A short piece on Gravity


Gravity. Everyone knows about it, everyone experiences it, yet not many people know what it is. Don’t worry though - the scientific community is also not sure about this one. Even though gravity was the first fundamental force to be discovered and named, it is the one we know the least about (the others being the strong nuclear force, the weak nuclear force, and electromagnetism - if you’re interested in those, I suggest you google them).newton2 The theory of gravity stems from Isaac Newton’s famous encounter with an apple, which supposedly fell upon his head and made him question what had caused the apple to fall. This encounter led Newton to develop a theory of gravity in 1687, which was successful at explaining the motion of the stars in the sky, serving as evidence for his new theory. In it gravity is modelled as an inverse-square law, which means that being twice the distance away from a gravitational source (i.e the earth) would result in a force only a quarter the original strength, being three times the distance away would result in a force one-ninth the strength, and so on.


Up until the 20th century, our understanding of gravity was unchanged. This has mainly to do with how weak gravity is, when compared to any of the other three fundamental forces. Think about it: to overcome the entire gravitational pull that the earth has on you, all you have to do is jump- the muscles in your legs are powerful enough to overcome the attractive force between you, and the earth. In direct contrast to that, think about how much force would be needed to separate two magnets; one the size of the earth, the other the size of you.


This gives you a rough idea about how weak gravity is, and why it is so hard to study it compared to the other three fundamental forces of nature- we simply do not feel its effect on objects much smaller than planets, making the effect practically non- existent on the atomic scale. Another way of understanding the difference in strength between electromagnetism and gravity is by looking at their mathematical formulation (no calculation involved, I promise!). Both forces have the same mathematical relationship governing them, the only difference being the value of the constant used.Coulequation-1.force-gravity
To make the comparison clearer, note that ‘mass (m)’ for the gravitational force (left) is the same as ‘charge (q)’ for the electromagnetic force (right). ‘r’ is the distance between the two objects. This means that the difference in strength of the forces is proportional to the magnitude of the constants k and G.

G = 6.67×10-11 m3kg-1s-2                                                                   k = 8.99×109 N mc-1 [2,3]

That is a difference of 20 orders of magnitude- or in other words gravity is 100,000,000,000,000,000,000 times weaker than electromagnetism!


Another thing setting gravity aside from the other three forces is how we describe it. The other three forces can be described through the interaction of particles, via the exchange of force carriers (bosons). Each force has a boson that carries its force- for example electromagnetism occurs through the exchange of photons between charges/poles (yes, Photons are light, and allow you to see but they are also the reason for any magnetic/electric interaction!). Gravity does not have an experimentally verified boson (the graviton), and the only thing we have that explains gravity adequately is Einstein’s famous theory of General Relativity.


Gravity represented as a distortion of Spacetime
Gravity represented as a distortion of Spacetime

Here, gravity is explained through the warping of the ‘fabric’ that our universe is made from, called spacetime. As this video explains, gravity (as described by Einstein) is the result of mass distorting spacetime causing things with mass to attract one another. Even though this is an adequate explanation which we have a lot of supportive evidence for, it is apparent that is cannot be the complete picture. One of the holy grails of physics is to reconcile general relativity with quantum mechanics for a more complete theory which describes all four fundamental forces in a similar fashion, and quantum gravity is becoming an increasingly active research area to this end.


A new paper accepted for publication in the journal Physical Review D claims that it should be possible, with current technology, to create measurable distortions in spacetime [4]. This would offer scientists the opportunity to study gravity up close in the lab, as opposed to relying on cosmological observations, which could lead to new, crucial insights into how gravity works as a force. In the paper, professor André Füzfa of Universite de Namur in Belgium outlines a method in which tiny warps in spacetime could be created using extremely powerful electromagnets. This should work thanks to the famous equation, E=mc2. What this equation tells us, is that mass (m), and energy (E) are equivalent - they are two sides to the same coin. This means that instead of relying on as object with a lot of mass to create distortions in Spacetime (like a planet, or a star does), it should be possible to achieve the same effect by concentrating the equivalent amount of energy in one place, and Füzfa calculated that extremely powerful magnetic fields should contain enough energy to do this. Even though this will require a huge amount of funding and is very far from being realised, it is nevertheless exciting to think about the fact that gravity may one day (soonish) be studied in a laboratory, which will offer valuable new insights into the nature of gravity in the pursuit of the holy grail - a quantum theory of gravity.


The featured image for this article is an example of ‘Gravitational lensing’, which is an amazing consequence of Einsteins description of gravity. The distortions seen in this image are due to light being bent around heavy objects (such as galaxies) as it makes its way from its source to us.


A visual representation of gravitational lensing
A visual representation of gravitational lensing

[1] - https://en.wikipedia.org/wiki/Gravity
[2] - https://en.wikipedia.org/wiki/Gravitational_constant
[3] - https://en.wikipedia.org/wiki/Coulomb%27s_constant
[4] - http://arxiv.org/pdf/1504.00333v3.pdf

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