This essay is part of a series called The Big Ideas, in which the writers answer a single question: what is reality? You can find out more by visiting The Big Ideas serial sheet.
When I was 8 years old, a revelation changed my life forever.
It was 1955 and the headlines announced the death of a famous scientist. A photo accompanied an article, showing his desk littered with papers and books. If I remember correctly, the photo caption indicated that among the piles of documents was an unfinished manuscript.
I was captivated by this discovery. What could be so difficult that this man, often hailed as one of the greatest scientists of all time, could not complete this work? I had to find out, and over the years I’ve visited libraries to find out more about him.
His name was Albert Einstein. His unfinished work explored what would be known as the Theory of Everything, an equation, perhaps no more than an inch long, that would allow us to unify all the laws of physics. This would give us, as Einstein had hoped, a glimpse into the mind of God. “I want to know his thoughts,” he said. I was hooked.
Today, many of the world’s best physicists are embarking on this cosmic quest, the profound implications of which span our understanding of reality and the meaning of existence. It would be the culmination of thousands of years of scientific investigation, since ancient civilizations also wondered how the universe was created and what it is made of. The ultimate goal of Theory of Everything is to combine Einstein’s theory of relativity with the bizarre world of quantum theory.
Essentially, the theory of relativity delves into the most massive phenomena in the cosmos: things like black holes and the birth of the universe. The realm of relativity is nothing less than the entire cosmos. Quantum theory, on the other hand, explores the behavior of matter at the lowest level. Its domain encompasses the tiniest particles of nature, those hidden deep within the atom.
The unification of these two spheres of thought into a single and coherent theory is an ambitious undertaking, which builds on and adds to the work started by Einstein. But to do that, scientists must first determine a crucial truth: where the universe came from.
This is where our two spheres of thought ostensibly diverge.
If we subscribe to Einstein’s theory of relativity, the universe is a sort of expanding bubble. We live on the skin of this bubble, and it burst 13.8 billion years ago, giving us the Big Bang. This gave rise to the singular cosmos as we know it.
Quantum theory is based on a radically different picture – that of multiplicity. Subatomic particles, you see, can exist in multiple states simultaneously. Take the electron, a subatomic particle that carries a negative charge. Wonderful devices in our lives, such as transistors, computers, and lasers, are all possible because the electron, in some sense, can be in many places at the same time. His behavior defies our conventional understanding of reality.
Here’s the key: in the same way that quantum theory requires us to introduce multiple electrons simultaneously, applying this theory to the entire universe requires us to introduce multiple universes – a multiverse of universes. By this logic, the solitary bubble introduced by Einstein now becomes a bubble bath of parallel universes, constantly splitting into two or colliding with other bubbles. In this scenario, a Big Bang could occur perpetually in distant regions, representing the collision or merging of these universe bubbles.
In physics, the concept of the multiverse is a key part of a cutting-edge field of study based on the theory of everything. This is called string theory, which is the focus of my research. In this image, subatomic particles are just different notes on a tiny vibrating string, which is why we have so many of them. Each string vibration, or resonance, corresponds to a separate particle. String harmonies correspond to the laws of physics. String melodies explain chemistry.
According to this thought, the universe is a symphony of strings. String theory, in turn, postulates an infinite number of parallel universes, of which our universe is just one.
A conversation I once had with theoretical physicist and Nobel laureate Steven Weinberg illustrates this. Imagine sitting in your living room, he told me, listening to the radio. In the room are the waves of hundreds of different radio stations, but your radio is tuned to only one frequency. You can only hear the station consistent with your radio; in other words, it vibrates in unison with your transistors.
Now imagine that your living room is filled with the waves of all the electrons and atoms vibrating in this space. These vibes could hint at alternate realities — ones with, say, dinosaurs or aliens — right there in your living room. But it is difficult to interact with them, because you do not vibrate coherently with them. We have detached ourselves from these alternate realities.
There is an exercise that my colleagues and I sometimes present at our doctorate. theoretical physics students. We ask them to solve a problem by calculating the probability of waking up on Mars tomorrow. Quantum theory is based on what is known as Heisenberg’s uncertainty principle, allowing for a low probability that we can exist even in distant places like Mars. So there is a tiny but calculable probability that our quantum wave will make its way through spacetime and end up there.
But when you do the math, you find that for that to happen, you’ll have to wait longer than the lifetime of the universe. In other words, you will most likely wake up in your bed tomorrow, not on Mars. To paraphrase the great British geneticist JBS Haldane, reality is not only stranger than we suppose, but stranger than we box assume.
More than six decades have passed since Einstein’s death, but I keep coming back to that photo of his desk I saw when I was 8, the work he left unfinished and its far-reaching implications. . In the quest to merge two opposing perspectives of the universe, we are left with a host of deeply troubling questions. Could we also exist in several states? What would we do if we had chosen another career? Married someone else? What if we could somehow change important episodes of our past? As Einstein once wrote, “The distinction between past, present, and future is but a stubbornly persistent illusion.”
Maybe there are copies of us living completely different lives. If this theory of everything is correct, then maybe there is a parallel universe where we are billionaires plotting our next getaway, or where we subsist as vagabonds desperately searching for our next meal. Who knows? A simple quantum fork in the road could have made all the difference.
Michio Kaku is a professor of physics at the City University of New York and author of “The God Equation”.