This essay is part of a series called The Big Ideas, in which writers respond to a single question: what is reality? You can read more by going to The Big Ideas series page†
When I was 8 years old, a revelation changed my life forever.
It was 1955 and headlines announced the death of a renowned scientist. A photo accompanied an article, which showed his desk littered with papers and books. As I recall, the photo caption noted that among the piles of material was an unfinished manuscript.
I was captivated by this discovery. What could be so challenging 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 visited libraries to learn 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. It would, as Einstein hoped, give us a glimpse into the mind of God. “I want to know his thoughts,” he famously said. I was addicted.
Today, many of the world’s top physicists embark on this cosmic quest, the far-reaching reverberations of which span our understanding of reality and the meaning of existence. It would be the culmination of thousands of years of scientific research, as ancient civilizations also questioned how the universe came to be and what it was made of. The ultimate goal of the 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 of the cosmos: things like black holes and the birth of the universe. The domain of relativity is nothing less than the whole cosmos. Quantum theory, on the other hand, examines the behavior of matter at its most minuscule level. His domain encompasses the tiniest particles of nature, which are hidden deep within the atom.
Uniting these two fields of thought into a single and coherent theory is an ambitious undertaking, one that builds on and contributes to the work Einstein started. But to do this, scientists must first determine a crucial truth: where the universe came from.
This is where our two spheres of thinking clearly diverge.
If we subscribe to Einstein’s theory of relativity, the universe is like a bubble that is expanding. We live on the skin of this bubble and it exploded 13.8 billion years ago, which gave us the Big Bang. This spawned the unique cosmos as we know it.
Quantum theory is based on a radically different image – one of multiplicity. Subatomic particles, you see, can exist in multiple states at the same time. 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 can, in a sense, be in several places at once. His behavior defies our conventional understanding of reality.
Here’s the key: In the same way that quantum theory forces us to introduce multiple electrons at once, applying that theory to the entire universe forces us to introduce multiple universes — a multiverse of universes. By that logic, the lone bubble introduced by Einstein now becomes a bubble bath of parallel universes, constantly splitting in two or bumping into other bubbles. In this scenario, there could be a continuous big bang in distant regions, representing the collision or merging of these bubble universes.
In physics, the concept of a multiverse is a key element of a leading field of study based on the theory of everything. It’s called string theory, which is what my research focuses on. In this photo, subatomic particles are just different tones on a small, vibrating string, which explains why we have so many. Each string vibration, or resonance, corresponds to an individual particle. The harmonies of the string correspond to the laws of physics. The melodies of the string explain the chemistry.
This thinking makes the universe a symphony of strings. String theory, in turn, posits an infinite number of parallel universesof which our universe is only one.
A conversation I once had with theoretical physicist and Nobel laureate Steven Weinberg illustrates this. Imagine sitting in your living room, he said, listening to the radio. In the room are the waves from hundreds of different radio stations, but your radio is tuned to only one frequency. You can only hear the station that is coherent with your radio; in other words, it vibrates in harmony with your transistors.
Now imagine that your living room is filled with the waves of all the electrons and atoms vibrating in that room. These waves can point to alternate realities — the ones involving dinosaurs or aliens, for example — right there in your living room. But it’s hard to deal with because you don’t vibrate coherently with them. We have detached ourselves from these alternate realities.
There is an exercise that my colleagues and I sometimes present to our Ph.D. students of theoretical physics. We ask them to solve a problem by calculating the probability that someone will wake up on Mars tomorrow. Quantum theory is based on what is known as the Heisenberg uncertainty principle, which gives a slim chance that we could exist even in distant places like Mars. So there is a small but calculable chance that our quantum wave will work its way through space-time and end up there.
But if you do the calculation, you find out that you would have to wait longer than the lifetime of the universe for this to happen. That is, most likely you will wake up tomorrow in your bed, not on Mars. To paraphrase the great British geneticist JBS Haldane: reality is not only stranger than we think, but also stranger than us can Suppose.
It’s been more than six decades since Einstein’s death, but I keep going back to that photo of his desk I saw when I was 8, the work he left unfinished and its profound implications. In the quest to merge two opposing perspectives of the universe, we are left with a host of very troubling questions. Could we also exist in multiple states? What would we do if we had chosen a different profession? Married to someone else? What if we could somehow change important episodes in our past? As Einstein once wrote, “The distinction between past, present, and future is but a stubbornly persistent illusion.”
Perhaps there are copies of us who live very different lives. If this theory holds true, then maybe there’s a parallel universe where we’re billionaires plotting our next escapade, or where we live as bums desperate 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 the author of “The God Equation.”