فصل 02کتاب: ستاره شناسی برای مردمی که در عجله هستند / فصل 2
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On Earth as in the Heavens
Until Sir Isaac Newton wrote down the universal law of gravitation, nobody had any reason to presume that the laws of physics at home were the same as everywhere else in the universe. Earth had earthly things going on and the heavens had heavenly things going on. According to Christian teachings of the day, God controlled the heavens, rendering them unknowable to our feeble mortal minds. When Newton breached this philosophical barrier by rendering all motion comprehensible and predictable, some theologians criticized him for leaving nothing for the Creator to do. Newton had figured out that the force of gravity pulling ripe apples from their orchards also guides tossed objects along their curved trajectories and directs the Moon in its orbit around Earth. Newton’s law of gravity also guides planets, asteroids, and comets in their orbits around the Sun and keeps hundreds of billions of stars in orbit within our Milky Way galaxy.
This universality of physical laws drives scientific discovery like nothing else. And gravity was just the beginning. Imagine the excitement among nineteenth-century astronomers when laboratory prisms, which break light beams into a spectrum of colors, were first turned to the Sun. Spectra are not only beautiful, but contain oodles of information about the light-emitting object, including its temperature and composition. Chemical elements reveal themselves by their unique patterns of light or dark bands that cut across the spectrum. To people’s delight and amazement, the chemical signatures on the Sun were identical to those in the laboratory. No longer the exclusive tool of chemists, the prism showed that as different as the Sun is from Earth in size, mass, temperature, location, and appearance, we both contain the same stuff: hydrogen, carbon, oxygen, nitrogen, calcium, iron, and so forth. But more important than our laundry list of shared ingredients was the recognition that the laws of physics prescribing the formation of these spectral signatures on the Sun were the same laws operating on Earth, ninety-three million miles away.
So fertile was this concept of universality that it was successfully applied in reverse. Further analysis of the Sun’s spectrum revealed the signature of an element that had no known counterpart on Earth. Being of the Sun, the new substance was given a name derived from the Greek word helios (“the Sun”), and was only later discovered in the lab. Thus, helium became the first and only element in the chemist’s Periodic Table to be discovered someplace other than Earth.
Okay, the laws of physics work in the solar system, but do they work across the galaxy? Across the universe? Across time itself? Step by step, the laws were tested. Nearby stars also revealed familiar chemicals. Distant binary stars, bound in mutual orbit, seem to know all about Newton’s laws of gravity. For the same reason, so do binary galaxies.
And, like the geologist’s stratified sediments, which serve as a timeline of earthly events, the farther away we look in space, the further back in time we see. Spectra from the most distant objects in the universe show the same chemical signatures that we see nearby in space and in time. True, heavy elements were less abundant back then—they are manufactured primarily in subsequent generations of exploding stars—but the laws describing the atomic and molecular processes that created these spectral signatures remain intact. In particular, a quantity known as the fine-structure constant, which controls the basic fingerprinting for every element, must have remained unchanged for billions of years.
Of course, not all things and phenomena in the cosmos have counterparts on Earth. You’ve probably never walked through a cloud of glowing million-degree plasma, and I’d bet you’ve never greeted a black hole on the street. What matters is the universality of the physical laws that describe them. When spectral analysis was first applied to the light emitted by interstellar nebulae, a signature was discovered that, once again, had no counterpart on Earth. At the time, the Periodic Table of Elements had no obvious place for a new element to fit. In response, astrophysicists invented the name “nebulium” as a place-holder, until they could figure out what was going on. Turned out that in space, gaseous nebulae are so rarefied that atoms go long stretches without colliding. Under these conditions, electrons can do things within atoms that had never before been seen in Earth labs. Nebulium was simply the signature of ordinary oxygen doing extraordinary things.
This universality of physical laws tells us that if we land on another planet with a thriving alien civilization, they will be running on the same laws that we have discovered and tested here on Earth—even if the aliens harbor different social and political beliefs. Furthermore, if you wanted to talk to the aliens, you can bet they don’t speak English or French or even Mandarin. Nor would you know whether shaking their hands—if indeed their outstretched appendage is a hand—would be considered an act of war or of peace. Your best hope is to find a way to communicate using the language of science.
Such an attempt was made in the 1970s with Pioneer 10 and 11 and Voyager 1 and 2. All four spacecraft were endowed with enough energy, after gravity assists from the giant planets, to escape the solar system entirely.
Pioneer wore a golden etched plaque that showed, in scientific pictograms, the layout of our solar system, our location in the Milky Way galaxy, and the structure of the hydrogen atom. Voyager went further and also included a gold record album containing diverse sounds from mother Earth, including the human heartbeat, whale “songs,” and musical selections from around the world, including the works of Beethoven and Chuck Berry. While this humanized the message, it’s not clear whether alien ears would have a clue what they were listening to—assuming they have ears in the first place. My favorite parody of this gesture was a skit on NBC’s Saturday Night Live, shortly after the Voyager launch, in which they showed a written reply from the aliens who recovered the spacecraft. The note simply requested, “Send more Chuck Berry.”
Science thrives not only on the universality of physical laws but also on the existence and persistence of physical constants. The constant of gravitation, known by most scientists as “big G,” supplies Newton’s equation of gravity with the measure of how strong the force will be. This quantity has been implicitly tested for variation over eons. If you do the math, you can determine that a star’s luminosity is steeply dependent on big G. In other words, if big G had been even slightly different in the past, then the energy output of the Sun would have been far more variable than anything the biological, climatological, or geological records indicate.
Such is the uniformity of our universe.
Among all constants, the speed of light is the most famous. No matter how fast you go, you will never overtake a beam of light. Why not? No experiment ever conducted has ever revealed an object of any form reaching the speed of light. Well-tested laws of physics predict and account for that fact. I know these statements sound closed-minded. Some of the most bone-headed, science-based proclamations in the past have underestimated the ingenuity of inventors and engineers: “We will never fly.” “Flying will never be commercially feasible.” “We will never split the atom.” “We will never break the sound barrier.” “We will never go to the Moon.” What they have in common is that no established law of physics stood in the their way.
The claim “We will never outrun a beam of light” is a qualitatively different prediction. It flows from basic, time-tested physical principles. Highway signs for interstellar travelers of the future will justifiably read:
The Speed of Light: It’s Not Just a Good Idea It’s the Law.
Unlike getting caught speeding on Earth roads, the good thing about the laws of physics is that they require no law enforcement agencies to maintain them, although I did once own a geeky T-shirt that proclaimed, “OBEY GRAVITY.”
All measurements suggest that the known fundamental constants, and the physical laws that reference them, are neither time-dependent nor location-dependent. They’re truly constant and universal.
Many natural phenomena manifest multiple physical laws operating at once. This fact often complicates the analysis and, in most cases, requires high-performance computing to calculate what’s going on and to keep track of important parameters. When comet Shoemaker-Levy 9 plunged into Jupiter’s gas-rich atmosphere in July 1994, and then exploded, the most accurate computer model combined the laws of fluid mechanics, thermodynamics, kinematics, and gravitation. Climate and weather represent other leading examples of complicated (and difficult-to-predict) phenomena. But the basic laws governing them are still at work. Jupiter’s Great Red Spot, a raging anticyclone that has been going strong for at least 350 years, is driven by identical physical processes that generate storms on Earth and elsewhere in the solar system.
Another class of universal truths is the conservation laws, where the amount of some measured quantity remains unchanged no matter what. The three most important are the conservation of mass and energy, the conservation of linear and angular momentum, and the conservation of electric charge. These laws are in evidence on Earth, and everywhere we have thought to look—from the domain of particle physics to the large-scale structure of the universe.
In spite of this boasting, all is not perfect in paradise. It happens that we cannot see, touch, or taste the source of eighty-five percent of the gravity we measure in the universe. This mysterious dark matter, which remains undetected except for its gravitational pull on matter we see, may be composed of exotic particles that we have yet to discover or identify. A small minority of astrophysicists, however, are unconvinced and have suggested that there is no dark matter—you just need to modify Newton’s law of gravity. Simply add a few components to the equations and all will be well.
Perhaps one day we will learn that Newton’s gravity indeed requires adjustment. That’ll be okay. It has happened once before. Einstein’s 1916 general theory of relativity expanded on the principles of Newton’s gravity in a way that also applied to objects of extremely high mass. Newton’s law of gravity breaks down in this expanded realm, which was unknown to him. The lesson here is that our confidence flows through the range of conditions over which a law has been tested and verified. The broader that range, the more potent and powerful the law becomes in describing the cosmos. For ordinary household gravity, Newton’s law works just fine. It got us to the Moon and returned us safely to Earth in 1969. For black holes and the large-scale structure of the universe, we need general relativity. And if you insert low mass and low speeds into Einstein’s equations they literally (or, rather, mathematically) become Newton’s equations—all good reasons to develop confidence in our understanding of all we claim to understand.
To the scientist, the universality of physical laws makes the cosmos a marvelously simple place. By comparison, human nature—the psychologist’s domain—is infinitely more daunting. In America, local school boards vote on subjects to be taught in the classroom. In some cases, votes are cast according to the whims of cultural, political, or religious tides. Around the world, varying belief systems lead to political differences that are not always resolved peacefully. The power and beauty of physical laws is that they apply everywhere, whether or not you choose to believe in them.
In other words, after the laws of physics, everything else is opinion.
Not that scientists don’t argue. We do. A lot. But when we do, we typically express opinions about the interpretation of insufficient or ratty data on the bleeding frontier of our knowledge. Wherever and whenever a physical law can be invoked in the discussion, the debate is guaranteed to be brief: No, your idea for a perpetual motion machine will never work; it violates well-tested laws of thermodynamics. No, you can’t build a time machine that will enable you to go back and kill your mother before you were born—it violates causality laws. And without violating momentum laws, you cannot spontaneously levitate and hover above the ground, whether or not you are seated in the lotus position.†
Knowledge of physical laws can, in some cases, give you the confidence to confront surly people. A few years ago I was having a hot-cocoa nightcap at a dessert shop in Pasadena, California. Ordered it with whipped cream, of course. When it arrived at the table, I saw no trace of the stuff. After I told the waiter that my cocoa had no whipped cream, he asserted I couldn’t see it because it sank to the bottom. But whipped cream has low density, and floats on all liquids that humans consume. So I offered the waiter two possible explanations: either somebody forgot to add the whipped cream to my hot cocoa or the universal laws of physics were different in his restaurant. Unconvinced, he defiantly brought over a dollop of whipped cream to demonstrate his claim. After bobbing once or twice the whipped cream rose to the top, safely afloat.
What better proof do you need of the universality of physical law?
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