Gravity

12
Sep

Why Is Gravity Such a Weakling?

We think of gravity as weighty—its omnipresent grasp pulling us down to the ground. Try to lift a piano up a flight of stairs and you can feel gravity’s resistance. (Laurel and Hardy showed this best!) Yet in a match with the other fundamental forces of nature—electromagnetism, the weak force and the strong force—gravity gets pummeled.

You can see gravity’s relative weakness simply by using an ordinary bar magnet to pick up paper clips from a desk. Battling the gravitational pull of all of Earth, the tiny magnet wins! In fact, gravity is a staggering 1040 times weaker than electromagnetism. But why, among the fundamental forces, is gravity the runt of the litter? Explaining gravity’s relative feebleness is a profound challenge for physics, and an essential milestone on the road to a unified theory of all four forces.

Uniting the four fundamental forces into a single unified theory is a longstanding scientific dream. On the face of it, these forces are very different. They each operate across different distances, with different strengths, determined by the properties of a special class of particles, called “exchange particles,” whose job it is to convey the forces.

Exchange particles are like Frisbees thrown between particle players; the process of tossing the Frisbee brings ordinary particles together—or in some cases pushes them apart. In electromagnetism, for example, electrons interact by exchanging photons. Because the photons have no rest mass, they travel through the vacuum at the speed of light, making electromagnetism a long-range force. Long-range means that it can operate over great distances. For example, terrestrial receivers can pick up radio signals (a type of electromagnetic radiation) transmitted by Voyager 1, situated at the edge of the solar system more than 11 billion miles away. The weak force, in contrast, is conveyed by massive exchange particles called the W+, W- and Z bosons. Because these exchange particles are so heavy, particles feel the weak force only on very short ranges—within atomic nuclei. The strong force, too, operates only on short ranges. But gravity, which theorists believe is carried by particles called gravitons traveling at the speed of light, is a long-range force.

Physicists have made great strides toward unifying electromagnetism with the weak interaction and, to some extent, with the strong interaction, by looking at how the forces behave at very high energies—energies that existed just moments after the Big Bang. Theoretical and experimental discoveries of the past few decades suggest that at these energies—above about 100 GeV (gigaelectronvolts)—the weak and electromagnetic forces behave as a single type of interaction, called electroweak, with identical range and strength. During this brief, hot period, the “Frisbees” that convey the weak force had no mass at all. But as the universe cooled, interactions with the Higgs field caused the W+, W- and Z bosons to acquire mass, differentiating them from massless photons and splitting what was once one force, the electroweak force, into two, electromagnetism and the weak force.

Although physicists have yet to develop a “grand unified” model that includes the strong force, they believe that, at even higher energies, it too may merge with the weak and electromagnetic forces.

But what about gravity? If you could draw the evolutionary family tree of the physical forces, you might see the split between electromagnetism and the weak force as something like the division of primates into various species. Gravity, though, represents a far more radical diversification. Reconciling gravity along with the other forces is something like showing how viruses and whales have common ancestry. Spanning the differences is possible, but tricky.

How far up the energy scale must we climb to find a point at which gravity could be unified with the other forces? The answer is a number called the Planck energy: around 1028 eV. That’s about 1017 times greater than the energy of electroweak unification—an enormous difference. One would need a machine almost a quintillion times more powerful than the Large Hadron Collider or Fermilab’s (retired) Tevatron to probe the Planck energy, putting experimental tests of this kind of unification well out of reach for the conceivable future! The sheer magnitude of the Planck-to-electroweak energy ratio, related to the stark weakness of gravity, is called the hierarchy problem.

String theorists have proposed an innovative solution to the hierarchy problem: Perhaps gravity is weakened through its ability to travel across an extra dimension. What sounds like science fiction has become an active branch of scientific inquiry called the “braneworld hypothesis.” The braneworld hypothesis suggests that the observable universe lives within a four-dimensional (three dimensions of space and one dimension of time) membrane, or “brane.” Beyond this brane, extended along a fifth dimension, is a region called the “bulk.” Unique among exchange particles, gravitons are free to wander the bulk; everything else is stuck on our brane. So while most particles are like ants confined to the surface of a wooden picnic table (the brane), gravitons are like termites that can bore within (and visit) the bulk. (In the language of string theory, the difference is that gravitons are represented by closed strings; other particles are open strings and have ends that stick to our brane like a curved handle attached to a door.) Because photons cannot enter the bulk, it remains invisible.

An advantage of explaining gravity’s weakness through its dilution into the bulk and making the brane the venue for the other forces is that unification can take place at energies not much higher than the electroweak scale, rather than at the Planck scale, rendering the search for a unified theory much easier.

There are a number of variations of the braneworld idea. One model, proposed in 1998 by physicists Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali envisions a “large extra dimension” of about one millimeter throughout which gravity is spread. (String theorists are typically concerned with such tiny size scales that one millimeter does indeed qualify as “large.”) A second brane, parallel to ours, would constitute the opposite boundary of the bulk, confining gravitons to the region between the two limits. You can imagine this second brane as something like the underside of the picnic table. The extra dimension would then comprise the thickness of the table—the distance between its top and bottom—limiting the graviton “termites” to travel through a finite amount of wood.

However, this proposal modifies the law of gravity in measurable ways, which have since been ruled out experimentally. Another version, proposed by physicists Lisa Randall and Raman Sundrum, includes only one brane and posits that the warping of the structure of the bulk, along the direction of the extra dimension, would be enough to confine gravity to a limited region and dilute its strength. The theory predicts a measurable leakage of gravitons from our brane into the warped bulk, which could potentially be detected in collisions at the Large Hadron Collider through unexpected energy loss that finds no other explanation. Researchers have sought such telltale clues to test the Randall-Sundrum idea and other braneworld approaches. The tricky part is using statistical models to rule out more mundane effects that could mimic the leakage of energy (for example, the release of neutrinos). While LHC results have not yet confirmed the braneworld idea, the jury is still out as to whether or not gravity’s weakness is a result of its slick ability to leave the visible universe and travel through a higher dimension.

Go Deeper
Editor’s picks for further reading

CERN Courier: The Nobel path to a unified electroweak theory
In this article, CERN marks the anniversary of the 1979 Nobel Prize in physics, which honored the foundations of electroweak unification.

The Particle Adventure: What holds it together?
Explore the physics of force-carrying particles on this interactive web site.

Warped Passages: Unraveling the Mysteries of the Universe’s Hidden Dimensions
In this book, physicist Lisa Randall explores extra dimensions, braneworlds, and the bulk.

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phalpern-big

Paul Halpern

    Paul Halpern is Professor of Physics at the University of the Sciences in Philadelphia. A prolific author, he has written thirteen science books, including "Einstein's Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics" (Basic Books). His interests range from space, time and higher dimensions to cultural aspects of science. The recipient of a Guggenheim Fellowship, Fulbright Scholarship, and an Athenaeum Literary Award, he has appeared on the History Channel, the Discovery Channel, the PBS series "Future Quest," and "The Simpsons 20th Anniversary Special." Halpern's books include "Time Journeys," "Cosmic Wormholes," "The Cyclical Serpent," "Faraway Worlds," "The Great Beyond," "Brave New Universe," "What's Science Ever Done for Us?," "Collider," and most recently "Edge of the Universe: A Voyage to the Cosmic Horizon and Beyond" (Wiley 2012). More information about his writings can be found at phalpern.com.

    • http://www.facebook.com/profile.php?id=1683224644 Brett Adams

      Is it weird that I read everything on this page in Neil deGrasse Tyson’s voice?

      • Paul Halpern

        Great to think of Dr. Tyson when pondering the mysteries of the cosmos!

    • Ritamarie949

      I just finished reading “Warped Passages.” It’s a terrific book.

      • Paul Halpern

        Prof. Randall is a truly remarkable physicist.

    • Anonymous

      Thank you for your clear explanations of incredibly complex subject matter, Dr. Halpern: fascinating reading.

      • Paul Halpern

        My pleasure! I’m glad you enjoyed the article!

    • http://twitter.com/GDTosman G D Tosman

      Excellent walk through of one of the most relevant questions. Did you see the http://theoryofsomething.com (the ToS) release http://bit.ly/Ov4bRS the day after you wrote this?

      In the ToS section 5.5.1 (http://bit.ly/OcPPFt) the gravity constant G is derived as 4*c^4*RadiusOfUniverse/TotalDarkEnergyOfUniverse from electromagnetism! Yes, unification came yesterday…

      Use the 46 billion light years for the radius of the universe and take the 72.8% of critical density for a flat universe as the dark energy and you get our gravitational constant as the weak 6.6738×10^-11 we know.

      The explanation to the weakness is in the Theory of Something. In short, gravity as well mass are mechanisms – not qualities of particles. Those mechanisms make both gravity and the mass about 10^-20 weaker than the electromagnetic force in itself. These mechanisms follow from that the Grid that fills space (like a Higgs field, a brane…) as well as all particles in our universe only have negative charges (the proton just looks positive due to a hole-mechanism).

      For a better explanation, check ToS sections 3-5 (Summary at http://bit.ly/ROyJJf), where the force accelerating distant galaxies also is derived (thought to be by the dark energy in the universe).

      • Paul Halpern

        Thanks for your comments & for sharing links to your ideas!

    • Thomas Kiefer

      Will the Theory of Something be evaluated by competent physicists anytime soon? I am concerned that the secretive author may not believe in peer review.

      • Adam Wiemers

        I am very excited about this theory and totally agree with your request but don’t understand your concern. The author of the ToS has published it to the Internet and seems to be trying to garner attention for it, even requesting that person’s with more advanced mathematical skills review it. How long did it take Einstein, a patent clerk with modest mathematic skills, to get traction for his theory of relativity among the scientific community? Your criticism seems baseless.

    • Camp Colombo

      I enjoyed the article. You know I never thought of gravity in a manner of being weak. In day to day life gravity on a personal level, of our weak human bodies, gravity can seem like a big bully. Thanks for the interesting read.

      • Paul Halpern

        Glad you enjoyed the article! Yes gravity seems strong for us, but is weak compared to the other forces.

    • Anonymous

      Has Paul or anyone come back regarding the Theory of Something by GD Tosman below?
      You MUST READ THAT! I found it 3 days ago via a WOW remark at Yahoo News and on the Q&A page at the theoryofsomething.com site I found in an answer:
      “In the ToS, the classical mechanical laws are derived from electricity and energy is explained (ToS 6.3) – That has not happened before and is proof of the ToS. E.g. we know that F=ma, but did not know why (ToS 5.4.2) and did not understand how that force could be equal to the gravity force (ToS 5.5.1).”
      Quickly looking at the referenced sections in the 100 page document – It looked like Tosman does what he says and much more. He actually puts electricity on top and then gets the mechanical laws from there. INCLUDING GRAVITY! (Picture from theoryofsomething.com site attched)
      And now I cannot stop reading, and wondering what known scientists say. Everything falls more and more into place, and the last 100 years Quantum Mechanics based theories seems like a gigantic ghost hunt. Tosman outlines everything and provides numbers and proofs that I cannot see how they can be disputed. He calculates Planck’s constant from the dark energy and shows that mass is not anything in a particle – just consequence of his Grid!
      kg is not even an SI base unit anymore – and there are many more amazing things shown – with numbers in place – than the gravity constant he mentions below. Just that the gravity constant includes the dark energy is a sensation. No one has even had a clue what dark energy is before.
      I’m not through all yet – want to hear what scientists say!
      Paul, where is your feedback? – Are you as “shocked” as I am?

    • Wild Will

      The reason gravity appears so weak is simple to explain. Particles are EM energy localized by propagation in a curved metric at the particle scale. They resemble soap bubbles in that the main force lies in the boundary, and for particles this is at the Strong Force scale. The curved boundary metric area is orthogonal to all particle radii in two dimensions, so the radial effect of the circulating energy is E/c^2. We call this mass.
      The strong force essentially acts between energies and is proportional to E^2, i.e. Fs = (1/hbar.c)E^2. Obviously a mass interaction is proportional to m^2 = (E/c^2)^2, so its c^4 weaker than the strong force.
      This is why the Large Number for electrons is (alpha^-2/3. c^2)^2, where c is in cgs units. In short, gravity is the observer domain manifestation of the strong force acting in particle metric boundaries. Disagree, need more detail?

      Look up ‘Rethinking Physics’ on the web, or at Redleadbooks.com

      Wild Will

    • Nathan Carte

      This is an interesting article. Gravity effects everything in the universe. It is easy for humans to apply gravity this way, because were taught that it keeps us planted on the earth. Electromagnetism on the other hand isnt talked about as much outside the physics table. I feel like it is because, even though both are invisible to the naked eye, gravity is easier to “notice”. I believe if more is taught, then more will be know about electromagnetism and how it affects our everyday life. Very good question though, really enjoyed the post.

    • Clarissa Kowalczyk

      Generally when thinking of gravity, you would not think of
      it as a “weakling”; it’s what keeps me and everything else planted to the ground. All throughout childhood, the force teachers tend to focus on is gravity. It takes some higher education until you can relate other forces to gravity and then you finally realize just how many forces are greater than it. I did not realize that gravity was 10^40 times weaker than electromagnetism though. It is fascinating to me that the universe cooling interacting with the Higgs field can cause the bosons to acquire mass, causing one force to split into two completely separate forces, one long-range and one short-range. To unify gravity with any other force seems rather implausible considering the lack of technology available to create the amount of energy required. I find it
      hard to wrap my mind around theories that cannot actually be scientifically tested. However, my lack of previous knowledge on the subject obviously does not make me the most valid critic. I agree with the theory that gravity most likely is weakened due to the fact that it is a force that can travel between different dimensions unlike the other forces. Only makes sense that as a force spreads out, it becomes weaker compared to the short –range forces.

      • Arnold

        on earth gravity might not seem like much. if you approach a black hole however gravity would be strong enough to crush all of the so called strong forces. the magnetic force would not even appear to exist as the magnets themselves would be crushed. the strong nuclear force between atoms would be stretched past it’s limits and torn apart. still think gravity is weak, lets ask again what force is really stronger then gravity?

    • fury

      Perhaps it is not that gravity is “leaking” into extra dimensions but that rather gravity is a finite sum in the universe and is instead spread out across the whole of time. This would explain why it appears weak to us as we are measuring it from our point in time rather than across the sum.

    • dovhenis

      What Holds In The Real World

      The Nobel Prize Committee Is Wrong. There is
      Definitely NO Higgs Particle. Gravitons are the elementary particles of the
      universe.

      Origin and nature of “may be gravitational
      waves” are continuously released gravitons since the last big-bang as
      singularity mass reconverts to energy.

      ============================

      On The Essence And Matrix Of The Universe-Life

      The following three sentences are the shortest
      data-based TOE. Seriously. Very seriously.

      The clearer the shorter

      Natural Selection to Self-Replication is Gravity

      - Self-replication is the ultimate mode of natural selection
      is the essence and drive and purpose of the universe. Period.

      - The pre-Big-Bang singularity is the ultimate
      self-replication (SR) of the cycling mass-energy universe. Period. (mother of
      universal SR mode…)

      - Earth’s RNA nucleotides life is just one of the myriad
      modes of self-replication.

      Dov Henis (comments from 22nd century)

      http://universe-life.com/

      http://universe-life.com/2012/11/14/701/

      -The 20yrs development, and comprehensive data-based
      scientism worldview, in a succinct format.

      -The Genome is a base organism evolved, and continuously
      modified, by the genes of its higher organism as their functional template.

      - Everything in the universe derives from mass-energy
      duality, from the universe cycle between
      its two poles all-mass/all-energy.

      - The Origin Of Gravitons is the ONLY thing
      unknown-unexplained in the Scientism Universe.

      PS: Spoon feeding

      The universe is a (circa 20 hillion yrs?) cyclic affair
      between all-mass and all-energy poles. NATURAL SELECTION of a mass format
      mandates energy intake because since the big-bang the resolved mass is
      reconverting at a constant rate from inert mass to energy, to moving mass. The
      mass that reconverts to energy SELF-REPLICATES to mass, in black holes, for the
      eventual re-singularity. The energy-to-mass SELF-REPLICATION process is
      GRAVITY. All this is enabled and goes on and mandated by/due to the small size
      and shape and inter-attraction of the gravitons that enable zero distance
      between them to re-form singularity. Black holes extract the gravitons from
      matter and store them at low energy level. Singularity is attained only ONCE
      per circa 20 billion years when ALL the gravitons of the universe are together
      at zero inter-gravitons space because it takes the totality of their combined
      low inter-attraction force to form the universal singularity.

      I hope that now it is understood what gravity is and why it
      is the monotheism of the universe…DH

      =================================================

      Black Holes Whence and
      Whither

      A.

      Black Holes Whence

      http://www.sciencenews.org/view/generic/id/345481/title/Cohabiting_black_holes_challenge_theory

      http://www.sciencenews.org/view/generic/id/351747/description/Milky_Ways_black_hole_pulling_in_gas_cloud

      From

      http://universe-life.com/2011/12/13/21st-century-science-whence-and-whither/

      http://universe-life.com/2012/09/02/all-the-mass-of-the-universe-formed-at-the-pre-big-bang-singularity/

      Galactic clusters formed by conglomeration?

      No. Galactic clusters formed by Big-Bang’s fragments
      dispersion, the released built-in singularity’s stresses, evidenced by their
      Newtonian behavior including their separation acceleration.

      The big bang is the shattering of the short-lived
      singularity mass into fragments that later became galactic clusters. This is
      inflation. The shattering is the start of movement of the shatters i.e. the
      start of reconversion of mass into energy, which is mass in motion. This
      reconversion proceeds at a constant rate since the big bang as the resolution
      of gravitons, their release from their shatters-clusters, proceeds at constant
      rate due to their weak specific force due to their small size.

      B.

      Black Holes Whither

      http://www.sciencenews.org/view/generic/id/345421/title/Team_glimpses_black_hole%E2%80%99s_secrets

      From

      http://universe-life.com/2011/12/10/eotoe-embarrassingly-obvious-theory-of-everything/

      A commonsensible conjecture is that Universe Contraction is
      initiated following the Big-Bang event, as released moving gravitons (energy)
      start reconverting to mass (gravity) and eventually returning to black holes,
      steadily leading to the re-formation of The Universe Singularity,
      simultaneously with the inflation and expansion, i.e. that universal expansion
      and contraction are going on simultaneously.

      Conjectured implications are that the Universe is a product
      of A Single Universal Black Hole with an extremely brief singularity of ALL the
      gravitons of the universe, which is feasible and possible and mandated because
      gravitation is a very weak force due to the small size of the gravitons, the
      primal mass-energy particles of the universe.

      This implies also that when all the mass of the presently
      expanding universe is consumed by the present black holes, expansion will cease
      and be replaced with empansion back to THE Single Universal Black Hole.

      Dov Henis (comments from 22nd century)

      ===========================

      The Universe, Some Updates

      What Big Banged To Produce The Universe

      From : http://universe-life.com/2011/12/10/eotoe-embarrassingly-obvious-theory-of-everything/

      A commonsensible conjecture is that Universe Contraction is
      initiated following the Big-Bang event, as released moving gravitons (energy)
      start reconverting to mass (gravity) and eventually returning to black holes,
      steadily leading to the re-formation of The Universe Singularity,
      simultaneously with the inflation and expansion, i.e. that universal expansion
      and contraction are going on simultaneously.

      Conjectured implications are that the Universe is a product
      of A Single Universal Black Hole with an extremely brief singularity of ALL the
      gravitons of the universe, which is feasible and possible and mandated because
      gravitation is a very weak force due to the small size of the gravitons, the
      primal mass-energy particles of the universe.

      This implies also that when all the mass of the presently
      expanding universe is collected and stored at very low energy level in black
      holes, expansion will cease and be replaced with empansion back to THE Single
      Universal Black Hole.

      Dov Henis (comments from 22nd century)

      ====================

      Universe Inflation And Expansion

      Inflation on Trial

      Astrophysicists interrogate one of their most successful
      theories

      http://www.sciencenews.org/view/feature/id/342219/title/Inflation_on_Trial

      Commonsense:

      Inflation and expansion are per Newton.

      Since the Big Bang galactic clusters loose mass at constant
      rate. Mass, gravitons, continue escaping at constant rate from their Big Bang
      fragments-clusters thus becoming energy, mass in motion, thus thrusting the
      clusters. Constant thrust and decreasing galactic clusters weight accelerate the
      separation of clusters from each other.

      Common sense.

      Dov Henis (comments from 22nd century)

      http://universe-life.com/

    • jonnychek

      Maybe gravity is turned off by electromagnetism.