Gravity

14
May

What Are Gravitons?

In a contest for the least contentious statement a person can make, “What goes up must come down” is surely a strong contender. Of the four known fundamental forces—gravity, the electromagnetic force, and the strong and weak nuclear forces—we have the most intuitive understanding of gravity. From our first experiments dropping Cheerios from our high chair, we spend our lives coming to grips with the limitations that gravity imposes on us.

gravitons_620
Credit: Christoph Zurbuchen/Flickr, adapted under a Creative Commons license.

In the late 1600s, Isaac Newton devised the first serious theory of gravity. He described gravity as a field that could reach out across great distances and dictate the path of massive objects like the Earth. Newton’s theory was stunningly effective, yet the nature of the gravitational field remained a mystery. In 1915, Albert Einstein’s theory of general relativity gave theorists their first look “under the hood” of gravity. What we call gravity, Einstein argued, is actually the distortion of space and time. The Earth looks like it’s rounding the Sun in an ellipse, but it’s actually following a straight line through warped spacetime.

Einstein’s theory of gravity is very good at explaining the behavior of large objects. But just a few years later, physicists opened up the world of the ultra-small, revealing that the other fundamental forces are due to the exchange of specialized force-carrying particles: photons convey electromagnetism, the strong nuclear force is transmitted by gluons and the weak nuclear force is imparted by the movement of the W and Z bosons. Is gravity due to the same kind of particle exchange?

We actually don’t know the answer to that question, but we have a name for that hypothetical particle if it does exist: It is called the graviton. And even though we have never observed a graviton, we know a great deal about them, if they are real. First, since the range of the force due to gravity is infinite and the force due to gravity weakens as one over the square of the distance between two objects (i.e. 1/r2), the graviton must have zero mass. We know this because if the photon had mass, it would change the “2” in the exponent and that “2” has been established with incredible precision. Like massless photons, gravitons should travel at the speed of light.

General relativity also gives us some insight into the nature of gravitons. In general relativity, the distribution of mass and energy in the universe is described by a four-by-four matrix that mathematicians call a tensor of rank two. This is important because if the tensor is the source of gravitation, you can show that the graviton must be a particle with a quantum mechanical spin of two. Another nice fallout of this correspondence is that the graviton is the only possible massless, spin two particle. If you observe a massless, spin two particle, you have found the graviton.

So why hasn’t anyone found a graviton yet? The problem with searching for gravitons is that gravity is incredibly weak. For instance, the electromagnetic force between an electron and a proton in a hydrogen atom is 1039 times larger than the gravitational force between the same two particles. Perhaps a more intuitive example is the behavior of a magnet and a paperclip. A magnet will hold a paperclip against the Earth’s gravity. Think about what that means. A little magnet, like the one that held your art to your parent’s refrigerator when you were a kid, pulls the paperclip upwards, while the gravity of an entire planet pulls downward, and the magnet wins.

Individual gravitons interact very feebly, and we are only held to the planet because the Earth emits so many of them. Because a single graviton is so weak, it is impossible for us to directly detect individual classical gravitons.

However, there are new and innovative ideas about gravity in which other forms of gravitons might exist. Some of these exotic gravitons might be detectable, but they require significant modifications to our understanding of our universe. This is where things get a bit mind-bending.

If “what goes up, must come down” might be a catch phrase for Captain Obvious, “we live in three dimensions” could be the rallying cry of his sidekick, Lieutenant Duh. However, some scientists have proposed the idea that gravity might have access to more than three dimensions. In that case, gravity might not actually be as weak as we think it is. It only appears weak because, unlike the other fundamental forces, it has extra dimensions into which it can “spread out.”

On the face of it, this seems silly. The 1/r2 nature of gravity is an incontrovertible sign that gravity operates in three dimensions, and this behavior has been directly verified down to distances smaller than a millimeter. But this leaves open the possibility of extra dimensions smaller than 150 micrometers or so. One can imagine these small dimensions by thinking of a tightrope. To a tightrope walker, who can only walk forward and backward on the rope, the rope is one-dimensional. But to an ant, which can also crawl around the rope’s circumference, the rope seems to be two-dimensional. What appears to be one-dimensional to a large being is two-dimensional to a smaller one. These smaller dimensions are cyclical in that if you travel around the outside of one, you will end up back in the same place.

Quantum mechanics tells us that every particle is also a vibrating wave, and it has been proposed that gravitons could vibrate in these extra dimensions, wrapping around the small dimension like bracelets encircling a slender wrist. However, the cyclical nature of the extra dimension imposes limits on how a graviton can vibrate. Only an integer number of wavelengths can fit evenly in the extra dimension. And this brings us to a couple of interesting consequences. In theories with extra dimensions, more than one type of graviton can exist. One way to see that is to imagine taking a sine wave and wrapping it around a cylinder. In order for it to fit perfectly, you must use one wavelength or two or three or any integer number of wavelengths. Each of these instances is a distinct graviton; the ones with more vibrations can actually have mass. Particles of this kind are called Kaluza-Klein gravitons after physicists Theodor Kaluza and Oskar Klein, who first proposed the idea of additional small spatial dimensions. On tiny scales, Kaluza-Klein gravitons can have mass, but on larger scales, they reduce to the familiar massless gravitons of classical theory.

Using particle accelerators like the Large Hadron Collider, physicists are already searching for these small extra dimensions, in part by looking for the expected decay products of massive gravitons. They haven’t found anything yet, which means that if extra dimensions exist, they must be a thousand times smaller than a proton, although there are many caveats to how one interprets the data.

Gravity is the one known fundamental force that has resisted study in the quantum realm and finding gravitons of any kind would be a huge step forward in our understanding of the phenomenon. Devising a successful theory of quantum gravity is one of the hottest goals of modern physics and ongoing experimental searches for gravitons will play a central role.

Go Deeper
Our picks for further reading

Nature of Reality: What Is Gravity Made Of?
In this video blog, physicist Greg Kestin describes the 2014 results from the BICEP2 experiment and their implications for gravitons and quantum gravity.

The Physics Teacher: Extra Dimensions of Space
Author Don Lincoln explains what physicists talk about when they talk about extra dimensions.

Poincare Prize Lecture: Is a Graviton Detectable?
In this technical lecture, eminent theorist Freeman Dyson asks whether it will ever be possible to detect gravitons.

Warped Passages: Unraveling the Mysteries of the Universe’s Hidden Dimensions
In this popular book, Harvard physicist Lisa Randall explains why theorists believe extra dimensions may exist, and how we might find them.

Tell us what you think on Twitter, Facebook, or email.

dlincoln

Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

    • Scott McKay

      is it possible that gravitons are the gray matter that we search for?

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        No…I believe you mean dark matter, but gravitons don’t have mass and don’t cause gravity…they transmit gravity. Dark matter, if it exists, causes gravity. In some sense it is like ordinary matter, except it doesn’t interact with light. So no go.

        • Scott McKay

          yes thats what I meant. Dark matter. Thank you for correcting that. It was just a thought.

    • john

      so what is the difference between Graviton and the bending of the fabric of space-time?

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        The difference is similar to a water molecule and a wave of water. Both H20 and a surfer’s delight are the same thing, but qualitatively very different.

        Concentrations of energy (i.e. mass) emit many gravitons and, in so doing, distort space.

        Of course, since we aren’t even sure if gravitons exist, there is no reason to be confident of any statements made here. What I am describing is the qualitative features of a theory, not established fact.

    • Kort Beck

      But, if gravitons doesn’t have mass , how does exist? , this article refers to the string theory i think, that theory is true? the vibrating string exist?

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Well photons don’t have mass and they surely exist.

        String theory is certainly not verified and some would call it pie-in-the-sky speculation. Personally, I like the elegance of the superstring idea, however I do not allow my sense of aesthetics get in the way. Until the idea is proven, superstrings must be relegated to the “not yet proven” category.

    • Aiden

      Faraday first postulated fields. Not Newton

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        True to a degree. However, you have made a historical statement. The gravitational field as we understand it is wholly contained within Newton’s work. We just think of it differently nowadays. Thus I stand by the statement in the article as it hinged on the understanding of gravity rather than the field concept. Essentially the Newton insight was more central to the point that the field one.

    • Anonymous

      Thanks for another great article Dr. Lincoln. The notion of quantized gravity in the spotlight due to the recently published research has me asking questions about how we came to understand any of the forces as quantized. In my head I envision a wall being peppered with pellets like in the double-slit experiment. I’m not quite sure even how to ask the question…but what does it mean to be quantized? What actually separates one particle from another? Empty space? We know now space isnt empty. What does this mean for what is happening in the empty space between particles in a stream? I guess this questions begs more fundamental question…why is anything quantized? Is the quantum nature of reality objectively real or have we imposed this “nature” upon reality because, by definition, to study something you must have a discrete “thing”/particle to study?

      Regarding gravity’s relative weakness in comparison to the other forces, I am reminded that in the singularity prior to the Big Bang the forces were united. If a large portion of gravity’s strength is bound in smaller dimensions could it in fact be appearing or manifesting as, say, the strong nuclear force?

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Just some brief thoughts.

        If you drop a pebble in water, you’ll see a localized wave pattern travel across the water. This is a quantized wave. This is in contrast to the continuous stream of waves that hits a shoreline. And we certainly know that matter is quantized. We can manipulate individual atoms. Further, we can extract individual electrons, protons or neutrons. It’s harder to see when talking about energy, but not fundamentally different.

        Regarding the gravity moving into extra dimensions being the strong nuclear force…I dunno…probably not, but we don’t have a theory of quantum gravity, so it’s hard to say anything definitively. I guess I think your implied proposal is unlikely, but I’ll hedge with a shrug and a beatstheheckouttame. We’ll have to wait for a clever lad or lass to come up with a good theory of quantum gravity and go from there.

        • Anonymous

          LOL! Sounds like a plan. Re: quanta, thank you, your analogy helped me understand the concept much better. Am I correct though in understanding that the distinction between the causes of these waves, the pebble and the wind respectively, and the waves themselves being particles of water moving in a certain pattern, is where the analogy breaks down?Because, at the subatomic level we’re discussing, in some mysterious way both the particle (localized “center” of the wave) is made of the same ‘stuff’ as the wave itself and they are, in fact, one? Or maybe I should just understand it the way we talk about electromagnetism – the force we care all most familiar with – that with charge comes a field called magnetism that behaves in a wave like way, and that it doesn’t make sense to speak of one without the other? Is this the more accurate way to conceptualize the wave-particle duality?

          • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

            I’m afraid I didn’t understand that well enough to answer.

            • Anonymous

              I was just trying to point out that in the analogy the pebble and the waves it creates are two separate things whereas, at the subatomic level, the particle and wave are the same thing, two sides of the same coin so to speak. And, in fact, that a single particle is also a wave. I’m referencing the paradox in the double-split experiment of how a single photon/particle can produce a wave pattern when, accd to everyday experience, it takes multiple particles to create a wave pattern. As far as I understand it – or think I understand it – the wave is the field (or perhaps put another way, the “range of influence” of the particle) by which the particle interacts with other particles. And that the particle, per your analogy, is at the “center” of the wave pattern. I was just wondering if I am understanding this wave-particle duality correctly, making the concept more difficult than it is or if there is still something I am missing.

            • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

              It is true that the pebble and the ripple are different, but they do not well represent the particle/wave dichotomy. A traditional wave goes on forever and ever…like a sine wave. A particle has a fixed position. A ripple is a localized wave. Thus the ripple has both particle and wave properties. That was the analogy I was trying to make.

            • Anonymous

              Ahhh…that really makes sense. I get it now.

              Thanks so much for your time and continuing engagment with the public in this forum. I appreciate very much that a scientist of your stature is willing to help the public understand the wonders of the universe.

            • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

              Glad it helped. I can’t always answer every question, but it’s nice when I can. Keep learning.

    • hardi sura

      do antigravitons exist or are gravitons their own anti particles like photons?

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        It is expected that gravitons are their own antiparticle.

    • Lochandubh

      Little square fried pieces of bread you sprinkle on soup n salads ?

    • George Raina
    • Mario

      What goes up… What about escape velocity?

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Yes, yes…we can all be pedantic. But a little literary flair adds to how interesting a story can be. My advice is to chill and enjoy the vibe. If you’re smart and educated enough to know about escape velocity, you’re smart and educated enough to let it go when it is clear that the author also understands a point and has made a literary choice.

    • Andriod Khan

      Can a black hole destroy gravity

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Nope.

    • science lover

      What if we think that every body releases energy and the energy covers the body.when another body comes to its energy field the energies of the two bodies interact together to form gravity…………….its just my thinking.I may be wrong¤¤¤¤¤

    • G

      Do gravitons exist outside of galaxies, for example, within intergalactic space or the “voids” between them?

    • G

      Do Gravitons exist outside of galaxies, for example, in intergalactic space or the “voids” in between them?

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Gravitons are thought to exist whereever there are gravitational fields, thus, to all intents and purposes, gravitons are everywhere.

    • AiyaOba

      Don Lincoln, your humility exemplifies the mind of a true giant.-Aiya-Oba (Philosopher).

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Well thank you.

        • AiyaOba

          I am honored.

    • Kierra Sinclair

      I think we’re looking the wrong way. We should be looking for what pushes an object towards a large object rather than what pulls it.

    • flamestar

      Gravitons can’t escape from a black hole but changes in space time can, Einstein is right.

    • carlpet

      but how do you twerk

      • Boss Ross

        hobbit this is science go away

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Very badly.

    • the hobbit

      My milkshake brings all the boys to the yard

      • carlpet

        hobbit this is science go away

    • WXGT

      sir can i ask does photon is anti particle or has anti particle sorry im not good at science

      • http://www.facebook.com/Dr.Don.Lincoln Don Lincoln

        Photons are their own antiparticle. It’s analogous to the idea that there is a +/-1 and +/-2, but +/-0 is just 0.

    • Mike P

      one more convienient uni-tasker particle that doesn’t provide a better reason that gravity exists.
      HERE’S MY WEBPAGE LINK. I LIKE TO REINTERPRET EXPERIMENTAL RESULTS SO WE CAN HAVE MORE USEFUL KNOWLEDGE TO GUIDE US IN PROGRESS

      https://www.facebook.com/mikepscience/timeline