yet again, discover that seemingly disparate forces are unified and unified with the elegance of simple mathematics. Einstein was inspired to find a unified theory for everything, a reasoning that explained both electromagnetism (as it was now called) and his new understanding of gravity (i.e., general relativity).
But gravity and electromagnetism are pretty different. For one, gravity is a pretty wimpy force compared to electromagnetism, by a factor of billions of billions. Just think of it: gravity keeps me sucked down to the ground and not drifting off into space. But if I were to jump off the Empire State building, the electrical charge that would be generated by the atoms in the sidewalk below would sufficiently (and fatally) repel the electrical charge of the atoms of my body and keep me from crashing through and plummeting to the center of Earth. Said another way, the electromagnetic forces in a teeny piece of sidewalk is enough to override the entire gravity of the Earth.
To make matters worse, in the 1920’s, physicists Nils Bohr and others discovered that the atom was not the smallest particle and that neither Newton’s nor Einstein’s theories explained the behavior of the relationships of these newly discovered neutrons, protons, and electrons. Observations showed that this microscopic world is so turbulent that it defies common sense. Space and time are so twisted and distorted that conventional ideas of linearity break down. There’s no way to tell for certain the position of an object at that level or the time of its arrival. You can’t pin much down. Evolving theories of quantum mechanics did a beautiful job of explaining the behavior of microscopic world by asserting that events are largely random and predictable only within margins of certainty. Not at all like Einstein’s orderly view of the world.
Further investigation into the workings of atoms revealed that gravity and electromagnetism weren’t the only two games in town. There were actually two other forces in play: the one that keeps protons bound to neutrons (aka the “strong nuclear force”) and the other allowing neutrons to turn into protons (aka the “weak nuclear force”), giving off radiation. In our everyday lives, gravity pales in comparison to their influence. (To help get the picture … in 1945,the first atomic bomb, only about 5 feet across, packed a “strong nuclear force” punch equivalent to 20,000 tons of TNT by breaking the superglue bond of the protons and neutrons, thereby splitting the atoms.) Every event in the universe is simply these four forces interacting with matter.
In the tradition of two seemingly different things turning out to be the same deal, experiments then showed that, under the very hot conditions that would have been similar to an earlier post-Big Bang phase, the electromagnetic force and the weak nuclear force merge. And it is theorized and widely accepted that if sufficiently intense conditions were created, the electromagnetic/weak nuclear force would merge with the strong nuclear force, thereby creating a “unified” theory for the strong forces at play. But, if the laws of quantum mechanics perfectly explain the workings of the world on the microscopic level (including electromagnetism and strong and weak nuclear forces) and should apply everywhere and yet the laws of general relativity (i.e. gravity) should also apply but only give us accurate coverage on the macroscopic level, what’s a theoretical physicist to do?
Einstein, by this point in history, became reclusive and disengaged from the continual revelations of quantum mechanics, and single-mindedly focused the rest of his life on solving unification. In 1955, he died, at age 76, after the scientific community at large had already abandoned the quest for unification. Physics was split into two camps: general relativists that study the universe as a whole and quantum mechanists that study the universe at its microscopic level.
Enter black holes. First theorized by Karl Schwarzschild in 1916 and observed decades later by satellite telescopes probing deep into space, black holes are formed by a piece of matter that is so very, very dense and yet so very, very small that it warps the space-time fabric (remember the trampoline?) that nothing can escape its pull, not even light. So, how do we go about understanding black holes? Do we use general relativity because the star is incredibly heavy or do we use quantum mechanics because it is incredibly tiny? Since we can’t have one theory for stars and one theory for atoms, we gotta use both at the same time. But together they produced non-sensical and frustrating results, much like attempting to navigate by car in a city that had two, simultaneously applicable yet conflicting sets of traffic laws.
OK. So much for the quick trip through 300 hundred years of physics. Buckle your seat belts because here’s where things really get wiggy. Or, maybe wiggly…
Just like mathematical equations can cryptically but powerfully capture the shape of a circle, a parabola, or some logarithmic curve, they have revealed that there are these mysterious things–tiny little massless particles, even smaller than electrons, neutrons, protons, or the quarks that the latter two are made up of–so small in size that they are as comparable to an atom as a tree is to the solar system. And the equations show that these things are shaped like strings and operate like vibrating strands of energy and that all forces and all matter in the universe are made up of them. The different ways the strings wiggle dictates the unique properties of atomic particles (such as mass and charge) and therefore represent the different kinds of elementary particles.
In order to accommodate all the different kinds of wiggles, the mathematics show that there must be not 3 dimensions, not even 3 dimension plus time, but a whopping 11 dimensions (also termed “degrees of freedom”) in our reality. Picture one of these extra dimensions as a thread wrapped around a spool. The spool is our visible 3-dimensional world but if you look close it has this little aspect that curls in and around us. Say this thread is actually 6-dimensional and around this 6-dimensional thread is wrapped another, 5-dimensional thread and so on on down. It is thought the way these dimensions curl back on each other influence the way strings move and vibrate, changing their presentation much like how pressing a valve on a french horn changes the sound it produces by changing the path through which the air travels inside it. It’s these extra dimensions that give one string its particular vibration, thereby producing in one case a proton or an electron in another.
String theory was first suggested by Leonard Susskind in the late sixties (or early seventies?) and dismissed by the academic community as unsound until a couple of diligent physics, John Schwarz and Michael Green, worked out the mathematical anomalies of the theory in 1984. For the next 10 years, these strings were ruled by only (!) 10 dimensions but had the pesky side effect of having 5 different theories that explained their operation. In 1995, Ed Witten narrowed this all down to one theory (called “M” theory) by adding an 11th dimension. By flipping the 10-dimensional string on end and looking at it from an 11-dimensional point of view, the shape of the string changes, as if rolled out like cookie dough, such that from this perspective it looks more like a membrane. This membrane (or, ‘brane, as it’s called for short) can itself be multidimensional and so very, very large that you could fit a whole universe on it.
If you think of each of these ‘branes as adjacent slices in a loaf of bread, our universe resides on just one slice. We might even have neighboring universes that don’t have to obey the same laws of physics (which, remember, are dictated by the way the set of strings in any one universe vibrate) or that may or may not have matter, planets, or even beings. Although these universes are less than a millimeter away, there’s no touching them from here and so they might as well be on the other end of the universe.
But what’s this all got to do with unifying our understanding of those four forces affecting matter in our universe? Recall that the problem with quantum mechanics was that it couldn’t account for gravity’s weakness relative to the electromagnetic and strong and weak nuclear forces. But what if gravity really is strong and we just can’t feel it because its effect seeps off our slice of the loaf the way that sound waves generated by billiard balls colliding leave the pool table or the way that powdered sugar falls off my french toast while the butter sticks to it.
Why should gravity be so different? Because the strings that govern the three “strong” forces, as well as governing matter and light, are mathematically shown to be open-ended strings whose ends are attached to our 3-dimensional membrane. But gravity’s string (also called a “graviton”) is shaped more like a rubber band. Without any loose ends to tie it down, gravitons are free to escape to other dimensions, making it seem weaker to those of us stuck in this membrane. What this suggests is that, if we do live on a membrane and there are parallel membranes near us, we may never see them but we might be able to feel them through gravity, by exchanging gravitational bursts.
What’s more, some string theorists have proposed that our universe arose by two wiggling membranes bumping into each other, thereby solving that lingering “What banged?” problem of the Big Bang theory. All of the energy of that collision had to go somewhere and it went into heating up all of the particles in our universe to the point where today, they’ve coagulated into stars and planets, and we are human beings living on one little planet zipping through the solar system in a galaxy that is rapidly sailing away from its neighboring galaxies. And chances are darn good this isn’t the first time, or even the last, that something like this has happened.
Well, that seems to kind of tidily sum everything up. Sound great? There’s a wee problem that keeps most physicists from sleeping soundly at night. These “strings” are so very, very tiny that we have no way of actually observing them and proving that they really exist. Yet, the level of elegance of the mathematics behind them strongly suggest that we are definitely on to something.
String theory holds the promise of understanding questions that are not considered scientific questions. Questions of how the universe began and why it is the way it is. But strings, if they exist, are so small that there is little hope of ever seeing one. “And if we can’t test string theory in the normal way we test scientific theories (by experiment and observation), then it’s not science, it’s philosophy. And that’s a real problem.”
To entertain life in the 10th dimension, go here. Thanks to for the link.