Antigravity

Antigravity

by : Aria Ratmandanu 

       What is antimatter ? It seems strange that nature would double the number of subatomic particles in the universe for no good reason. Nature is usually quite sparing, but now that we know about antimatter, nature seems to be supremely redundant and wasteful. And if antimatter exists, can anti-universes also exist ? 

        To answer these questions, one has to investigate the origin of antimatter itself. The discovery of antimatter actually dates back to 1928, with the pioneering work of Paul Dirac, one of the most brilliant physicists of the twentieth century. He held the Lucasian Chair at Cambridge University, the same chair held by Newton, and the chair currently held by Stephen Hawking. Dirac, born in 1902, was a tall, wiry man who was in his early twenties when the quantum revolution broke open in 1925. Although he was studying electrical engineering at that time, he was suddenly swept up in the tidal wave of interest unleashed by the quantum theory.

        To the young Dirac, the challenge was to reformulate the Schrödinger equation to accommodate the theory of relativity. In 1928 Dirac proposed a radical modification of the Schrödinger equation that fully obeyed Einstein’s relativity theory. The world of physics was stunned. Dirac found his famous relativistic equation for the electron purely by manipulating higher mathematical objects, called spinors. A mathematical curiosity was suddenly becoming a centerpiece for the entire universe. (Unlike many physicists before him, who insisted that great breakthroughs in physics be firmly grounded in experimental data, Dirac took the opposite strategy. To him pure mathematics, if it was beautiful enough, was the sure guide to great breakthroughs.

      In developing his new equation for the electron, Dirac realized that Einstein’s celebrated equation, E = mc2, was not quite right. Although it is splattered over Madison Avenue ads, children’s T-shirts, cartoons, and even the costumes of superheroes, Einstein’s equation is only partially correct. The correct equation is actually E = ± mc2. (This minus sign arises because we have to take the square root of a certain quantity. Taking the square root of a quantity always introduces a plus or minus ambiguity.)

     But physicists abhor negative energy. There is an axiom of physics that states that objects always tend to the state of lowest energy (this is the reason that water always seeks the lowest level, sea level). Since matter always drops down to its lowest energy state, the prospect of negative energy was potentially disastrous. It meant that all electrons would eventually tumble down to infinite negative energy, hence Dirac’s theory would be unstable. So Dirac invented the concept of the “Dirac sea.” He envisioned that all negative energy states were already filled up, and hence an electron could not tumble down into negative energy. Hence the universe was stable. Also a gamma ray might occasionally collide with an electron sitting in a negative energy state and kick it up into a state of positive energy. We would then see the gamma ray turn into an electron and a “hole” develop in the Dirac sea. This hole would act like a bubble in the vacuum; that is, it would have a positive charge and the same mass as the original electron. In other words, the hole would behave like an antielectron. So in this picture antimatter consists of “bubbles” in the Dirac sea.

Just a few years after Dirac made this astounding prediction, Carl Anderson actually discovered the antielectron (for which Dirac won the Nobel Prize in 1933).”

What is the antimatter counterpart of gravity? Do anti-universes exist ?

       As we discussed, antiparticles have the opposite charge of ordinary matter. But particles that have no charge at all (such as the photon, a particle of light, or the graviton, which is a particle of gravity) can be their own antiparticle. We see that gravitation is its own antimatter; in other words, gravity and antigravity are the same thing. Hence antimatter should fall down under gravity, not up. (This is universally believed by physicists, but it has actually never been demonstrated in the laboratory.) Dirac’s theory also answers the deep questions: Why does nature allow for antimatter ? Does that mean anti-universes exist ?

       In some science fiction tales, the protagonist discovers a new Earth-like planet in outer space. In fact, the new planet seems identical to Earth in every way, except everything is made of antimatter. We have antimatter twins on this planet, with anti-children, who live in anti-cities. Since the laws of anti-chemistry are the same as the laws of chemistry, except charges are reversed, people living in such a world would never know they were made of antimatter. (Physicists call this the “charge-reversed or C-reversed universe, since all charges are reversed in this anti-universe, but everything else remains the same.)

       In other science fiction stories scientists discover a twin of the Earth in outer space, except that it is a Looking Glass universe, where everything is left-right reversed. Everyone’s heart is on the right side and most people are left-handed. They live out their lives never knowing that they live in a left-right reversed Looking Glass universe. (Physicists call such a Looking Glass universe a parity-reversed or P-reversed universe.)

      Can such antimatter and parity-reversed universes really exist? Physicists take questions about twin universes very seriously, since Newton’s and Einstein’s equations remain the same when we simply flip the charges on all our subatomic particles or reverse the left-right orientation. Hence, C-reversed and P-reversed universes are in principle possible.

       Nobel laureate Richard Feynman posed an interesting question about these universes. Suppose one day we make radio contact with aliens on a distant planet but cannot see them. Can we explain to them the difference between “left” and “right” by radio? he asked. If the laws of physics allow for a P-reversed universe, then it should be impossible to convey these concepts.

      Certain things, he reasoned, are easy to communicate, such as the shape of our bodies and the number of our fingers, arms, and legs. We can even explain to the aliens the laws of chemistry and biology. But if we try to explain to them the concept of “left” and “right” (or “clockwise” and “counterclockwise”), we would fail each time. We would never be able to explain to them that our heart is on the left side of our body, in which direction the Earth rotates, or the way a DNA molecule spirals.

       So it came as a shock when C. N. Yang and T. D. Lee, both at Columbia University at the time, disproved this cherished theorem. By examining the nature of subatomic particles they showed that the Looking Glass, P-reversed universe cannot exist. “For this earthshaking result, called the “overthrow of parity,” Yang and Lee won the Nobel Prize in Physics in 1957.

       To Feynman, this conclusion meant that if you are talking to aliens on a radio, it is possible to set up an experiment that could enable you to tell the difference between left- and right-handed universes by radio alone. (For example, electrons emitted from radioactive cobalt-60 do not spin in equal numbers in a clockwise or counterclockwise fashion, but actually spin in a preferred direction, thereby breaking parity.)

      Feynman then envisioned that a historic meeting finally takes place between the aliens and humanity. We tell the aliens to stick out their right hand when we first meet, and we will shake hands. If the aliens actually stick out their right hand, then we know that we have successfully communicated to them the concept of “left-right” and “clockwise-counterclockwise.

       But Feynman then raised an unsettling thought. What happens if the aliens stick out their left hand instead ? This means that we have made a fatal mistake, that we have failed to communicate the concept of “left” and “right.” Worse, it means that the alien is actually made of antimatter, and that he performed all the experiments backward, and hence got “left” and “right” mixed up. It means when we shake hands, we will explode!

      That was our understanding until the 1960s. It was impossible to tell the difference between our universe and a universe in which everything was made of antimatter and was parity-reversed. If you flipped both the parity and the charge, the resulting universe would obey the laws of physics. Parity by itself was overthrown, but charge and parity was still a good symmetry of the universe. So a CP-reversed universe was still possible.

      This meant that if we were talking to aliens on the phone, we could not tell the difference between an ordinary universe and one that was both parity- and charge-reversed (left and right are interchanged, and all matter is turned into antimatter).

     Then in 1964 physicists received a second shock: the CP-reversed universe cannot exist. By analyzing the properties of subatomic particles, it is still possible to tell the difference between left-right, clockwise-counterclockwise if you are talking by radio to another CP-reversed universe. For this result, James Cronin and Val Fitch won the Nobel Prize in 1980.

      (Although many physicists were upset when the CP-reversed universe was shown to be inconsistent with the laws of physics, in hindsight “in hindsight the discovery was a good thing, as we discussed earlier. If the CP-reversed universe were possible, then the original big bang would have involved precisely the same amount of matter and antimatter, and hence 100 percent annihilation would have taken place, and our atoms would not have been possible! The fact that we exist as a leftover from the annihilation of unequal amounts of matter and antimatter is proof of CP violation.)

        Are any reversed anti-universes possible? The answer is yes. Even if parity-reversed and charge-reversed universes are not possible, an anti-universe is still possible, but it would be a strange one. If we reversed the charges, the parity, and the march of time, then the resulting universe would obey all the laws of physics. The CPT-reversed universe is allowed.

        Time reversal is a bizarre symmetry. In a T-reversed universe, fried eggs jump off the dinner plate, reform on the frying pan, and then jump back into the egg, sealing the cracks. Corpses rise from the dead, get younger, turn into babies, and then jump into their mother’s womb.

        Common sense tells us that the T-reversed universe is not possible. But the mathematical equations of subatomic particles tell us otherwise. Newton’s laws run perfectly well backward or forward. Imagine videotaping a billiard game. Each collision of the balls obeys Newton’s laws of motion; running such a videotape would make for a “bizarre game, but it is allowed by the laws of Newton.

       In the quantum theory things are more complicated. T-reversal by itself violates the laws of quantum mechanics, but the full CPT-reversed universe is allowed. This means that a universe in which left and right are reversed, matter turns into antimatter, and time runs backward is a fully acceptable universe obeying the laws of physics!

      (Ironically, we cannot communicate with such a CPT-reversed world. If time runs backward on their planet, it means that everything we tell them by radio will be part of their future, so they would forget everything we told them as soon as we spoke to them. So even though the CPT-reversed universe is allowed under the laws of physics, we cannot talk to any CPT-reversed alien by radio.)