Physics is quantitative

Physics is more quantitative than most other sciences. That is, many of the observations experimental results in physics are numerical measurements. Most of the theories in physics use mathematics to express their principles. Most of the predictions from these theories are numerical. This is because of the areas which physics has addressed are more amenable to quantitative approaches than other areas. Physical definitions, models and theories can often be expressed using mathematical relations, as early as 1638, when Galileo published the law of falling bodies in his Two New Sciences.

A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by comparing the predictions of its theories with data procured from observations and experimentation, whereas mathematics is concerned with abstract logical patterns not limited by those observed in the real world (because the real world is limited in the number of dimensions and in many other ways it does not have to correspond to richer mathematical structures). The distinction, however, is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as mathematical physics.

Relation to mathematics and the other sciences
Physics relies on mathematics to provide the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories can be succinctly expressed using mathematical relations.

Whenever analytic solutions are not feasible, numerical analysis and simulations can be utilized. Thus, scientific computation is an integral part of physics, and the field of computational physics is an active area of research.

Beyond the known universe, the field of theoretical physics also deals with hypothetical issues, such as parallel universes, a multiverse, or whether the universe could have expanded as predominantly antimatter rather than matter.

In the Assayer (1622), Galileo noted that mathematics is the language in which Nature expresses laws, to be discovered by physicists. Physics is also intimately related to many other sciences, as well as applied fields like engineering and medicine. The principles of physics find applications throughout the other natural sciences as they depend on the interactions of the fundamental constituents of the natural world. Some of the phenomena studied in physics, such as the phenomenon of conservation of energy, are common to all material systems. These are often referred to as laws of physics. Others, such as superconductivity, stem from these laws, but are not laws themselves because they only appear in some systems. Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other disciplines (biology, chemistry, geology, material science, engineering, medicine etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of collections of matter (such as gases and liquids formed of atoms and molecules) and the processes known as chemical reactions that result in the change of chemical substances. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (called in this case quantum chemistry), thermodynamics, and electromagnetism.

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Applications and influence

Applied physics is a general term for physics which is intended for a particular use. An applied physics curriculum usually contains a few classes from the applied disciplines, like chemistry, computer science, or electrical engineering. It usually differs from engineering in that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem. The approach is similar to that of applied mathematics. Applied physicists can also be interested the use of physics for scientific research. For instance, people working on accelerator physics might seek to build better particle detectors for research in theoretical physics.

Physics is used heavily in engineering. Statics, a subfield of mechanics, is used in the building of bridges or other structures; the simple machines such as the lever and the ramp had to be discovered before they could be used; today, they can be taught to schoolchildren. The understanding and use of acoustics will result in better concert halls; similarly, the use of optics creates better optical devices. An understanding of physics makes for more realistic flight simulators, video games, and movies, as well as in forensic investigations (what do we know and when do we know it; what did the subject know and when did the subject know it).

Because of its historical relationship to the development of scientific method, physics reasoning can handle items which would ordinarily be mired in conundrums or uncertainty. For example, in the study of the origin of the Earth, one can reasonably model Earth's mass, temperature, and rate of rotation, over time. From these values, the chemical composition of Earth at differing epochs can be posited. Even if a precise linear timeline might be problematic, qualitative statements can then be made about the history of Earth, which are still founded in the laws of physics.

There are many fields of physics which have strong applied branches, as well as many related and overlapping fields from other disciplines that are closely related to applied physics.

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Molecules at the movies

Atoms and molecules are not the most photogenic of subjects. Once you’ve zoomed in on their miniscule size, you still have to find a way of snapping electrons hurtling around at breakneck speeds of up to 10 million km/h.

Physicists at Imperial College London have however made hydrogen and methane molecules into unsuspecting movie stars thanks to a revolutionary new measurement technique.

Using a process known as high harmonic generation (HHG), this technique can capture ‘films’ of chemical processes at timescales that almost defy the human imagination.

The principle bears similarities to strobe photography, where rapid motion is freeze-framed by leaving the camera’s shutter open in a darkened environment and then producing a bright flash of light. The shorter this flash, the faster the motion that can be recorded.

Freezing the movement of a speeding bullet, for example, requires a flash of light no longer than 1 microsecond (see image above).

In HHG, pulses of laser are fired into a jet of gas and the high intensity wave of light sweeps an electron off each atom. But the electron quickly smashes back into the resulting ion. ‘In this collision, the electron’s kinetic energy is released in an extremely short but very bright burst of x-ray radiation,’ says Dr John Tisch, who worked with Imperial College colleagues Dr Sarah Baker and Professor Jon Marangos to develop this new technique.

Blink and you’ll miss it

This x-ray ‘flash’ can be squeezed down to a staggering 100 attoseconds long - pretty impressive when you consider that in the time it takes for you to blink, a whopping 300 million billion attoseconds have already slipped by.

By analysing certain characteristics of the x-ray beam, researchers build up a ‘movie’ of the molecule, for example as it changes its structure after being rapidly ionised. The time interval between the individual ‘frames’ in these ‘movies’ is also 100 attoseconds - resulting in an effective framing-rate of 10 million billion frames per second, compared to around 2000 frames per second for the super-slow motion used in TV sports coverage.

This new measurement system and related techniques being developed by a number of groups around the world could revolutionise our fundamental understanding across science.

‘The kind of dynamics that we are now beginning to be able to capture control the function of many molecules', comments Dr Tisch. And putting more complex molecules under the spotlight in years to come could yield ‘footage’ of processes that have never been seen before.

So it might not be long before scientists attend the world premiere screening of photosynthesis or entangled electron states. Such understanding could help to create quantum computers, faster electronics, designer materials or artificial photosynthesis.

Imperial College’s physicists may be the current world record holders for the fastest ever observation of molecular dynamics, but, as Dr Tisch puts it, ‘the relentless drive for shorter and shorter measurement techniques’ is still on. With molecules swiping all the leading roles, it’s clear that slow motion takes are truly no longer the preserve of film stars and football players.

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Boobs, babes and blood

For the average physicist, you might expect typical study subjects to include atoms or distant galaxies. Visit the medical physics department at University College London however, and you’d be more likely to stumble upon premature babies or breast cancer patients in the lab.

Dr Adam Gibson and his team are developing a new medical imaging technique, known as optical tomography, which uses near infrared light to take a peek at what blood is getting up to inside the human body.

The instrument, called MONSTIR, shines light through the body and counts individual photons as they emerge at the other side. ‘Blood absorbs light very strongly,’ says Gibson. ‘So the amount of light that gets through is an indicator of the amount of blood.’

By using two different wavelengths of light and comparing how the two are absorbed, the colour of the blood - a telltale sign of its oxygen levels - can also be deduced. ‘Bright red, tomato ketchup blood carries lots of oxygen, and dull, brown blood has less,’ explains Gibson.

In itself, this is already an exciting development for medical imaging. Most imaging techniques take a snapshot of the body which then needs to be interpreted, but MONSTIR’s images provide not just pretty pictures but hard numbers.

A rush of blood to the head

Oxygenated blood fuels many processes in the body and following its path can deliver precious insight to doctors. The team at UCL has already used MONSTIR to explore what’s on the mind of premature babies. ‘The part of the brain that’s active uses more blood,’ explains Gibson. ‘So you can use this to map out brain activity.’

As well as investigating babies’ perception of pain and the way they learn language, the team have used the technique to detect bleeding inside the brain.

Breast cancer patients have also come under MONSTIR’s scrutiny. ‘Cancer is associated with an increase in blood volume to the tumour, and sometimes also changes in blood oxygenation,‘ says Gibson.

Whilst the relatively poor resolution of the images produced means the technique is not best suited to screening for cancer, it has advantages over alternative imaging techniques for other applications in the fight against cancer. In particular, MONSTIR could come in handy for monitoring the effectiveness of a particular drug which aims to starve off cancer by disrupting blood flow to the tumour.

‘With x-rays, you get ionising radiation, and MRI is big, expensive and clumsy, so it’s not convenient to use it over and over again, whereas we can use this as often as we want to,’ comments Gibson.

Finding parents willing to lend their newborn babies for a day can sometimes be harder than spotting far flung planets, but Gibson wouldn’t have it any other way. ‘One of the nice things about medical physics is the patient contact. It’s nice for us to be reminded of what we’re doing our research for.’

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Enigma of the singing dunes

When the thirteenth century explorer Marco Polo encountered the weird and wonderful noises made by desert sand dunes, he attributed them to evil spirits. But 700 years later, scientists still don’t completely understand the causes of this eerie phenomenon.

Sand dunes can be heard ‘singing’ in more than 30 locations worldwide, and in each place the sounds have their own characteristic frequency, or note. In reality the sounds produced are less like singing and more like a low-frequency drone (low frequency corresponds to low notes; bass as opposed to treble). The sounds are emitted when sand cascades down the face of a dune in an avalanche, the cause of which can be the wind, people walking on the top of the dune or even sliding down it.

In 2001 a team of French physicists, including Stéphane Douady and Bruno Andreotti, went to Morocco to study the shape and motion of sand dunes. They became fascinated by the singing of the dunes and began to investigate it in addition to their other research. They found that avalanches they triggered manually produced the same sound as those that occurred naturally, which suggests that the wind doesn’t play a part. They also concluded that the sound is not produced by the dune resonating, as happens in the case of a musical instrument for example, because the frequency of the sound produced is the same for different sizes of dune. Thus the team focused their investigation on the motion of the sand grains, rather than on the properties of the entire dune.

Douady and Andreotti both came up with the idea that the sounds must be produced by sand grains becoming synchronised – moving in definite patterns as they move down the surface of the dune. Their hypotheses differed in that Douady believed the sounds, which after all are just vibrations of air molecules, were produced by air being squeezed out from between the synchronised grains. Andreotti proposed that the sound was due to the surface of the avalanche vibrating the air around it like a large hi-fi speaker. The pair began to follow very different lines of inquiry and ended up in complete disagreement. This, combined with a subsequent quarrel over how best to publish their findings, led to the two researchers falling out. So much so, in fact, that they now avoid each other, despite working in the same small field of physics. Their scientific adventures and disagreements were the subject of an award-winning article in the November 2006 edition of Physics World, the Institute of Physics members’ magazine.

There is still no consensus as to the exact explanation of the singing dunes. In fact, scientific papers published more recently suggest that the large-scale structure of the dune does play a part after all. The story of the differences between Douady and Andreotti, and the twists and turns in the wider investigation into the exact cause of the dunes’ song, highlight the fact that the progress of science can be affected by the vagaries of human nature. For example, why does a scientist choose a certain path of investigation? That’s something that your average physics textbook won’t ever answer, even if we do finally get to the bottom of why dunes sing.

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Latest Features

Keep your ears peeled

What do bats, submarines and doctors have in common? They all rely on sound to ‘see’ better.

Most of the time, our eyes do a pretty good job of telling us about our surroundings. Seeing in the dark or in murky water is much trickier, but bats and dolphins have found a way around this problem, relying on sound waves to find their bearings and even to catch prey. The secret behind this ability lies in an everyday phenomenon - the echo.

All about echoes

You’re in a cave. Shout. A moment later you hear your voice’s ghostly twin - an echo. This happens because the sound waves produced by your vocal cords travel through the air, bounce off the cave walls and are picked up again by your ears.

Sound travels relatively slowly, about 343 metres per second in air, which is why there’s a slight delay before you hear the echo. But this delay is what allows bats to make use of echos.

Bats have vocal cords and ears that are fine-tuned for producing and hearing very high frequencies of sound, known as ultrasound. They emit high-pitched ‘chirps’, which are inaudible to humans, and then listen out for the resulting echos. Keeping track of how long it takes for each cry to be reflected back enables them to work out how far away an object is.

Other characteristics of the echo can also give bats an idea of the object’s size and the direction it’s moving in. By piecing together this information, they can build up an accurate image of their surroundings, and spot tasty insects in the dark.

Ultrasound waves enable bats to get a clear picture of their environment as they don’t spread out around obstacles as much as lower frequency sounds like the human voice. Imagine trying to find your way around a dark cave just by listening to the echos of your shouts.

Sound in medicine

It wasn’t long before scientists caught on to this nifty idea and adapted it to enable submarines to locate their targets and to give doctors a peek into their patients’ bodies.

Ultrasound imaging has been used in medicine for over 50 years. Specialised equipment directs pulses of high frequency sound waves into the body and, as these waves meet different layers of tissue, they are reflected back and analysed.

Millions of pulses and echoes are produced and received each second, providing information which is then processed to create an image on screen. Ultrasound scans are commonly used to check on unborn babies as well as muscles and internal organs including the heart, liver and kidneys, without harming the patient.

The applications of ultrasound don’t end there. Adapting the frequency and intensity of the waves makes them suitable for a multitude of uses, including cleaning teeth, locating oil reservoirs, destroying cancerous tumours and killing bacteria in water.

With all that high pitched racket going on, we should probably be grateful that we can’t hear ultrasound.

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Setting hearts aflutter

Rappers and butterflies agree on at least one point: when you want to deliver a show-stopping performance, the more bling the better. While rappers have taken a shine to the glitz and glamour of diamonds, some butterflies, birds and beetles have evolved iridescent, luminous colours of their own.

Most of the colours we see in nature are produced by pigments, but a completely different mechanism underlies the shimmering hues of iridescent butterflies. Robert Hooke and Isaac Newton were the first to hypothesise that these creatures possessed miniscule physical structures which manipulated light. Fast forward a few hundred years and researchers are still busy unravelling the mysteries of what is known as structural colour.

Standing out from the crowd

A typical iridescent butterfly’s wings are carpeted in hundreds of thousands of tiny scales. ‘These scales are anything from 1 to 3 microns thick and laid out like roof tiles,’ explains Dr Peter Vukusic, a physicist who studies iridescence in nature at the University of Exeter. Each of these scales is in turn composed of several layers of ultra thin film.

‘When light hits the top surface of the film, some of the wave is reflected, and some is transmitted downwards,’ says Vukusic. The transmitted wave then strikes the lower surface of the film and bounces back up, rejoining the light that was reflected from the top surface (see diagram).

This means that the transmitted wave travels further than the reflected wave and, depending on the thickness of the film, the two waves can become out of phase – the peaks and troughs of the waves no longer line up exactly. Any phase difference causes interference between the two waves.

Vukusic compares the process to ocean waves coming into a harbour. Upon meeting, two coinciding crests can form a ‘super crest’, or a crest and a trough can cancel each other out. Similarly, the butterfly’s scales will amplify certain wavelengths of the reflected light, whilst dampening others, affecting the colour that we ultimately see.

More than just producing unusual shades, structural colours bring added intensity. ‘Pigmentary colour is often not very bright, as incoming light is not reflected in any particular direction’, says Vukusic. Structural colour, on the other hand, directs light, grabbing viewers’ attention with flashes of brilliant colour. Depending on the species, this can serve to attract the opposite sex, intimidate a rival, or ward off predators.

The Butterfly Effect

It may seem that science is finally catching up with nature’s nifty designs, but there is still plenty left to learn. ‘It’s an exciting field to be working in’, says Vukusic. ‘Every single day we are looking at species that have never been investigated before.’

An increased understanding of these systems has already inspired a number of applications, from antifraud devices on banknotes to shimmering eyeshadows. It can only be a matter of time before rappers catch on as well.

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Recreating the Big Bang

Or almost. Switch on for the Large Hadron Collider at CERN is due to happen on 10 September, 2008, marking the beginning of a physics experiment which intends to recreate conditions last seen a trillionth of a second after the Big Bang.

The experiment involves smashing particles together in a 27 km ring, built 100m underground on the French-Swiss border. Racing at 99.99 per cent of the speed of light, particles will collide at the heart of massive detectors, allowing scientists a fleeting glimpse of tiny subatomic particles which may answer fundamental questions about the building blocks of nature.

As the universe was forming, hadrons bound together to make the larger particles that scientists have been studying since the Greek Democritus coined the term ‘atom’.

We have a relatively clear understanding of some kinds of hadrons like protons and neutrons but many of our universe’s tiniest constituents have yet to reveal their secrets.

One particle which will have theorists sitting on the edge of their seats is the Higgs Boson – if its existence is confirmed then physicists can truly claim to understand the origins of mass: in other words, why some particles are ‘heavier’ than others.

ATLAS is one of the four particle detectors being used to capture images of the fleeting, tiny but very high-energy collisions. ATLAS has been designed to take ‘pictures’ of 600 million proton collisions every second and will be the machine, if physicists are currently correct, that confirms the existence of the Higgs Boson.

Surprises in store

However, other less predictable phenomena are also causing pulses to race as we get closer to switch-on and the day the data from the detectors starts to pour in.

At present, we understand three dimensions: up and down, back and forth, and left and right. Looking into the smallest nooks and crannies of space could reveal extra dimensions, almost unimaginable, which would force us to completely rewrite today’s physics textbooks.

Physicists also hope that the LHC will elucidate some of the mysteries surrounding dark matter. This enigmatic substance makes up the majority of our universe’s mass yet we know very little about it. Physicists have deduced from gravitational effects on visible matter that it’s out there, but only by recreating the conditions present at its birth do we stand a chance of really knowing what it is.

The most exciting findings however are those that cannot yet even be predicted. As Professor Antonio Ereditato, Director of the Laboratory for High Energy Physics in Bern, says, ’This is like opening a window on an unknown view. You expect to see mountains but you may see a sea shore.’
As the LHC steers particles onto a collison course with the equivalent energy of a 400 tonne train travelling at 150 km per hour, the detectors will spew out huge amounts of data. It will then be up to physicists to diligently pore over these results until a mountain, a sea shore or just a very, very Big Bang comes into view.

From : www.physics.org

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