Gravitational Waves,OK. But Where Are Gravitons? (Very Detailed)


Present-day physics cannot describe what happened in the Big Bang. Quantum theory and the theory of relativity fail in this almost infinitely dense and hot primal state of the universe. Only an all-encompassing theory of quantum gravity which unifies these two fundamental pillars of physics could provide an insight into how the universe began. Einstein and his successors, who have been searching for this for almost one hundred years.

Space consists of tiny elementary cells or “atoms of space” in some modern theories of quantum gravity trying to unify General Relativity and Quantum Mechanics. Quantum gravity should make it possible to describe the evolution of the universe from the Big Bang to today within one single theory.Our world is ruled by four fundamental forces: the gravitational pull of massive objects, the electromagnetic interaction between electric charges, the strong nuclear interaction holding atomic nuclei together and the weak nuclear force causing unstable ones to fall apart. Physicists have quantum theories for the last three of them that allow very precise calculations of phenomena on the smallest, subatomic scales. However, gravity does not fit into this scheme. Despite decades of research, there is no generally accepted quantum theory of gravity, which is needed to better understand fundamental aspects of our universe.


Gravitons are tiny particles that carry the “force” of gravity. They are what brings you back down to Earth when you jump. So why have we never seen them, and why are they so impossibly complicated we need string theory to figure them out?

Even without observing gravitons, scientists know a few things about them. They know, because gravity is a force with an infinite reach, that gravitons would have to be massless. This technically makes them “gauge bosons,” and puts them in the company of photons and gluons. Scientists also know that gravitons have a spin of two, which makes them unique among particles. The combined properties mean that, if scientists were able to pin down an event involving a mysterious particle with no mass and a spin of two, they would know they were looking at a graviton.

Where String Theory Jumps In.

In our everyday lives, we experience three spatial dimensions, and a fourth dimension of time. How could there be more? Einstein’s general theory of relativity tells us that space can expand, contract, and bend. Now if one dimension were to contract to a size smaller than an atom, it would be hidden from our view. But if we could look on a small enough scale, that hidden dimension might become visible again. Imagine a person walking on a tightrope. She can only move backward and forward; but not left and right, nor up and down, so she only sees one dimension. Ants living on a much smaller scale could move around the cable, in what would appear like an extra dimension to the tightrope-walker.

How could we test for extra dimensions? One option would be to find evidence of particles that can exist only if extra dimensions are real. Theories that suggest extra dimensions predict that, in the same way as atoms have a low-energy ground state and excited high-energy states, there would be heavier versions of standard particles in other dimensions. These heavier versions of particles – called Kaluza-Klein states – would have exactly the same properties as standard particles (and so be visible to our detectors) but with a greater mass. If CMS or ATLAS were to find a Z- or W-like particle (the Z and W bosons being carriers of the electroweak force) with a mass 100 times larger for instance, this might suggest the presence of extra dimensions. Such heavy particles can only be revealed at the high energies reached by the Large Hadron Collider (LHC).

Why is gravity so much weaker than the other fundamental forces? A small fridge magnet is enough to create an electromagnetic force greater than the gravitational pull exerted by planet Earth. One possibility is that we don’t feel the full effect of gravity  because part of it spreads to extra dimensions. Though it may sound like science fiction, if extra dimensions exist, they could explain why the universe is expanding faster than expected, and why gravity is weaker than the other forces of nature.

There is, however, a major problem. To understand it, let’s go back to photons and electrons. When an electron falls from one level to another, out pops a photon. When that photons falls, or otherwise moves, it produces no second photon. Electron movement produces photons. Photon movement does not produce more photons. There are occasional times when photons can do odd things. They can split into electron and positron pairs, which can produce more photons, and which then recombine into a photon again. Although this burst of particles may get hectic, it doesn’t produce an endless branching chain of photons. Because of this, photons and electron interactions are said to be renormalizable. They can get weird, but they can’t become endless.

Gravitons are not so tame. While photons are spawned by movement in electrons, gravitons are whelped by energy and mass. Gravitons are massless, but they do carry energy. This means a graviton can create more gravitons.

Like other quantum particles, gravitons can carry a lot of energy, or momentum, when confined to a small space. A graviton is confined to a small space when one graviton is popping out another graviton. At that moment, two gravitons are in a tiny space, one right next to the other. That huge amount of energy causes the newly-created graviton to create yet another graviton. This endless cycle of graviton production makes gravitons nonrenormalizable.

String theory is invoked in these situations part because nonrenormalizable gravitons are points. Strings are longer than points, and so the creation of the stringy graviton isn’t so confined in time and space. That bit of wiggle room keeps the creation of a graviton from being so energetic that it necessitates the creation of yet another graviton, and makes the theory renormalizable.

A graviton is really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really hard to detect.

The LIGO detector and team??

Quantum gravity is a theory that has been the target of decades of study by physicists worldwide. If this idea is proven, it would tie together the General Theory of Relativity (which governs gravitational fields) with quantum mechanics, and the bizarro-world of subatomic particles.

Gravitational waves, produced by accelerating objects, ripple through space-time, according to most interpretations of the General Theory of Relativity penned by famed physicist Albert Einstein. Researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) have announced they detected these disturbances in the fabric of time and space for the first time.

Certain aspects of subatomic behavior is quantized – they can only move or exist in particular whole number states. This characteristic may be thought of like steps to walk up into an apartment. Many physicists believe gravitational waves are similarly quantized, made up of individual quantum particles of gravity – gravitons.

Although it is not certain, many physicists believe that these particles join together, forming the gravitational waves that travel through space. Like photons of light, these gravitons, would have no rest mass, and move at the speed of light.

The effects of quantum gravity are predicted to be quite pronounced in the region immediately surrounding the center of black holes. However, it is impossible to collect data from events near a singularity. The events witnessed by astronomers are LIGO-recorded activity from just outside the event horizons of a pair of black holes as they collided.

The LIGO detector cannot detect single gravitons, and cannot, by itself, test the theory of quantum gravity. However, there is reason to believe that either LIGO, or a future gravitational wave detector, could be used to find evidence of quantum gravity by examining the emission spectrum of energy seen surrounding the event horizons of black holes.

According to some theories, even at the event horizon, the effects of gravitons could cause gravitational waves to be more powerful, and less regular, than they would be without their influence.

“Certain scenarios with strong quantum modifications in a region extending well outside the horizon are expected to modify classical evolution, and distort the near-peak gravitational wave signal, suggesting a search for anomalies such as decreased regularity of the signal and increased power,” Steven Giddings of the University of California, Santa Barbara, said.

Any variation seen between observations and graviton-free theories of gravitational waves, such as Einstein’s, could assist physicists seeking to understand the ultimate units of gravity.

As astronomers use LIGO and other detectors to search for elusive ripples in space-time, they may also come across evidence of other strange features of space, including cosmic strings, theoretical one-dimensional strings of energy, which may have been created long ago, when the universe was young.

Moreover, as additional findings of gravitational waves are recorded, physicists will search the data for behavior of the ripples which might suggest the presence of gravitons. If they are found, the discovery could herald a new age of understanding how gravity works. Such a finding could suggest that other notions of gravity, such as string theory, could prove to be the basis of future work on the nature of gravity.

But until such variations are seen, the existence of gravitons remains strictly theoretical.

A Small Packet Of Gravity?

Some theorists suggest that a particle called the “graviton” is associated with gravity in the same way as the photon is associated with the electromagnetic force. If gravitons exist, it should be possible to create them at the LHC, but they would rapidly disappear into extra dimensions. Collisions in particle accelerators always create balanced events – just like fireworks – with particles flying out in all directions. A graviton might escape our detectors, leaving an empty zone that we notice as an imbalance in momentum and energy in the event. We would need to carefully study the properties of the missing object to work out whether it is a graviton escaping to another dimension or something else. This method of searching for missing energy in events is also used to look for dark matter or supersymmetric particles.

Quantum(Microscopic) Black Holes

Another way of revealing extra dimensions would be through the production of “microscopic black holes”. What exactly we would detect would depend on the number of extra dimensions, the mass of the black hole, the size of the dimensions and the energy at which the black hole occurs. If micro black holes do appear in the collisions created by the LHC, they would disintegrate rapidly, in around 10-27 seconds. They would decay into Standard Model or supersymmetric particles, creating events containing an exceptional number of tracks in our detectors, which we would easily spot. Finding more on any of these subjects would open the door to yet unknown possibilities.

Photons easy to detect…So why not gravitons?

You have to appreciate just how weak gravity really is. Gravity is 10^-36 times weaker than the electromagnetic force. Trying to detect individual gravitons will be impossible with the tools we have at hand. That is why scientists are trying to detect gravity waves generated during the early stages of the universe. The effect of gravity on mass is independent of the size of the mass unless the masses involved are of roughly equivalent size. If one mass is very much smaller than the other then its influence is negligible. The tides, whilst involving a substantial amount of mass are a very bad choice. Firstly, they are composed of liquids which are not only reacting to the gravitational force, but to the kinetic energy of the molecules that constitute the oceans. Secondly, overall they compose only a fraction of the mass of the whole earth.

Detecting a photon, for example, is extremely easy. There many types of devices that are able to detect single photons, such as photomultipliers, used in labs around the world. In fact, you don’t even need any fancy technology; the human eye can, in principle, detect a single photon. (that isnt quite the topic so i wouldnt go into depths)

Possible Way To Detect A Graviton

However, detecting gravitons is much (much much etc…) harder. A famous example  considers an ideal detector with the mass of the planet Jupiter, around 10271027 kilograms, placed in close orbit around a neutron star, which is a very strong source of gravitons. A back-of-the-envelope calculation reveals that even in this extremely unrealistic scenario, it would take 100 years to detect a single graviton!

Okay, you say, so let’s just make that detector (sometime in the far future when we have the technology to do so) and wait for 100 years. There’s a crucial detail that I forgot to mention, however. The star also emits neutrinos in addition to gravitons; in fact, many more neutrinos than gravitons. And neutrinos are much easier to detect than gravitons. In fact, we can calculate that for every graviton that is detected in this scenario, around 10331033neutrinos will be detected. So we will never be able to find the one graviton among the 10331033 neutrinos.

Ah, you say, but we can build a neutrino shield and block the neutrinos! But such a shield would need to have a thickness of several light years, and if you try to make it more dense in order to fit between the star and the detector, it would collapse into a black hole…

In conclusion, even with insanely advanced futuristic technology, it would simply be impossible to detect a graviton.

What we have been able to detect, though, are gravitational waves. This amazing discovery by the LIGO experiment was announced on February 11 2016. Gravitational waves are made of lots and lots of gravitons, just like electromagnetic waves are made of lots and lots of photons. A typical gravitational wave is composed of roughly 1,000,000,000,000,000 gravitons per cubic centimeter, therefore it is obviously much easier to detect than a single graviton.

On the other hand, we definitely do not have the technology to detect individual gravitons, and unless some new ingenious way to detect them is found, we will never be able to do so even with much more advanced technology.

What are the consequences of this technological impossibility to detect gravitons? As it turns out, it doesn’t really matter! Let me explain.

First, where exactly do gravitons appear in physics? Theoretical physicists are trying to combine general relativity and quantum mechanics into a single theory, called quantum gravity. We do not have a final theory of quantum gravity yet, but we are working very hard on it, and we already understand many aspects of what such a theory should be.

In a theory of quantum gravity, gravitons are the quanta of the gravitational field. Therefore, quantum gravity will use gravitons as part of its formulation, just like the theory of quantum electrodynamics uses photons, which are the quanta of the electromagnetic field.

However, we did not confirm quantum electrodynamics experimentally by detecting photons. Quantum electrodynamics produces predictions that are different from those of classical electrodynamics, and by experimentally testing these predictions we have been able to confirm that the electromagnetic field is indeed quantized.

In a similar way, when we finally have a good candidate for a theory of quantum gravity, it will produce predictions that are different from those of classical gravity. By experimentally testing these predictions, we will be able to confirm that the gravitational field is quantized.

In other words, what we need to do is not detect gravitons; we need to test the predictions of a theory of quantum gravity, as soon as we have such a theory. This will indirectly confirm the existence of gravitons.

Credits to:

Me (Mainly)

LHC website

Gizmodo website for some part of photons info.

🙂 Hope u enjoyed


4 thoughts on “Gravitational Waves,OK. But Where Are Gravitons? (Very Detailed)

  1. Pingback: Gravitational Waves,OK. But Where Are Gravitons? (Very Detailed) | Leen Noordzij

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