On the morning of April 18, 1955, a pathologist named Thomas Harvey performed an autopsy on Albert Einstein at Princeton Hospital and made a decision that would haunt him for the rest of his life. Without permission from Einstein's family, he removed the brain, wrapped it in a cloth, and took it home. For decades, Harvey carried Einstein's brain across the country in a cider box, periodically slicing off samples for researchers who hoped to find, hidden somewhere in its folds and fissures, the physical secret of genius. They never found it. The organ that had conceived of relativity, reimagined the nature of time, and peered into the structure of the cosmos looked, by most measures, remarkably ordinary. Whatever Einstein had been, it seemed, could not be located in the tissue. It lived instead in the ideas he left behind, which have proven considerably harder to contain than a brain in a box.
No equation has reshaped our understanding of reality more profoundly than E=mc². When Albert Einstein (1879–1955) published his theory of special relativity in 1905, he did not merely advance physics. He demolished the foundations on which it had rested for two centuries and rebuilt them entirely, giving us radical new conceptions of time, space, mass, and energy. What followed over the next decades confirmed that this was no isolated breakthrough. From quantum mechanics to cosmology, Einstein's insights continued to ripple outward, their implications compounding with every generation of scientists who inherited them. Nearly seventy years after his death, those implications are still unfolding.
The scale of that inheritance raises an obvious and urgent question: where does physics go from here? Einstein himself spent the final thirty years of his life chasing an answer, searching for a single unified theory that would reconcile the laws governing the very large with those governing the very small. He never found it. That unfinished dream has since become the defining obsession of theoretical physics, a quest that has consumed some of the sharpest minds of the 20th and 21st centuries and produced some of its most astonishing ideas, among them string theory, parallel universes, and the possibility that space and time are not fundamental at all.
Few scientists have pursued that dream more boldly, or communicated its stakes more vividly, than Michio Kaku. A theoretical physicist at the forefront of string field theory, Kaku has dedicated his career to completing what Einstein started, while simultaneously ensuring that the revolutionary spirit of that pursuit reaches well beyond the academy. As a bestselling author, BBC host, and tireless public communicator, he has a rare gift for transforming the most vertiginous ideas in modern physics into thrilling intellectual adventures. In this interview, Kaku reflects on Einstein's unfinished vision, the frontiers of theoretical physics today, and why the grandest questions our universe poses are only just beginning to be asked.
Casa Carlini: What were Einstein’s major contributions to physics besides the theory of relativity?
In 1905, Einstein published the theory of relativity along with several other groundbreaking breakthroughs. One major achievement was his experimental proof of the existence of atoms, amidst skepticism from contemporaries. This year also saw Einstein explain the photoelectric effect, detailing how a light beam striking metal would eject electrons and generate a tiny current.
However, Einstein sought more; he aimed to formulate a general theory of relativity that could address gravitation and acceleration, which special relativity could not explain. From 1905 to 1915, he dedicated himself to achieving this goal.
Einstein introduced the concept of a particle of light, which was later named the photon, laying the groundwork for quantum theory. As a result, he is often regarded as the godfather of quantum theory, a significant framework of the 20th century.
In the same miraculous year of 1905, Einstein explained the photoelectric effect, detailing how a light beam striking metal would eject electrons and generate a tiny current. He introduced the concept of a particle of light, later named the photon, laying the foundation for quantum theory. Thus, Einstein is often regarded as the godfather of quantum theory, the other great theory of the 20th century. The photoelectric effect and photon principles are fundamental to today’s technology, including solar cells, TV cameras, lasers, and modern electronics.
Also in 1905, Einstein's miracle year, he explained the photoelectric effect, how a light beam falling on metal would eject electrons and create a tiny current. Einstein introduced a particle of light, later called the photon, which forms the basis of the quantum theory of matter and light. Einstein is thus the godfather of the quantum theory, the other great theory of the 20th century. (The photoelectric effect and the photon are used today in solar cells, TV cameras, lasers, and modern electronics.)
CC: What is the difference between the special theory of relativity and the general theory?
MK: Einstein's special relativity, proposed in 1905, aimed to explain light's behavior, where light travels at the same speed regardless of motion, and time slows down at higher speeds. This insight alone would have guaranteed him fame as a great physicist.
MK: We apply Einstein's theories daily. He predicted light bending around stars, resulting in distorted images of distant galaxies, known as Einstein's rings. We observe these rings through telescopes, using them to study the universe. Furthermore, astronomers have cataloged thousands of black holes, confirming Einstein's predictions about space and time behaviors around them.
In 1915, he created general relativity, based on the idea that empty space could be curved. Anyone passing through curved space would have the illusion that a force was acting on them. In this way, Einstein explained the true nature of gravity. (For example, imagine ants walking on a crumpled sheet of paper. They are mysteriously tugged to the right and left, they claim, by an unseen force. But we know that there is no force acting on the ants. They are tugged in different directions because they are walking on curved space.)
Unlike special relativity, general relativity can explain large-scale astronomical phenomena, such as black holes, bending starlight, and the Big Bang theory.
CC: How does a physicist apply Einstein’s theories to modern science?
MK: We apply Einstein's theories every day to modern science. For example, Einstein predicted that when light passes by a star, the light beam bends (as if it were moving in glass). But if a light beam passes around distant galaxies, then we see a galaxy's image distorted into the shape of a ring, called Einstein's rings. Today, we see Einstein's rings via our telescopes and use them to explore the universe. Also, astronomers have cataloged thousands of black holes in outer space. One lies at the very center of our Milky Way galaxy, weighing about 2 million suns. We can now show experimentally that these black holes obey the predictions made by Einstein decades ago. For example, Einstein said that space-time was like thick molasses that swirled around a black hole, dragging space along with it. We can now confirm this prediction by Einstein. Lastly, Einstein also introduced the concept of Bose-Einstein condensates. He showed that, when matter is cooled down to near absolute zero, atomic motion almost disappears, and atoms seem to coalesce into one gigantic superatom that vibrates in unison. Thus, tiny and strange quantum effects, which are usually too small to be seen in the lab, can be seen in a BE condensate. About 70 years or so after Einstein and Bose predicted the existence of this strange form of matter, it was finally found in the lab. In the future, perhaps laser beams made of atoms (and not light) and also quantum computers (and perhaps even invisibility) may be byproducts of BE condensates.
MK: Although Einstein spent the last 30 years of his life pursuing an even greater theory, called the unified field theory, he ultimately failed in this mission. Yet, he may have been onto something significant. Today, many physicists, including myself, strive to fulfill his dream of unifying all physics into a single equation. The leading candidate for this unification is string theory, which can seamlessly combine Einstein’s relativity with quantum theory.
CC: How do Einstein’s theories relate to your own work (string field theory, etc.)?
MK: Einstein spent the last 30 years of his life (from 1925) searching for an even greater theory, which he called the unified field theory. It was to be his crowning achievement. He wanted a theory of everything, i.e., a theory which could unite all the fundamental forces of the universe into a single theory, which would allow him to "read the mind of God."
Einstein failed in this mission, but perhaps he was onto something. Today, there are scores of physicists (myself included) who are trying to complete his dream of unifying all of physics into a single equation. The leading candidate is called string theory, which can unite Einstein’s theory of relativity with the quantum theory. Remarkably, these two theories contain the sum total of all physics at the fundamental level.
For decades, anyone trying to unify relativity with the quantum theory was met with serious mathematical problems. Any naïve union of the two blows up in your face. Today, string theory is the only theory that can combine Einstein’s theory of gravity with the quantum theory and still yield finite, meaningful results.
CC: During one of your TV appearances, you spoke about “dark energy,” a mysterious force that is causing the universe to fly apart faster and faster. You mentioned that Einstein was onto something called the “cosmological constant,” but he thought he had made a mistake. Today, this “mistake” has become an integral part of astrophysics. Can you elaborate on that?
MK: Just in the last five years, physicists have realized that there is a strange energy permeating all of space, called dark energy, which is causing the galaxies to accelerate away from each other. The universe, instead of slowing down (as was universally thought), is actually in a runaway mode, accelerating until perhaps we hit the Big Freeze. Temperatures will drop to near absolute zero, and the entire night sky will be totally black. (Dark energy, or the energy hidden in empty space, was introduced by Einstein in 1916, but he later called it his greatest blunder. Strangely, Einstein’s blunder is perhaps the most important factor in determining the ultimate fate of the universe.)
At present, no one knows where this dark energy comes from. If one naively tries to calculate dark energy, one finds a huge mismatch. The theory is off by a factor of 10 raised to the 120 power! This is the largest mismatch in the history of science. Obviously, there are still huge gaps in our understanding of the universe if we cannot calculate dark energy (which makes up 73% of the matter and energy of the universe. By contrast, the higher elements, which make our bodies, only make up .03% of the universe.)
CC: Which, if any, of his theories have been refuted since his death, and which are still being debated?
MK: Einstein’s most controversial belief was his criticism of the quantum theory. The quantum theory is the most successful theory of all time, but it is based on probabilities and chance. He did not believe that God played dice with the world.
Even today, physicists debate the philosophical questions raised by Einstein. For example, according to the quantum theory, a cat placed in a box is neither dead nor alive until you look at it. To describe it, you have to add the wave function describing a dead cat, with the wave function of a live cat. (Only when you open the box and make a measurement does the cat suddenly spring into existence as we know it.) So before you look at a cat in a box, it is neither dead nor alive, but exists in a netherworld as the sum of the two.
Philosophers used to ask, does a tree fall in a forest if there is no one there to listen to it? For centuries after Newton, scientists firmly believed that the universe existed independent of humans, and hence the tree either did or did not fall. But the quantum theory says otherwise. It says that until you actually see the tree, the tree exists in all possible quantum states (burnt, firewood, splinters, toothpicks, sawdust, a sapling, and an ordinary tree). Only when you look at it does the tree suddenly spring into existence.
Remarkably, Nobel Laureates have split on this question. Nobel laureate Eugene Wigner believed that observations determine existence, and observations require a conscious mind; hence, the existence of the universe meant that there was a cosmic consciousness permeating it. In some sense, he was offering this as proof of the existence of God.
According to Niels Bohr, a pioneer of quantum theory, “Anyone who is not shocked by the quantum theory does not understand it.” For instance, quantum theory states that electrons can occupy two locations simultaneously, a concept foundational to all of chemistry. In chemistry classes, students often draw orbitals—football-shaped clouds surrounding atoms—but rarely learn about the underlying nature of these clouds. They represent the wave function of an electron that can exist in many places at once, allowing it to bind atoms together. Consequently, the ability of electrons to occupy dual locations is fundamental to the integrity of molecules. Without quantum theory, atoms would disintegrate, and our reality would cease to exist as we know it.
Einstein thought that this was silly, and he said so. Although the quantum theory is accurate to one part in 10 billion and makes possible all the miracles of lasers and modern electronics, it is based on a philosophical foundation of sand. Einstein once said that the more successful the quantum theory becomes, the sillier it looks.
Today, there is still no universal consensus on the questions raised by Einstein back in the 1920s concerning the quantum theory’s outrageous paradoxes. But one idea is gradually catching on, and this is “decoherence.”
For example, perhaps the cat splits into two cats or two universes. In one universe, the cat is dead. In the other universe, the cat is alive. So, at each juncture in the history of the universe, it constantly splits into two. The key is that we have “decohered” from these other universes, and hence can no longer communicate with these parallel universes. Nobel Laureate Steve Weinberg compares this to listening to a radio in your living room. You can hear many, many stations on your radio, so you know that many invisible radio waves are permeating your living room. But your radio is tuned to only one frequency, and it has decohered from all the others. The radio frequency you are hearing no longer interacts with all the other radio frequencies, and you hear only one sound.
CC: While Einstein is primarily known for his theory of relativity, he was also a humanist and philosopher. What are some lesser-known facts about him that may reveal his character outside the scientific realm?
Likewise, in your living room are the wave functions of alternate universes. There are the wave functions of dinosaurs, pirates, aliens, exploding stars, etc., in your living room. But you cannot interact with these parallel universes. Your universe no longer vibrates in unison with them, and hence has decoupled from them. (So, perhaps in one universe Elvis is still alive, but you can no longer interact with that universe.)
CC: While Einstein is best known for his theory of relativity, he was also a humanist and a philosopher. What are some of the lesser-known facts about him that might shed light on the kind of person he was outside of the science lab?
MK: In writing a biography titled Einstein’s Cosmos, I was struck by how Einstein maintained his sanity amidst chaos, hatred, and war. He received the adulation of millions without succumbing to it. Despite the Nazis denouncing him and burning his books, he boldly criticized them publicly, becoming a figurehead for anti-Nazi opposition within the scientific community.
A less resolute individual might have buckled under the ceaseless barrage of hate from the Nazis. Yet, Einstein's resolve only strengthened as he publicly condemned them, understanding the potential danger this posed to him personally.
CC: In your opinion, if he were alive today, what inventions/discoveries that have occurred since his death would he likely find the most amazing?
MK: Quantum teleportation, derived from his work, may be the most astonishing. Fans of Star Trek are familiar with teleportation—the ability to vanish and reappear elsewhere. In the laboratory, physicists have teleported individual photons and cesium atoms, though teleporting a human remains far beyond our current capabilities.
Quantum teleportation actually uses a thought experiment that Einstein devised to try to destroy the quantum theory. It's called the EPR experiment (after Einstein, Podolsky, and Rosen). Imagine two electrons or photons shooting out in opposite directions. Originally, they were vibrating in phase with each other. Even after they are separated, their wave functions are still vibrating in phase, so there is an invisible "umbilical cord" that still connects them. We say that these two electrons are in quantum coherence. For example, if one electron is vibrating in the up direction, then the other electron should be in the down direction (so the sum is zero, as before). Now let these two electrons travel for many light-years. Then measure one electron. Let's say that it spins down. You now know, FASTER THAN LIGHT, that the other electron is spinning up. Because nothing can travel faster than light, Einstein reasoned, all this is nonsense, and hence quantum coherence was ridiculous.
Actually, this experiment has now been done many times, and each time, Einstein is wrong, and the quantum theory is correct. (But this does not violate relativity, since it is only random information that is traveling faster than light. You cannot send Morse code or a meaningful message using the EPR experiment. No useful information can be sent this way, so relativity is still not violated).
Today, physicists re-adapt the EPR experiment to create quantum teleportation. Objects that are vibrating in phase with each other are connected by quantum coherence, and we use this to teleport information about one atom into another, distant atom. (This means, however, that the original atom must be destroyed, so if Capt. Kirk teleports across space; his original body is destroyed in the process.)
CC: What, in your view and based on your own work, still lies ahead in the way of amazing discoveries that are directly derived from Einstein’s theories?
MK: Recently, scientists have been building a series of fantastic instruments that may further our understanding of Einstein's pioneering work. First of all, in 2008, the Large Hadron Collider will be turned on outside Geneva, Switzerland, the most powerful instrument of science ever built. It is 27 kilometers in circumference and will create beams of protons with trillions of electron volts in energy. These are energies not seen since the instant of the Big Bang itself. In fact, we call it a window on creation. We hope to find entirely new particles with this powerful atom smasher, including mini-black holes that are the size of sub-atomic particles. (They are so small that these black holes do not pose a danger. In fact, cosmic rays from outer space hit the earth all the time with more energy than the LHC, and nothing happens.)
We also hope to find new particles with the LHC, called sparticles, or super particles. These are higher vibrations of the string, which are so heavy that they have not been seen so far. Some of these sparticles have no charge and are, in fact, totally invisible. (These sparticles are the leading candidate for dark matter, an invisible form of matter which surrounds the galaxies, making up 23% of the matter-energy content of the universe. With dark matter and dark energy, we now realize that most of the universe is, in fact, dark, i.e., invisible, and that atoms like hydrogen and helium in the stars make up only 4% of the universe.)
Then, perhaps around 2015, NASA will send a new type of satellite into space to probe the heart of the Big Bang itself. LISA (Laser Interferometry Space Antenna) will detect gravity waves in space, i.e., shock waves of gravity caused by colliding black holes and even the instant of creation. (Gravity waves were predicted by Einstein decades ago.) It consists of three satellites, connected by laser beams, separated by 3 million miles. If a gravity wave still circulating the universe from the Big Bang hits LISA, it will jiggle the laser detectors, and we will measure its intensity and frequency.
CC: In your opinion, what discoveries or inventions since his death would likely amaze him today?
LISA (or its successors, such as the Big Bang Observer) might be sensitive enough to shed light on the pre-big bang universe. At present, no one knows where the Big Bang came from, or what happened before it. But string theory makes predictions as to what might have preceded the Big Bang, and can predict the radiation emitted from these pre-big bang scenarios. Therefore, scientists hope that by analyzing gravity waves from the Big Bang, we will be able to compare this radiation with the predictions made by these pre-big-bang theories. In this way, we might be able to determine which model is correct, and therefore what most likely happened before the Big Bang. (One serious possibility is that our universe is a bubble floating among billions of other bubble/universes in 11-dimensional hyperspace. Occasionally, these bubble/universes collide, split in half, sprout baby bubbles, or pop into existence. Einstein gave us the fourth dimension. Now, physicists are going beyond four dimensions and investigating 11-dimensional space-time.)
CC: For students (and people in general) who are not Einsteins, how can his concepts be made easier to understand?
MK: One of my favorite Einstein quotes is that unless a theory can be explained to a child, the theory is probably useless. By this, he meant that the essence of a theory has to be a simple, elegant physical picture or principle that even children can grasp. All too often, physicists get lost in a thicket of mathematics that eventually leads to nowhere. The guiding principle must always be pictorial and simple.
For example, the essence of special relativity can be summarized in one picture. When he was 16-years-old, he visualized racing alongside a light beam. Since light is a wave, the light beam should appear frozen as you move neck-and-neck with the beam. But no one had ever seen a frozen wave before, and hence Einstein, as a boy, was led to believe that it was impossible to outrun a light beam. In fact, he came to a radical conclusion that light always travels at the same speed, no matter how fast you move.
Similarly, general relativity can be explained by pictures that children can understand. Imagine a large funnel, and then throw a marble along the surface. The marble will circulate in the center of the funnel because the surface is curved. Now replace the marble with the earth, replace the center of the funnel with the sun, and we see that the earth orbits around the sun, not because gravity pulls on the earth, but because the space around the sun pushes the earth. In other words, "gravity does not pull, space pushes." This, in one phrase, is the essence of general relativity.
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