Wonders of the Universe

CHAPTER 3

FALLING

FULL FORCE

For all its scale and grandeur, the Universe is shaped by the action of just four forces of nature. Two of these, the weak and strong nuclear forces, remain hidden from everyday experience inside the atomic nucleus. The third force, electromagnetism, is perhaps most familiar to us, as it is the one we marshal to power our lives – electric currents flow because of the action of this force. Finally, there is gravity, the great sculptor – the force that acts between the stars. Gravity shapes the cosmos on the largest distance scales. From the formless clouds of hydrogen and helium that once filled our universe, gravity forged the first stars, sculpted the first planets and arranged them into the exquisite shapes of the galaxies. Having assembled countless billions of solar systems, gravity drives their cycles and rhythms. It is the invisible string behind the revolution of every moon around every planet and every planet around every star. Gravity keeps our feet on the ground and the Universe ticking over.

Gravity is more than a mere gentle presence; it is relentless, and for the largest agglomerations of matter in the Universe – the stars – it is both creator and destroyer. Stars shine in temporary resistance to gravitational collapse, but when they run out of nuclear fuel and the other three forces can no longer rearrange the matter in their cores in order to release energy and resist its inward pull, gravity crushes the most massive of them out of existence. In doing so, it creates the least understood objects in the Universe.

Before embarking on the voyage of their lives, astronauts are prepared for the flight and the sensation of weightlessness in aircrafts such as this C-131 at Wright Air Development Center, which flies at a ‘zero-g’ trajectory. These flight simulators are dubbed ‘vomit comets’ because of the nausea they often induce.

NASA

Soviet cosmonaut Gherman Titov is perhaps not the luckiest of men. In 1960 he was selected alongside Yuri Gagarin for the Soviet manned space programme. Out of the twenty men who started the programme, only these two made it through a fierce selection process that tested their physical and psychological resilience to the limit. Throughout training the two fighter pilots matched each other point for point, but someone had to be first, and Gagarin was given the ticket into the history books. On 12 April 1961, Gagarin became the first human to travel into space, completing a single orbit in 108 minutes before returning to Earth first in Vostok 1 and then by parachute. In one of those interesting bits of space trivia, Gagarin actually arrived back on Earth after his spacecraft, because he ejected at an altitude of 7,000 metres (23,000 feet) due to worries about the safety of the capsule on landing. Vostok 1 arrived safely on the ground 10 minutes before he did.

This bus ride to the Vostok launch on 12 April 1961 was the first part of the journey that was to make Yuri Gagarin a Soviet hero and worldwide celebrity.

NASA

On his return, Gagarin became a Soviet hero and a worldwide celebrity, leaving Titov to become the second man to orbit our planet. Titov’s name will be unfamiliar to most, although to this day he remains the youngest man ever to make the journey into space, at just under 26 years old. He piloted Vostok 2 on 6 August 1961, completing 17 orbits of Earth. Titov also claimed a rather less glamorous place in the history books; on the 25.3-hour mission, he not only became the first man to sleep in space (snoozing for a couple of hours as his spacecraft orbited the planet), but also the first to suffer the symptoms of a condition that has affected almost half of those who have experienced weightlessness for an extended period of time. Titov was the first victim of Space Adaption Syndrome. Known more usually as space sickness, this condition includes a variety of symptoms such as nausea, vomiting, vertigo and headaches as a common reaction to the odd sensations of space travel. Although weightlessness remains one of the great thrills of being an astronaut, it is also one of the most difficult to prepare for. Since Titov introduced medics to Space Adaption Syndrome, space agencies around the world have employed the only method they can of creating weightlessness here on Earth. How is it possible to remove the effects of gravity? The answer is by doing the same thing that Gagarin and Titov did: by falling towards Earth.

The American response to the Vostok programme was Project Mercury, a series of six manned launches which included the historic flights of Alan Shephard, the first American in space, on 5 May 1961, and John Glenn, the first American to orbit Earth. The astronauts selected for the programme, known as the ‘Mercury Seven’, became celebrities in the United States, and all of them eventually flew into space. The final flight of the Mercury Seven was John Glenn’s Space Shuttle mission in 1998, which he completed at the age of 77. The Tracy brothers in the TV series Thunderbirds were named after five of the Mercury Seven: Scott (Carpenter), Virgil (‘Gus’ Grissom), Alan (Shephard), Gordon (Cooper) and John (Glenn). Wally Schirra and Deke Slayton missed out. (I think Wally and Deke would have been great names for Thunderbirds pilots. The days when astronauts were bigger than rock stars are sadly missed.)

Astronauts prepare for Extravehicular Activity by practising techniques on a Hubble Space Telescope mock-up in the Neutral Buoyancy Laboratory. Underwater conditions simulate the weightlessness experienced in space.

NASA

During training for Project Mercury, perhaps after hearing about the experiences of Titov, NASA developed a way of flying a regular military aircraft to take would-be astronauts on an unusual ride. Using a C-131 aircraft, weightlessness was achieved by flying an unconventional flight path. This parabolic path creates a brief period of around 25 seconds during which all the occupants of the plane experience the sensation of weightlessness. This is because they are actually weightless; it may be brief, but when repeated twenty or thirty times in succession, the physiological effects are just as intense as those felt in space. This led to the C-131 being named the ‘Vomit Comet’, a name that has stuck with every plane used for this task ever since.

I’ve known about the Vomit Comet since I was a child, because I was, and still am, passionate about the space programme. Imagine my delight when I heard we were going to ride in it for our film on gravity. Who cares if it makes you feel rough, if the Mercury Seven could face it, so could I.

The Vomit Comet is the perfect place to experience the two related aspects of the force of gravity that hold the key to understanding what gravity actually is. Firstly, it is possible to completely cancel out the effects of gravity by simply falling towards the ground. This sets gravity apart from all the other forces of nature; it is not possible to negate the effect of electric charge, other than by adding more electric charge of the opposite sign. The Comet achieves the removal of gravity simply by flying along the trajectory that a cannon ball would take when fired out of a gun. The plane doesn’t just drop to the ground like a lift with a severed cable, of course (because then it would be impossible to control), but the acceleration of the plane towards the ground is exactly the same as the acceleration you would experience in a falling lift or a parachute jump (if you neglect air resistance). In numbers, the plane must accelerate towards the ground at 9.81 metres per second squared to cancel out the force of gravity. In order to keep the plane under control, it also flies forward at its usual flight speed. This results in the plane flying along a parabolic path. The fact that the effects of gravity are completely removed in freefall is very interesting, and the converse is also true: it is also possible to add to Earth’s gravitational pull by accelerating.

The race for space was on in the 1960s, as the US and Soviet nations battled to be the first to launch a human being into space.

NASA

Everyone knows that astronauts in space are weightless and float around inside their spacecraft, but not everybody knows why. It is not because they are a long way from Earth that gravity is absent (they are in fact only a few hundred miles above Earth’s surface, and the strength of Earth’s gravitational field in near-Earth orbit is not too different to the strength on the surface), it is that the effects of gravity are removed by falling, which is important point number one.

We flew in a modified Boeing 727-200, which is still used today for training shuttle astronauts. During the flight I was also able to demonstrate another strange but equally important and related aspect of gravity. Isaac Newton knew it when he wrote down his theory of gravity in 1687, as did Galileo many decades before him. The strange thing is this: all objects fall at the same rate under the force of gravity, even though gravity acts on objects in proportion to their mass. Newton and Galileo knew this to be the case because they did experiments and noticed that it was true, but they had absolutely no idea why. If you think about it for a moment, it is very odd indeed. Newton found that the gravitational force between two objects, such as Earth and you, is proportional to the product of their masses. So the force you feel due to the pull of Earth’s gravity is proportional to the mass of Earth multiplied by the mass of you. If you were to double your mass, the force between you and Earth would double. But, the rate at which you accelerate towards Earth because of its gravitational pull is also proportional to your mass, and when you work everything out it turns out that your mass completely cancels out, so therefore all things fall at the same rate under gravity. This looks very strange and was famously demonstrated by Apollo 15 Commander Dave Scott on the surface of the Moon in 1971. Scott dropped a feather and a hammer to the ground and, of course, both hit the ground at the same time. The reason you can’t do this on Earth is because air resistance slows the feather down, but in the high vacuum of the Lunar surface the only force acting on the falling objects is gravity. No matter how much physics you know, this is entertaining to watch because it isn’t in accord with common sense! Surely a cannon ball should fall to the ground faster than a single atom? The answer is, no, it doesn’t, and here is something to think about for later on: even a beam of light falls to the ground at the same rate as a cannon ball. Understanding this concept is key to understanding gravity.

As we accelerate away from Earth we experience a gravitational pull 1.8 times the strength of Earth – so I weigh nearly twice as much as I do back down on the ground.

I was able to demonstrate this for myself in the Vomit Comet armed with a model of Einstein. When we were weightless, I let a little plastic Albert float beside my head. One way of understanding why we floated next to each other is to simply state that we were both weightless, so we floated, but think about what this looks like from outside the plane. To someone on the ground looking up at us, the plane, myself and plastic Albert are all falling towards the ground under the action of Earth’s gravity, and obviously we are falling at the same rate. If I fell faster than Einstein, he wouldn’t float next to my head. Indeed, if the much more massive plane fell faster than both plastic Albert and myself, we’d both bump into the ceiling! The fact that we all floated around together is a beautiful demonstration of the fact that all objects, no matter what their mass, fall at the same rate in a gravitational field.

This simple fact inspired Albert Einstein to construct his geometric theory of gravitation, called General Relativity, which to this day is the most accurate theoretical description of gravity that we possess. We shall get to Einstein’s beautiful theory later on, and in doing so we’ll arrive at a very simple explanation of why everything falls at the same rate, and why gravity can be removed by the act of falling

Gravity holds the water in our oceans and hugs the atmosphere close to the planet. It’s the reason why the rain falls and the rivers flow; it powers the ocean currents and drives the world’s weather; it’s why volcanoes erupt and earthquakes tear the land apart. Yet gravity also plays a role on an even grander stage. Across the Universe, from the smallest speck of dust to the most massive star, gravity is the great sculptor that created order out of chaos.

© NASA/Corbis