"Where did we come from? How did the universe come into being? Are we alone in the universe? Is there alien life out there? What is the future of the human race?" Such questions are at the heart of renowned physicist and cosmologist Stephen Hawking's life's work. They also drive this year's blockbuster hit The Theory of Everything.
The film, directed by James Marsh and starring Eddie Redmayne, tells the story of Hawking's inspirational and dramatic personal and professional journey. While it offers a riveting and detailed portrait of Hawking's life, health and relationships, it glosses over many intricacies of his scientific ideas.
And understandably so. The details of Hawking's work, which are rooted in relativity and quantum mechanics, are obscured in a labyrinth of equations and abstractions. In his career, Hawking focused on three fundamental questions: How was the universe created? What happens inside a black hole? What is unifying physical law that governs the entire universe? For the layman, understanding these can seem a daunting task.
However, if you loved The Theory of Everything and are intrigued yet confused by the actual science the film touches on, there's hope. Here is the essence of Hawking's work, explained in 10 gifs:
How was the universe created?
Near the start of The Theory of Everything, a bullish young Hawking declares, "Time is my topic!" What he meant was that his Ph.D. — his first major scientific investigation — would focus on one of the most enduring and profound questions humans have asked throughout the history of our existence: Does the universe have a beginning, or is it eternal?
An expanding universe
Scientists first started answering this question early in the 20th century when Albert Einstein devised the laws of relativity. These laws predicted that space itself must be expanding, a property that was confirmed by the observations of several scientists.
Belgian scientist Georges Lemaître then asked: If all the galaxies in the universe are rushing away from each other, what happens when you rewind the clock? He reasoned that the further back in time you go, the smaller the universe was. So at one point in time, all of the universe must have been squeezed into one point in space, called a singularity.
Lemaître was alluding to the Big Bang. During his Ph.D., Hawking would try to show that this was a correct model of the universe, according to math. To do this, he would need to account for two competing models that proposed that the universe was eternal: the Steady State universe and the "bouncing" universe.
A steady sate universe
Fred Hoyle, one of Hawking's peers at Cambridge, was a strong advocate of a Steady State universe. In this model, new matter is created as the universe expands, so over time, space would look more or less the same in every direction. Steady State theory avoids the need for the universe to have a beginning.
A bouncing universe
Another theory favored by several scientists including Evgeny Lifshitz and Isaak Khalatnikov stated that the universe could not collapse into a single point because matter is distributed irregularly. Instead, the universe bounces back and expands any time it shrinks to a small enough volume:
A Big Bang universe
In response, Hawking worked with London physicist Roger Penrose to show that singularities could be real features of the universe, not just mathematical artifacts. Using the equations of relativity, first they showed that stars didn't need to be perfect spheres to collapse into a single point. They then applied the math to the entire universe itself and showed that a Big Bang singularity existed, no matter how they modeled the distribution of matter inside of it.
At the same time, other scientists made several independent observations that also supported the Big Bang model, confirming that time had a beginning.
What happens inside a black hole?
The strange properties of black holes enticed Hawking to study them. Black holes arise from the collapse of heavy stars into an infinitely dense point. The gravity in black holes is so strong that light cannot escape from them. Time ends inside black holes because they warp the universe so that space becomes indistinguishable from time, and the laws of physics break down.
The space around a black hole
To understand how black holes behaved, Hawking realized that he needed to examine the properties of space surrounding them. He did this by applying quantum theory, a branch of physics that explains how nature behaves at a subatomic level. Quantum theory predicts that particle and anti-particle pairs are continually being produced due to energy fluctuations in space. These tiny "virtual" particles annihilate each other quickly after appearing because they have "opposing energies."
The edge of a black hole
Hawking realized that when virtual particles appeared right at the edge of a black hole, one of the particles could fall in before annihilation occurred. This phenomenon became known as "Hawking radiation."
The discovery of Hawking radiation had two amazing implications. First, it meant that because escaping virtual particles carry energy, black holes can give off a small amount of heat. Particles that fall into the black hole will have negative energy because energy in the universe is conserved. This interaction reduces the mass of the black hole, meaning that, over an incredibly long time, black holes can disappear.
Hawking's work on black holes was significant because was one of the first times a theory elegantly brought together two of the main laws governing our universe: gravitation and quantum mechanics. These discoveries marked an important step towards finding a theory of everything.
What is unifying physical law that governs the entire universe?
A theory of everything
Modern science still lacks a unifying theory that can describe how everything in our observable universe behaves, from the vastness of space to the fuzzy world inside of an atom, in a simple mathematical equation. Currently the closest model we have is the Standard Model, which states that the basic structure of the universe is comprised of subatomic particles. However, the Standard Model is fundamentally an incomplete theory because its equations cannot explain some important features of our universe such as gravity.
A theory of strings
Given these limitations, Hawking and many theoretical physicists have embraced String Theory as a contender for a theory of everything. The theory predicts that there is a fundamental level of structure where all matter and forces are made of one-dimensional strings. The strings can vibrate in different patterns to give rise to a different types of particles. Unlike the Standard Model, this vibration can produce a particle that is responsible for gravity.
A theory of dimensions
But there's a big caveat to String Theory: Its mathematics predicts that the universe contains extra spatial dimensions, up to 11. We can't see these extra dimensions because they are compacted into extremely tiny scales. Physicists have calculated that the extra dimensions can be compressed into an incredible number of different shapes:
A new theory of the Big Bang
A big problem for String Theory is that it offers no real way to determine exactly which of the many shapes of the extra dimensions is correct. Hawking and his peers believe that in reality, all shapes exist. The implication of this is that our universe is one among many multiverses.
Hawking believes that String Theory can provide further insight than his earlier work on singularities, which could not explain how the Big Bang's "bang" was actually started. If multiverses exist, String Theory predicts that our universe could have been created from a collision or splitting of other universes.