by Marcus Chown
Notes
1 ‘Life Is a Braid in Spacetime’ by Max Tegmark (Nautilus, 9 January 2014: http://nautil.us/issue/9/time/life-is-a-braid-in-spacetime).
2 ‘The Tyger’ by William Blake (Songs of Experience, 1794).
3 Massive: The Hunt for the God Particle by Ian Sample (Virgin Books, London, 2010).
4 ‘Broken Symmetries, Massless Particles and Gauge Fields’ by P. W. Higgs (Physics Letters, vol. 12, 13 September 1964, p. 132).
5 ‘Peter Higgs in Conversation with Graham Farmelo’ at the Centre for Life, Newcastle, 1 November 2016: https://www.youtube.com/watch?v=LZh15QK_TFg.
6 A Children’s Picture-Book Introduction to Quantum Field Theory by Brian Skinner (https://www.ribbonfarm.com/2015/08/20/qft/).
7 Thanks to Jon Butterworth for this analogy.
8 A Zeptospace Odyssey by Gian Francesco Giudice (Oxford University Press, Oxford, 2010).
9 Higgs’ inspiration for the mechanism by which spontaneous symmetry-breaking endows particles with masses had come from the phenomenon of superconductivity, in which a metal cooled to close to absolute zero (–273ºC) loses all resistance to the flow of an electrical current. The American physicist Philip Anderson had pointed out that inside a superconductor, the collective field from all the particles breaks the symmetry of electromagnetism, giving the photon a longitudinal oscillation and so an effective mass. Because photons have a mass, a magnetic field – which, according to quantum field theory, is composed of photons – has a short range and can penetrate only a short way into the superconductor. This ‘Meissner effect’ is a perfect analogue of how the Higgs field breaks the symmetry of a gauge theory and gives the massless force carriers a short range. The insight that something like the superconductor mechanism might operate elsewhere in physics and, in particular, be responsible for giving gauge particles mass came from Yoichiru Nambu – the Japanese–American physicist was a great influence on Higgs’ thinking.
10 ‘Conceptual Foundations of the Unified Theory of Weak and Electromagnetic Interactions’ by Steven Weinberg (Nobel Lecture, 8 December 1979: https://www.nobelprize.org/prizes/physics/1979/weinberg/lecture/).
11 ‘Broken Symmetries and the Masses of the Gauge Bosons’ by P. W. Higgs (Physical Review Letters, vol. 13, no. 16, 19 October 1964).
12 ‘Evading the Goldstone Theorem’ by Peter Higgs (Nobel Lecture, 8 December 2013: https://www.nobelprize.org/prizes/physics/2013/higgs/lecture/).
13 Einstein showed that whether you see an electric or magnetic field depends entirely on your velocity, revealing that neither is fundamental and in fact both are aspects of a single electromagnetic field.
14 ‘A Theory of the Fundamental Interactions’ by Julian Schwinger (Annals of Physics 2, 1956, p. 407).
15 In the Standard Model, the strong force results from a gauge theory based on an unbroken SU(3) symmetry called quantum chromodynamics, whereas the weak and electromagnetic forces arise from a gauge theory based on a broken SU(2) × U(1) symmetry.
16 If Englert’s collaborator Robert Brout had not died on 3 May 2011, he would probably have shared the Nobel Prize in Physics.
17 ‘Facts and Figures about the LHC’: https://home.cern/resources/faqs/facts-and-figures-about-lhc.
18 The Infinity Puzzle by Frank Close (Oxford University Press, Oxford, 2013, p. 342).
19 The W+, W– and Z0 particles were discovered at CERN’s ‘Super Proton–Antiproton Synchrotron’ in the early 1980s. Weighing 80.4, 80.4 and 91.2 times the mass of a proton respectively, each was almost as massive as an atomic nucleus of silver. Carlo Rubbia and Simon van der Meer won the 1984 Nobel Prize in Physics for the discovery.
* How can something appear the same to everyone, no matter what their velocity? Imagine a rainbow. The colours are known to be a measure of the distance between successive peaks of light waves, and there are waves with both shorter and longer ‘wavelengths’ than visible light. By convention, a rainbow is said to contain seven colours. Think of them as numbered from one, representing the longest wavelength, to seven, denoting the shortest. It turns out that an infinite number of ‘colours’ are possible. Imagine labelling them ‘–[infinity]’ to ‘+[infinity]’. Now imagine that they all exist in space. If you fly through them at constant velocity, all the waves will appear scrunched up, or ‘Doppler shifted’, so one will become two, two will become three, and so on. The result of shifting all the colours in this way, however, will still be a set of colours that span the range from –[infinity] to +[infinity]. Consequently, it will be impossible to tell that you are moving with respect to the light. In a sense, all colours do exist in space because, according to quantum theory, every vibration ‘mode’ of the electromagnetic field must contain a minimum amount of energy. And what is true of the electromagnetic field is true of every field, including the Higgs field.
† Another aspect of the weak force is that it affects only particles that spin one way. Think for a minute how weird this is. Imagine there is a Category Five hurricane and lots of couples are dancing in it. Those spinning clockwise are instantly blown away, while those spinning anticlockwise are unaffected. This remarkable – in fact, scarcely believable – aspect of the weak force – that it violates so-called left–right symmetry – was discovered by Chinese–American physicist Chien-Shiung Wu in 1956.
‡ The building blocks of matter – the quarks and leptons – are fermions, whereas the force-carrying particles that glue them together are bosons. All normal matter is built of only four of these particles – the up- and down-quark and the electron and electron neutrino (a proton in an atomic nucleus consists of two ups and one down, while a neutron consists of two downs and one up). The other quarks and leptons are merely heavier versions of these. It is a complete mystery why nature has chosen to triplicate its basic building blocks in this way.
§ Sigma is a probability measure: the larger its value, the more certain physicists are that a result is real and not just a random fluke in their data. At a ‘five-sigma’ level of confidence, physicists know there is a one in two million chance that nature has hoodwinked them, which is why it counts as a ‘discovery’.
¶ Because the strong force between quarks gets stronger the further apart they are, it follows that it gets weaker the closer together they are. Inside protons and neutrons, the force is so weak that the quarks behave like free particles. They are said to be ‘asymptotically free’.
9
The voice of space
If you ask me whether there are gravitational waves or not, I must answer that I do not know. But it is a highly interesting problem.
ALBERT EINSTEIN
Ladies and gentlemen, we have detected gravitational waves. We did it!
DAVID REITZE, 11 FEBRUARY 2016
In a galaxy far, far away, at a time when the most complex organism on Earth was a bacterium, two monster black holes were locked in a death spiral. They whirled around each other one last time. They kissed and coalesced. And, in that instant, three times the mass of the Sun literally vanished. It reappeared a moment later as a tsunami of tortured space–time, surging outwards at the speed of light.
For a brief instant, the power in the ‘gravitational waves’ was fifty times greater than the power radiated by all the stars in the universe combined. In other words, had the black hole merger created visible light rather than violent convulsions of space–time, it would have shone fifty times brighter than the entire universe.
The gravitational waves spread outwards like concentric ripples on a pond. They buffeted a million galaxies. They jiggled a million million million stars. They tickled planets and moons and asteroids and comets without number.
On Earth, great tectonic plates bucked and spun about and crashed together, rearing up into towering mountain ranges, which were ground back down to nothing by wind and rain and ice. Life, which had been stalled at the single-celled stage for three billion years, made the improbable leap to multicellular organisms. Plants and animals proliferated, spreading across
the face of the planet, extinguished repeatedly by impacting chunks of interplanetary rubble, only to rise again. The dinosaurs came and went. Ice spread down from the poles, before returning whence it came over and over, ebbing and flowing like a mile-deep white tide. An upright ape arose and left footprints in the volcanic ash of Laetoli in Tanzania, and whose descendants, barely an instant of geological time later, left bootprints in the dust of the Moon’s Sea of Tranquility.
The ripples in space–time rolled onwards. They lapped at the outer shores of the Milky Way. They surged inward to the Orion Spiral Arm. They jostled the cloud of icy comets at the fringes of the solar system. They sped past the gas giant planets and their mega-moons and on towards the rocky planets that huddled close to the fires of the Sun. They tickled Mars, the Moon and the top of the Earth’s atmosphere. And finally, after their immense 1.3-billion-year journey across space, they ran into something that had been patiently waiting for them.
Hanover, Germany, 14 September 2015
It was around midday when a ‘ping’ announced the arrival of an email. Marco Drago, sitting at his computer, did not immediately look to see what the email said because he was busy writing a scientific paper. Every day at this time, an email like this came in, and always it was routine and of no particular significance.
For Drago, Monday 14 September 2015 had started out as an ordinary day. On a sunny autumnal morning, he had left his flat in Nordstadt, a quiet district close to the centre of Hanover, and walked for ten minutes to the Max Planck Institute for Gravitational Physics. Employing about 200 people, the establishment consisted of a pair of modern rectangular buildings, separated by a roadway and connected by a glass corridor. Drago had arrived at his first-floor office at 9am, taken off his coat and sat down at his laptop to check the emails that had arrived overnight. About one thousand people were involved in the LIGO–Virgo collaboration, and time differences between their different countries meant that the electronic chatter was non-stop.
Drago had come to Germany the year before from the University of Trento near Verona. His postdoctoral job at what was informally known as the Albert Einstein Institute required him to monitor one of the algorithms that analysed the outputs of the two LIGO gravitational wave detectors in the US and the Virgo detector in Europe, and which extracted any ‘trigger signals’ that might conceivably be candidates for gravitational waves.
Drago saved what he had written of his paper, flipped to his email inbox and clicked on the alert. The Virgo detector, near Pisa, was not yet working, so there were just two attachments. One came from Livingston in Louisiana, and the other from Hanford in Washington state.
At Livingston was a four-kilometre ‘ruler’ made of laser light, and three thousand kilometres away in Hanford was an identical four-kilometre ruler made of laser light. When Drago clicked on the two attachments and displayed them on a split screen one above the other, he saw that, at 5.51am Eastern Standard Time, a shudder had gone through the Livingston ruler, and seven milliseconds – less than a hundredth of a second – later, an identical shudder had gone through the Hanford one. They were displayed as wiggly lines, proceeding from left to right, their up-and-down excursions mirroring perfectly the stretching and squeezing of the giant rulers. For about a tenth of a second, the wiggles became faster and more frantic before reaching a crescendo and abruptly dying away.
It was the unmistakable signature of a passing gravitational wave. A passing gravitational wave from a pair of merging black holes. But that was simply too ridiculous for Drago to accept. And not for a moment did he allow himself to believe that that was what he was seeing.
Although Advanced LIGO – the Laser Interferometric Gravitational Wave Observatory – had been operational for a month, it was still undergoing engineering upgrades to boost its sensitivity and was not scheduled to begin scientific work until four days later, on 18 September. The LIGO project had begun in the 1980s, but only now, in its ‘Advanced’ incarnation, was it approaching the sensitivity necessary to make a detection. What was the chance, thought Drago, that so soon after being switched on, it would pick up a gravitational wave? Next to zero.
This was not the only thing that set alarm bells ringing in Drago’s mind. Gravitational waves are incredibly weak.1 The reason is simple: the force of gravity is astonishingly feeble – 10,000 billion billion billion billion times feebler than the electric force that glues together the atoms in the human body.2 ‘You think Earth’s gravity is really something when you’re climbing the stairs,’ says physicist Rainer Weiss of MIT. ‘But as far as physics goes, it is a pipsqueak, infinitesimal, tiny little effect.’
An equivalent way of saying this is that space–time is incredibly stiff – a billion billion billion times stiffer than steel. It is easy to vibrate a drum skin, but incredibly hard to vibrate the drum skin of space–time. Although waving your hand in the air creates ripples in the fabric of space–time, only the most violent movements of mass imaginable, such as the merger of black holes, create gravitational waves powerful enough to be detected by twenty-first-century technology.
Black hole mergers, however, are likely to be extremely rare, which means that any whose gravitational waves are arriving at Earth are likely to have occurred immensely far away across the universe. Having spread through a mind-cringingly large volume of space, they would be diluted to infinitesimally small ripples. Everyone therefore expected that the first signal to be picked up by LIGO–Virgo would be barely perceptible amid the background ‘noise’. But the signal Drago was staring at on his computer screen was far from weak and feeble. It was powerful and unmistakable. It virtually jumped out of the screen at him. So there was little doubt in his mind. It had to be a false alarm.
In search of a second opinion, Drago walked along the corridor to the office of a Swedish postdoctoral colleague. Andrew Lundgren pulled up the email on his computer and clicked on the links. When the pair of identical waveforms from Hanford and Livingston popped up on his screen, he was as unconvinced as Drago. The two men were of one mind: it was a fake.
There were two logical possibilities. Occasionally, in order to test the response of their instrument, the LIGO–Virgo engineers would inject a fake signal into the system. But it was their practice to flag up such a ‘scheduled injection’, and Drago and Lundgren could find no mention of such an event.
The second kind of fake signal was designed to ensure that the physicists would be able to distinguish a bone fide gravitational wave from a spurious artefact. A small team of physicists was responsible for such ‘blind injections’. Sworn to secrecy, it would confess to creating a signal only if it was caught out or if a scientific paper claiming a detection was on the verge of appearing in a journal. On 16 September 2010, a blind injection had led to such a paper, which was about to be submitted to Physical Review Letters when it was revealed as a fake.
After an hour of discussion, Drago and Lundgren concluded that a blind injection was the most likely explanation for the signals they were looking at. The only thing to do was to phone Hanford and Livingston and check that everything was operating properly. It was 3.30am at Hanford, and Drago was only able to get a response from the control room at Livingston, where it was 5.30am. But it was quite enough. William Parker, the technician on duty, assured him that there were no problems with the instrument and confirmed that there had been no scheduled injection.
Drago and Lundgren decided to email the entire LIGO–Virgo collaboration about the alert and see what other people thought about it. ‘Hi all,’ typed Drago. ‘Very interesting event in the last hour. Someone can confirm this is not a hardware injection?’
The rest of the day went by in a blur. In the US, people began to wake up and started to discuss the signal. There were so many emails for Drago to deal with that it was impossible to for him to do any other work. He was too busy to even feel excited, but he was still convinced that the signal was a fake.
Everything changed two days later, when a piece of heart-stopping ne
ws came through from the team of physicists whose job was to keep the scientists on their toes. There had been no blind injection.
The Albert Einstein Institute, not surprisingly, is adorned with more than one image of its famous namesake. That evening, as Drago left the building to go home, he passed a metre-high portrait of Einstein on the corridor wall. It seemed as if the great man, who had predicted the existence of gravitational waves almost exactly a century earlier, was smiling down at him mischievously and saying, ‘I told you so.’
Princeton, New Jersey, July 1936
For ten minutes, Einstein’s new Polish assistant, seated on the other side of his desk and partially eclipsed by teetering stacks of papers, had been singing Howard Percy Robertson’s praises. Leopold Infeld appeared to have struck up quite a friendship with the man on his recent return from a sabbatical at Caltech. In fact, Einstein had that day watched the pair on the lawn from his office window. They had been in animated conversation in the bright July sunshine, Robertson puffing at regular intervals on his pipe before departing, his briefcase swinging, down the sweeping driveway of the Institute for Advanced Study.
Einstein knew who Howard Percy Robertson was. The young professor at Princeton University had impressed him the previous year when he had taken Einstein’s own theory of gravity, made some plausible assumptions about the uniformity of matter throughout the universe and managed to obtain a ‘cosmological solution’ that perfectly explained Edwin Hubble’s 1929 discovery of the expanding universe.
