So what happens if these two physics can’t be fused? Well, then there are numerous key questions that physicists will not be able to address, some of which they know already and perhaps some others of which they have only a hazy inkling. Many of these questions have to do with some fairly fundamental issues—the true nature of mass, of time, of the beginning of the universe, of why it is like it is. At this juncture, physics intersects cosmology in a field that has come to be called astrophysics, where asking questions about the universe out there, way out there, turns out to be useful for asking questions about physics right here. The astronomical universe has the kind of laboratory conditions that could never be found on the earth. To the extent that we can observe what’s going on out there—and to the extent that we are confident that physics in the region of the earth and this solar system is no different (that is, obeys the same laws) from what it is at the farthest boundaries of the universe—then the physicist can pose fundamental questions about the nature of matter and energy in the laboratory of the cosmos.
Sounds great, but here are some serious problems associated with being an astrophysicist who uses the universe as laboratory. One is that you can’t actually get there. Two is that you can’t do experiments on “it.” Three is that you are a part of it, so measuring it objectively can be awkward. Four is the time limit, the speed of light, which puts a horizon on time so that you can’t see there, or then either. Actually, you can only see “there” “then”; that is, looking out into space is looking back in time, so the whole history of the universe is laid out in front of you—look out the right distance and you can see whatever “then” you want (just not “here”). This maddening confusion of time and distance is only one of the mind benders that cosmology offers. Five is that there is only one of it, so you can’t have a large sample size. And just in case any of those get solved, there are more problems waiting.
Well then that seems to be more than enough ignorance to go around, which should by now suggest to you that this is an ideal system. In fact, astronomy and astrophysics have provided tests, proofs, and solutions to problems ranging from gravity to calendars in the ancient world to the chemical periodic table to Einstein’s general relativity. Their power is undeniable. From the unseen ignorance they spew out proofs and data like a dark neutron star throwing off particles. Astronomy is a true laboratory because it is home to deeper and deeper layers of questions.
Scientists who relish these unanswerable questions divide up into two main approaches: theoretical and experimental. This division, once very competitive, is no longer so distinct, as they have become mutually reliant on each other. It is still true that theorists mostly manipulate mathematical equations and experimentalists collect data, but theorists depend on experimental data to support assumptions, validate predictions, and in some cases distinguish which of several mathematically equivalent models is physically realistic. And experimentalists depend on theorist-derived models to design experiments and to interpret data. For this case history we have three scientists who span the range from theoretical to experimental, but I caution the reader not to be too parochial about these definitions.
Brian Greene is a theoretical physicist who is also well known for his elegantly written descriptions of the hard-to-get-your-head-around concepts that make up the Alice in Wonderland worlds of relativity and quantum mechanics. He is a unifier. His physics, which is primarily expressed in mathematics, is a search for the true description of the universe we inhabit based on what we observe and some guesses about what we should be able to observe if we had the technology. And this universe is likely to be quite unintuitive. He believes it won’t be complicated, in the sense of having lots of moving parts to keep track of; it will be sophisticated and it may use difficult mathematics, but it needn’t be complicated.
One of Greene’s strategies is to try to work out the metaphorical explanation of the mathematical insights that are at first so other worldly. “To get over experience,” as he puts it, is to develop an intuitive (that is to say, unintuitive) view of relativity and quantum mechanics. This may not be as improbable as it sounds. You could imagine that if Einstein had come before Newton, we all might have a different view of the world. It would be normal to think of time as malleable and gravity as a geometrical feature of space. It might then seem that our everyday Newtonian views of gravity, with its massive bodies “attracting” each other, are hopelessly complicated (as many high school physics students will tell you) and full of arbitrary-seeming formulas. Greene works hard at this, at understanding the world from uncommon and unintuitive points of view. You might say this ability was the key to Einstein’s breakthrough: he was willing to get past experience, in this case a Newtonian perspective, and imagine life as a photon riding a beam of light.
How counterintuitive might Greene’s universe be? He thinks that the ideas of space and time will not appear in the fundamental description of the universe. These will be derivative concepts in the way that temperature is a derivative unit—it is a result of molecules in a gas or liquid moving fast, but one fast-moving molecule doesn’t have any heat. Thus, even though his favorite theory is based on strings occupying 11 dimensions, they may not include the familiar 4 of space and time. We don’t even have the language to ask the questions without invoking paradoxes and logical inconsistencies. “Time will disappear at the beginning of the universe,” but what then do we mean by “beginning”? In an improbable conflation of Bill Clinton and Gertrude Stein, you might ask of the beginning of the universe, “Is ‘is’ there?”
How does one think thoughts that are so far outside our language that framing them is difficult and expressing them to others invites frustration for both speaker and listener? Well, there’s mathematics, because it doesn’t have the same conceptual limits of spoken languages. In mathematics there are computational rules and you follow them and get the results that you get. Sometimes you are forced to make some pretty strange assumptions in the math, because if you don’t, you get results that are not interpretable—they’re mathematically correct, but meaningless. So Greene and his colleagues do math, make assumptions that sometimes seem fanciful but that they nonetheless have to defend, get improbable results, and then try to understand how these could describe the universe. All in a day’s work. Although most days it doesn’t work, and they go back and attack from another angle, with another set of mathematical strategies. They get their glimpses through equations that challenge and force their imaginations.
In fact, Greene has a somewhat cavalier approach to all of what he does. This is especially striking because he is the very public face of the controversial String Theory model, and you might think he has a lot invested in its eventual success as the prevailing model for unifying physics. And while he is quite confident that it will work out, that he will be proven correct, that he is on the right track—he is ready to be shown otherwise, to be completely wrong, to find that he has been chasing down one of those cats that aren’t there. “If string theory is wrong, I want to know right away”; no sense wasting any more time on it. Of course, one is almost never completely wrong or irrelevant. Just as one is rarely completely right. Engagement with the universe is the critical issue—right and wrong are for gamblers, politicians, judges, and the like. Science lies elsewhere, with the engagement. From the engagement comes the unexpected, the sudden intuition, the abruptly obvious comprehension, and the new ignorance.
Astronomer David Helfand, more on the experimental than theoretical side, is waiting for a nearby star to blow up. When I say nearby, it is important to realize that terms like “near” and “far” in astronomy are not the same as when you give directions to your friend’s house for a party. Helfand is hoping for something local, here in the Milky Way, in our hundred thousand light year neighborhood. An especially bright supernova appeared in 1181, and it was recorded as a “guest star” in religious and historical reports and personal diaries from China and Japan. (As an interesting sidelight on what we don�
�t know we don’t know, the event went largely unnoticed, or at least unrecorded, in Medieval Europe, where Aristotelian views of a never-changing perfect celestial sphere still held sway. A fairly bright new star in the sky was apparently not considered an important enough event to record; it wasn’t likely to be any more than some “disturbance.” You see how easy it is to miss things. In contrast, another supernova that could be detected by the naked eye in the night sky occurred in 1572. Now well into the Renaissance and more liberated from the tyranny of classical authority, it was recorded by astronomers throughout Europe. Indeed, this one was used as a case against the Aristotelian view of immutable heavens.) Stars blowing up leave traces that can tell you something about what they were made of and how they operated and why they came to an end. A stellar death is a kind of astronomical crime scene, and the science uses a forensic approach, analyzing what’s left behind to reconstruct and understand the causes of an event that couldn’t easily have been predicted beforehand. The critical thing is to know exactly when the star exploded, the time of death if you will, because interpreting all the rest of the evidence will depend in one way or another on that. And it also helps to be there as close to the event as possible. Right now Helfand uses historical reports of stellar mishaps that appear in ancient texts to date his explosions, but to be there when it really happened, to have the telescopes and instruments trained on the star within seconds of it actually blowing up, that would be the real prize. (There was a supernova in 1987, but, unlike the 1572 event, it was in another galaxy and too far away to be quite as useful as one here in our neighborhood.) The data that could come from that sort of observation would clear up many nagging questions. How fast do things cool off, that is, dissipate energy? What happens to atomic nuclei and their quarky constituents under conditions that could never be replicated on earth? What happens to space and time in the immediate region of an exploding star? All great questions. It’s also clear, because he can barely suppress it, that Helfand would simply like to have the astronomically good luck to be around when one of those epochal events takes place. Who wouldn’t?
Of course, one probably has, but we don’t know about it. For example, the red star that makes up Orion’s right shoulder (Helfand admits it’s the only constellation he knows) is ready to go, but since it’s 400 light years away we are seeing the star that was there when Shakespeare was producing plays. It may have actually blown up the night Hamlet was premiered, but word (or rather light) of it will not have reached us just yet. The strange thing about the universe is that we know better how it looked in the past than how it looks now. Betelgeuse may be long gone in a spectacular fireball, but we are none the wiser. In the deep universe we only see the past; we can never see now.
As it is, Helfand, like many astronomers and cosmologists, lives in a warped time zone where things happen very slowly and then very rapidly and often without much warning, even though they are sometimes cataclysmic. For example, Helfand studies an astral event that is just over 800 years old, using techniques that confine him to making observations every 10 years or so (depending usually on the launch of some new satellite with new instruments), and then the data stream out in millions of bits per second and can sometimes be interpreted in minutes, followed by long days to complete the deeper analysis, and weeks or months to publish it.
Something not often appreciated is the tremendous range of the time scale of science. Scientists study some events that unfold over eons and some that last millionths of a millionth of a second. Although science continues over generations, it is rare that an individual scientist picks a problem that cannot be resolved within his or her lifetime. Helfand admits, sardonically but perhaps a little sadly, that for one of the current problems he is working on he now realizes that he will likely die without the answer, simply because NASA changed plans and now doesn’t expect to launch the required instrumentation for about 30 years.
How much does our 50-year working human life span govern the way scientists organize their questions? I think it must, but most of us rarely consider it consciously when we talk about experimental programs or when we identify the important questions that we would like to work on. Those longer range goals get a brief mention at the end of review articles, they are the futuristic imaginings, but they have little effect on the day-to-day work of most scientists.
Helfand makes a quick list of the questions that astronomy could impact in the next 400 years, clearly not concerned with his life span or anyone else’s. These are the centers of ignorance: nuclear matter at densities greater than those in a normal nucleus; the identity of dark matter; the polarization of the microwave background radiation leading to measurements of quantum fluctuations (more on this scary-sounding phrase later); the reconciliation of general relativity and quantum mechanics (that’s the big unified field theory); the identity of the dark energy; gravity waves as a possible test of string theory and higher dimension universes. Quite a list. I started this section by saying that physics could be considered among the most successful of the sciences for all that it has explained. It could also be the most successful for all the questions it has spawned. That list of Helfand’s couldn’t have even been conceived by some of the great minds in physics a mere hundred years ago.
Amber Miller, a true experimental astrophysicist, practices a kind of archaeology of the universe. She looks for fossil traces of the very early universe in the remnants of the initial explosive event that brought the whole thing into existence. Using these fossils, she asks detailed questions about the early universe so that we can understand why it has come to be what it is, whatever that is.
The numbers that astronomers use are big—everything is unimaginably vast out there. It’s one thing to hear these numbers, and see them plugged into the theorist’s equations, often using special notations that allow you to state a big number without actually writing out all those zeros. This also makes it easier to overlook their true size. The number 1021 simply doesn’t register on the mind in the same way as a 1 followed by 21 zeros: 1,000,000,000,000,000,000,000. But for an experimental astrophysicist, someone who actually makes the measurements and builds equipment to specifications that are determined by those numbers, it all becomes more real—or curiously less real because you realize how truly incomprehensible these distances and times are.
Miller is desperate to find out what happened at the very beginning of the universe. The very, very beginning—say within the first 10–35 seconds of its existence. Now that’s a very small number, but in the mirror world of astrophysics that means looking out at a great distance, a very great distance, because in astrophysics distance is time. Light from those very early moments in the universe must travel to us from what is now the very edge of expanding space. Long ago means far away.
The universe creates space ahead of it. The first explosion, the Big Bang as it is now called, created time and space and as the edge of that explosion moves outward it continues to create time and space. The universe is not so much expanding; rather space is being created and the universe, the things in it, galaxies and the like, are simply filling up the expanded space. Talking about these things reminds you how important it is to ask the questions properly. No sense asking what’s on the other side of that expanding edge of the universe. It isn’t there; it hasn’t been created yet. Just because a question can be asked doesn’t make it a meaningful question.
In trying to understand what is happening on that edge of creation, Miller looks for cosmological fossils by sending balloons into the outer atmosphere loaded with sophisticated instruments that can measure something called the cosmic background microwave radiation. In the 1940s it was proposed that if there were a Big Bang–type explosion that started the universe, then even all these billions of years later there should still be some trace of it permeating the universe. This trace was calculated to be a very low-frequency hum, like noise on your radio when you can’t quite get the station. In one of the great stories of serendipity in science, precis
ely this noise was discovered by two scientists at Bell Labs while they were trying to get rid of a troubling and persistent hum that plagued a new radio-telescope instrument they were testing. That hum was not a fault in the instrument but the cosmic background radiation left behind by the Big Bang:—a 13.7-billion-year-old fossil.
What this fossil shows is a bit curious and leads to a conundrum in cosmology. The universal hum, the cosmic background radiation (CMB), is nearly the same in all directions. This suggests that things out there are pretty homogenous, which would be okay but remember when we say “out there” we are really saying “back then” because in astrophysics we can only see what has had time to reach us traveling at the speed of light. This is why astronomers measure distance with a unit called a light year. In this Red Queen world it is easier to see the past than the present. Looking at “back then” leads to a subtle question. The universe we are seeing now is much larger than the universe we could see some time ago. Even in the last 10 years the observable universe has increased in size by a radius of 10 light years, so the parts of the universe we are seeing now couldn’t have been seen 10 billion years ago. If they couldn’t have been seen then, they couldn’t have interacted with each other. So how come they’re the same now? It’s as though we have a condition now that must have been caused after the parts were able to interact—that is, the result seems to have come before the cause. Not sensible at all. Smacks of the Red Queen telling Alice that she could believe six impossible things before breakfast.
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