Either the bullet hit the president in the back, came out of his neck, then struck the governor in the armpit, came out below his right nipple, went through his wrist, lodged in his thigh, and then turned up with a perfect nose and a slightly compressed tail on the governor’s stretcher; or else there were two assassins. For the former scenario – the single-bullet theory posited in the Warren Commission Report – to hold up, ‘it would oblige the bullet, angling downward as determined at the official autopsy, to reverse direction inside Kennedy’s body and reflect backward up from inside his back toward his neck bones, striking a vertebra, reflecting again at a high angle before exiting just below his Adam’s apple,’ Paul Chambers writes in Head Shot: The Science behind the JFK Assassination, published in 2010 and now appearing in an expanded edition. Chambers, a shock physicist, has worked for Nasa (optics branch), the Naval Surface Warfare Center (energetic materials and detonation department) and the Naval Research Laboratory (condensed matter and radiation division). He knows a lot about what happens to something when something else hits it very hard (high-velocity impacts, deformation of solids), and how the air behaves when an object goes faster than the speed of sound (sonic booms, echoes, acoustical signatures). He’s not interested in pinning the murder on Sam Giancana, Lyndon Johnson, J. Edgar Hoover, Castro, Khrushchev, Howard Hunt, Earl Warren, George H.W. Bush, Duong Van Minh, the John Birch Society, the Freemasons or Aristotle Onassis. ‘I am not a conspiracy theorist,’ he begins. ‘I am a conspiracy empiricist.’ He wants to know the truth because without it ‘another president could once more be cut down in his or her prime.’ One chapter of the book recounts the history of the scientific method, with digressions on Eratosthenes, Kepler, supernovas and the invention of the transistor. ‘In science,’ Chambers writes, ‘it’s important to keep an open mind and let the data speak for itself.’

I try to keep an open mind. There are books on my desk from big publishers that use science to explain the meaning of life, why people worship God, kill each other, sleep with each other and write hit songs. Most of these explanations seem to my open mind either obvious or irrelevant, but I wouldn’t mind knowing if an oscilloscope can prove that there was a second shooter on the grassy knoll. Chambers is 96 per cent certain that sounds recorded on a Dictabelt microphone attached to a policeman’s motorcycle, with the switch unintentionally stuck in the ‘on’ mode, fit with the timing of the shots as reproduced in the Zapruder film, and 50 per cent certain that one of the four sound events on the Dictabelt recording, according to the acoustical signature of its echoes and the way they match up with a 1978 reconstruction staged in Dealey Plaza, is a shot from the grassy knoll.

The single-bullet theory, which Chambers rejects but refrains from calling the ‘magic bullet’, was advanced by Arlen Specter and other staff lawyers on the Warren Commission. Specter went to Dallas and interviewed doctors on orders from Warren to clear up the question of whether the hole in Kennedy’s throat was an exit wound or an entrance wound. The doctors said it could have been either, but they hadn’t figured out the bullet’s path. Specter also oversaw live fire tests of the .26-calibre Mannlicher-Carcano rifle found in the book depository, conducted by Alfred Olivier, a veterinarian and a ballistics expert. Olivier fired shots through gelatin moulds, boxes of horsemeat and goatmeat, an anaesthetised goat and the wrist of a cadaver. The sacrificial goat was dressed like Kennedy in a jacket, shirt and undershirt. The wrist shot produced a wound far worse than the one the governor suffered. ‘My feeling,’ Olivier said, ‘is that it would be more probable that it passed through the president first.’

Chambers faults the Warren Commission for having mixed motives – both discovering the truth and putting a stop to rumours that might ignite a Third World War – and for being a homogeneous group of lawyers, many of them young and inexperienced. He contrasts it with the commission that investigated the Challenger explosion, which included Richard Feynman, one of Chambers’s heroes, and had the sole purpose of preventing other space shuttles from exploding. Chambers also notes that Gerald Ford, a single-bullet booster, changed the wording of the report from ‘a bullet had entered his back at a point slightly above the shoulder’ to ‘a bullet had entered the base of the back of his neck’. The edit was made public in 1997, and Ford told the Assassination Records Review Board he’d done it ‘for the sake of clarity’. ‘This,’ Chambers writes, ‘was inexcusable.’

That’s about as angry as Chambers gets. Throughout Head Shot, he writes very clear prose that takes on a dizzy, endearing lyricism only when he dwells on the wonders achieved by science. He is mostly sober and commonsensical and forgiving to people he perceives to be making honest mistakes. In addition to Ford and Vincent Bugliosi, proponent of the single-bullet theory and prosecutor of Charles Manson, Chambers has it in for Aristotle, who reversed the advances of predecessors like Aristarchus by putting the earth at the centre of the universe and neglecting to test out his theories with experiments. ‘At least Aristotle believed in his own work,’ Chambers writes. ‘The majority of the Warren Commission didn’t believe in theirs.’ Two of Kennedy’s aides who were riding in the motorcade assented to the Commission’s view but later told Tip O’Neill they thought shots came from the grassy knoll. Chambers devotes a chapter to the fifty so-called grassy-knoll witnesses even though their testimony, not being unscientific, isn’t essential to his argument. ‘There are photos taken moments after the shooting showing people racing toward the knoll in a bold effort to find the assassin,’ he writes. ‘These people must have been full of adrenaline, clouding their judgment, because anyone who had just killed Kennedy with a rifle could easily do the same thing to them.’

Most of what Chambers recounts is not new, and the possibility of a gunman on the grassy knoll was admitted by both the pre-Warren FBI investigation and the late 1970s reopening of the case by the House Select Committee on Assassinations. His contribution is to dispel the notions that Kennedy’s head moved ‘back and to the left’ after the headshot seen on the Zapruder film either because a bullet hit him from behind and caused a jet effect when it came out on the right side of his head, sending the head back in the direction of the impact (a theory tested by shooting melons); or because of a muscle spasm. Rather, Chambers relies on the laws of conservation of momentum, conservation of energy and conservation of mass (‘this is rocket science’), as well as Occam’s razor, and embarks on a proof that the head recoiled ‘back and to the left’ because a frangible bullet was fired from a Winchester .220 Swift in front of Kennedy and to his right, i.e. on the grassy knoll. At the end of the proof he can’t help getting a bit emotional:

The energy of the incoming bullet was 2289 joules. Where did all the energy go? … It simply went into other forms, some apparent, some hidden. Part of the energy of the bullet went into creating the high-speed jetting of blood and tissue in frame Z313. Some of it, three joules, went into lifting Kennedy’s head against gravity … The bulk of the energy, however, tragically, was deposited in Kennedy’s brain. As the bullet and skull fragments penetrated his dura, the outer layer of the brain inside his skull, they transferred energy into Kennedy’s neural tissues, breaking physical and chemical bonds and producing a rise in temperature. The kinetic energy of the fragments was converted into mechanical and thermal energy inside Kennedy’s brain, obliterating a lifetime of thoughts, dreams, ambitions, hopes and plans and leaving a lifeless lump of useless tissue. It was this violent deposition of energy directly into his head, over two thousand joules (equivalent to a small explosive charge), that ended Kennedy’s life.

Chambers turned six in 1963, so it’s understandable that he’s sentimental about JFK. His nuttier comments come when he addresses questions of probability:

Kennedy’s presidency lasted roughly a thousand days. No one shot at him until 22 November 1963. If he were around other people, appearing in crowds, or sitting near a window for about 10 per cent of this time, or a little over two hours a day on average, his potential exposure would be about 8,640,000 seconds. The probability is therefore 1 in 1,440,000 that both shooters would attempt to fire at him by chance in the same six-second period. It is more probable that civilisation will come to an end next year due to an asteroid impact.

Indeed, multiple simultaneous shooters would indicate something more than a coincidence. Chambers takes Oswald at his word that he was a patsy and believes it ‘improbable’ that he even fired a shot from the book depository, citing police paraffin tests that found no nitrates on his cheeks.

Like Santa Claus, the Tooth Fairy and the poor, the second shooter will always be with us. Every now and then a new theory comes round to fulfil our wish for there to be something grander at work than one lonely loser with a rifle. You can pick whatever villain you like: the Mob, the Cubans, the oilmen, the Soviets, the Vietnamese or the CIA. Chambers’s book is full of entertaining illustrations, some of them quite gruesome. My favourite is a blow-up of the Polaroid that a bystander called Mary Moorman took of Kennedy’s limo and the grassy knoll right at the moment of the murder. Behind the picket fence on the knoll, there’s a tiny blob and on it four tinier spots that look like a pair of squinty eyes, a nose and a mouth. ‘Some claim that the image of a face is apparent,’ Chambers writes. ‘The evidence for this is insufficient to meet the rigorous scientific standards of this analysis.’ Even though he was a boarding school student in Massachusetts at the time, whenever I look at the blob I can’t help thinking that its squinty eyes belong to George W. Bush.

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