by Shane L. Larson
Obi-wan Kenobi, in perhaps one of the most famous utterances in cinematic history, claimed that the Force “is an energy field, created by all living things. It surrounds us, it penetrates us, it binds the galaxy together.” This propagated rapidly through popular culture when it was realized that Obi-wan must have been talking about duct tape, which after all has a light side, a dark side, and also binds our world together.
But an astute citizen of the Cosmos may grow curious at Kenobi’s observation, and ask “what does bind the galaxy together?” As it turns out there is a force that penetrates the fabric of the Universe, in a way it is the fabric of the Universe. We call it gravity.
Many of us have heard the idea that there are four fundamental forces in Nature: gravity, the electromagnetic force, the weak nuclear force, and the color force (the “strong nuclear force” is a faint bit of the color force that “leaks” out of atomic nuclei to be detectable by our experiments). Why is gravity The Force? Why not the others?
In order to fill the Cosmos, a force must be a long range force — the Cosmos is a BIG place! The weak nuclear force and the color force are short range — they act very strongly over very tiny distances, in atomic nuclei and in the nuclear particles that comprise nuclei. The electromagnetic force is a long range force, but it acts in the presence of electrically charged particles, which come in two flavors — positive (+) and negative (-). It is easy to make separate positive and negative charges and to locally generate strong electromagnetic forces (lightning is a prime example from Nature), but by and large the Cosmos is electrically neutral — opposite charges are attracted to each other, and they quickly neutralize and cancel each other out, leaving no free charge behind. Gravity is also a long range force, but it has only one kind of “charge,” which we call “mass.” There is no negative mass, so gravity cannot be shielded or canceled, and it acts over vast distances.
Gravity is the only game in town when it comes to forces acting on cosmic scales, despite being so incredibly weak. I can see the skepticism on your face! I said gravity binds the Cosmos together, and in the same breath said it was incredibly weak! Whatever do I mean?
I mean that gravity is weak compared to the other forces of Nature, a fact you can easily demonstrate in your kitchen. Pick up an apple. What is holding an apple together? It is mostly intermolecular forces between the molecules that make the apple up, and those forces are electromagnetic in nature. Now,using your bare hands, try to break the apple half. Not so easy, is it?
Now, stand up and jump up in the air. How high did you get? Even if it was just a couple of inches consider this fact: you were able to momentarily over come gravity. Using a little bit of chemical energy, gleaned from that rabbit food you ate at lunch (perhaps an apple you ate), you were able to overcome the gravitational pull of the ENTIRE EARTH! Gravity is weak (and you are strong).
While these kinds of deep machinations are fascinating questions into the deep nature of Nature, you might still be scratching your head wondering what good is this knowledge? The first widely understood law of Gravity was Newtonian gravity, described by Isaac Newton in 1687. It was used almost immediately to begin describing the motion of heavenly bodies, but by and large the world went about its business more or less oblivious to this stunning achievement of the human intellect. The practical application of Newtonian gravity, using it for something that humans build or use, was not for almost 270 years: in 1957, the Soviet Union launched Sputnik, requiring a detailed understanding of orbital dynamics, which is derived from Newtonian gravity. By a similar token, Albert Einstein wrote down the modern description of gravity, general relativity, in 1915. There were immediate applications of general relativity to astrophysics (a trend that has only grown since), but practical applications to human affairs did not seriously arise until the late Twentieth Century. Let me tell you some stories about how gravity, general relativity, is changing our world.
GRACE. Our society is engaged in much teeth-gnashing about the nature of the Earth’s changing climate, but most scientists are doing what scientists do best — they put their heads down, they collect data, then they figure out what the data is telling them. Of particular importance to climate studies is the hydrological cycle on Earth. Gram for gram, water is a bigger player in thermodynamics than any other substance on Earth. It is extremely effective at cooling and heating, which is why you use it to cool off in the summer and warm up in the winter! The movement of water on Earth, in the oceans, the clouds, the rivers, and the atmosphere has enormous impacts on climate worldwide. But the hydrosphere is HUGE! We can’t possibly hope to monitor water levels and water flow in lakes and rivers and oceans worldwide by placing individual sensors. So how are we to learn about the water on Earth and how it moves and changes? The answer is we use gravity.
Satellite geodesey can make precision measurements of the Earth’s gravitational field. As a satellite flies over the Earth, the changing mass below the satellite changes the strength of gravity, which alters the satellite’s trajectory in its orbit. We monitor the orbit to know how the gravity (and the mass creating the gravity) is changing! In 2002, NASA launched a mission called GRACE (Gravity Recovery and Climate Experiment), consisting of two satellites flying about 220 km apart, monitoring each others’ orbit using a microwave signal. For over 5 years, GRACE monitored the Earth’s gravitational field and was able to see how it changes as water and ice move around our planet. Just one example is shown below, illustrating how the gravity in the Amazon basin goes up and down with the coming and going of the rainy season. Similar results illustrate the changing ice around the planet, particularly in the Arctic and Antarctic.
GPS. Perhaps the most ubiquitous use of gravity in your everyday life is the global positioning system. Once relegated to navigation on planes and automobiles, the advent of GPS built into smartphones has enabled an explosion of location services that allows you to find friends, local restaurants, comic book stores, and concert venues in unfamiliar cities.
Fundamentally, GPS works by triangulation. Satellites send out timing signals that are received by your smartphone or GPS navigator. The signals are broadcast in synch with one another. This means that if you are an equal, fixed distance from two satellites, you’ll get the same time from both (this is like using headphones — the sound from the L and R side are synchronized so you hear all the right parts of the song and the same time!). If you are closer to one satellite, then you receive a time from that satellite sooner than a distant satellite (this is like watching a track meet from the stadium — runners hear the starting gun before you do, because they are closer). Your navigator compares your local time to the time received from the satellites, allowing the determination of distance to each satellite. Since the position of each satellite is known, your location can be computed.
The satellite timing signals must be modified, using general relativity. Why? The satellites are much higher in the Earth’s gravitational field than you are, and general relativity tells us their clocks tick at a different speed. How much different? Over the course of a day, the general relativity correction to the clock times is about 38 microseconds — 38 millionths of a second! You may be thinking “But that is so tiny!” Yes it is tiny, but GPS works based on how far light travels in a given time. In 38 microseconds, light travels 11.4 kilometers (7 miles)! When you are trying to find a sushi restaurant, or the soccer field for your kids next game, 11 kilometers is a long way off!
Gravitational waves. Let me tell you one last story, not about the practical uses of gravity, but about our dream of using gravity to reveal the secrets of the Cosmos. In 1918, while exploring the implications of general relativity, Einstein discovered that there exists a kind of gravitational radiation, where the gravity from an astrophysical system carries energy away and into the far reaches of the Universe. He calculated the strength of this radiation, and very quickly decided that it would be exceedingly difficult (if not impossible) to experimentally measure.
But fast-forward the blu-ray to today, and we have technology at our disposal that Einstein could never have imagined — high precision, high power lasers; GPS positioning systems to accurately locate anything anywhere on the planet; high performance computers capable of performing billions of computations per second; a globe girdling network that passes information from one continent to another as easily as one might shout down the hallway to a colleague; and most importantly, a vast community of scientists well-trained and well-versed in wresting secrets from Nature, the best minds our planet has to offer. You add that all together, and we find ourselves in the land of Einstein’s dreams, poised to measure the faint echoes of gravity bathing the Earth from distant corners of the Cosmos.
Nearly a century of thinking on the matter of gravitational radiation has coalesced around a magnificent machine called LIGO — the Laser Interferometer Gravitational-wave Observatory. Using lasers shining up and down 4 kilometer long beam arms, a new generation of astronomers — gravitational wave astronomers — hope to detect the dance of neutron stars and black holes spiralling toward collision, the constant drone of young pulsars spinning down into their final rest in the stellar graveyard, and maybe (if we are lucky) the cataclysmic supernova explosion of a star dying, a process that synthesizes most of the atoms that comprise what we are all made of.
Gravitational wave astronomy is a way of asking anew the questions about who we are and what our place in the Cosmos is; it is a way of once again indulging in the unique gift to our species, an insatiable sense of curiosity and wonder. But are there practical outcomes from this remarkable feat of human imagination? Perhaps not obvious ones, because the practical outcomes were not the driving force in the creation of the experiment. But as with all great feats of science and engineering, from the Manhattan Project to Apollo to LIGO, there are always beneficial outcomes. Already LIGO’s technology is pushing the frontiers of optics and laser technology, environmental monitoring, and computer network capabilities. But changes you see in your living room may be 7 or 70 or 270 years away.
This has always been the case for gravity; the timescale is simply a matter of how creative our engineers and scientists get!