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The weakness of gravity

July 14, 2011

The story goes that a young Newton was sitting beneath a tree when an apple fell to the ground, which he famously attributed to a force of attraction between the apple and the Earth called gravity. He went one step further to suggest that it is the same force that is responsible for the orbits of the moon around the Earth and the Earth around the Sun.

Whereas the story is most likely fictional, it illustrates an important concept that ideas are borne out of curiosity. Learning to ask the right questions is one of the cornerstones of scientific advancement. The lack thereof, I believe, has played a fundamental role in scientific decline in the Muslim world (as I’ve previously discussed here).

One of such questions and indeed one of the biggest unsolved mysteries in our understanding of the Universe is; why is gravity so much weaker than the other forces of nature? It is questions like this that the Large Hadron Collider, the most powerful particle accelerator in the world, is designed to solve. This particular curiosity is also one of my primary research preoccupations and the reason for my somewhat long hiatus from the blogosphere.

Our current understanding of the Universe is embodied in a theoretical framework called the Standard Model. The theory describes the fundamental particles and their interactions with each other via four forces, the weakest of which is gravity. The weakness of gravity may come as a surprise since we hold gravity responsible for the Moon orbiting around the Earth, the Earth around the Sun, the motion of the galaxies and the fact that we’re not floating in space. However, it is actually extraordinarily weak compared to the other forces in nature.

 How weak is gravity? It takes the entire mass of the Earth to pull that apple to the ground! The force of gravity becomes stronger the more mass objects have and the closer they are to each other. Its effects only become visible when we talk about objects as large as the Earth and are virtually unnoticeable if you consider for instance, the gravitational attraction between the individual apples on the tree.

So, what makes gravity so much weaker than all the other forces?

In 1998, Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali proposed the scenario of Large Extra Dimensions to solve this puzzle. The basic idea is that there are more dimensions in space than the three-dimensions that we live in and experience.

How does this explain why gravity is so weak? While the other forces of nature are constrained to our three-dimensional world, gravity is thought to be free to propagate in these extra dimensions, so its effect in our three-dimensional world is somewhat diluted. Hence we see it as being weak since it is thinly spread over all the dimensions, whereas its strength is probably comparable to the other forces.

It is proposed that these extra dimensions are un-observably small, since we don’t witness objects vanishing into these higher dimensions and that they are compactified, for instance they may be tightly curled in a loop, such that even if you enter one of these dimensions, you won’t get very far and end up right where you started.

At the Large Hadron Collider experiment at CERN, often referred to as the ‘Big Bang machine’, we accelerate bunches of protons in opposite directions around a 27 km ring and then collide them head-on. The collision produces an immense amount of energy that can create new particles. One of these could be the graviton, the particle thought to be responsible for transmitting the gravitational force. If gravity permeates through all of the extra dimensions, there may be times when graviton is produced in these high energy collisions and then escapes immediately into these other dimensions.

This disappearing act of the graviton would therefore be a tell-tale sign of the existence of extra dimensions and would produce an imbalance of energy in our detectors as the energy of the graviton would be ‘missing’.

We searched through scores of proton-proton collisions to find cases where a graviton may have been produced and then vanished immediately into these higher dimensions. So far, with the data available at the end of 2010, we have not found any evidence for these particles and as a result we were able to place constraints on the size and the number of these extra dimensions.

However, so far in 2011 we have accumulated 30 times more data than was available in 2010. With so many proton-proton collisions at the highest energies ever achieved, the likelihood of a scientific breakthrough has increased tremendously. Who knows what we may discover and what Pandora’s Box it might open. The beauty of science is that there will always be one more question to answer.

Dr Sarah Alam Malik is a postdoctoral associate in Particle Physics. She works on the Large Hadron Collider experiment at CERN, Switzerland and the Tevatron experiment at the Fermi National Accelerator Laboratory, USA. Her research interests can be found here.

 

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