Blockbuster Science

by David Siegel Bernstein

  As mentioned in the first interlude, the Standard Model explains most of what we observe in the universe but not everything (i.e., gravity). So the Standard Model is not quite a theory of everything; rather, it is a theory of most things. Some physicists believe we can go deeper than quarks (the constituent parts of protons) and electrons. To do this, we must depart the known and head for the speculative.

  This is where string theory comes in. Instead of thinking of quarks and electrons as single-dimension particles, string theory suggests they might really have two or more dimensions. These dimensions might be tiny, curled-up ones, or so large that our three-dimensional universe can comfortably dwell within it.

  Imagine the four strings of a violin. Each string is tuned (stretched) differently so that when they are bowed (an excitation), a different musical note is produced. This isn't too much different in string theory where the elementary particles (quarks, electrons, and their antimatter equivalent siblings) are the musical notes of strings. However, unlike our violin, which anchors the strings so they can stretch in different ways, the strings in the theory float in spacetime. They are tied to nothing, and yet they have tension.

  Something to ponder: if they exist, where do the strings come from?

  A violin's music comes from its strings vibrating in three dimensions. When we draw these vibrations on a two-dimensional sheet of paper, it looks like a sine or cosine wave (math terms for wavy lines drawn across a flat screen or paper). The strings in string theory are strumming their music in ten, eleven, or twenty-six dimensions.2 The fundamental particles in the Standard Model arise from these vibrations.

  Chapter 1 described four of these dimensions: three spatial (length, width, and height) and one of time. The other six or seven or more, if they exist, must be hidden. Otherwise we would be able to experimentally detect them. A good hiding place would be to compact them to a size that is so small that they become Planck-length small—a millionth of a billionth of a billionth of a billionth of a centimeter. Named for Max Planck who defined the base units (length for example) used to define quantum mechanics, Planck-length is so small that classical ideas about physics are no longer valid. Quantum mechanics dominates.

  I know this is hard to imagine, so let me help. Consider the edge of a piece of paper that is one millimeter thick. Now imagine a character named Ralph. He is insecure because of his size. He stands one-tenth the height of the paper's edge (0.1 mm). If his size were to represent the size of the entire observable universe, then Planck size would equal 0.1 mm relative to him.

  In the early 1990s, physicists realized that string theory faced an uncomfortable dilemma: there was no single string theory. Five unique versions each successfully describe phenomena under certain conditions, and each theory requires an additional dimension or two to describe a particle in the Standard Model, but each breaks down while explaining other particles. If only the five could be united into a single theory then almost everything could be described.

  This is where M-theory comes in. M-theory gives us an explanation for why so many dimensions are necessary. It treats each of the five string theories as subsets and serves as a road map to connect them. Of course, another dimension had to be added for M-theory to work. But who's counting? Don't ask me what the M stands for. It is a mystery in physics. I have heard many suggestions but nothing conclusive.

  If there are fewer dimensions than quantum events then negative probabilities must be included. Trust me, if this is true then things get ugly. Scientists do not like ugly. So it is better to add them in than subtract them out. If extra dimensions do exist, they might be really small and rolled up into their space. Or possibly the extra dimensions might be very large and contain all of matter and gravity within them.

  These large ones are called membrane dimensions, sometimes called branes by physicists. In brane theory, our three-dimensional universe might be a stretched brane floating through a four-dimensional background called the bulk. Imagine a two-dimensional sheet of paper riding the winds of our three-dimensional world. Add a dimension to both (along with a few other considerations), and you get a brane floating in the bulk.

  Within the M-theory framework, a brane is required as an attachment point for all of the strings. The tiny dimensions are squashed down into a particular dimensional shape called Calabi-Yau space, from which they are able (mathematically) to produce all of the physics we are able to see. An atom's fundamental qualities depend on this geometry.

  Science fiction has plenty of room for these extra dimensions. In Liu Cixin's novel The Three-Body Problem, Earth is invaded by technology hidden in curled dimensions.3 China Meiville's The City & the City deals with a conflict between overlapping dimensions.4 Sunborn by Jeffrey A. Carver uses a lot of the science ideas in this book. He has ancient AIs living in compact dimensions inside a black hole.5 Now that you know all about hidden dimensions, all you need to do is read the chapters about black holes and AIs.

  MEET THE COMPETITION

  Loop quantum gravity (LQG) theory is the chief competitor in the search for a grand unified theory. Where string theory attempts to explain everything in the Standard Model and bring gravity into the family of universal forces, LQG is much more modest. It seeks only to reconcile quantum gravity with spacetime.

  General relativity treats gravity as a property of the geometry of spacetime, while quantum mechanics treats gravity as a quantum force. LQG theory holds that spacetime itself might be quantized, meaning it treats space as granular rather than continuous as Einstein believed.

  So, if you kept zooming in on an area of space, say the distance between you and this book, with an impossibly powerful microscope, you would begin to see space itself pixilate and appear granular. The theory holds that these grains are woven together by finite loops of gravity. This is profound because it means space might be discrete (individual grains) and not continuous.

  Unlike string theory, there might be a way to test for loop quantum gravity. All you need to do is study the radiation a black hole evaporates. Researchers believe that if quantum gravity exists, measurable discrepancies will appear in the types of radiation evaporating from a black hole.6

  One of the biggest challenges for researchers is to find an evaporating black hole. So far, no one has detected one. The same technique might also be used to find evidence of quantum gravity in background radiation left behind after the beginning of the universe. Don't worry if you have no idea what black hole evaporation means. The topic is absorbed into chapter 6. For now, just know that this theory is testable.

  CAN QUANTIZED SPACE SOLVE A PARADOX AND HURT A VILLAIN?

  Loop quantum gravity theory might answer the paradox of infinite distance. Allow me to unjustly turn you into a criminal mastermind having a bad day. You, the criminal, spot the Green Arrow, a hero of DC Comics, just in time to see the arrow launching toward your head. Greeny is in a take-no-prisoners state of mind.

  If we believe Zeno of Elea (who lived during the 400s BCE), you are safe.7 The arrow passes halfway across the warehouse you call a lair, then half the remaining distance, then half again, and so on. I've divided its journey into infinite numbers of shorter and shorter segments. A half, then a half of the half, then half of the half of the half, and so on. The arrow never hits you because it must pass through an infinite number of points that make you infinitely far away. The arrow always gets closer but never strikes. Math has saved you!

  Only it won't. Not really. According to physics, you are about to feel a sharp pain and then probably nothing ever again. I will tell you why. There are a few philosophical and mathematical explanations I could provide, but let's go with the quantum one. At any given time after the shot is taken, only a finite number of quantized grains of space are between you and the arrow. Sorry, infinity is not going to help a criminal.

  PARTING COMMENTS

  The idea behind all the different versions of string theory is that strings are the most fundamental unit of nature. They strum their music in
dimensions of the universe we cannot perceive, and yet they create all that we can see. To us, these dimensions might only exist as mathematical constructs. The tiniest of them might be curled into the tiniest of scales, which is Planck length. This size is so small that length might not matter anymore. The largest dimension might be a brane that contains our entire universe.

  The five string theories are internally consistent, but separately they fail as an explanation of everything. M-theory is an umbrella theory that unites them. It is a road map for which theory is best at explaining which type of phenomenon.

  Loop quantum gravity theory takes the more modest approach of not attempting to explain all particles. Instead it focuses only on gravity. If it can connect gravity to quantized spacetime, then it will have unified relativity and quantum mechanics.

  FOR THE RECORD

  My favorite intersection of Murphy's Law and string theory: anything in string theory that theoretically can go wrong will go wrong, but if nothing does go theoretically wrong, then experimentally, it is ruled out.

  If you wish to make an apple pie from scratch, you must first invent the universe.

  —Carl Sagan

  The science of studying the whole universe is called cosmology. Cosmologists are big-picture scientists. And, as you might have guessed, the universe is pretty big. It also comes with 13.8 billion years of history. This chapter tackles a few of the big questions in cosmology, including the universe's size and how it began.

  HOW BIG IS THE UNIVERSE?

  The answer: pretty big.

  Okay, to start things off, start thinking big…really big, because the observable universe is about ninety-three billion light-years in diameter. A light-year is about six trillion miles. I will let you in on something extra: tomorrow the universe will be bigger.

  How much bigger?

  The universe is growing at approximately seventy kilometers per second per megaparsec.1 This complicated-sounding rate is known as Hubble's constant. Allow me to attempt to untangle it for you.

  Astronomers use parsecs to express stellar distances. A parsec represents 3.26 light-years. A megaparsec is a million parsecs (3,262,000 light-years). By the way, parsec is shorthand for parallax of one arcsecond. If you are planning on writing your own space adventure, I suggest sticking to using parsec.

  So if you peer out at the night sky using the powerful telescope NASA loaned you, at a distance of about 3.3 million light-years out into space, the galactic objects will appear to be receding from Earth at about seventy-one kilometers per second.

  The deeper into space you set your telescope, the faster the expansion will appear. Look out far enough, and objects will vanish. The reason that distant areas of space appear to (and actually do) move faster than the speed of light (and vanish) relative to our galactic location is due to the geometric nature of the expansion.

  The next time you watch Star Wars: A New Hope and Han Solo says, “You've never heard of the Millennium Falcon? It's the ship that made the Kessel run in less than twelve parsecs,” remember that a parsec is a unit of distance, not one of time.2 The only science fiction solution that might make this statement scientifically consistent is if Han meant that he was able to find the shortest route in and out of hyperspace.

  Take a moment to appreciate how big the universe is and how fast it is expanding. Traveling at the speed of light, a photon (quantized light) leaving our sun takes a bit more than eight minutes to get here. To reach Pluto, the little guy needs five and a half hours. If the photon is interested in taking a road trip, the journey to the next closest star (Proxima Centuri) is roughly 4.2 light-years.

  So, there is a lot of space out there in space. If we could travel at the speed of light (we can't), we would need over four years to get to our next stellar neighbor, Proxima Centuri. So, gallivanting around the Milky Way is time prohibitive. Given current technology, for the foreseeable future, humans will not make it far from Earth.

  The news is not all bad. The science (if not the politics or the funding) exists now to create space stations or planetary bases in our solar system. If humanity is more ambitious, a lot of good ideas can be found for terraforming a few of the moons and planets in our solar system. This topic is covered in more detail in chapter 12.

  If you imagine escaping the solar system, a sprinkle of science fiction dust might come to your rescue. In the Star Wars universe, ships travel in hyperspace lanes. These are wrinkles in spacetime that allow ships to jump from one point to another without traveling directly toward their destination.

  In the Star Trek universe, warp drive technology, powered matter/antimatter annihilation mediated by dilithium crystals, is used. (See the first interlude for a definition of antimatter.) The best technobabble description of the warp engine is that it is a gravimetric field displacement engine powered by matter/antimatter reaction. This is supposed to mean that warp fields are generated around a starship to form a subspace bubble. The bubble distorts local spacetime, allowing the starship to slide through the distortion at velocities (warp factors) exceeding the speed of light. A more scientific description of a warp drive is found in chapter 17.

  The speed of a warp factor is never clearly defined in any permutation of Star Trek. The different warp speeds do not make scientific sense to me, so I won't try to explain the exponential nature of the factors.

  The best part: no time dilation when zipping around in either the Star Wars or Star Trek universes.

  HOW IS THE OBSERVABLE UNIVERSE DIFFERENT FROM THE ACTUAL UNIVERSE?

  When describing the universe's size, I deliberately used the term observable. I did this because the universe we live in is different from the universe we can see.

  This is a big deal in cosmology. To astronomers, observable refers to the ability to see the light emanating from distant regions of the universe. However, some regions of the universe are so distant that the light from their stars has not had time to reach us yet. When it finally does, I suspect we will be long gone, but our descendants might get an eyeful.

  Then there are other regions that are expanding away from us faster than the speed of light (according to general relativity, anything containing mass might not exceed the galactic speed limit while traveling through space, but this does not apply to space itself), so their light will never reach our pale blue dot. As the universe expands, the horizon will get smaller. The universe we can interact with will also get smaller.

  The shrinking horizon isn't the only consequence of the galactic speed limit. It also means that nothing we see is current. The greater the distance, the greater the time differential. Time differentials are surprisingly commonplace and accepted in our perception of reality. I will not know what is going on in the world at the moment you read this sentence. Your now is my future.

  When you look out at the stars, you see yesterday. Actually, what you see is yesterday's great-great-great-grandmother. The Milky Way is about 100,000 light-years wide, and our sun is pretty far out from the galactic center, so any light emitted from a star at the opposite end of the Milky Way would need 100,000 years to travel here. If you see it, then it is old light, a long-gone yesterday. Here's the kick in the pants: as far as the universe is concerned, this is more or less a recent snapshot. Any light you see from Andromeda, the next closest spiral galaxy, is about three million years old.3

  Yes, it is sad that there are parts of the universe we'll never know about, but this isn't necessarily bad news for science fiction.

  HOW ARE GALACTIC DISTANCES CALCULATED?

  So, how do we know the distance between stars or between galaxies? Astronomers use cosmic yardsticks, also called standard candles, to measure distances. They also use a lot of geometry. For galaxies that are very far away, the yardstick is a supernova. Supernovas are ridiculously bright stellar explosions. Their observed brightness and a measurement of red-shifting due to expansion can be used to determine distance. More on red-shifting in a moment.

  Closer to home, Cepheid stars are used t
o gauge distances. These are very luminous stars that pulsate (changing diameter, temperature, and brightness) in a predictable pattern. Henrietta Leavitt discovered the period-luminosity relationship in 1912.4 Astronomers are able to measure distance because of the relationship between the brightness (seen by a telescope) and the pulsation period. Bonus 4 of this chapter provides a more detailed description of how Cepheid stars are used to measure distance.

  AND IN THE (OR RATHER A) BEGINNING…

  And now for an origin story…our origin story. I'm talking about the beginning of the observable universe, the big bang. The name is a misnomer because the event was neither big nor bangy.

  The name started out as an insult by English astronomer Fred Hoyle made on BBC radio. He believed there was no such thing as a spectacular beginning. Instead, he believed in the competing theory called steady state. In that theory, the universe doesn't change over time, but the stuff within it, like galaxies, can move around. He was wrong, but he still gets credited for coming up with the name.5

  So, what exactly is the big bang? It was an amazing event where all the matter that ever was and ever will be arose from a very (very) small point called a singularity. Recall from chapter 1 that a singularity is an infinitely small and dense point in spacetime. And 13.8 billion years ago, everything we observe spewed from one. Oh, and the big bang also created time.

  You want evidence for a big bang? Good! You are thinking like a scientist. Below are three lines of evidence. There are more, but this is a good start.

  1.Observation

  Through observation, Edwin Hubble discovered evidence for an expanding universe from the red-shifting of galaxies. When light from an object moving away from an observer shifts to the red end of the electromagnetic spectrum with respect to the observer, redshift has occurred. The color of the visible portion of the electromagnetic spectrum, in order of least to most energetic (frequency), is red, orange, yellow, green, blue, indigo, and violet. These are the colors of a rainbow.