What’s the Matter With Gravity?

Of the four fundamental forces of physics, gravity is the one you know in your bones. Gravity owns you. Try to cross a downhill street slick with ice and you slide helplessly; whatever is in control, it’s not you. First you appreciate friction, then you understand the full omnipotence and omnipresence of gravity.

Gravity pulled the first matter in the earliest universe into the largest structures and inside them, spun up galaxies in which stars coalesced. Gravity regulates stars’ lives and when some die, it compacts them into neutron stars or black holes. Gravity sets the orbits of planets around their stars. On Earth, it drags down mountains, moves glaciers, creates the tides, and drives all convecting systems, from the earth’s fluid mantle to the weather to a pot of soup. Physicists understand gravity in great detail and with great accuracy, but they suspect they’re missing something—something big enough to change or even unify our most comprehensive theories of the universe.

Gravity was the first force to be studied quantitatively, says David Kaiser, historian of science at MIT, most notably by Isaac Newton, who was trying to understand motion. The force that drives a moving body equals the body’s mass times its acceleration, Newton said: The larger it is and the faster it accelerates, the bigger the force. Newton put that equation together with the laws proposed by Johannes Kepler—in particular, the law that says the planets’ distances from the sun are related to the time they take to orbit it. And then, astoundingly—”It’s famously difficult to figure out what led Newton from A to B in his head,” says Kaiser—Newton proposed a law that describes the force between his putative, famously falling apple and the Earth. He then extended that law to the force between any two celestial bodies. Newton called this universal gravitation, the revolutionary idea that, whether you’re an apple or a planet, whether you’re falling from a tree or orbiting in space, you obey the same rules.

Using Newton’s equations, scientists could for the first time measure masses that are otherwise immeasurable. “What amazes me,” says Gabriela Gonzalez, physicist at Louisiana State University, “is that we can weigh the sun that way.” Universal gravitation allows us to “weigh” planets, binary stars, black holes, and even the invisible dark matter which floats otherwise undetectably through the universe and about whose nature, no one has a clue.

Newton’s law had a flaw: It did not explain how one thing could act on another instantly, across any distance, with nothing in between. Nobody liked this “action at a distance,” including Newton. “It sounded occult,” says Kaiser, “like alchemy.”

Einstein didn’t like it either, and found an alternative. In his general theory of relativity, he proposed that gravity is the result of the nature of space-time. Space-time can be thought of as a continuous three-dimensional fabric which a body warps according to its mass; the more massive the body, the deeper the warp. A smaller body is not attracted to a larger one; it’s just rolling into the deeper valley.

But Einstein’s theory of gravity contains a flaw, or maybe just a puzzle. Gravity doesn’t fit in with the universe’s other three fundamental forces: the electromagnetic, the weak, and the strong. The other three can all be described by quantum mechanics, which explains the three forces as fields created and carried by waves which are also particles. To date, gravitational waves remain undetected and gravitational particles called gravitons are probably undetectable. So at bottom this force that’s so familiar, whose quantification you read every day on your bathroom scales, is—what?

This is where gravity becomes odd. If gravity is, as physicists say, mass telling space how to curve and space telling mass how to move, then space is inextricably related to mass. And mass, says Einstein’s E = mc2, always implies energy. So space must have energy too. And it does: In quantum theory, even empty space—a vacuum—has energy. The amount of energy in the vacuum, say quantum theorists, is so enormous that space should be curved so tightly that the universe would fit into a proton.

You could be forgiven for thinking this last is the ravings of theoretical physicists. But vacuum energy also crops up in another problem. For the last 14 years, astronomers measuring the universe’s expansion have found that the universe is not, as they’d expected, being slowed by the pull of its own gravity. Instead, the expansion is accelerating, speeding up; some push is countering gravity’s pull. A physicist with a sense of poetry, Michael Turner at the University of Chicago, called the push “dark energy.” And Turner and other physicists say that the simplest, most elegant explanation for dark energy is the energy in the vacuum.

Except the universe isn’t curled up inside a proton: Simple and elegant or not, vacuum energy doesn’t make the dark energy problem go away. “The mystery of dark energy,” says Leonard Susskind, physicist at Stanford University, is that compared to the calculated amount of vacuum energy, “there’s so little of it.” Maybe the calculations are wrong. Maybe dark energy won’t be understood until the excruciatingly complex supersymmetric and/or string theories get worked out. By this point, however, the reasoning is so mathematical—”Oh boy, is it mathematical!” says Susskind—that it’s hard even for physicists to follow.

An ordinary human hardly knows what to make of it. First, you can believe that Newton and Einstein between them described gravity exquisitely. Second, theorists don’t have the last word, and experimentalists are looking for gravitational waves. Gravitational waves are created when accelerating bodies distort space-time, says Gonzalez, “though the distortions are very, very small.” But something like two neutron stars coalescing into a black hole should create intense waves that should be detectable.

The most sensitive experiment is the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which can measure distortions smaller than 10-18 meters. LIGO hasn’t yet found gravitational waves, but it’s being upgraded to be able to detect waves over a larger volume of sky.

In fact, a number of different experiments, proposed and operating, stationed all over the world and out in space, are trying in differing ways to find gravitational waves of differing wavelengths coming from astronomical objects that range from binary white dwarf stars in our own galaxy, to the echo of the big bang itself. None of these experiments have found gravitational waves either. But if they do, the waves will carry new kinds of information from the hearts of the universe’s most turbulent creatures.

So what’s the matter with gravity? It may or may not be related to dark energy and it doesn’t fit in with the other forces. If it’s not a particle or a wave, then what else it might be is unclear. “And that’s where it sits now,” Turner says. “But wouldn’t you rather have no answer than the wrong answer?”

Go Deeper
Editor’s picks for further reading Introduction to Newton’s Law of Gravity

Astronomical Review: Why Gravity is So Weak
In this essay, Martin Rees compares the strength of gravity to the strength of the other fundamental physical forces.

FQXi: The Myth of Gravity
An article on a new model in which gravity is not a fundamental force.

NOVA: Relativity and the Cosmos
In this essay, Alan Lightman explores the history and meaning of general relativity.

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Ann Finkbeiner

    Ann Finkbeiner is a freelance science writer who usually writes about astronomy and physics and who runs a small, jewel-like graduate program in science writing at The Writing Seminars at Johns Hopkins University. She's written three books: "After the Death of a Child," "The Jasons," and "A Grand and Bold Thing." And she's proud co-owner of a science blog, The Last Word on Nothing. Really, when you're done here, go read it.

    • Excellent insight. Gravity is such a mysterious force. Its enlightening to know that nature always has an ace up its sleeve to keep things interesting for us.

    • “But wouldn’t you rather have no answer than the wrong answer?” Yes. Unless you can test the wrong answer and thus discover the right one.

      • Michael Snyder

        Often refuting an “answer” does not lead to a right conclusion. Refuting a theory does not yield an answer in the affirmative, it just means you go back to the drawing board and look for ways to revise your theory (or construct a new one all together) in light of your new findings. And in fact, the “no answer” in favor of “wrong answer” is the avoidance of type I error, a false positive, e most undesirable type of error you can get in a study.

    • Pbarnes

      A Mohammad Kahn posted a link to a peer reviewed article purporting to refute Special Relativity, one of his claims was that gravity was emergent from the electromagnetic forces. It boggles my mind that no one has reviewed this and commented on it. The post was under George Musser’s posting “Could Simple Experiments Reveal the Nature of Spacetime”. Is anyone other than new age hippies looking for spiritual meaning reading this blog? George Mussser?, Nova staff? Do you people exist?

    • Noswad11

      “space should be curved so tightly that the universe would fit into a proton.” Sounds similar to the universe before the big bang. Somehow gravity may have been “broken” resulting in our current situation of expansion and the dificulty in understanding gravity

    • Ujjal Goraya

      If Newton and Einstein’s Theories have “flaws” what chance we mortals have ?
      In my humble opinion Gravity is Elastic. it can let objects “Roam about” or it can pull objects back!

    • Azniteman

      Find the “weight” and nature of “dark matter” and more will be revealed about gravity. I believe dark matter is matter that has been stripped of all
      atomic orbits and has no known observational properities. It is “unexicited
      matter”. Black holes could be responsible for creating dark matter. This dark matter is responsive to gravity but not the other forces.

    • Timebridge

      Gravity is the by-product of the expansion of the universe. If the universe was not expanding, there would be no gravity. (At least thats what I hypothesize… – now to test it!)

      • Exactly
        my thought as well!

        riding an ever accelerating elevator, you’ll always feel a downward force.

        Gravity can’t be measured, in much the same way one can’t
        measure a shadow. A shadow, like gravity is a “result” of a
        “cause”. It has no substance unto itself and thus doesn’t really

        (by measure I mean “to its fullest extent”.
        Yes you can measure a shadow’s area and its absents of light but it doesn’t
        describe “fully” the higher dimensional source that created it.)

    • Mgriffith

      very interesting–just so technical. my question has always been– how far does space go– & what is it out there for??? mars.

    • Michael Brement

      Comments on the Theory of Everything
      Remember Einstein’s thought experiment? He imagined a man falling off the roof of a building. As the man fell to the ground, Einstein tried to imagine how gravity was acting on the man’s mass, as the man’s mass was attracted to the Earth’s mass. Einstein knew that his General Theory of Relativity described Space as a fabric that moves and is affected by mass. The revelation of this experiment was that Einstein realized that the man wasn’t falling through Space, he was falling with Space. The accepted scientific view in the 21st Century is that Space and Time are not separate. Physics students are learning about Space-time, not Space and Time. Whether it’s accepted or not; Space-time is the 4th Dimension. The General Theory of Relativity describes another very important point. Time is not constant, it’s very fluid. As velocity increases time slows down. Accepting that you can’t separate Space from Time, that means that as velocity increases, Space-time slows down.
      What has this to do with the Theory of Everything, let alone a way to quantify Gravity? Since Einstein came up with “Energy equals Mass times the speed of light squared”, it has been accepted that for Mass to reach the speed of light it would have to increase to infinite size, which would guarantee to be impossible. Scientists have been seeing this guarantee in only one direction. We live on a planet that exists in a Solar System that exists in a Galaxy that exists in a Cosmos that is part of a very big Universe. Another accepted scientific view is that the Universe is expanding. It stands to reason that Space-time is moving if you accept that all Mass moves with Space-time. When you are standing in one place, are you unmoving? We stand on a planet that rotates at approximately 1000 miles per hour. This planet also orbits the Sun at about 67 thousand miles per hour. The Sol System orbits the center of the Milky Way at what could be half of the speed of light (a guess). It’s anyone’s guess how fast the Milky Way is moving with Space-time in the Cosmos. To me, this means that as Mass increases, velocity increases.
      We need to consider one more axiom of accepted 21st Century science. Absolute Zero is considered the point at which Mass has no energy and no movement. Atoms line up in an orderly fashion and minimize the space between them. Since the entire Universe and everything in it is always moving, it’s impossible for any amount of mass to ever completely stop moving. Scientists know that even in deep space background energy keeps the temperature above Absolute Zero. Considering the definition of Absolute Zero and the idea that Space-time never stops moving, if an atom could reach Absolute Zero it could only happen if the entire Universe stopped moving. If the Universe stopped then Space-time would have to stop also. With no movement there would be no Space and there would be no Time. Think about this on a Quantum level. The volume of an atom is mostly space and an atom is always moving. If the atom stops moving then it has no space. The analogy Scientists have used is the one in which you consider the mass of the Empire State Building and remove all the space out of all the atoms in that building. The resulting volume of that building’s mass would fit in one grain of sand. This effect can be seen in the Universe when a Super Nova collapses in on itself. The ultimate would be the formation of a Black Hole. This means that the ultimate destructive force in the Universe is an area that can come the closest to reaching Absolute Zero.
      In the recent past, Scientists thought that all matter, light and energy were destroyed in a Black Hole. Dr. Steven Hawking proved that information is not lost and is retained at the Event Horizon. All this brings me to a logical assumption; a Black Hole is an area where Space-time has been removed from the trapped matter. Since a light photon exhibits properties of mass, it also gets trapped in the Black Hole. Energy is the only thing that exists as information on the Event Horizon.
      How does this all relate? What if there was a force that is always trying to remove Space-time from matter? What if the Strong Force, the Weak Force and the Electromagnetic Force where the only things that prevented it? Would this be a way to quantify Gravity? I’m not a mathematical genius so proving or disproving any of this will have to be left to those with the knowledge and desire to make the effort.
      As a “Theory of Everything” goes, this could be just as valid as any of the others that need to be proved or disproved. Keep this in mind before you dismiss this out of hand. Einstein was the one who saw the relationship between matter and Space-time. He ultimately didn’t have the time to pursue that revelation to the next level. Also, he was hampered by Religion, Politics, and Society’s conventions that prevented him from seeing what he could have.
      One more question. If this could be the way to quantify Gravity, would this be the way to connect the Macro world to the Quantum world? I can’t do the calculations but there has to be someone out there who can! Now that the “God particle” has been found what will it show science about the fabric of the Universe? I just thought of one more consideration. Another General Theory of Relativity truth that Einstein proved was the universal speed limit of light. If you accept the idea that all matter moves with Space-time and no matter can move faster than the universal speed limit, what does that tell you? Remember, Physics 101 teaches that a photon of light exhibits properties of both a wave and a particle. A particle has mass and mass is affected by Space-time (as well as Space-time is affected by mass). Take the logical leap; the universal speed limit isn’t the speed limit of light, it’s the speed limit of Space-time.

      • Me

        ^ Better than the article.

    • Doctor Who

      PBS wants to explore science.
      Commerical TV wants Big foot and UFO’s Bleech.

    • Weldon Vlasak

      Quantum physics was originated by Max Planck in which he considered the atom to be an electronic system.The hydrogen atom has two electric charges, thus being an electronic device. As a highly experienced engineer, I spent 20 years studying this little device, and discovered the secret of gravity. The sum of the four Coulomb forces between two atoms produces a force that is slightly higher than the force of gravity throughout all space. It has nothing to do with Einstein’s theory, which has a fatal flaw due to the Minkowski misinterpretation of radiation. For details, see my website: . To learn about Planck’s theory, read his own description: “Planck’s Columbia Lectures” delivered at Columbia University in 1908. You will learn more about physics there than any physics course.

    • Wayki
    • ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

      Everyone has the wrong idea of what energy, forces and fields are.
      Energy is a particle vibration or movement.
      You cannot have energy without a mass, energy is mass vibrating.
      Energy cannot be out on its own. (a supposed mass-less particle is a particle nonetheless, but there are no mass-less particles, so that’s irrelevant)

      Same thing goes for forces.
      A force is a group of particles arranged in a field pulling each other… and all of the particles absolutely have to be physically connected.

      A force (a group of connected particles) can only push very short distances and in rare circumstances like same pole magnets.

      But the point is… a force has to have particles involved.
      A force cannot be out on its own.

      Most of mainstream physics is a misconception.

      There is no such thing as pure energy.
      Again… Energy is a vibration on a particle (or particle movement).

      Can energy be converted into mass?
      Ummm… no, energy already has mass involved, it is a particle vibration or movement. There is no pure energy and you are not going to convert energy into mass.

      Think of a guitar string. If you pluck it… that is the energy. If you remove the guitar string from the scenario… can you still have the energy? No, of course not.

      Can you convert the guitar string vibration into mass? No… that is ridiculous.

      Look at what everything really is…

      Dimensions and units…
      mass = [M] = kilograms
      length = [L] = meters
      time = [T] = seconds
      frequency = [T^-1] = seconds ^-1
      speed = [L] / [T] …… = m/s
      acceleration = [L] / [T^2] …. = m / s^2
      momentum = [M] [L] / [T] … = kg_m / s
      force = [M] [L] / [T^2] . = kg_m / s^2
      energy = [M] [L^2] / [T^2] = kg_m^2 / s^2
      power = [M] [L^2] / [T^3] = kg_m^2 / s^3

      Notice mass [M] is not equal to energy [M] [L^2] / [T^2] …the vibration is missing

      Here is what Einsteins famous equation really looks like…

      [M] [L^2] / [T^2] = [M] [L^2] / [T^2]

      Energy already is a mass times speed^2.

      If you could just lop-off parts of an equation and claim whatever is left is equal… i.e. “energy equals mass” then you could also say that “power equals mass” and so does momentum and force. It is really stupid to think like that.
      Speed is NOT equal to length. Speed is equal to length divided by time.
      Energy is NOT equal to mass. Energy is equal to mass times speed squared.