1957 Nov 03: Russians launch dog into space

The Soviet Union has launched the first ever living creature into the cosmos. The dog, described as a female Russian breed, was projected into space this morning from Baikonur Cosmodrome aboard the artificial space satellite Sputnik II. Sputnik I, launched on 4 October, is still circling the globe. The dog has been fitted with monitors to check its heartbeat and other vital signs and was reported to be calm during the first hours of the flight. Russian scientists are particularly interested in the effects of solar radiation and weightlessness on living organisms. Fury of animal lovers Moscow Radio reported the second satellite was launched to commemorate the 40th anniversary of the October Revolution and gave details about the spacecraft's contents and orbit. Sputnik II weighs half a ton (508kg) and carries instruments for studying solar and cosmic rays, temperature and pressure, two radio transmitters and a hermetically-sealed container with "an experimental animal" inside, as well as oxygen and food supplies. It is travelling more than 900 miles, (nearly 1,500 km) above the Earth - higher than Sputnik I - and is orbiting at about five miles (8km) a second. It will take one hour and 42 minutes to circle the Earth. The satellite is transmitting telegraphic signals that are being picked up from receiving stations around the globe. Animal welfare organisations expressed outrage at news that the Russians have sent a dog into outer space. The National Canine Defence League is calling on all dog lovers to observe a minute's silence every day the dog is in space. The RSPCA said it received calls of protest even before the Moscow Radio announcement of the launch had ended. It has advised those who wish to protest to do so at the Russian Embassy in London. 'Dog was trained for mission' It is believed the Russians are planning to catapult the dog back to Earth although there has been no official announcement confirming this. One British scientist told newspaper reporters the dog had probably been trained for the journey but was unlikely to survive. "A terrified dog would be useless scientifically," said Dr William Lane-Petter, Director of the Laboratory Animals Bureau of the Medical Research Council. "It would not give them the information they want. This dog will have been trained long for the task and subjected to similar simulated conditions, and this flight is just another experience of the same sort."

In Context

The following day, after several inquiries from Western journalists, Russian officials confirmed the dog's name as Laika. The Soviet authorities said Laika died painlessly after a week in orbit but in 2002 new evidence revealed the dog died from over-heating and panic just a few hours after take-off. Laika's "coffin" burned up in the Earth's atmosphere over Barbados on 14 April 1958, five months after launch. Three years later, the Russians achieved another space first by sending Yuri Gagarin into orbit on 12 April 1961. The Sputnik II flight made Laika one of the world's most famous animals and allowed Russian scientists to learn much about the prospects for human space travel.

1957 Oct 04: Sputnik satellite blasts into space

A Russian satellite has been launched into space - the first man-made object ever to leave the Earth's atmosphere.
The Russian news agency, Tass, said the satellite Sputnik was now 560 miles (900 kilometres) above the Earth and circling it every hour-and-a-half.

Scientists predict the metal sphere will eventually burn up in the atmosphere but they hope it will send important data back to Earth before doing so.

The Soviet Union and the USA have both committed to launching satellites for research as part of the International Geophysical Year (IGY).

Delegations from both countries' IGY committees were at a reception at the Russian embassy in Washington when news of Sputnik's launch came through.

The chairman of the American IGY committee, Dr Joseph Kaplan, congratulated the Russians on a "remarkable achievement".

The leader of the Russian delegation, Dr A A Blagonravov, who is believed to have been closely involved with the preparations for the launch, described Sputnik as "the simplest kind of baby moon".

He attributed its weight - 180lb (83.5kg) - largely to heavy batteries.


'Nothing to fear'

The satellite's weight has led some American experts to speculate that the rocket which launched it might also be capable of carrying a nuclear weapon thousands of miles.

The fact that Sputnik is expected to fly over the US seven times a day has also caused unease.

There have already been calls for an immediate review of US defences, given the implications of the technological leap ahead by a political enemy.

But Dr Blagonravov said no-one had anything to fear from the Soviet satellite programme.

"It will keep everyone too busy watching the instruments to think about anything else," he said.

President Eisenhower has been informed of the Russian success.

But he said the news would not lead the US to accelerate its own satellite programme.

In Context

Sputnik transmitted information via radio signals to Soviet scientists for three weeks. The signals fascinated both radio enthusiasts and Western scientists. Sputnik II was launched in November 1957 with a passenger aboard - a dog called Laika. The flight allowed Russian scientists to learn much about the prospects for human space travel. The Soviet authorities said Laika died painlessly after a week in orbit but in 2002 new evidence revealed the dog died from over-heating and panic just a few hours after take-off. In December 1957 the US programme suffered a setback when a rocket carrying a test satellite into space exploded. Finally, in February 1958, the US successfully launched its first satellite, 'Explorer'. But the Americans' achievements were eclipsed in 1961 when the Soviet Union put the first human, Yuri Gagarin, into space.

Bibliography and References

References


1. Chanda, Rajat. Quantum Mystery. 1/Ed. National Book Trust, India. 1997. ISBN 81-237-2180-3

2. Halliday, Resnick, et al. Fundamentals of Physics. 5/Ed. John Wiley & Sons, Inc. 1997. ISBN 9971-51-2289

3. Uses articles from The Grolier International Encyclopedia. Grolier Inc., Connecticut, MD USA. 1993. ISBN 0-7172-9646-6

4. Uses articles from The Encyclopaedia Britannica. Encyclopaedia Britannica, Inc. 2001. ISBN 0-8522-9787-4



Suggested Reading


The authors suggest the following books in case you are interested in further exploring this topic. They provide information on the quantum world for the non-physicist.

1. Fritjof, Capra. The Tao of Physics

2. Gell-Mann, Murray. The Quark and the Jaguar: Adventures in the Simple and the Complex

3. Greene, Brian. The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory

4. Gribbin, John. In Search of Schrödinger's Cat: Quantum Physics and Reality

5. Gribbin, John. The Matter Myth: Dramatic Discoveries That Challenge Our Understanding of Physical Reality

6. Hawking, Stephen. A Brief History of Time

7. Kaku, Michio and Thompson, Jennifer Trainer. Beyond Einstein: The Cosmic Quest for the Theory of the Universe

8. Thorne, Kip S. Black Holes and Time Warps: Einstein's Outrageous Legacy



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10. Quantum Physics Today

Applications of Quantum Physics


Inspite of all the fights and debates, quantum physics has been successful not only in explaining many physical phenomena, but also in predicting newer ones. For example, it correctly predicted that hydrogen could exist in two types, depending on the relative orientation of the angular momentum of the nucleus. The physicist Paul Dirac was also able to employ it in 1927 to make a novel prediction of the existence of particles similar in mass and spin to electrons, but with opposite electric charge. The particles, which have come to be known as positrons, were discovered subsequently by Carl Anderson in 1932. They were the first example of antiparticles, many of which would be discovered in the coming decades and are still being discovered.

Research that employs quantum physics remains at the centre of contemporary physics. One aspect of this research involves the search for approximate methods that can be applied with the basic principles of quantum physics in studies of situations that are so complex that they cannot be dealt with exactly. Much of the research in condensed-matter physics is of this nature. An important discovery in this area is that in some situations the discreteness of physical quantities that usually occurs on the subatomic level can also occur on the macroscopic level. The quantised Hall effect, a property of electrical resistance of certain substances under the influence of electrical and magnetic forces, is a recently discovered example of this.

Today, quantum physics plays an increasingly important role in our life. We have already named some of the applications in their context in the previous chapters. Lasers, for instance, are a direct application of Bohr's theory. If you are reading an electronic version of this page on a computer, then the microprocessor on which it runs is ultimately based on Bohr's theory. Much of the microelectronic devices, indeed the electronic revolution itself, is indebted to quantum physics. Scanning tunnelling microscope (STM) is another very useful device based on a concept known as barrier tunnelling, which is derived from quantum theoretical principles.


Quantum Theory, Relativity and the Standard Model


Quantum theory, nevertheless, has not been successful everywhere. An important area of research involves the attempt to include gravity among the phenomena that can be described by quantum physics.

It has been learnt that there are three fundamental forces (also known as interactions) that govern the world of the very small: electromagnetic force, strong nuclear force, and weak nuclear force. The zoo of subatomic particles, along with the understanding of these three fundamental forces, combined with the laws of the quantum world today make up what is commonly called the Standard Model of Particle Physics. For a very specific reason it is clear that the Standard Model is not a complete theory.

In order for the Standard Model to be a complete theory, it would have to be able to account for all objects, events, and forces which come under its purview. There is one force which is not incorporated into the Standard Model: Gravitation. The gravitational force is so weak at subatomic levels that it can safely be ignored in most calculations. Nevertheless, there are circumstances under which the gravitational force is strong enough on very small scales that it cannot be ignored. Two of these cases that have been frequently noted are the singularity at the centre of a black hole, and on the very small scale but high density conditions of the Big Bang at the beginning of the Universe.

Gravitation is very well described by Albert Einstein's General Theory of Relativity; it works so well, in fact, that even though it has been tested under very many conditions in very many experiments, virtually no exceptions have been found to the theory. On the other hand, relativity is a 'classical' theory, in the sense that it deals with smooth continuities and so has not been 'quantised.' Every attempt so far to bring quantum theory and relativity into agreement has failed for various mathematical reasons.

One possible exception can be made to this rule. There is a branch of particle physics called Superstring Theory, which describes subatomic particles not as dimensionless points but as one-dimensional strings (of various lengths with various properties, some of the open like a line, others closed in a loop). Superstring theory is not only very elegant in a mathematical sense; it also includes a mathematical equivalent of Einstein's General Theory of Relativity. The only problem is that Superstring Theory cannot be tested with any existing experimental equipment -- in fact, nobody can even imagine what sort of equipment can be used to test it.

For the present, the Standard Model in general and quantum physics must remain incomplete. Still, there is much work to be done within the framework that physicists already have, and indeed one day someone working on the edge of physics (as Planck, Einstein and others were doing a century ago) may make a breakthrough discovery that will lead to yet another new age of our understanding of the physical world. Meanwhile, there is no doubt that quantum physics is the most successful theory of physical phenomena yet invented by the human mind.



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9.Alternative Interpretations

In the last chapter we told you that Bohr's explanation of quantum physics faced criticism from several scientists; we also discussed two paradoxes arising out of this interpretation. In this chapter we shall look at two important alternative interpretations of the quantum theory.


Hidden Variable Theory

David Bohm wrote a text book on quantum physics as a young scientist at Princeton University. Although he focussed on the Copenhagen interpretation in his book, he was not satisfied with the inherent probabilistic and subjective nature of this interpretation.

Because an individual radioactive nucleus will eventually decay at a specific time, Einstein and others believed that a complete physical theory should allow this time to be predicted exactly, rather than just statistically. Bohm became a leading proponent of the 'hidden variable' theory, which restores determinancy at the quantum level. Whereas in Bohr's interpretation quantum events are predicted probabilistically, in hidden variable theories there exist 'hidden' properties, that is properties which we cannot measure, which determine events about quantum physics.

In Bohm's interpretation, the Schrödinger wave function is treated almost like a classical force field, such as a magnetic force field or gravitation. A term called 'quantum potential' is also introduced, just as we know about electric and magnetic potential. The wave function guides the particles along their paths, just as a magnetic field causes magnetic substances to move in a particular fashion.

The mathematician John von Neumann proved long ago that no hidden variable theory can agree exactly with the predictions of quantum physics. Not all the predictions have been examined, however, so there exists some possibility a hidden variable theory could be formulated that agrees with all observations that have been made. None has yet been produced that physicists find satisfactory.


The Many-Worlds Interpretation


As we have seen earlier, the central problem of the measurement process in quantum physics is to understand how a measurement selects one of the many options the system has at that point.

Based on a work of H. Everett in 1957, J. A. Wheeler and D. DeWitt suggested that all available choices are indeed realised at measurement -- the universe splits into copies of equally real universes, each containing one of the possible choices! Additionally, these almost identical parallel universes coexisting in time and space cannot communicate with each other. This proliferation of copies of the entire universe is repeated each time a measurement is made!

At its face value this incredible suggestion is untestable.

The point of all this is to avoid the 'reduction' or 'collapse' hypothesis of the standard interpretation of quantum physics. Another possible virtue is to address the 'fine-tuning problem' which tries to understand why many of the parameters in our universe are adjusted in their values to generate the observed properties of matter. A many-worlds interpretation hypothesis would be that all possible values of these paramters actually occur in parallel universes; we happen to reside on the one with finely-tuned parameters. But the other universes with different parameters will have very different properties and evolution processes and cannot be the almost identical copies needed to avoid the reduction hypothesis!


Quantum physics is now over 70 years old and has been very successful in providing explanations for physical phenomena, with the prevailing Copenhagen interpretation. Much of the criticism it has faced derives from the radical change from earlier theories that quantum physics represents. Some of it involves problems that arise within quantum physics itself. The question of the correct interpretation of the mathematical formalism has remained something of a problem up to the present. For now, we shall leave the matter there, and take up the current status of this subject in the next chapter.



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8. Quantum Paradoxes

The Copenhagen interpretation of quantum physics, introduced in the previous chapter, met with considerable resistance. Many physicists, including Einstein and Schrödinger, who accepted the mathematical formulation of quantum physics, were uncomfortable with this interpretation, and criticised it. The question of the correct interpretation of the mathematical formalism has remained something of a problem even today.

Much of the criticism to the Copenhagen interpretation came in the form of 'thought experiments', which could only be conducted in theory. These experiments were designed to demonstrate 'flaws' in the probabilistic interpretation, and formed the themes of intense debates. In this chapter, we shall study two such important paradoxes arising from the Copenhagen interpretation.


Schrödinger's Cat Paradox

In 1935, Schrödinger wrote a criticism of the standard view of quantum mechanics and illustrated with the example of a cat. This has since been known as the Schrödinger's cat paradox. In order to understand the concept easily, we shall explain the setup stepwise.

1. Imagine a perfectly sealed box containing:
* a cat, and
* a device with a deadly poison (say potassium cyanide)

2. A quantum event, namely the disintegration of a radioactive atom [1], triggers the poison device.

3. There is a 50% chance that the radioactive substance decays in one hour.


Fig: The Schrödinger Thought Experiment


The question is: What is the state of the cat inside the box?

This whole system with the cat in the box which could be either alive or dead and the radioisotope and the poison can be described by a wave-function (in which the dead/alive states of the cat are mathematically superposed). According to Bohr, the cat is neither dead nor alive, but in an indeterminate state which is neither. The act of opening the box and looking in actually causes the wave-function to collapse into one determinate state -- dead cat or live cat. It is actually the act of observation which determines the poor cat's fate. But in reality the cat can be either dead or alive; there is no indeterminate state as such.


Einstein, Podolsky and Rosen Paradox (EPR Paradox)

Einstein attempted to demonstrate incompleteness of the standard quantum mechanical description of physical reality in a famous paper written with B. Podolsky and N. Rosen, where a thought experiment is described with persuasive reasoning.


Fig: The EPR Thought Experiment

1. Let us consider a particle at rest. Let us say this particle is inherently unstable, and disintegrates into two particles. (In reality, there exist particles of this kind. An example is pi-zero, which decays into two photons and has a lifetime of a tiny fraction of a second.)

2. The fundamental laws of conservation, which are valid for all systems, guarantee that momentum and angular momentum [2] must be conserved. That is, the sum total of these quantities for the two particles remains the same before and after the disintegration.

3. Hence we can say that the momentum and angular momentum of the two resulting particles must be equal and opposite, since before disintegration they were equal to zero.

4. Therefore, measurement of the momentum of one particle can be used to deduce the value of the momentum of the partner. Alternatively one can deduce the position of the target particle with certainty and unlimited accuracy by making a position measurement on its partner.

5. This trick would bypass the uncertainty principle and show that the quantum theory is incomplete.

The statement that measurements done on one part of the separated system should not affect the other part of the system has been known as the reality condition.


The EPR paper was a powerful challenge to the Copenhagen group. Bohr refuted EPR by reiterating his philosophy: what is important is the whole set of conditions under which the measurement is made. He said that in the EPR scenario, the two particles form an irreducible quantum system with one wave function which incorporates both particles. Although no direct signal can travel between them, still one cannot ignore the influence of measurements on one or the other.

Einstein could never agree with this idea, calling it 'ghostly action at a distance'. But a physicist named John Bell proved in 1964 that Einstein's reality condition necessarily implies a relation among the results of a series of measurements. Numerous experiments have since been carried out to test Einstein's reality condition as well as Bell's theorem, and the results have proved that Einstein was wrong, and quantum physics was right.

Nevertheless, Bohr's Copenhagen interpretation was (and still is) not the only explanation for quantum phenomena, and even today scientists do not seem to agree on any one interpretation. In the next chapter, we shall look at some of the alternative theories proposed for quantum mechanics.

Footnotes

[1.] Radioactivity is a natural phenomenon in which an atom disintegrates by emitting certain particles. Since the resulting atom has a different configuration, it exhibits the properties of a different element. For example, uranium disintegrates into thorium by emitting what is called an alpha-particle.

[2.] Momentum is defined as the product of the mass and velocity of a body. It is a measure of the energy of a moving body, and is zero for a body at rest. Angular momentum is a similar measure for rotating objects.



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7.Wave Mechanics and Wave Particle Duality

Schrödinger's Wave Equation

We introduced you to the wave-like nature of matter particles, or matter waves, in Chapter 5. Erwin Schrödinger, an exceptionally gifted Austrian physicist, explored the consequences of these matter waves and proposed a wave equation for them, analogous to the known equations for other wave motions in nature. This wave equation describes how a wave associated with an electron or other subatomic particle varies in space and time as the particle moves under various forces.


Erwin Schrödinger


Schrödinger's theory of the quantum world is called wave mechanics. He worked out the exact solutions of the wave equation for the hydrogen atom, and the results perfectly agreed with the known energy levels of these atoms, seemingly without any of the complications and metaphysical speculations associated with the uncertainty principle. Moreover, the equation could also be applied to more complicated atoms, and even to particles not bound in atoms at all. It was soon found that in every case, Schrödinger's equation gave a correct description of a particle's behaviour, provided it was not moving at a speed near that of light. [1]

In spite of this success, the very meaning of the waves remained unclear. Schrödinger believed that the intensity of the wave at a point in space represented the 'amount' of the electron that was present at that point. In other words, the electron was spread out, rather than concentrated at a point. However, it was soon found that this interpretation was untenable, because observations revealed that particles never spread out. For example, it follows from the wave equation that when a wave, representing an electron, strikes a target, it spreads out in all directions. Experimentally, on the other hand, the electron scatters in some specific direction but never breaks up.

Max Born's Interpretation of the Wave Equation



Max Born


Max Born, another German physicist and a good friend of Einstein, interpreted this result as follows: the wave associated with the electron is not a tangible 'matter wave', but one that determines the probability of scattering of the electron in different directions. Where the associated wave has a large amplitude, the probability of finding an electron there is high. In any case, one would find the whole particle, and not parts of it.

This indicated that the simple interpretation of the wave equation as a description of the physical matter waves in ordinary space, as was originally assumed by Schrödinger himself, is incorrect. Born noted the following conclusions:

1.Matter waves represent probability amplitudes associated with the occurrence of an event.

2.If there are multiple alternatives for the event to happen, the total amplitude is the sum of the alternative amplitudes.

3.Finally, the absolute square of the overall amplitude, the intensity of the wave, must be interpreted as the probability that the event will happen.

Thus Max Born reduced Schrödinger's equation completely into probabilistic terms, although Schrödinger himself was reluctant to accept this interpretation, and retained this reservation for the rest of his life.

This difference between the world of the very small and the everyday world we experience cannot be understated. When you are walking across a street and an automobile is approaching you, it is at a definite place at any given time, and it is possible to describe with a great deal of precision where exactly the automobile is, and where it will be at another moment in time if it continues on the same path. For objects in the realm of the very small, this is simply not possible.

These discoveries are at the heart of quantum physics. The importance of the introduction of statistical probabilities into physical law cannot be understated. In earlier physics, that is to say the classical mechanics of Isaac Newton, probabilities had no place. In the Newtonian world-view, an object of any kind could at least in theory be described as having a definite position at any given time. This was not the case at all in the new quantum theory: the probabilities describing material particles could never be replaced by old-style definite positions.


A Probabilistic World


Applying these points to an electron in an atom, nevertheless, we see that the electron is no longer a point in space moving along a definite path, but a cloud of probabilities [2]. In other words, one cannot say that an electron will be in this position at this point of time, but one can only say like this: 'It is 95% probable that the electron will be at this position at this point of time.' Similarly, to every point in a light wave we can attach a numerical probability (per unit time interval) that a photon can be detected in any small volume centred on that point, and light is viewed as a probability wave.



Fig: Electron probability clouds for the s-, p- and d-orbitals


With de Broglie's principle, we can also extend this to electrons and all other matter particles, and hence account for their wave nature (and hence the double-slit experiment). They have been experimentally found to be correct; in 1994, interference fringes, a typical characteristic of waves, were generated with beams of iodine molecules, which are about 500,000 times more massive than electrons.

By 1930s, this indeterministic interpretation of quantum physics, mainly put forward by Niels Bohr with support from Born's probability interpretation and Heisenberg's uncertainty principle, came to be known as the Copenhagen interpretation, mainly because Bohr ran an influential physics institute there during this period. What happened next is dealt with in the next chapter.

Footnotes

[1.] At speeds comparable to that of light, particles begin to behave strangely. That, however, is a part of the relativity theory, and we shall not touch upon the matter.

[2.] It is impossible to give an everyday world analogy for this 'cloud of probabilities'. Nevertheless, this is the best description of how these particles exist and behave


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6.Veiled Reality

In 1927, a German physicist by name Werner Heisenberg proved rigorously from quantum theory a very unsettling truth. He discovered that:
Because of the very nature of subatomic objects, it is impossible, even in theory, to know with arbitrary precision both the position and velocity of any given wave / particle; position and velocity can only be described in terms of probabilities.

It cannot be overstated that this limitation is not caused by any technological or theoretical failure; it is simply the nature of the quantum world to be indeterminate. Suppose we try to pin down the exact location of an electron through a microscope. If we have to 'see' the electron, it means the electron should scatter the light into our eyes, or some other light-detector. But we learnt that if the electron absorbs a photon, it gains energy and is disturbed. It induces an uncertainty in the particle's velocity. On the other hand, if we try to use a low-energy beam of light, we cannot determine the position of the electron with surety.


Karl Heisenberg


Thus the fundamental laws of nature conspire to keep the objective reality of the atomic and sub-atomic world veiled from us. For example, electrons around an atomic nucleus appear to be in a hazy cloud. The sharp paths or orbits remain forever hidden from us.

This principle applies equally to any object, but its effect is negligible for an object of everyday size. In fact, ordinary experimental uncertainties harm the observation much more than Heisenberg's principle! This is the reason why Newtonian classical physics was so successful in explaining real-world phenomena.

Any observation disturbs a system under study; in non-quantum systems one can reduce the disturbance arbitrarily. In any case, one can compute, at least in principle, effects of the disturbance dictated by deterministic laws and subtract them out. This is impossible in the quantum domain even in principle.

Many physicists working at the time of the quantum revolution found this fact very difficult to accept. One notable figure was Albert Einstein himself, who refused even to his dying day that any part of the universe could be indeterminate by nature. Part of the reason for this difficulty is that most of the physicists of that time had been trained in classical mechanics, which very much shaped their thinking. The other part of the difficulty is that it is impossible for the human mind to imagine something that is actually indeterminate. In the world of human experience, we only find determinate things. For example, you will never find a cat which is neither alive nor dead, but in an in-between indeterminate state. All cats are either alive or dead; indeed, all macroscopic objects have a determinate state which can be observed and described (at least in theory). This is in no way true of the quantum world which had been discovered.

In time physicists learnt to accept the fact of indeterminacy, even though they could not imagine it. The ones who could not accept it eventually died. At the beginning of the twenty-first century, there are virtually no physicists who reject quantum physics. There is certainly disagreement about the meaning of quantum physics, but that is really more a matter of philosophy.

Once physicists realised that quantum hypothesis is not compatible with determinism -- this was the breakthrough -- they tried to find answers to the objections raised in Bohr's model differently. The next chapter explains how.






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5.Matter and Waves

In the previous chapter, we saw how Bohr's model was remarkably successful in explaining the origin of line spectra, and how it was able to explain the stability of the atom. Yet, over time, it came to be laden with the following drawbacks.

1.It worked only for hydrogen and hydrogen-like atoms and radiation. Attempts at extending the hydrogen theory to more complex atoms exposed new difficulties.

2.Some of the spectral lines were shown to possess fine structures. At first some refinements to the Bohr theory seemed to account for such results, but their brightness could not be accounted for.

Some people began suspecting that the problems could not be resolved by tinkering with the Bohr model with its well-defined electronic orbits. A basic change in worldview, a paradigm shift, was needed.


The Double-Slit Experiment

Meanwhile, another simple experiment baffled physicists, because classical physics fails to explain the results. This well-known experiment is called the 'double slit experiment'. To simplify the discussion a bit we shall talk about holes instead of slits. Briefly, the experiment is as follows.

1.The setup comprises a stream of electrons hitting a wall with two small holes. A heated filament is a good source of electrons. A constant voltage applied between the source and the wall with two holes ensures that electrons arrive at the holes with the same momentum. An electron detector at the screen detects the rate of arrival of electrons.

2.If we move the detector very slowly up or down, it will be found that the rate goes through maxima and minima reminiscent of the interference pattern [1] associated with waves described above, but the detected individual electrons are always point-like, never smeared out. If we close one of the holes, the interference pattern is lost, and an image of the open hole is seen.

3.Suppose we drastically reduce the electron emission rate, to such a level that the source emits only one electron at a time, at random intervals. We expect that each electron passes through only one of the two holes. For the electron that passes through one hole, whether the other hole is open or closed should not matter. Astonishingly, interference fringes still build up on the screen if the experiment runs long enough.


Thus we see that electrons, considered to be particles, show properties of waves like interference. As you may recall, light, well-known to be a wave, also acts as particles in some cases, notably the photoelectric effect. This suggests something, as De Broglie found out.


De Broglie's Explanation

In 1924 Louis de Broglie, an imaginative French physicist, asserted that material particles would behave as waves under certain conditions, and vice versa. He argued that just as light waves behave as particles to give rise to photoelectric effect, matter also would behave as waves under certain conditions. He even gave a simple formula to calculate the wavelength [2] associated with electrons of a given momentum.


Prince Louise-Victor Pierre Raymond De Broglie


It might surprise you that the de Broglie wavelength associated with photons is equal to the usual electromagnetic wavelength. Ordinary objects, such as balls and bullets, also have de Broglie waves associated with them, but the wavelengths are so small that they can be neglected. Only we enter the sub-atomic domain do they gain significance.

This idea of matter waves at first seemed very speculative, and failed to attract attention. In 1927, however, it was experimentally confirmed to be true by Davisson and Germer at the Bell Laboratories, after conducting studies on scattering of electron beams from crystals.

Decades later, today, the wave nature of matter is taken almost for granted. Diffraction studies involving beams of electrons or neutrons are used routinely to study the atomic structures of solids and liquids. Matter waves are a valuable supplement to X-rays in such studies. Electron microscopes, which employ electron beams, have revolutionised microbiology and significantly improved our understanding of living organisms. High-energy particle accelerators probe extremely small space and time intervals using the same principle.


Returning to our atom, de Broglie waves clarify the ad hoc quantum condition of Bohr's atom. Allowed orbits are only those for which the orbit length equals an integral multiple of electron wavelengths. All other orbits disappear due to destructive interference.

Nevertheless, objections to Bohr's theory continued to persist due to its drawbacks mentioned above; a large number of generalisations and modifications to Bohr's theory were tried, but they were all complicated and unsatisfactory.

The breakthrough came soon, as the next chapter uncovers.

Footnotes

[1.] Interference is a phenomenon in which two or more waves superpose to yield a resultant wave whose amplitude or intensity may be greater or less than the component waves. The striking colours on soap bubbles and oil slicks are due to interference of light waves.

[2.] Wavelength is a measure of the length of a wave, and is given by the distance between two consecutive 'crests' or 'troughs' of the wave. If you measure the distance between two 'heads' of a ripple in a pond as 10 cm, then its wavelength is said to be 10 cm.


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4.Niels Bohr and the Quantum Atom

So far we have seen that quantum physics deals with electromagnetic radiation -- that is to say, light. But at the beginning we said that quantum physics tells us that material things are described by quantum physics. So what happened to matter?

Niels Bohr happened to matter. Bohr was a Danish physicist whose parents were both scientists. Apparently young Niels grew up in an atmosphere that was favourable to science. He received his doctorate in 1911 from the University of Copenhagen.



Niels Bohr


One can see just by looking at Niels Bohr's early academic life (though there are many other indicators) that the 'atomic theory' of matter -- that is to say that matter is made up of tiny atoms -- had gone from being scientifically marginalised to being at the heart of physics in a very short time. Bohr's doctoral thesis was concerned with the inadequacy of classical (that is to say, Newtonian) physics for describing the behaviour of matter at the atomic level.

In the 19th century, physicists who saw some value in the atomic theory thought of the atom as a tiny undivided and indivisible unit of matter, the smallest possible unit into which something could be broken down. Experiments dealing with the photoelectric effect, along with other observations, strongly suggested that the atom had some internal structure, since particles called 'electrons' were being emitted from them.

Rutherford's Model and Its Drawbacks


A model of the atom was described by the British physicist Ernest Rutherford in 1911, and is known as the Solar System model. It is very simple, and is still used to teach elementary atomic structure to school children.

An atom consists of a central nucleus. This nucleus is composed of positively charged protons, and electrically uncharged (neutral) neutrons.

Negatively charged electrons revolve round the nucleus in definite orbits.
The orbits themselves can be at any distance from the nucleus.

In any atom, the number of protons is equal to the number of electrons, and hence it is electrically neutral.


Fig: Rutherford's Atomic Model


Rutherford used the laws of motion that had been set forward by Sir Isaac Newton to describe the atom. According to Rutherford's description, the electrons of an atom could occupy one of an infinite number of orbits, in accordance with Newton's laws. There were problems with Rutherford's description of the atom from the beginning. Let us find out two drawbacks of Rutherford's theory.


1. Inherent Instability of the Atom

According to Rutherford's theory, electrons could orbit the nucleus at any distance. When the electrons circle round the nucleus, they are constantly changing their direction. According to classical electrodynamics (which deals with the motion of electrons), such electrons which either constantly change their direction or their velocity or both should continuously emit radiation. While doing so, they should lose energy, and thus spiral into the nucleus. This means every atom is unstable, quite contrary to our observation! [1]


Fig: Rutherford's atom is inherently unstable


2. Atomic Spectra


Rutherford's description of the atom could not be entirely correct because it did not account for some observations that had already been made. Perhaps the most important of these observations concerned the behaviour of certain gases. These gases at low pressure emit light in a set of discrete bands of the electromagnetic spectrum. This is quite different from the radiation emitted by solids, which is spread evenly across the electromagnetic spectrum. The radiation emissions of these gases were important because they showed that at least under some circumstances, the orbits of the electrons could not be at just any distance from the nucleus, but were confined to discrete distances (or energy states).




Fig: A. Continuous spectrum and B. line spectrum of hydrogen


If the electrons in these gases were free to orbit at any distance, then the light emitted from them would have been spread evenly across the electromagnetic spectrum. Instead, what experimenters saw was that the light from these gases showed a distinct line pattern. That is to say that the light being emitted was only seen in a certain set of wavelengths, with empty spaces in between.

These line-spectra were different for each gas, and was found to be the characteristic of its atom. Today, astronomers use line-spectra to detect the elements present in stars.

Bohr's Explanation

Niels Bohr quickly seized upon this problem and used it to propose a quantised description of the atom.

1.Bohr proposed that while circling the nucleus of the atom, electrons could only occupy certain discrete orbits, that is to say energy levels [2]. Bohr used Max Planck's equations describing quanta of radiation to determine what these discrete orbits would have to be. As long as electrons stay in these energy levels, they are stable.

2.Further, Bohr said electrons give or take energy only when they change their energy levels. If they move up, they take energy (say from light), and if they move down, they release energy. This energy itself is released in discrete packets called photons, which were introduced in the previous chapter.

3.Furthermore, Bohr also said that an electron which is not in its native energy level (in other words, which has been excited to a higher energy level) always has to fall back to its original, stable level.
Bohr interpreted the lines in the spectra of gases as formed by the transitions of electrons to and from various energy-levels. This has been verified thoroughly with the hydrogen atom, and found to be correct. Bohr's formulae agreed excellently with observed line positions.


Fig: Bohr's Explanation of Line Spectra


Imagine that you are taking a walking along a beach. As you walk along, you see a sand-castle that someone has built. As you get closer to the sand castle, you discover that you can only stand three meters, two meters, or one meter from the sand castle. You cannot stand at one and a half meters, nor can you stand at two and three quarters of a meter from the sand castle. No matter how hard you try, some mysterious force keeps you at one of those three distances. In everyday life such a situation is absurdly impossible. In the physics of the very small, it is a necessity.

This description in which electrons can only occupy certain orbits is called the shell model of the atom, because Bohr described the possible orbits of the electrons as orbitals or shells. When an atom of a gas released energy, an electron would move down to a lower orbit (requiring less energy), and when an atom acquired energy, an electron would move up to a higher energy level. But these orbits or shells were discrete, like the distances from the sand castle. The orbits were not a smooth, continuous series of possibilities as one finds in the everyday world, but rather a set of distinct states separated from each other, much like the separation of the quanta of electromagnetic radiation that Planck had discovered. This caused the distinct lines in the spectrum.

For the first time, quantum physics had been applied to matter.

Footnotes

[1.] Extending the same analogy, can we say the solar system is also unstable? Do planets going round the Sun also lose energy and spiral into the Sun in the same manner?
The answer is yes. Radio-wave analysis from a dense double-star system called PSR 1913+16 confirms energy loss at exactly the rate of spiralling-in corresponds to the theoretical prediction. But there is no cause to worry: unlike electrostatic forces, gravitational forces are the weakest forces in nature, and the Sun will die its natural death (in about 5 billion years from now) much before the Earth plunges into it!

[2.] Though this was just a postulate in Bohr's theory to aid the understanding of the atom, it would eventually be clarified. The next chapter explains how


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3.Einstein Demystifies Photoelectric Effect

In the previous chapter we told you about Max Planck and how he introduced the 'quantum' concept to radiation. This resolved the radiation problem but initially it did not attract much attention, mainly because the idea was so out of line with the then current thinking, and nobody had practically found independent evidence for the existence of the quantum. That was left to the genius of Einstein. He was responsible for the second coming of the quantum.

One of the most recognised faces of the last century is that of Albert Einstein. Very many people know a little about Einstein, the eccentric old genius with the fright-wig hairstyle and rumpled clothing. Many people are also aware of his great intelligence. In some parts of the English-speaking world, the word 'Einstein' has become a generic term referring to a very intelligent person.



Albert Einstein


People who know something about science are also aware of his theories of relativity: the Special Theory of Relativity (published in 1905) and the General Theory of Relativity (published in 1915). What many people do not know is that Einstein was the second person to make a major contribution to the quantum revolution, in a paper also published in 1905 [1]. In fact, this paper won him a Nobel prize.

The paper in question was about the nature of light [2]. Albert Einstein's contribution to this discussion was a theoretical explanation of a certain phenomenon, which showed that light was a stream of particles (which would later be named photons).

This phenomenon is called the photoelectric effect. When you shine a light upon certain metals, a stream of particles (later found to be electrons [3]) is emitted from that metal. The emission has been found to have certain properties.

1.The number of electrons emitted by the metal depends on the intensity of the light beam applied on the metal; more intense the beam, higher the number of electrons emitted.
2.The emitted electrons move with greater speed if the applied light has a higher frequency.
3.No electron is emitted until the light has a threshold frequency, no matter how intense the light is.

These observations baffled physicists for many decades, since they cannot be explained if light is thought of only as a wave.

If light were to be a wave, both the energy and the number of the electrons emitted from the metal should increase with an increase in the intensity of light. Observations contradicted this prediction; only the number, and not the energy, of the electrons increased with the increase of the intensity of the light.



Fig: Einstein's Explanation of Photoelectric Effect


What Einstein showed was that the photoelectric effect as it had been observed could be explained if individual particles (or quanta) of light were penetrating the metal and knocking electrons loose from atoms. According to Einstein's paper, increasing the intensity of the light increased the number of photons, while the energy of each individual photon remained the same, as long as the frequency of the light remained the same. Therefore the number of electrons emitted would increase, but the energy transmitted to them by the particles of light would remain the same.

In one stroke Einstein showed that light is a stream of particles, and also that there was solid evidence for the existence of quanta. His theory could satisfactorily explain all the known properties of the photoelectric effect, and was the first result derived from quantum theory of the interaction between radiation and matter.

The same theory also raises a fundamental question: is light ultimately a wave, or a stream of photons?

The answer is: both. Light behaves as a wave under certain conditions, and as a stream of particles under others. It is said to have a dual nature: we can understand it as either wave or particle, depending on our context of observation. If you are wondering why this is so, rest assured that this matter will be clarified later, in Chapter 7.

Footnotes

[1.] Einstein's paper was titled, in German, Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt or, in English, On a Heuristic Viewpoint Concerning the Production and Transformation of Light.

[2.] Normally when we use the word light, we are actually referring to visible light. Visible light, however, is only a small part of the electromagnetic spectrum, which ranges all the way from radio waves to gamma radiation. Every kind of radiation on the electromagnetic spectrum is really the same thing, only with different wavelengths and frequencies.

[3.] Electrons will be explained in the context of atomic structure in the next chapter


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2.Max Planck's Revolutionary Hypothesis

Over three hundred years ago, Sir Isaac Newton revolutionised the study of the natural world by putting forth laws of nature that were stated in mathematical form for the first time. Newton's book, The Mathematical Principles of Natural Philosophy [1] forever changed how scholars would study the physical world. Newton's formulation of physical laws was so powerful that his equations are still in use today. [2]
By the start of the 20th century, physicists had worked with Newton's laws so thoroughly that some of them thought that they were coming to the end of physics. In their opinion, not much was left to do to make physics a complete system. Little did they know that the world they described was soon to be understood in a completely different way. The quantum revolution was about to happen.

This revolution was begun by a very unlikely person, a physicist named Max Planck, who was very conservative in all his views. It speaks well of Planck's intellectual honesty that he was able to accept the reality of what he discovered, even though he found the consequences of his discoveries distasteful and unpleasant for the rest of his life.


Max Planck

Born in 1853, Max Planck came from a conservative and respectable family in Kiel, Germany. Young Max was very bright, and had a variety of fields from which to choose to study for his professional life. Planck chose physics because he felt that it was the field in which he was most likely to do some original work. At the young age of 21, he received his doctorate in physics from the University of Munich.

Planck was investigating the properties of heat- and light-emitting bodies. Classical physics had theories which predicted that the brightness of a body increases continuously as the frequency [3] of its electromagnetic radiation [4] is increased.

Fig: Intensity vs Frequency Plot

Unfortunately, experiments revealed a totally different picture. The brightness did increase initially, but only upto a limit. Then, actually, it began to fall. We thus get a bell-shaped curve if we plot frequency against brightness.

Besides, another observation was made: as bodies become hotter, their maximum brightness shifts towards higher frequencies. This is why an object, heated to 300-400 C, emits mostly infra-red or heat waves. As the temperature is increased, the object appears to be red, then orange, and finally white or even blue.

Classical theories totally failed to explain this discrepancy between the known facts and the observations. Then, in the winter of 1900, Max Planck found a solution to this problem. Planck ushered in the quantum era by making a bizarre assumption:

Emission and absorption of energy can occur only in discrete amounts.

This might seem totally unsurprising to you, but believe me, it shook the scientists of that period. Planck himself did not know he would end up with this statement!

Imagine for a moment that you are a sculptor, and you have obtained a piece of stone in the shape of a cube. To begin your sculpture, you take a chisel and place its edge against the stone, and then strike the chisel with a hammer. What do you imagine would happen? I think you would imagine that a piece of the stone would be split off, as well as some smaller splinters and pieces of stone. Imagine instead that when you struck the stone, it broke into hundreds of small cubes, each one of them exactly the same size, 3 centimeters per side. Wouldn't you be surprised, even shocked? Imagine furthermore that no matter how hard you tried, these smaller cubes could not be broken into smaller pieces at all!

We think that the reaction that a sculptor would have in such a circumstance would be similar to what Planck and other physicists felt upon discovering that energy only occurred in discrete amounts. It was a completely unexpected discovery, and yet it was only the beginning of what would come later.

Planck called these discrete lumps as quanta. This was against the entire world-view that had been built from the time of Newton onward. In the physics that had been built up since the time of Newton, and indeed in the minds of most thinkers before Newton, matter and energy were thought to be smooth and continuous. Even by the time of Planck, the idea that matter could be ultimately broken down into tiny indivisible 'atoms' was only held by a few physicists.

And so the quantum revolution began.

Footnotes

[1.] Natural Philosophy was the term used in Isaac Newton's time for what we now call 'the sciences'. 'Science' would only become distinct from philosophy much later. The term 'scientist' was only coined in the middle of the 19th century.

[2.] Newton's physics gave a very good understanding of the working of gravity, as well as other natural phenomena. Although Newton's interpretation of his discoveries has mostly been abandoned, the mathematical laws that he discovered are close enough to what is actually found in the real world that they are still useful for many things, from engineering to astrophysics.

[3.] Frequency is a term associated with waves, and refers to the number of waves that pass through a point in a given time interval. For example, if you can count three 'heads' of ripples pass by you in a pond in ten seconds, then the frequency is 3/10 Hz. Hertz is the unit of frequency.

[4.] Electromagnetic radiation consists of fluctuating electric and magnetic fields. In the increasing order of frequency, radio waves, microwaves, infra-red radiation, the visible light (red through violet), ultra-violet radiation, X rays and gamma rays are all electromagnetic waves. They are caused by changes in the electric field or the magnetic field, and they travel at the speed of light, which is 300,000 km per second.

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1.Introduction to Quantum Physics

For centuries, man has wondered on phenomena and processes happening around him. As time passed, he was successful in applying his intuition and common sense in comprehending the stars, galaxies and their behaviour, but they fail in the microscopic world of molecules, atoms and sub-atomic particles.

Quantum theory provides us with the rules and regulations of the miniature world. These rules are phenomenally successful in accounting for the properties of atoms, molecules, and their constituents, and form the basis of understanding the fundamental properties of all matter. In fact, one may say that the greatest success story of the 20th-century physics is to confirm that this theory works, without a single exception, in spite of critical examination by some of the best minds spanning decades of time.
Yet, the conceptual foundation of quantum theory is mysterious. It led to intense debates among scientists, and confused many. Niels Bohr, one of the most prominent scientists in this domain, once remarked, 'You have not studied quantum mechanics [1] well if you aren't confused by it.' Albert Einstein, the greatest physicist of the 20th century, never approved of this theory. Bizarre though it may seem, quantum physics has led physicists step by step to a deeper view of the reality, and has answered many fundamental questions.

In the following chapters we shall touch upon several fascinating areas of the quantum world. Our attempt is to help the reader understand the fundamental concepts of quantum physics without the underlying mathematics. We try not to be physicists, but only to have an understanding of what physicists know of our world. That goal is within our grasp.

Welcome aboard, to the quantum world!

Footnotes

[1.] Usually one sees the term quantum mechanics used rather than quantum physics. This is directly related to Isaac Newton's original ideas. Newton conceived of the universe as a vast machine, and so in Newtonian physics one could speak of the 'mechanics' or machine-like workings of the universe. 'Quantum mechanics' is simply the same idea applied to the world of atoms and sub-atomic particles -- though the machine-like workings of the quantum world are very different from those of the Newtonian world.





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