Understanding the Gravity of a New Theory
Both the motions of planets orbiting around the sun and a pencil rolling off the table onto the floor are described by the gravity. However, despite this phenomena being so easily recognized, why and how it happens is still very much up to debate. Theories of gravity have been updated before due to new observations that better characterize the motions of objects, but a recent theory proposed by Erik Verlinde casts a new and very different light on gravity that has shaken the scientific community.
Keywords: gravity, Newton, Einstein, string theory, scientific paradigms.
Gravitational History
To really grasp the importance and novelty of the new theory proposed by Verlinde, let’s first take a look at the two canon theories of gravity.
Sir Isaac Newton’s theory is most likely what the majority of people think of when they think about gravity. For example, an apple falls off a tree and towards the Earth (but perhaps not before falling on someone’s head first). The principle idea behind this is that objects fall in a path that’s directed towards the heaviest object.
In Newtonian gravity, all masses attract other masses through a gravitational force. This force is proportional to the two passes and inversely proportional to the square of the distance between them.
On cosmic scales, this force is exerted both on the smaller mass, like the earth, as well as the larger object, like the sun. The difference is, because the earth is much smaller, the gravitational force from has a greater impact on our motion, thus keeping Earth more or less in it’s elliptical trajectory. When a relatively small object is very close to the surface of the earth, the gravitational force reduces down to the constant that we can see when an apple falls.
There are, however, many instances where Newtonian mechanics breaks down. To be clear, this does not mean that physics is fundamentally broken, just that this theory can’t explain everything. For example, Newtonian mechanics cannot explain events on extremely small and large scales. For this reason, and many others, Einstein’s theory of general relativity has been widely adopted when describing gravity especially at larger cosmic scales.
In this theory, matter and space interact into a thing called spacetime. When applied to gravity, it suggests that masses will affect the space around them. Imagine the universe as a large flat net, with a very heavy ball in the center. The ball will cause the net to dimple or curve in response to it’s weight.
Experiments using general relativity lead to the hypothesis that the universe is expanding. From the studying of light as it traveled across space came the observation of redshifts, or light’s path stretching, due to the increasing amount of distance it had to travel. This combined with the experience that the universe is not being torn apart leads to the hypothesis that there must be something holding it all together — dark matter.
Composing of 27% of the universe, dark matter is allegedly responsible for, not only keeping the universe in one piece, but for the cosmic radiation that persists throughout the universe since it’s beginning (it’s counterpart, dark energy, is 68% of the universe). Thus far, neither have been measured directly, but still many theories heavily rely on them to measure how far away and how big galaxies are with gravitational lensing. It’s a pretty difficult task to come to accept a phenomena that has not been directly observed. Indeed, it is observation that drives science and research experiments. Even though there has been some evidence that points to its existence, it is still uncomfortable for many scientists to accept.
Emergent Gravity Theory
A solution has been proposed by a new theory that does not introduce dark matter as a concept to explain the motion of the cosmos. Going from first principles, instead of tuning the theory to account for observations, Erik Verlinde proposes that the phenomena of gravity is not actually a principle in itself but instead comes from the physical interactions of nature.
This means that gravity and space would be emergent properties. Instead of being a property of the interactions of masses as in Newtonian mechanics, or of mass with the space around it as in Einstein’s general relativity, gravity is intrinsically tied to the nature of existing things and their relationship with each other. This relationship is based on the ability of information to be transmitted, where this information could be mass, energy, spacial coordinates, etc. Such ideas were derived directly from string theory.
Some Strings Attached
String theory starts by assuming that subatomic particles, like electrons for example, are small vibrating sources of energy instead of being point-like as they are in Newtonian or quantum mechanics. These strings, or sources of energy, vibrate at different rates such that an electron would have a different frequency than a proton.
When the strings vibrate, they are transmitting information about their identities. By weaving together strings of various vibrations, you can create particles and eventually the whole universe. A useful analogy might be the electron’s vibration creates a tone or note that is unique and able to harmonize with other particles to create the symphony of a molecule where the tones communicate information to your ears. It is in the sending and receiving of information that gravity then emerges, like the chard you hear from the combination of notes.
Similarly, dark matter too appears to be emergent property. Even though it does not explain where it comes from, this theory, instead of assuming that it exists to justify data, comes straight from mathematical and physical principles. As with any new idea, a lot more has to be fleshed out before it’s completely understood, but this new way of looking at dark matter could shine some light onto what and why it exists.
Even though emergent gravity is a very new and peculiar theory, a team of researchers at the Leiden Observatory in the Netherlands has tested his theory and found that the data verifies Verlinde’s predictions. They did this by using gravitational lensing, which is a phenomena where light bends around very large bodies in space, namely entire galaxies. They measured the distribution of gravity around more than 33,000 galaxies and found that the force of gravity is significantly stronger that Einstein’s theory predicts and is much closer to Verlinde’s.
Conclusions: Paradigm Shift towards a Unified Theory
These apparent rise and fall of accepted theories and paradigms has occurred throughout the course of scientific history. This occurrence is perhaps best described by scientist and philosopher, Thomas Kuhn who claims that there are two types of science: normal science and scientific revolutions. Normal science progresses incrementally where hypotheses and experiments attempt to discover something new within a given model system, or paradigm. Scientific revolutions, on the other hand, are times where that model is called into question, typically with a proposition of what to put in it’s place, based on a new theories or observations.
As new information is discovered, either through more observational data or more sensitive measurements via technology, our conceptions about how the universe work must adapt. Especially during an era where there are competing and incompatible theories attempting to explain the physical world, new theories that provide the potential for a unified theory are much needed.
Currently, general relativity is very much at odds with quantum mechanics. Using string theory, the idea that everything is communicating energy, potentially sets the stage for non contradictory theories, or perhaps a “theory of everything.” Perhaps before we go that far, maybe we can finally come to an understanding of gravity and dark matter first.
Sarah is in a graduate program for Chemical Biology at the University of Michigan. She writes for her blog Annotated Science and soon will be published with Michigan Science Writers! You can follow her on Twitter at annotated_sci for more content on science and open access news.