How to Destroy the Universe (21 page)

The idea that sound can inflict pain has led researchers to develop sonic weapons. Since 2004, the US military has been fitting its warships with devices called LRADs (long range acoustic devices), which can project a beam of sound with a loudness of up to 150 dB over a distance of up to 600 m (2,000 ft). The weapons work using a dish about 1 m (3 ft) across, which keeps the sound in a tightly collimated beam. This narrow beam allows it to send intense sound over a considerable range.

LRADs are non-lethal, but the pain they induce is meant to repel attackers in small boats, which might be hard to hit using conventional gunfire. Since 2004, the devices have also been mounted on civilian and commercial vessels to repel pirate attacks. Police forces around the world are now employing them for use in crowd control. Some acoustics researchers even believe that lethal sonic weaponry could be a real possibility in the years and decades to come. Any sound above about 150 dB is considered seriously hazardous to health, and there are already devices used in research applications that can generate sound waves with amplitudes of up to 180 dB.

At present these devices only produce ultrasound, which is sound with a frequency above the upper limit of human hearing (about 20,000 Hz). This makes the devices safe to work with. Ultrasound is now being investigated as a replacement for the motor bearings in computer hard drives, where the hard disc that stores the data inside the computer is levitated on a cushion of rapidly vibrating air. Ultrasound is also used in some electronics factories, where a beam of high-intensity ultrasound can generate enough heat to melt solder and thus fix electrical components in place, and do so with extremely high precision.

Lower-intensity ultrasound is used for medical scans—especially of unborn babies, where sound waves offer a less harmful method of scanning compared to, say, X-rays. The technique works by beaming high-frequency sound waves into the region to be scanned and measuring the time it takes for the waves to bounce back from each point to build up a picture of the tissue within.

At the other end of the scale is infrasound—sound with a frequency below the lower limit of human hearing (which is about 20 Hz). Experiments have shown that infrasound can induce feelings of anxiety, fear and sadness—and for this reason it has been cited as the explanation for many reports of paranormal activity. Studies by the late British researcher Vic Tandy have found high levels of infrasound in sites reputed to be haunted—including the Edinburgh Vaults, a system of tunnels that are said to be haunted.

Infrasound has also been at the center of one of the great urban myths of acoustics research. That is, the existence of something known simply as the “brown note.” This is meant to be a frequency of sound, between 5 and 9 Hz, that corresponds to the resonant frequency of the human colon. So the story goes, if you subject someone to a loud enough blast of the brown note it
will force them to evacuate their bowels immediately. Attempts to prove this idea—including one high-profile investigation on the TV show
MythBusters
—have failed to turn up any credible evidence. And for this reason the brown note is generally regarded as an old wives' tale.

However, the physical phenomenon that the brown note myth is based on,—resonance—most certainly exists. If you strike a tuning fork on a surface and let it ring, the note you hear is the fork's so-called resonant frequency. Next put the tuning fork on a speaker and gradually increase the frequency of the sound played through the speaker. The amplitude of the fork's vibrations will gradually increase until—at the resonant frequency—they become especially strong, reaching a peak, before subsiding again as the frequency grows larger. Resonance is an especially important consideration in earthquake zones, where tremors at the resonant frequency of a building can cause major damage.

Resonance is a way by which the amplitude of a sound wave can change dramatically. But the frequency can alter too—when the source of the sound and the person observing it are moving relative to one another. This is called the Doppler effect, and it's the reason why the pitch of an emergency vehicle siren sounds
slightly higher when the vehicle is moving toward you and lower when it's moving away from you.

Imagine a sound source moving toward you. After each wave crest is emitted the source moves a short distance so that when the next crest is given off the distance between the two is shorter than it would have been were the source stationary. Because the frequency of the sound is given by the wave speed (which is unchanged no matter how fast the source is moving) divided by the wavelength, the decrease in wavelength translates into an increase in frequency—the pitch goes up. Similarly, when the person listening to the sound is moving toward the source, their motion increases the number of wave crests passing them every second, thus increasing the frequency of the sound. For the same reason, the sound's frequency decreases when they travel away from the source.

The Doppler effect, named after the 19th-century Austrian physicist Christian Doppler who discovered it, has a host of applications. These include mapping blood flow in ultrasound scans by the shift in frequency of an ultrasound wave bouncing off a moving mass of blood. And in submarine warfare, the shift in the frequency of the sonar pings that are sent out by a submarine and then reflected back by surface warships reveals the speed at which the warships are traveling.

It is possible for objects to travel faster than the speed of sound in a medium. In ordinary air, this is 343 m/s, or 1,235 km/h (767 mph). When anything, say an aircraft, travels faster than this, the Doppler effect is the least concern. Sound trying to travel away from the front of the aircraft literally cannot move fast enough to get away from it. As a result the sound waves pile up to form a shock wave—a discontinuous jump in pressure, which anyone nearby hears as a “sonic boom.”

Aircraft aren't the only things to create shock waves. Nuclear explosions generate supersonic blast waves that propagate outward from the detonation point. Thunder is the sonic boom created as a lightning bolt instantaneously heats the air around it to tens of thousands of degrees.

The sonic boom from a nuclear explosion can reach levels of well over 200 decibels—though if you're that close to an atomic detonation, a bit of noise will probably be the least of your worries. A space shuttle launch is fairly deafening too, coming in at as much as 170 dB. And even an accelerating dragster manages a fairly ear-popping 160 dB—easily enough to cause permanent hearing damage if you stand too close or don't wear proper protection.

It turns out that nature can generate some fairly cacophonous noises ofits own. The loudest animal on the planet is also the largest: the blue whale. Its mating call can be heard from hundreds of kilometers away, and reaches a peak level of nearly 190 dB. The loudest natural sound on record was the eruption of the Indonesian volcano Krakatoa in 1883. It was heard 4,800 km (3,000 miles) away in Alice Springs in central Australia. If you had been standing right next to it, the noise level would have been somewhere in the vicinity of 280 dB.

This still wasn't the loudest noise to have ever rocked the planet during the tenure of human beings. That honor goes to the eruption of the supervolcano under Lake Toba, in Sumatra, 74,000 years ago. Probably the largest volcanic eruption in the last 25 million years, it flung some 2,800 km
³
(670 cu miles) of material into the air and devastated 20,000 km
²
(8,000 sq miles) of land. Estimates suggest this mighty volcanic outburst may have clocked in at a staggering 320 dB—louder than pretty much anything else that's graced the planet since.

• The cosmological constant

• Hubble's law

• Weighing the Universe

• Dark matter

• Last entertainment

• The Big Rip

Nearly 14 billion years ago, our Universe was born in a superheated fireball known as the Big Bang. Space, time, matter and radiation all popped into existence, expanded and cooled to form the stars and galaxies we see adorning the heavens on a clear night. But how might this grand cosmic tableau come to an end? Scientists foresee several possible scenarios for the Universe's ultimate demise. None of them will be particularly pleasant for anyone around to see it.

The fate of the Universe rests entirely on the answer to a single question: how much matter does it contain? The Universe is in essence one great big gravitational
system. Of the four forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—only gravity is important for describing the large-scale motion of the galaxies and the fabric of space and time itself. Our best model of gravity is Albert Einstein's theory of general relativity. The theory ascribes the gravitational motions of planets, stars and galaxies to curvature of the space and time on which they rest. This curvature is fixed by the mass of these objects—plus all the other mass and energy in its various forms that the Universe contains.

Einstein himself was the first person to try to apply general relativity to the whole Universe. On balance, he probably wished he hadn't. This was in 1917, just two years after he had first formulated the theory. He immediately ran into a problem. The equations of relativity were suggesting to him that space cannot sit still. It should either be contracting, falling in on itself under its own gravity; or expanding, with galaxies flying apart from one another fast enough to beat their mutual inward pull.

The best astronomical observations of the time suggested that the Universe was neither expanding nor contracting, but was instead static. As a result, a baffled Einstein did what many other physicists have done before and altered his theory to fit the observations. He introduced a fudge factor, which he called the
“cosmological constant,” which effectively made gravity repulsive rather than attractive at long range. This repulsion would counteract the short-range attractive force, and so hold the Universe in a static configuration. Physically, the cosmological constant amounted to positing the existence of energy locked away in empty space. It seemed like a great idea for about 10 years. Then, in 1929, US astronomer Edwin Hubble and his assistant Milton Humason made a discovery that would change everything.

Hubble and Humason had noticed the work of another US astronomer Vesto Slipher working at the Lowell Observatory in Arizona. Slipher had carried out a study of the light reaching Earth from faraway galaxies, and had noticed something odd. When the light from a galaxy is broken up into its spectrum to reveal the brightness at each particular wavelength, it should show a pattern of bright and dark lines. This is caused as the atoms in the stars that a galaxy is made of absorb and emit light at particular wavelengths, determined by the energy levels of their electrons (see
How to be everywhere at once
). Slipher saw this characteristic pattern in the light from his galaxies, but it wasn't where it should be. The sequence of lines had been shifted to longer wavelengths, in other words toward the red end of the spectrum. The light from the galaxies was thus said to be “redshifted.”
We see the same redshifting effect in sound waves here on Earth, caused by the Doppler effect (see
How to make the loudest sound on Earth
). When a sound source is moving away from you, the sound's wavelength gets stretched out to become longer (making its pitch, or frequency, lower).

The implication was clear: these galaxies are all moving away from us. Hubble and Humason decided to investigate further. They studied Slipher's galaxies looking for a certain type of star known as a Cepheid. These are variable stars, the brightness of which changes over time and with a period that's directly linked to their average brightness. So measuring the periods of Cepheids in the galaxies told them how intrinsically bright these stars were. Then, by measuring the apparent brightness of the stars through a telescope, the astronomers were able to calculate by how much the light had dimmed with distance and so how far away each one of them was. Next they plotted the newfound distance to each galaxy against the degree of redshift in its light. When they did this a distinct pattern emerged: the redshift increased with distance. This is exactly what you would expect in an expanding Universe. A good analogy is the surface of a balloon. Blow up a balloon slightly and then draw dots on its surface with a marker pen. Now blow the balloon up fully. As it inflates all the dots move away from every other dot, and the rate at which any two dots recede
from each other is proportional to their separation. Applied to the Universe, this relationship between expansion speed and cosmic distance is known as Hubble's law.

Einstein was left with egg on his face following this discovery. It turned out that he had modified a perfectly good theory. If he'd played his cards right, he could have used general relativity to predict cosmic expansion ahead of it being observed, and so moved even higher up the echelons of physics greatness. But if Einstein thought he had problems, that was nothing compared to what this meant for the Universe. With the idea of a safe, static cosmos gone for good the question was: where might this cosmic expansion lead in the long term? To answer this, astrophysicists needed to determine whether the expansion would continue forever, or whether gravity might one day put the brakes on and bring space caving back in on itself. To do that they would need to measure how much gravity-generating mass and energy there is in our Universe.

It soon became clear that even this wasn't going to be a simple task. You might think it simply boils down to having a look through a telescope and totting up how
much bright matter you can see in the Universe. But astronomers soon realized that most of the matter in the Universe cannot be seen. In fact, what we see turns out to be just a tiny fraction of what's actually out there. Astronomers know this because mass makes its presence felt through the force of gravity. In the 1970s, astronomers noticed that spiral galaxies—galaxies like our own Milky Way that consist of a flattened, rotating disc of gas and dust—are rotating too fast to be held together by the gravity produced by their visible material alone. The laws of gravity predict that the rotation speed should gradually diminish to zero as you move outward from the galaxy's center. But astronomers found that once clear of the bulge at a galaxy's center, the rotation speed was more or less constant. The only way around this seemed to be that the galaxy was embedded within a halo of invisible material. Further evidence had already been seen in galaxy clusters, where the masses of the galaxies seemed insufficient to gravitationally bind them together into a group given the speed of each galaxy's own random motions. So it was that astronomers got their first whiff of what has since become known as “dark matter.”