Matter and Antimatter, and Two Universes

The universe is made of matter. The galaxies, stars, and planets we observe, and even our own bodies, are all made of matter. Modern physics has also established the existence of antimatter corresponding to this matter. Yet the very names matter and antimatter invite misunderstanding. They give the impression that one is real matter and the other is somehow fake, or that when matter and antimatter meet, nothing remains. In reality, antimatter is just as physically real as matter. An electron has a negative electric charge, while its corresponding antiparticle, the positron, has a positive charge. The two have the same mass but opposite electric charges. Likewise, a proton and an antiproton have the same mass, although their electric charges and certain other quantum properties are opposite. What, then, does it mean when we say that matter and antimatter annihilate each other?

When an electron and a positron meet, the two particles can disappear and photons can be produced. What disappears is the individual identity of the electron and the positron as particles. Energy does not disappear. The energy corresponding to the rest masses of the two particles is converted into photon energy, while total momentum and angular momentum are also conserved. The electron's negative charge and the positron's positive charge add up to zero. A photon has no rest mass, but it possesses energy and momentum, as well as angular momentum properties associated with spin. The annihilation of matter and antimatter, therefore, is not an event in which physical reality returns to nothingness, but a process in which particular particles are transformed into other particles. Matter and antimatter are not so much opposing substances as entities possessing symmetrical properties under the same physical laws.

Why, then, has so much matter remained in the universe? Modern cosmology generally holds that matter and antimatter existed in nearly equal amounts in the early universe. As they annihilated each other, only a tiny excess of matter remained, eventually forming the universe we see today. This is often explained by saying that for every billion antimatter particles, there was one additional matter particle. A billion sounds like an enormous number. Yet even an ordinary quartz grain of sand approximately 0.5 millimeters in diameter contains around five quintillion atoms. If we imagine a billion atoms as a single group, one grain of sand contains about five billion such groups. Saying that one particle out of a billion survived describes a ratio. If that proportion of matter remained throughout the universe, the absolute quantity could still be immense enough to form all the galaxies and stars that exist today.

That figure was not obtained by directly counting particles in the early universe. Scientists observe the cosmic microwave background and the relative abundances of primordial elements to calculate the proportions of matter and radiation. They then interpret those results in terms of an asymmetry between matter and antimatter in the early universe. Yet why such an asymmetry arose in the first place remains unknown. To understand this problem, scientists search for antiparticles in space and create antimatter in laboratories to measure its properties. Samuel Ting, a Chinese-American physicist who received the Nobel Prize in Physics in 1976, led a major experiment to search for antimatter in space. In May 2011, the Alpha Magnetic Spectrometer, AMS-02, developed by his research team, was carried aboard the space shuttle Endeavour and installed on the International Space Station. The instrument analyzes the electric charges, momenta, and other properties of high-energy particles arriving from space to distinguish matter particles from antiparticles. Positrons and antiprotons are indeed detected in space. They can also be produced by cosmic-ray collisions. The discovery of antiparticles alone, therefore, does not establish the existence of stars or galaxies made of antimatter. At CERN, the European Organization for Nuclear Research, experiments are also conducted in which antiprotons and positrons are combined to produce antihydrogen atoms, which are then confined using electromagnetic fields. Researchers compare the properties of matter and antimatter with great precision, searching for differences that have not yet been discovered.

Everything up to this point concerns research actually being conducted in modern physics. Now let us consider another possibility. Suppose matter and antimatter existed in exactly equal amounts in the original universe, and an amount of antimatter corresponding to all the matter remaining in our universe still exists somewhere. What if that antimatter exists not in some distant galaxy, but in another universe? One universe is made of matter, the other of antimatter. The two universes originated from the same source, and each possesses its own space and time. Beings in the other universe may also observe stars and planets and may have developed life and civilizations. To them, the particles making up their bodies are ordinary matter. It would be the particles making up our bodies that they classify as antimatter. There is no absolute label attached to the universe determining which side is matter and which is antimatter.

The idea of two symmetrical universes is not entirely new. In 2018, Latham Boyle, Kieran Finn, and Neil Turok published a cosmological model based on CPT symmetry. They explored the possibility of another universe symmetrical to ours across the Big Bang. CPT refers to a combined symmetry involving C, which exchanges the properties of particles and antiparticles; P, which reverses spatial orientation; and T, which reverses the direction of time. In this hypothesis, the two universes evolve in opposite temporal directions away from the Big Bang. Yet observers in both universes would experience their own time as moving forward. The direction in which their memories form is the direction of time they experience.

Now let us add one more assumption. What if the two universes are not completely separated? Some of the antiparticles whose creation we observe in particle accelerators might actually be particles crossing over from the other universe. High-energy particle collisions momentarily disturb the boundary between the two universes, allowing particles to travel through a temporary rupture. The rupture closes immediately, preventing us from observing the other universe. We interpret the event as particle creation. Of course, particle production observed in actual accelerators is explained by quantum field theory, and there is no experimental evidence that a boundary between two universes opens in this way. This is an imaginative possibility based on the assumption that another universe exists but has yet to be discovered. What if a civilization developed the technology to open and close that boundary in a stable and controlled manner? They would be able to connect two spacetimes with opposing directions of time. Could the phenomenon of time reversal then be interpreted in an entirely new way?

Of all Christopher Nolan's films, Tenet is my favorite. It provoked considerable debate and was often described as difficult to understand, but I was far more strongly drawn to Tenet than to his other films. I have watched it dozens of times. Each time, I find myself thinking about the reversal of time, the causal relationships between events, and the difference between what the characters know and what actually happened. In the film, people pass through a special device that reverses their direction through time. The film explains this as a reversal of entropy. From the perspective of someone traveling backward through time, that person continues to think, move, and form memories normally. To everyone else, however, the person appears to move in reverse.

But if two symmetrical universes actually exist, the device in the film might not be a machine that turns time backward. It might be a device that connects two spacetimes progressing in opposite temporal directions and aligns the state of an object with the direction of time in the other universe. A person passing through the turnstile does not travel backward through their own time. They continue living toward their own future. The reason they appear to move backward in time to an observer in our universe is that the direction of time to which they belong is opposite to ours. To develop this hypothesis into a complete physical theory, we would need to explain how the time coordinates of the two universes are connected and how an object passing through the other universe could arrive in the past of our own universe.

There is another question worth considering. In the film, the people of Earth interpret the time reversal they observe as a reversal of entropy. They are trying to explain the phenomenon using the physical laws they know. But what would a civilization in the other universe think if it heard this explanation? They already know that two universes exist and have even developed the technology to connect their spacetimes. To them, the reversal of entropy might seem like a primitive explanation that fails to grasp the true nature of the phenomenon. What actually happened was movement between two universes, but the people of Earth conclude that an object's time is running backward based solely on what they observe within their own universe. Their observations may be correct while their interpretation of the cause is wrong. They believe they understand time-reversal technology without even knowing where that technology came from.

Once I began thinking this way, the story of Tenet started to look entirely different. In the film, humanity in the future attempts to intervene in the past to ensure its own survival. Sator, played by Kenneth Branagh, gains enormous wealth and power using gold and information sent from the future. He plans for his death to trigger the transmission of the algorithm's location to the future. But what if the beings sending Sator gold and controlling him are not future humans? What if they come from another universe symmetrical to ours? They call the universe they inhabit the Original Universe, and they call ours the Replica Universe. Time progresses in opposite directions in the two universes, and they have already developed the technology to manipulate the boundary between their spacetimes. They did not approach Sator in order to change their own past. They approached him to eliminate another universe that threatens their own.

For them, our universe has long been causing serious problems. As imagined earlier, suppose that every time a particle accelerator operates in our universe, microscopic damage accumulates in the spacetime of the other universe. Our scientists celebrate the discovery of new particles, while unexplained physical disturbances repeatedly occur in the other universe. After studying these phenomena for a long time, its inhabitants discover that another universe exists in a symmetrical relationship with their own, and that intelligent beings there are continually damaging the boundary between them. We conduct experiments to understand the universe. They must stop our experiments to preserve their own universe. Yet the civilization of our universe does not even know they exist. From their perspective, the cause of the catastrophe must be eliminated. Destroying a single particle accelerator would not be enough. As long as intelligent life exists in this universe and continues conducting physical experiments, the same problem could recur. Eventually, they decide to use their technology for controlling the boundary between the two universes to eliminate our entire universe.

Sator is a human hired to carry out that plan. He receives gold and power in exchange for helping complete the device desired by the civilization of the other universe. His death becomes the final signal needed to complete the operation. What the film calls the algorithm is not an invention that must be sent into the future, but the core of a technology capable of destroying or restructuring the boundary between the two universes. Who, then, were the protagonists of Tenet actually fighting? They believed their enemies were future humans using time-reversal technology to attack the past. They believed they had saved the world by stopping Sator and securing the algorithm. But in reality, they had not even discovered the identity of their enemy. The only person they encountered was Sator, a man hired by a civilization from another universe. He was merely a scout deployed at the front line of an immense war. The characters in the film believe they have defeated him and achieved victory, but what they confronted was not the enemy's main force. It was only their outermost agent. The civilization of the other universe would not consider itself an aggressor either. To them, this is not a war to conquer another world, but a war to protect their own universe. If they believe our experiments are damaging their world, they will consider their actions a legitimate form of defense.

And another question remains. If they claim that theirs is the Original Universe, how could we verify that claim? If the two universes originated from the same source, could we determine which one is the original? They, too, might be unable to prove that theirs is the original. The protagonists of Tenet believe they have won a war against the future, but they may have merely stopped the first attack launched from another universe. They still do not even know whom they are fighting.

Humanity on Earth may still be operating its particle accelerators, completely unaware of any of this.