雅思阅读 78: The Elusive Antimatter Puzzle（难解的反物质之谜）

改编自 CERN（European Organization for Nuclear Research）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://home.cern/science/physics/antimatter/

Reading Passage

A. In 1928, the British physicist Paul Dirac wrote down an equation that wove together quantum theory and special relativity to describe an electron moving at near the speed of light. The work would win him a Nobel Prize, but it carried a troubling consequence. Just as the simple equation x² = 4 has two solutions, so Dirac's mathematics yielded two possible descriptions: one for an ordinary electron with positive energy, and another for a particle with negative energy. Classical physics insisted that energy must always be positive, so the second solution seemed like a nonsense result. Physicists eventually interpreted it as the sign of a real, hitherto unseen particle — an "anti-electron" with the same mass as the electron but the opposite electric charge. Four years later, in 1932, the American experimenter Carl Anderson detected exactly such a particle in cosmic rays and named it the positron. What had begun as an awkward mathematical curiosity had turned out to describe a physical reality, opening a door onto a hidden mirror world that no one had suspected lay just beneath the familiar one. The timing is worth noting. Dirac wrote his equation on paper, expecting nothing more than a theoretical correction; within four years the particle it predicted had been found in apparatus built for other purposes. Such tight collaboration between theory and experiment — mathematics predicting what the microscope would later see — is one of the model's achievements, and it set a pattern that particle physics has followed ever since.

B. Today we know that Dirac's insight was far more general. Every kind of matter particle has a corresponding antiparticle, identical in mass but opposite in charge — the negatively charged electron is paired with the positive positron, and so on through the whole cast of subatomic particles. This symmetry has two striking consequences. First, particles and antiparticles are always created in pairs; you cannot make one without making the other at the same time. Second, when a particle meets its antiparticle, the two vanish in a flash, their entire mass converted into energy, mostly as light particles called photons. It is this mutual annihilation that makes antimatter so hard to handle: the instant it touches ordinary matter it is gone, releasing its energy in the process. Scientists must therefore trap it in vacuum, away from any surface, using magnetic fields that hold the charged particles suspended in mid-air.

C. The symmetry that produces and destroys matter and antimatter together leads directly to the field's deepest puzzle. In the first fractions of a second after the Big Bang, the hot, dense universe should have churned out particle-antiparticle pairs in perfect balance. If that symmetry had held to the end, one of two outcomes should have followed. Either matter and antimatter would today exist in equal quantities, locked in endless confrontation, or — because every pair ultimately annihilates — the cosmos should contain nothing but leftover radiation, with no atoms, stars or people at all. What we observe is neither. The universe is plainly not empty, yet it is also overwhelmingly made of ordinary matter; the antimatter, for all theoretical expectations, is nowhere to be found. This mismatch, known as the matter-antimatter asymmetry, is one of the great unsolved problems in physics. It is also, in a literal sense, the reason anyone is here to ask about it. A universe that had annihilated itself to nothing would contain only cooling radiation, no galaxies, no chemistry, and certainly no experimenters to wonder where the antimatter had gone. The existence of stars and people is itself evidence that something in the early universe broke the symmetry — the only question is what.

D. The best current explanation is that some unknown process in the early universe created a tiny excess of matter over antimatter — roughly one extra matter particle for every billion pairs. When the rest annihilated, that microscopic surplus was all that remained, and it is the raw material of every galaxy, star and living thing we can see. To explain how such a surplus could arise, physicists look for violations of the symmetries that say matter and antimatter should behave identically. In 1964, two American physicists, James Cronin and Val Fitch, found the first such violation in the decay of certain particles. Their discovery showed that nature is not perfectly symmetric between the two, but the effect they measured was far too small to account for the surplus observed. Researchers now suspect that other, still-undiscovered asymmetries must be at work — and that resolving them may require physics beyond the present Standard Model.

E. To chase the answer, physicists at CERN manufacture antimatter and study it with exquisite precision. The starting point is the Antiproton Decelerator, a machine that slows antiprotons down so their properties can be measured rather than blurring away in a flash. Experiments then combine these antiparticles with positrons to make antihydrogen — a complete atom of antimatter — and test whether it behaves exactly like ordinary hydrogen. Others, using the Large Hadron Collider, measure the subtle differences in how particles and their mirror counterparts decay. Whether any of this will uncover the missing asymmetry, or merely deepen the mystery, is unknown. Yet the question itself is profound: if the early universe had been perfectly symmetric, none of this — no physicists, no machines, no stars — would exist at all. Antimatter, in the end, is not just a curiosity from a laboratory; it is the ghost at the birth of everything. The amounts produced are tiny — an experiment might trap a few tens of thousands of antiparticles, a quantity so small that their annihilation energy would barely light a lamp. Yet those vanishingly few particles are enough, when measured with sufficient precision, to test whether the laws governing them are truly symmetric, or whether some faint imbalance written into nature is what let the universe survive its own explosive beginning.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. Dirac's equation and the discovery of the positron ii. How antimatter mirrors matter — and annihilates iii. The cosmic puzzle of the missing antimatter iv. A tiny excess and the search for asymmetry v. Measuring antimatter with machines vi. The invention of the positron microscope vii. Why matter is heavier than antimatter

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. What led Dirac to predict the existence of a new particle? A. His equation yielded a second, negative-energy solution. B. He directly observed antimatter in a cloud chamber. C. He had just discovered cosmic rays. D. He wanted to explain the Nobel Prize.

6. What happens when a particle meets its antiparticle? A. They merge to form a single larger particle. B. They annihilate, converting their mass into energy. C. They repel each other without interacting. D. They both turn into neutrons.

7. According to the best explanation, how much matter survived the early annihilation? A. Roughly one extra matter particle for every billion pairs. B. Exactly equal amounts of matter and antimatter. C. All the matter that was originally produced. D. No matter at all should have survived.

8. What do researchers use CERN's Antiproton Decelerator for? A. To speed antiprotons up to nearly the speed of light B. To slow antiprotons down so they can be studied precisely C. To destroy all antimatter as safely as possible D. To generate electricity for the laboratory

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. Carl Anderson discovered the positron in 1932.
10. Particles and antiparticles are usually created one at a time, independently.
11. The universe today contains roughly equal amounts of matter and antimatter.
12. Cronin and Fitch found the first violation of matter-antimatter symmetry.
13. Dirac was awarded the Nobel Prize jointly with Carl Anderson.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

When a particle meets its antiparticle, they (14) __________, converting their entire mass into energy, mostly in the form of (15) __________.

答案与解析

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