On Earth As in the Heavens

Natural History Magazine

by Neil deGrasse Tyson

From Natural History Magazine, November 2000

Until Isaac Newton wrote down the universal law of gravitation, there was little reason to presume that the laws of physics on Earth were the same as everywhere else in the universe. Earth had earthly things going on and the heavens had heavenly things going on. Indeed, according to many scholars of the day, the heavens were 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 branches 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 universaltiy 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 contained 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 whatever laws of physics prescribed the formation of these spectral signatures on the Sun, the same laws were 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. The 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, the farther away we look, the further back in time we see. Spectra from the most distant objects in the universe show the same chemical signatures that we see everywhere else in the universe. 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 process that created these spectral signatures remain intact. In particular, a quantity know 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 you've probably never greeted a black hole on the street. What matters is the universality of the laws of physics that describe them. When spectral analysis were first turned to the light emitted by interstellar nebulae, a signature was discovered that, once again, had no counterpart on Earth. By then, however, the periodic table of elements had no missing boxes; when helium was discovered there were several. So 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 with each other. 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 Chinese. You don't even know whether shaking their hands—if indeed they have hands to shake—would be considered and 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 the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft, the only ones given a great enough speed to escape the solar system's gravitational pull. Pioneer donned 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 diverse sounds from mother Earth including the sound of the human heartbeat, whale songs, and musical selections ranging from the works of Beethoven to 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 magazine cartoon that appeared shortly after the Voyager launch: pictured was 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 how strong the force will be, and 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 that the biological, climatological, or geological records indicate. In fact, no time-dependent or location-dependent fundamental constants are known—they appear to be truly constant.

Such are the ways and means of our universe.

Among all constants, the speed of light is probably 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, and there exist well-tested laws of physics that predict and account for that. These statements sound closed-minded. True, some of the most the embarrassing 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 fly faster than sound. We will never split the atom. We will never go to the Moon. You've heard them. 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. No doubt about it. Highway signs for interstellar travelers of the future will surely read:

The Speed of Light:
It's Not Just a Good Idea
It's the Law.

The good thing about the laws of physics is that they require no law enforcement agencies to maintain them, although I do own a nerdy T-shirt that says OBEY GRAVITY.

Many natural phenomena reflect the interplay of multiple physical laws operating at once. This fact often complicates the analysis and, in most cases, requires supercomputers to calculate things and to keep track of important parameters. When comet Shoemaker-Levy 9 plunged into and then exploded within Jupiter's gas-rich atmosphere in 1994, the most accurate computer model of what was to happen 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 anti-cyclone 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 are 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 in the universe—from the domain of particle physics to the large scale structure of the universe.

In spite of all this boasting, all is not perfect in paradise. It happens that we cannot see, touch, or taste the source of ninety percent of the gravity of 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 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. In 1915, Albert Einstein came up with the general theory of relativity, which reformulated the principles of gravity in a way that applied to objects of extremely high mass, a realm unknown to Newton's and where his law of gravity breaks down. The lesson here is that our confidence flows through the range of conditions over which a law has been tested and verified. The broader this range the more powerful that law becomes in describing the cosmos. For ordinary household gravity, Newton's law works just fine. For black holes and the large scale structure of the universe we need general relativity. They each work flawlessly in their own domain, wherever that domain may be in the universe.

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, school boards vote on the subjects to be taught in the classroom, and in some cases these votes are cast according to the whims of social and political tides. Around the world, varying belief systems lead to political differences that are not always resolved peacefully. And some people talk to bus stop stanchions. The miracle of physical laws is that they apply everywhere, whether or not you choose to believe in them. Furthermore, that it's not a function of your mental health. After the laws of physics, everything else is opinion.

Not that scientists don't argue. We do. A lot. When we do, however, we are usually expressing opinions about the interpretation of ratty data on the 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 laws of thermodynamics. No, you can't build a time machine that will enable you to go back and kill you 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. Although you could perform this stunt if you managed to let forth a powerful and sustained exhaust of flatulents.

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. I had 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. Since whipped cream has a very low density, and floats on all liquids that humans consume, 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 brought over a dollop of whipped cream to test for himself. After bobbing once or twice in my cup, the whipped cream sat up straight and afloat.

What better proof do you need of the universality of physical law?