- What is nuclear pasta?<\/strong><\/em><\/a><\/li>\n
- What is spacetime?<\/strong><\/em><\/a><\/li>\n
- How do we measure distance in space<\/strong><\/em><\/a><\/li>\n<\/ul>
Where is all the antimatter?<\/strong><\/h2>\n
![\""How\"]( "\\"\"How\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Could there be anti-planets out there, orbiting anti-stars? Credit: ESA\/Hubble & NASA<\/span><\/figcaption><\/div>\n It is a question whose solution lies in the very first moments of the Big Bang almost 14 billion years ago.<\/p>\n
Something happened back then to turn a Universe born with as much antimatter as ordinary matter into one strangely devoid of the stuff.<\/p>\n
Did the antimatter vanish as the result of some strange quirk in the laws of nature, or is it still out there somewhere, making up anti-planets in orbit around anti-stars?<\/p>\n
One thing is certain: despite its weirdness and rarity, antimatter is for real.<\/p>\n
The first hints of antimatter\u2019s existence emerged in 1928, when 26-year-old British physicist Paul Dirac announced his discovery of a single equation unifying no fewer than three of the key theories of physics: quantum theory, electromagnetism and Einstein\u2019s special theory of relativity.<\/p>\n
Paul Dirac\u2019s antimatter equation<\/b><\/h2>\n
![\""British\"]( "\\"\"Paul\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>British physicist Paul Dirac share the 1933 Nobel Prize for \u201cthe discovery of new productive forms of atomic theory.\u201d Credit: Bettmann\u00a0\/\u00a0Getty Images<\/span><\/figcaption><\/div>\n In 1928 Paul Dirac made his astounding claim, making antimatter the focus of unprecedented attention.<\/p>\n
It was a spectacular achievement and one that won Dirac a Nobel Prize, but its implications were perplexing.<\/p>\n
Like quadratic equations familiar from school maths, Dirac\u2019s equation had not one but two solutions.<\/p>\n
One of them made perfect sense and described the properties of that most familiar of particles, the electron.<\/p>\n
But the other solution seemed to describe another sub-atomic particle, whose properties were the exact mirror-image of the electron, with the same mass but opposite electrical charge \u2013 an anti-electron.<\/p>\n
![\""An\"]( "\\"\"Positron\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>An image of the first positron ever observed. Credit: Carl D. Anderson (1905\u20131991) \u2013 Anderson, Carl D. (1933). \u201cThe Positive Electron\u201d. Physical Review 43 (6): 491\u2013494. DOI:10.1103\/PhysRev.43.491.<\/span><\/figcaption><\/div>\n What worried Dirac was that no-one had ever seen such a particle, or anything like it.<\/p>\n
Even so, he decided to put his trust in his equation, and regard the new particle as a prediction.<\/p>\n
Dirac\u2019s faith was quickly rewarded. In 1932, physicist Carl Anderson at the California Institute of Technology had developed a new type of particle detector to study cosmic rays, incredibly fast particles hurtling through the atmosphere from deep space.<\/p>\n
A photograph taken with the detector revealed something utterly bizarre: the trace of a particle behaving as if it were the exact mirror-image of an electron.<\/span><\/p>\n
It was the first sighting of the particle predicted by Dirac\u2019s equation four years earlier: identical in mass to the electron, but with a positive charge. It was duly dubbed the positron.<\/p>\n
Yet the discovery only served to highlight the mystery of antimatter.<\/p>\n
While electrons are ubiquitous, it took a state-of-the-art particle detector to find even a single positron.<\/p>\n
The search for antimatter<\/strong><\/h2>\n
![\""Artist's\"]( "\\"\"Artist's\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>The Big Bang could have created 50:50 matter and antimatter. So why is there more matter than antimatter in the Universe? Credit: NASA\/WMAP Science Team<\/span><\/figcaption><\/div>\n There was nothing in Dirac\u2019s equation or in the known laws of physics to explain why the Universe should prefer matter to antimatter \u2013 so where was it all?<\/p>\n
One suggestion was that the Earth just happened to lie in a relatively antimatter-free region of the Universe.\u00a0Perhaps there were whole stars<\/a> made out of the stuff lurking out there.<\/p>\n
If so, it would be easy enough to tell, for as soon as antimatter comes into contact with ordinary matter, it is totally destroyed in a huge burst of radiation.<\/p>\n
Einstein\u2019s famous equation E = mc\u00b2 shows that if just a single gram of antimatter comes into contact with ordinary matter, the energy released is equivalent to the detonation of an atomic bomb.<\/p>\n
In fact, matter-antimatter annihilation is the most potent source of energy in the Universe, and its effects are hard to miss.<\/p>\n
![\""A\"]( "\\"\"A\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>A Hubble Space Telescope image showing a number of galaxies thought to host bright gamma-ray bursts. Credit: NASA, ESA, A. Fruchter (STScI), and the GOSH Collaboration<\/span><\/figcaption><\/div>\n So if there really are anti-stars or even whole anti-galaxies beyond our own, the Earth should be bathed in a constant glow of gamma radiation \u2013 the tell-tale sign of matter-antimatter annihilation.<\/p>\n
The reality could hardly be more different. Since the early 1960s, astrophysicists have sent ever more sophisticated gamma-ray detectors above the protective blanket of the atmosphere to see what is out there.<\/p>\n
And the message has been consistent: there are no signs of antimatter objects anywhere in the Universe.<\/p>\n
The lack of gamma-rays from our own Galaxy suggests that the nearest anti-star must be at least 10 lightyears away, with no more than one star in 10,000 being made from antimatter.<\/p>\n
- What is spacetime?<\/strong><\/em><\/a><\/li>\n
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