Why does gravity travel across dimensions?

The concept of gravity was introduced by Issac Newton in the year 1687 AD. Newton described gravity as the force of attraction between objects having mass. Albert Einstein drastically changed this view of gravity when he published his famous general theory of relativity in the year 1916. According to general relativity, gravity is not just the attraction between objects. Instead, it is the result of the curvature of spacetime produced by objects having mass. You can find more on general relativity here.

Gravity as the curvature of spacetime (Image: NASA)

General theory of relativity, as we know, does not go very well with quantum physics. Physicists have been working for decades to unite gravity with quantum physics and develop a Theory of Everything (ToE) that fully explains and links together all physical aspects of the Universe. One potential candidate for a ToE is the string theory.

According to string theory, at the most fundamental level, everything is made up of tiny identical strings that vibrate at different frequencies and in different ways to generate different fundamental particles that make our universe. The string that makes up an electron vibrates differently than a string that makes up a quark.

For the string theory to work, four dimensions (3 Space + 1 time) are not sufficient. Thus, string theory implies that our Universe must have additional ‘hidden’ dimensions. These hidden dimensions can either be large (non-compact) or tiny (curled up into a compact space). You can find more on string theory here.

String Theory: A Universe made up of strings?

In the context of string theory, gravity like the other three fundamental forces is mediated by a fundamental particle known as ‘graviton’. Gravitons like photons (particles that make up light and carry the electromagnetic force) are massless and chargeless. They always travel at the speed of light. One of the many objectives of the Large Hadron Collider (LHC) at CERN is to find out whether gravitons exist or not. But detecting gravitons is not so easy because they can travel across dimensions. If at all gravtions are produced during the high energy collisions at LHC, they would rapidly disappear into extra dimensions.

String theory works with two kinds of strings- closed strings and open strings. Open strings have endpoints and hence they are bound to branes. On the other hand, closed strings do not have any endpoints and hence they would not be bound to branes. Closed strings could move freely between branes! (A brane, in string theory, is an object that generalizes the notion of a point particle to any number of dimensions. Our Universe might be one of the many 3 dimensional branes floating in space.)

Open string (left) and closed string (right)

In string theory, graviton is a closed string. As a result, it is not bound to any branes and can easily travel between them as opposed to photon, which is an open string. Therefore, it is said that gravity can travel across dimensions but light cannot.

 

This figure shows how open strings have both their endpoints bound to branes

Gravity traveling across dimensions can explain why it is the weakest among the four fundamental forces in the universe as it would be leaking into the higher dimensions. It can also provide an explanation for dark matter- gravitons from other universes might be leaking to our own universe. However, if gravity were to move completely freely between branes, wouldn’t the gravity dilute too much? It turns out that 3-branes like our universe have their own gravitational pull which prevents too much gravity from leaking into higher dimensions.

Although string theory hasn’t been completely accepted as a ToE, it still is one of the most interesting theories that makes really interesting predictions.

Hope you enjoyed reading this article. Now you know why they say, “Gravity can travel across dimensions”. If you have any doubts feel free to ask in the comments, we’ll try our best to solve them.

Cover Image Credit: ESO/L. Calçada