Science in Space

You are about to step out of your own safe little world where time and space work in predictable and orderly ways. To understand the deepest reaches of space, or the tiniest corner of an atom you will have to forget everything you know about space and time. This is what theoretical physicists do everyday of the week. But they understand the Universe in another language – mathematics. Here, we have tried to show you with words what they understand with numbers. Don't be put off if you find it hard to imagine their concepts, some of them confess to having the same trouble themselves. We will show you how the giants of 20th century thought saw the Universe. And how, following on from their predecessors, physicists of the 21st century are forging on to find the final theory of the Universe – the one rule to rule them all. Ready? Jump!

At the beginning of the new millennium, physicists are perhaps closer than ever to defining a grand 'theory of everything'. It's known as the peculiarly labelled 'string theory'. The question of how the Universe works has perplexed some of the greatest minds in history; so far all have failed to find a comprehensive answer. The principal reason is that in order to do so, physicists must construct a framework that transcends unimaginable scales of size, time and dimension.

The Universe is a place of extremes. On the large scale, it is dominated by huge clusters of stars bound together by gravity – the galaxies. Our galaxy, the Milky Way, is just one of countless millions. Like stars, galaxies themselves bond to form clusters and super clusters which extend out far into the observable Universe. On the small scale is another realm entirely, composed of atoms and subatomic particles (electrons, protons and their smaller constituents quarks) glued together by strong and weak electromagnetic forces.

If you're wondering why bother with all of this, this is what Jim Gates, Professor of Physics at the University of Maryland, has to say:

'The kind of physics that my community engages in – trying to understand the most fundamental structure and issues for our Universe – is a birthright for all of us. For me there is a personal joy in participating in that adventure. It belongs to everybody, just like great art and great music belongs to everybody. Great science belongs to everybody.'

The 20th century was a fertile time for the understanding of these two very different realms, and it was this increasing understanding that initiated the search for a theory of everything. The first steps toward it were ushered in by a humble office clerk who worked in Bern, Switzerland.

Among the greatest achievements of Albert Einstein was his ability to fathom the workings of the Universe on a grand scale. In 1905, Einstein published his 'special theory of relativity' (or 'special relativity'), the essence of which was a new way of perceiving space and time. Einstein announced that space and time are not separate, as was previously believed, but are inextricably woven together – they are one and the same. He called this spacetime.

Put simply, special relativity binds together not three but four dimensions, three for space (forward-backward, left-right, up-down) and the fourth being time. We move through all of these dimensions everyday of our lives. Einstein proposed that whichever of these dimensions we are moving through, the sum total of the velocities of all of them must always be equal to the speed of light. In this sense, space and time are similar dimensions, and we can think of an object's speed through time in a similar manner as its speed through space.

As you move around here on Earth you do so through the three dimensions of space, but only at a fraction of the speed of light. But remember, your speed through all four dimensions must always equate to the speed of light. To compensate for your pathetically slow speed on Earth, you move through time at quite a jog, relatively speaking. In fact, almost all of your 'speed' is given over to your movement through time.

Of course, Einstein's theory provokes an apparently intriguing paradox. If you were travelling through space close to the speed of light, you would no longer be travelling so fast through the dimension of time because you are already almost fulfilling the golden rule of light-speed travel. Special relativity states that when travelling at velocities very close to the speed of light, the time dimension element of the sum equation becomes smaller. Hence the faster you travel through space, the slower time passes, as seen from a stationary observer (for whom time passes more quickly). When travelling through space at the speed of light, the passage through the time dimension is zero – time stops completely. Hence as photons (of which light is composed) travel at the speed of light, time does not exist for them – they are as young as they were on the day they were created during the Big Bang billions of years ago. Time waits for no man, unless you can travel very close to the speed of light.

The reason we find the implications of special relativity difficult to comprehend is because we live in an incredibly slow world, one that blocks out the true mechanics of spacetime. At the speeds we travel here on Earth, the effect of slowing time is so marginal that it's not worth bothering with.

Special relativity requires we look at the Universe in a way that we are not accustomed, but it reveals the true nature of space and time. In doing so it demolishes the Newtonian 'clockwork' Universe in which time and space were seen as separate entities.

Einstein's keen perception and ultimate understanding of relative motion and time led him to formulate his elegant special theory of relativity. Yet it threw up a further conflict, clashing again with the work of Isaac Newton – this time Newton's sacrosanct theory of gravity. Newton correctly theorised that a body exerts a gravitational pull on another body with a force determined by two properties – the mass of the bodies and their distance apart. Newton surmised that if you were to change one parameter for one body, the other body would instantly feel a change in the way it's tugged; the pull of gravity would instantly change. But special relativity states that nothing can travel faster than the speed of light (through the famous E=mc² equation, which shows an infinite amount of energy (E) is needed to accelerate a mass (m) to the speed of light (c).

Based on special relativity where nothing can travel faster than light, including the effect of gravity, it follows that if the Sun were to suddenly disappear, Earth would not be affected by the loss of the Sun's gravitational pull instantaneously, as Newton's understanding of gravity implied. On the contrary, the loss of the Sun would be felt a little after eight minutes – the time it takes light (and the 'transmission' of any force, such as gravity) to travel the 93 million miles between the two bodies. Such a blinding conflict led Einstein to his 'happiest thought' and the formulation of his general theory of relativity in 1915.

With general relativity, Einstein once again revolutionised the understanding of space and time. He showed through mathematics that space is warped by mass – the greater the mass, the greater the warping of space. Just as a bowling ball placed on a trampoline will 'dent' its surface, so the mass of our Sun (and any other object in space) dents the fabric of spacetime. Earth orbits the Sun as it is 'trapped' on the limb of the warped space, mass dictates how space is warped, the manifestation of this warping is the force we call gravity. Furthermore, since space and time are interwoven (the salient conclusion of special relativity), warping space warps time.

While perhaps a little perplexing at first, special and general relativity possess a degree of logic that, once grasped, is quite brilliant and relatively easy to follow. Quantum theory, on the other hand is more of a leap of faith. Quantum theory is a weird and wonderful theory that describes the workings of the Universe at the tiniest level.

As we start to shrink down to the atomic and subatomic scale we encounter a world that is very different from the one we are used to experiencing. The essence of quantum theory (also known as quantum mechanics) is that matter at subatomic scales takes on a Jeckyll and Hyde personality, having both wave and particle properties.

The idea stems from a series of discoveries made during the late 19th and early 20th century. In 1905, Einstein solved a riddle that had perplexed German scientist Max Planck. Planck deduced that atoms can absorb or emit energy, in the form of electromagnetic radiation (such as light or infrared 'heat' energy) but only in discrete packets. Planck surmised that such an observation was a consequence of the ability of atoms to emit energy only in separate chunks.

Then, Einstein proved that electromagnetic waves, such as light, are themselves composed of discrete packets of energy, or 'quanta'. Later, the quanta were named photons. Turn on a light bulb or electric fire and billions upon billions of photons stream out each second to illuminate or heat your room. Einstein had these ideas whilst he was trying to explain an observation called the photoelectric effect. Not only did he show that light comes in photons, but he also came up with the weirdest notion that light waves can actually act as if they were light particles.

The photoelectric effect happens when light bombards the surface of a metal, and electrons are ejected from the metal, only when light of a minimum frequency (or energy) is used. Above that frequency, increasing the intensity of light (or the number of photons) increases the number of electrons that are ejected. Hitherto, light had been thought of as light-waves that bend on hitting a solid surface. But it is easier to imagine that a particle rather than a wave could dislodge an electron from an atom by colliding with it. This is what Einstein suggested. Once the critical minimum energy or frequency of light is exceeded, the energy content of each photon is enough to cause one metal atom to lose one electron. Below the critical energy, no single photon has enough energy to dislodge an electron from an atom, but above it, more photons will dislodge more electrons. The theory explained the experimental observation to a tee. The governing idea of Einstein's photoelectric effect is that light sometimes has the properties of a wave and sometimes has the properties of a particle – a phenomenon known as wave-particle duality.

In 1923, a French prince named Louis de Broglie used Einstein's special theory of relativity to reason that since energy, which is propagated through wave motion (like the energy contained in ocean waves), and mass (matter) are interchangeable through Einstein's historic E=mc² equation, then matter too must display wave-particle duality. If light can be considered as sometimes having wave properties and other times having particle properties, he asked, why doesn't matter behave similarly?

De Broglie's assumptions were startlingly vindicated a few years later by scientists at the Bell Telephone Company. In a reversal of Einstein's observations of light 'particles', they demonstrated that electrons, miniscule particles of matter, also behaved like waves. The particles themselves are considered to be point particles – in effect having no spatial extent (remember this, as it's a very important point when dealing with string theory).

In essence, all matter has wave-like characteristics, not just electrons. Your computer, your computer screen and the chair you are sitting on have wave-like tendencies no matter how solid they may seem. But, if wavelengths are to be large enough to be measured, the mass must be infinitesimally small. So don't worry, you are unlikely to be suddenly swayed by the motion of waves in your chair. At the smallest subatomic level though, everything, all matter, has wave-like properties, including the particles that make up your chair. But herein is another weird twist in the quantum world, one that lies at the heart of quantum mechanics – the infamous 'uncertainty principle'.

Developed by Werner Heisenberg in the 1920s, the uncertainty principle is a result of matter having wave-particle duality. The premise is this. Think of a particle of matter, such as an electron. It is a tiny point of matter, and hence has a position in space – you can measure where it is. But the electron also has wave properties, and so is moving – you can measure where it's going. But the two measurements are in direct conflict, since if the electron is moving it doesn't have a stationary position; waves don't have a position in the same sense as particles, they only have a direction and momentum since all mass that moves carries a momentum force.

Heisenberg showed mathematically that you can't possibly know both the position and momentum of an electron at the same time. The uncertainty principle states that electrons, and all other point particles, can't be described as being in such a place at such a time with such a momentum. You can measure one, for example an electron's momentum, but not its position. In fact there is a trade-off between the two properties – the more precisely you know one property, the less precisely you know the other. The more you are able to ascertain the momentum of an electron (which way it is heading), the less you are able to know where it is, and vice versa. How weird is that?

You might be forgiven for thinking this is mad, after all it is perfectly possible to know the position of a moving car at a instant in time with a fairly high degree of accuracy. But in the tiny world of quantum mechanics this becomes more difficult as the numbers get smaller.

You may now be staring at your computer, thinking 'Well, I know exactly where my computer is.' But on the subatomic scale it is impossible to know instantaneously the velocity and position of any of the electrons that are part of the atoms in the material from which your computer is made – you could measure one quantity, the other you'd have to guess. Heisenberg's uncertainty principle is a truly bizarre and rather disturbing theory, but it lies at the heart of quantum mechanics. It also has some profound implications regarding general relativity.

Einstein's general theory of relativity (built upon his earlier formulation, special theory of relativity) is a theoretical framework that beautifully describes the Universe on the large scale – the interactions between stars, galaxies and clusters of galaxies, space and time.

Quantum theory provides a theoretical framework for understanding the Universe on atomic and subatomic scale. Both general relativity and quantum theory have been vindicated though years of thorough experimentation to levels of stunning accuracy. But there remains a problem – the two theories are incompatible.

In a nutshell, general relativity breaks down at the subatomic scale and uncertainty dominates. Not only is the position of particles unpredictable so is the whole fabric of spacetime. The perfect geometric shape of spacetime degrades into a chaotic unpredicted foaming mass at the quantum level. While the two theories explain the macro and micro Universe beautifully, it seems never the twain shall meet. This fact represents the major conflict of 20th century physics, the major stumbling block in the physicists' Holy Grail. But don't despair – physicists have been feverishly working to crack the nut. The result? String theory.