© Barnard Castle School Department of Physics 2008, 2009
Text and Questions © Paul McHarry / Keith Gibbs
Years 7-9
IGCSE
GCSE
Pre-U
A-level
Antimatter
Antimatter is often thought of as the stuff of science fiction. Spaceships of the future often have "antimatter drives". But antimatter is very real and in use every day.
As far back as 1928, English physicist Paul Dirac created an equation which showed that electrons could have negative as well as positive energy. Such "anti-electrons" would have the same mass as ordinary electrons but the opposite charge - positive instead of negative. Dirac suggested that such particles might actually exist and particle physicists began a search for them.
Four years later, Carl Anderson, examining the tracks of cosmic ray particles in a cloud chamber, found the track of a particle with the same mass as an electron but the opposite charge. He had discovered the anti-electron; the positron. Dirac won the Nobel Prize for his work.
If matter and antimatter come into contact, they annihilate each other, turning themselves into a flash of pure energy. Antimatter is used every day in our hospitals. When a patient has a PET scan, he is injected with a radioactive isotope which emits positrons. This is carried in the bloodstream to his brain. When the positrons annihilate with electrons in his brain, the energy produced is in the form of two gamma rays. Detectors around the patient's head pick up these gamma rays and an image of his brain is built up.
Antimatter is often thought of as the stuff of science fiction. Spaceships of the future often have "antimatter drives". But antimatter is very real and in use every day.
As far back as 1928, English physicist Paul Dirac created an equation which showed that electrons could have negative as well as positive energy. Such "anti-electrons" would have the same mass as ordinary electrons but the opposite charge - positive instead of negative. Dirac suggested that such particles might actually exist and particle physicists began a search for them.
Four years later, Carl Anderson, examining the tracks of cosmic ray particles in a cloud chamber, found the track of a particle with the same mass as an electron but the opposite charge. He had discovered the anti-electron; the positron. Dirac won the Nobel Prize for his work.
If matter and antimatter come into contact, they annihilate each other, turning themselves into a flash of pure energy. Antimatter is used every day in our hospitals. When a patient has a PET scan, he is injected with a radioactive isotope which emits positrons. This is carried in the bloodstream to his brain. When the positrons annihilate with electrons in his brain, the energy produced is in the form of two gamma rays. Detectors around the patient's head pick up these gamma rays and an image of his brain is built up.
Paul Dirac
The Higgs Boson
Why does anything have mass? A block of lead is heavy because it contains a lot of atoms and they all have mass. But why? Why is a proton more massive than an electron?
While walking through the Scottish Highlands in 1964, physicist Peter Higgs developed what he called his "one big idea". He said, particles seem massive because they are slowed down as a result of travelling through a force field. Particles which interact more strongly with the field, (now known as the Higgs Field), are slowed down more and we say they have more mass. This interaction with the Higgs field takes place by exchange of particles known as Higgs Bosons. The Large Hadron Collider, the world's biggest particle accelerator, which was switched on in August 2008, aims to try and discover if Higgs Bosons really exist.
Why does anything have mass? A block of lead is heavy because it contains a lot of atoms and they all have mass. But why? Why is a proton more massive than an electron?
While walking through the Scottish Highlands in 1964, physicist Peter Higgs developed what he called his "one big idea". He said, particles seem massive because they are slowed down as a result of travelling through a force field. Particles which interact more strongly with the field, (now known as the Higgs Field), are slowed down more and we say they have more mass. This interaction with the Higgs field takes place by exchange of particles known as Higgs Bosons. The Large Hadron Collider, the world's biggest particle accelerator, which was switched on in August 2008, aims to try and discover if Higgs Bosons really exist.
Peter Higgs
Relativity
Newton's laws of motion describe how most objects move: cricket balls, cars, planets. But Albert Einstein showed, in 1905, that when objects move so fast that they approach the speed of light, they increase in mass, they shrink in length and they age more slowly.
This is because the speed of light is a Universal speed limit and, as objects approach this, spacetime - the fabric of the Universe - distorts. Once Einstein had recognised that space and time are not two separate entities, as Newton had thought, but are part of the same fabric, it was clear that they must affect each other. The idea of slowing down time by travelling faster sounds like science fiction but, in 1971, four identical atomic clocks were sent on scheduled flights twice around the world. Two flew east and two flew west. Comparing their times with a fifth clock kept on the ground, it was found that they had all lost a fraction of a second compared with the grounded clock. In another experiment involving subatomic particles known as muons, it was found that their lifetimes were extended because of their enormously high speeds. Muons are created in the upper atmosphere by collisions between cosmic rays and particles in the atmosphere. These muons, travelling at close to the speed of light, have a well-defined lifetime or "half-life". It should allow them to travel only a relatively short distance down towards the Earth's surface before they decay. Again and again it was found that very large numbers reached the surface easily. Why? Because, from our point of view, they are aging much more slowly due to their high speed. (From the muon's point of view, it seems that time passes at the same rate but the distance they travel shrinks. So again they reach the surface easily before they decay).
Einstein developed two theories of relativity: the Special Theory describes objects moving at constant velocities. The General Theory is about accelerating objects.
Newton's laws of motion describe how most objects move: cricket balls, cars, planets. But Albert Einstein showed, in 1905, that when objects move so fast that they approach the speed of light, they increase in mass, they shrink in length and they age more slowly.
This is because the speed of light is a Universal speed limit and, as objects approach this, spacetime - the fabric of the Universe - distorts. Once Einstein had recognised that space and time are not two separate entities, as Newton had thought, but are part of the same fabric, it was clear that they must affect each other. The idea of slowing down time by travelling faster sounds like science fiction but, in 1971, four identical atomic clocks were sent on scheduled flights twice around the world. Two flew east and two flew west. Comparing their times with a fifth clock kept on the ground, it was found that they had all lost a fraction of a second compared with the grounded clock. In another experiment involving subatomic particles known as muons, it was found that their lifetimes were extended because of their enormously high speeds. Muons are created in the upper atmosphere by collisions between cosmic rays and particles in the atmosphere. These muons, travelling at close to the speed of light, have a well-defined lifetime or "half-life". It should allow them to travel only a relatively short distance down towards the Earth's surface before they decay. Again and again it was found that very large numbers reached the surface easily. Why? Because, from our point of view, they are aging much more slowly due to their high speed. (From the muon's point of view, it seems that time passes at the same rate but the distance they travel shrinks. So again they reach the surface easily before they decay).
Einstein developed two theories of relativity: the Special Theory describes objects moving at constant velocities. The General Theory is about accelerating objects.
Albert Einstein
Maxwell’s Equations
The four equations derived in the nineteenth century by Scottish physicist James Clerk Maxwell were described as the most important advance in Physics since the theory of Universal Gravitation. They formed the foundations on which twentieth century Physics was built and Einstein incorporated them into his theories of Relativity.
They describe the behaviour of electromagnetic waves, including light, and the fields that create them. It was Maxwell who realised that light is an electromagnetic wave and Einstein who recognised that the speed of electromagnetic waves in space forms a universal speed limit.
The first equation, Ñ.D = r, describes the shape and strength of the electric field around a charged object. The second, ÑxH = J + (dD/dt), describes the shape and strength of the magnetic field around a magnet. The third, Ñ.B = 0, describes how changing electric currents produce magnetic fields and the fourth, ÑxE = -(dB/dt), tells how changing magnetic fields produce electric currents.
The electromagnetic force is one of the four fundamental forces of nature. It describes all electromagnetic radiation from radio and television, through microwaves to X-rays and gamma rays. It is crucial to Chemistry since it is the force that holds all molecules together. It is the most important force in our everyday lives. When you touch someone, you do it by exerting an electromagnetic force on them!
The four equations derived in the nineteenth century by Scottish physicist James Clerk Maxwell were described as the most important advance in Physics since the theory of Universal Gravitation. They formed the foundations on which twentieth century Physics was built and Einstein incorporated them into his theories of Relativity.
They describe the behaviour of electromagnetic waves, including light, and the fields that create them. It was Maxwell who realised that light is an electromagnetic wave and Einstein who recognised that the speed of electromagnetic waves in space forms a universal speed limit.
The first equation, Ñ.D = r, describes the shape and strength of the electric field around a charged object. The second, ÑxH = J + (dD/dt), describes the shape and strength of the magnetic field around a magnet. The third, Ñ.B = 0, describes how changing electric currents produce magnetic fields and the fourth, ÑxE = -(dB/dt), tells how changing magnetic fields produce electric currents.
The electromagnetic force is one of the four fundamental forces of nature. It describes all electromagnetic radiation from radio and television, through microwaves to X-rays and gamma rays. It is crucial to Chemistry since it is the force that holds all molecules together. It is the most important force in our everyday lives. When you touch someone, you do it by exerting an electromagnetic force on them!
So famous, they appear on T-shirts
Olbers’ Paradox
If the Universe were infinitely big and had been around forever, then there should be a star in every direction. (This is a hard idea to grasp but remember that "infinite" means endless. If an infinite number of monkeys sat down at an infinite number of keyboards and began to type, one of them would type all the works of Shakespeare!)
If we could see a star in every direction, the night sky should be as bright as the Sun. But it isn't. It is mainly black, with only a few points of light.
When we look at the sky, we are seeing the whole history of the Universe. The deeper into space we look, the further back in time we are seeing. The fact that the sky is almost entirely black means that the Universe is not infinitely big and not infinitely old. Johannes Kepler had noticed this as far back as the 17th century but it was the German astronomer Heinrich Olbers who formulated the idea in 1823.
In fact, the oldest stars we observe with telescopes are about 13 billion years old. Much before that, there were no stars. the Universe was too young for them to have formed. Using a telescope as a time machine, however, has its limitations: there was a time, when the Universe was less than 400,000 years old, when it was opaque and a telescope, however powerful, would not be able to penetrate this time barrier. Physicists must use bigger and bigger particle accelerators now to look back to the first few seconds after the Big Bang.
If the Universe were infinitely big and had been around forever, then there should be a star in every direction. (This is a hard idea to grasp but remember that "infinite" means endless. If an infinite number of monkeys sat down at an infinite number of keyboards and began to type, one of them would type all the works of Shakespeare!)
If we could see a star in every direction, the night sky should be as bright as the Sun. But it isn't. It is mainly black, with only a few points of light.
When we look at the sky, we are seeing the whole history of the Universe. The deeper into space we look, the further back in time we are seeing. The fact that the sky is almost entirely black means that the Universe is not infinitely big and not infinitely old. Johannes Kepler had noticed this as far back as the 17th century but it was the German astronomer Heinrich Olbers who formulated the idea in 1823.
In fact, the oldest stars we observe with telescopes are about 13 billion years old. Much before that, there were no stars. the Universe was too young for them to have formed. Using a telescope as a time machine, however, has its limitations: there was a time, when the Universe was less than 400,000 years old, when it was opaque and a telescope, however powerful, would not be able to penetrate this time barrier. Physicists must use bigger and bigger particle accelerators now to look back to the first few seconds after the Big Bang.
The Universe is not infinitely old
Superconductors
At very low temperatures, just a few degrees above absolute zero, some metals and alloys conduct electricity without any resistance. Currents made to flow in these conditions could keep going for billions of years without losing any energy. In the laboratory, currents have been maintained for many years.
In ordinary conductors, the electrons which make up the electric current lose energy by colliding with the vibrating atoms of the metal as they flow through it. As the metal is cooled, the vibration of its atoms decreases until, at a certain critical temperature, electrons group together in pairs, (known as Cooper pairs), and behave more like photons of light. The pairs of electrons are pushed along in a wave-like ripple produced by the slight vibration of the metal atoms.
Today, superconductors are used to make the powerful electromagnets used in MRI scanners and in particle accelerators such as the Large Hadron Collider. The search is now on for superconductors which work without the need to be cooled to ultra-low temperatures. Currently, alloys have been produced which are superconducting at around -130 degrees Celsius.
At very low temperatures, just a few degrees above absolute zero, some metals and alloys conduct electricity without any resistance. Currents made to flow in these conditions could keep going for billions of years without losing any energy. In the laboratory, currents have been maintained for many years.
In ordinary conductors, the electrons which make up the electric current lose energy by colliding with the vibrating atoms of the metal as they flow through it. As the metal is cooled, the vibration of its atoms decreases until, at a certain critical temperature, electrons group together in pairs, (known as Cooper pairs), and behave more like photons of light. The pairs of electrons are pushed along in a wave-like ripple produced by the slight vibration of the metal atoms.
Today, superconductors are used to make the powerful electromagnets used in MRI scanners and in particle accelerators such as the Large Hadron Collider. The search is now on for superconductors which work without the need to be cooled to ultra-low temperatures. Currently, alloys have been produced which are superconducting at around -130 degrees Celsius.
The Meissner Effect
A magnet floating above a superconductor, cooled with liquid nitrogen. An electric current flows on the surface of the superconductor, repelling the magnetic field of the magnet.
A magnet floating above a superconductor, cooled with liquid nitrogen. An electric current flows on the surface of the superconductor, repelling the magnetic field of the magnet.
The Copenhagen Interpretation
Quantum Mechanics is the most powerful description of the physical Universe. It describes the whole of Physics and, therefore, Chemistry and Biology as well. Its equations produce all the right answers but what it actually means is still not clear. It prompted the Danish theoretical physicist Neils Bohr to remark, "Anyone who is not shocked by Quantum Theory has not understood it."
By 1927, one group of physicists, led by Erwin Schrödinger, argued that the quantum behaviour of particles could be described in terms of the physics of waves, summed up in his famous Wave Equation. Others, clustered around Werner Heisenberg, believed that matter and radiation had a particle nature and behaved according to Heisenberg's fundamental Uncertainty Principle.
Bohr, who was the head of Heisenberg's department at the University of Copenhagen, attempted to pull together all the theories and experimental results into a unified view, now known as the Copenhagen Interpretation. He realised that there was no such thing as objective science. The very act of measurement or observation of a system decides the way it will behave. Schrödinger's wave equation holds within it all the probabilities for the behaviour of a particle in a given situation and, when we have decided how to observe it, the equation collapses down so that everything except the observed outcome is lost. The idea that we can stand, God-like, outside the Universe and observe objectively what happens inside is fanciful.
So, why don't we observe this strange behaviour in everyday objects like people and rugby balls? For large objects, Schrödinger's equation just reduces to our more familiar Newton's laws of motion. Ultimately, Bohr's Copenhagen Interpretation simply said that for microscopic things, we use a quantum explanation but for large, macroscopic objects, classical physics is all that we need. This is the orthodox position of Physics but it is hardly satisfactory and there have been many other interpretations since Copenhagen. One such, known as GRW (after Ghirardi, Rimini and Weber), says that the wave equations or wave functions describing atoms randomly collapse. An atom's wave function may collapse maybe once in a hundred million years, so this is not seen in the normal course of an experiment. But for large objects like people and rugby balls, where each atom is linked to many others, the collapse of one atom's wave function would trigger the collapse of the whole object to one quantum state. There are so many atoms in a rugby ball that even if each one's wave function collapsed only once in a hundred million years, there would still be one collapse every trillionth of a second. The ball would thus behave like a ball and be located in one place at one time, unlike an electron, for example, which does not have an existence in just one place until the act of observing it creates it there.
Confused? You should be, but read about Schrödinger's cat and see if that helps. And remember this: quantum mechanics confused Einstein and he didn't like it. Yet quantum theory is the most tested theory in all of science and nothing it has ever predicted has been found to be wrong!
Quantum Mechanics is the most powerful description of the physical Universe. It describes the whole of Physics and, therefore, Chemistry and Biology as well. Its equations produce all the right answers but what it actually means is still not clear. It prompted the Danish theoretical physicist Neils Bohr to remark, "Anyone who is not shocked by Quantum Theory has not understood it."
By 1927, one group of physicists, led by Erwin Schrödinger, argued that the quantum behaviour of particles could be described in terms of the physics of waves, summed up in his famous Wave Equation. Others, clustered around Werner Heisenberg, believed that matter and radiation had a particle nature and behaved according to Heisenberg's fundamental Uncertainty Principle.
Bohr, who was the head of Heisenberg's department at the University of Copenhagen, attempted to pull together all the theories and experimental results into a unified view, now known as the Copenhagen Interpretation. He realised that there was no such thing as objective science. The very act of measurement or observation of a system decides the way it will behave. Schrödinger's wave equation holds within it all the probabilities for the behaviour of a particle in a given situation and, when we have decided how to observe it, the equation collapses down so that everything except the observed outcome is lost. The idea that we can stand, God-like, outside the Universe and observe objectively what happens inside is fanciful.
So, why don't we observe this strange behaviour in everyday objects like people and rugby balls? For large objects, Schrödinger's equation just reduces to our more familiar Newton's laws of motion. Ultimately, Bohr's Copenhagen Interpretation simply said that for microscopic things, we use a quantum explanation but for large, macroscopic objects, classical physics is all that we need. This is the orthodox position of Physics but it is hardly satisfactory and there have been many other interpretations since Copenhagen. One such, known as GRW (after Ghirardi, Rimini and Weber), says that the wave equations or wave functions describing atoms randomly collapse. An atom's wave function may collapse maybe once in a hundred million years, so this is not seen in the normal course of an experiment. But for large objects like people and rugby balls, where each atom is linked to many others, the collapse of one atom's wave function would trigger the collapse of the whole object to one quantum state. There are so many atoms in a rugby ball that even if each one's wave function collapsed only once in a hundred million years, there would still be one collapse every trillionth of a second. The ball would thus behave like a ball and be located in one place at one time, unlike an electron, for example, which does not have an existence in just one place until the act of observing it creates it there.
Confused? You should be, but read about Schrödinger's cat and see if that helps. And remember this: quantum mechanics confused Einstein and he didn't like it. Yet quantum theory is the most tested theory in all of science and nothing it has ever predicted has been found to be wrong!
Neils Bohr
Werner Heisenberg
Hubble’s Law
Edwin Hubble was the first astronomer to discover that galaxies outside our own Milky Way are moving away from us. By taking measurements of the brightnesses of stars in these galaxies, Hubble could work out how far away they are. By measuring the amount by which the wavelengths of the light from each galaxy has been stretched on its way here, (the so-called Red Shift), he was able to calculate how fast each galaxy was rushing away.
Plotting a graph of the speed of recession against the distance away, Hubble found it to be a straight line through the origin. In other words, the speed at which a galaxy is rushing away is proportional to its distance from us. The further away a galaxy is, the faster it is receding. This is known as Hubble's Law. The gradient of the graph is the Hubble Constant.
Hubble's discovery was the first evidence that the Universe is expanding and therefore the first evidence for the Big Bang origin of everything. From graphs such as Hubble's, we can work out the age of the Universe - about 13.7 billion years. We can also estimate its size: about 1026 metres across. In words, that would be a hundred million million million million metres.
Edwin Hubble was the first astronomer to discover that galaxies outside our own Milky Way are moving away from us. By taking measurements of the brightnesses of stars in these galaxies, Hubble could work out how far away they are. By measuring the amount by which the wavelengths of the light from each galaxy has been stretched on its way here, (the so-called Red Shift), he was able to calculate how fast each galaxy was rushing away.
Plotting a graph of the speed of recession against the distance away, Hubble found it to be a straight line through the origin. In other words, the speed at which a galaxy is rushing away is proportional to its distance from us. The further away a galaxy is, the faster it is receding. This is known as Hubble's Law. The gradient of the graph is the Hubble Constant.
Hubble's discovery was the first evidence that the Universe is expanding and therefore the first evidence for the Big Bang origin of everything. From graphs such as Hubble's, we can work out the age of the Universe - about 13.7 billion years. We can also estimate its size: about 1026 metres across. In words, that would be a hundred million million million million metres.
Werner Heisenberg
Schrödinger’s Cat
This is the most famous feline of all time, (with the possible exceptions of Top Cat and Tom from Tom and Jerry); certainly in all of science. Like Fred Hoyle's invention of the name Big Bang as a term of ridicule for an idea he could not accept, (namely the appearance of the Universe out of nothing), Erwin Schrödinger invented his cat to illustrate what he saw as the absurdity of the Copenhagen Interpretation of quantum mechanics. This said that there is no such thing as an isolated experiment; that the act of observation decides the outcome of experiments on the microscopic scale. When a photon of light triggers a detector, we know where it is. Until that time, the photon is everywhere at once.
Schrödinger thought that was absurd. Since quantum mechanics describes the whole world of Physics, not just things on the atomic scale, he argued that for large-scale objects, the same idea would have to apply. A real object, like you or me, would have to exist in all possible states until it was observed. Enter the cat.
Schrödinger imagined a cat shut in a steel box. Also in the box is a device, which the cat cannot tamper with, containing a tiny piece of a radioactive isotope and a Geiger counter. The half-life of the isotope is very long so that in one hour, maybe one of its atoms will decay but, equally likely, none will. If a decay does happen, the Geiger counter is triggered and the electric current it produces activates a relay, which releases a hammer. The hammer smashes a flask of cyanide and the cat is killed. After one hour, when the box lid is opened, there is a 50:50 chance of finding the cat alive or dead.
According to the Copenhagen Interpretation of quantum mechanics, however, before the lid is opened and the cat is observed, it can be neither alive nor dead. It is both. It exists in a state of all possibilities. Opening the lid decides whether the cat is alive or dead.
Schrödinger was not convinced but we cannot, today, simply dismiss the idea because experiment after experiment shows that, on the scale of atoms and photons, this is exactly how the Universe behaves.
This is the most famous feline of all time, (with the possible exceptions of Top Cat and Tom from Tom and Jerry); certainly in all of science. Like Fred Hoyle's invention of the name Big Bang as a term of ridicule for an idea he could not accept, (namely the appearance of the Universe out of nothing), Erwin Schrödinger invented his cat to illustrate what he saw as the absurdity of the Copenhagen Interpretation of quantum mechanics. This said that there is no such thing as an isolated experiment; that the act of observation decides the outcome of experiments on the microscopic scale. When a photon of light triggers a detector, we know where it is. Until that time, the photon is everywhere at once.
Schrödinger thought that was absurd. Since quantum mechanics describes the whole world of Physics, not just things on the atomic scale, he argued that for large-scale objects, the same idea would have to apply. A real object, like you or me, would have to exist in all possible states until it was observed. Enter the cat.
Schrödinger imagined a cat shut in a steel box. Also in the box is a device, which the cat cannot tamper with, containing a tiny piece of a radioactive isotope and a Geiger counter. The half-life of the isotope is very long so that in one hour, maybe one of its atoms will decay but, equally likely, none will. If a decay does happen, the Geiger counter is triggered and the electric current it produces activates a relay, which releases a hammer. The hammer smashes a flask of cyanide and the cat is killed. After one hour, when the box lid is opened, there is a 50:50 chance of finding the cat alive or dead.
According to the Copenhagen Interpretation of quantum mechanics, however, before the lid is opened and the cat is observed, it can be neither alive nor dead. It is both. It exists in a state of all possibilities. Opening the lid decides whether the cat is alive or dead.
Schrödinger was not convinced but we cannot, today, simply dismiss the idea because experiment after experiment shows that, on the scale of atoms and photons, this is exactly how the Universe behaves.
Erwin Schrödinger
Quantum Entanglement
It has been generally known for more than a century, (in fact since Einstein proposed his theories of Relativity), that nothing in the Universe travels faster than the speed of light; neither the particles we normally associate with matter nor the photons we usually identify as the carriers of radiation. This has consequences in the world of quantum physics: quantum theory tells us that objects exert forces on each other by exchanging particles called bosons. Some bosons travel at the speed of light, (like the virtual photons responsible for the electromagnetic force that holds you together); some travel more slowly, (like the W-particles responsible for the weak nuclear force which causes radioactive beta decay)> None travels faster. This means that no force can act faster than the speed of light. If the Sun suddenly disappeared so that the gravity force holding the Earth in its orbit were switched off, it would be 11 minutes before the Earth realised it and flew out of its orbit, even if gravity acted at the speed of light. We need to understand this in order to appreciate the weirdness of quantum entanglement.
In the microscopic world, particles have a property known as "spin". Particles with spin are not actually spinning but the term is related to their angular momentum. Different particles have different amounts of spin. Electrons, for instance, have the minimum amount and can have two types of spin which we can call clockwise and anticlockwise. Spin in a conserved quantity, like momentum, energy and charge. The total spin of a system cannot change.
Now imagine a process in which two electrons are created from a photon of radiation. To conserve the total spin in the process, one electron would have to be created with clockwise spin and the other with anticlockwise spin. Until one of the electrons is observed, they both exist in both spin states. (Quantum mechanics calls it a superposition of states - see The Copenhagen Interpretation and Schrödinger's Cat for more on this). Remember that it is the act of observation that decides the quantum state of the electron. Next, suppose that the two electrons, isolated in boxes, are moved very far apart - to opposite sides of the Universe if necessary. The lid of one of the boxes is opened and the electron is observed. The observation collapses the electron's wave function and it is observed to have clockwise spin. At that instant, if conservation of spin is not to be violated, the other electron, at the opposite side of the Universe, must also have its wave function collapsed and have anticlockwise spin. The observation which made the first electron have clockwise spin has instantly caused the other electron to have the opposite spin.
How is this possible if the "signal" from the first electron must cross the Universe (1026 metres) at no more than the speed of light, (300 million metres per second)? This would require a time of 10 billion years! Einstein called this action at a distance "spooky". Relativity has no explanation for it. Yet it can be demonstrated experimentally. The fact seems to be that the electrons, having once interacted, have wave functions that are somehow entangled. A change in one must necessarily cause a change in the other.
It has been generally known for more than a century, (in fact since Einstein proposed his theories of Relativity), that nothing in the Universe travels faster than the speed of light; neither the particles we normally associate with matter nor the photons we usually identify as the carriers of radiation. This has consequences in the world of quantum physics: quantum theory tells us that objects exert forces on each other by exchanging particles called bosons. Some bosons travel at the speed of light, (like the virtual photons responsible for the electromagnetic force that holds you together); some travel more slowly, (like the W-particles responsible for the weak nuclear force which causes radioactive beta decay)> None travels faster. This means that no force can act faster than the speed of light. If the Sun suddenly disappeared so that the gravity force holding the Earth in its orbit were switched off, it would be 11 minutes before the Earth realised it and flew out of its orbit, even if gravity acted at the speed of light. We need to understand this in order to appreciate the weirdness of quantum entanglement.
In the microscopic world, particles have a property known as "spin". Particles with spin are not actually spinning but the term is related to their angular momentum. Different particles have different amounts of spin. Electrons, for instance, have the minimum amount and can have two types of spin which we can call clockwise and anticlockwise. Spin in a conserved quantity, like momentum, energy and charge. The total spin of a system cannot change.
Now imagine a process in which two electrons are created from a photon of radiation. To conserve the total spin in the process, one electron would have to be created with clockwise spin and the other with anticlockwise spin. Until one of the electrons is observed, they both exist in both spin states. (Quantum mechanics calls it a superposition of states - see The Copenhagen Interpretation and Schrödinger's Cat for more on this). Remember that it is the act of observation that decides the quantum state of the electron. Next, suppose that the two electrons, isolated in boxes, are moved very far apart - to opposite sides of the Universe if necessary. The lid of one of the boxes is opened and the electron is observed. The observation collapses the electron's wave function and it is observed to have clockwise spin. At that instant, if conservation of spin is not to be violated, the other electron, at the opposite side of the Universe, must also have its wave function collapsed and have anticlockwise spin. The observation which made the first electron have clockwise spin has instantly caused the other electron to have the opposite spin.
How is this possible if the "signal" from the first electron must cross the Universe (1026 metres) at no more than the speed of light, (300 million metres per second)? This would require a time of 10 billion years! Einstein called this action at a distance "spooky". Relativity has no explanation for it. Yet it can be demonstrated experimentally. The fact seems to be that the electrons, having once interacted, have wave functions that are somehow entangled. A change in one must necessarily cause a change in the other.
Christopher Monroe, whose experiments on entanglement have made a major advance toward the long-sought goal of super-fast quantum computing
Chaos Theory
In a normally beating heart, the muscle tissue contracts and relaxes in a repetitive, periodic way. Electrical signals from the brain travel as a coordinated wave through the three-dimensional structure of the heart. When the signals arrive, each cell contracts then relaxes for a critical time. In heart attack victims, the wave breaks up and the heart's muscle tissue writhes in an uncoordinated way, like a bag of worms. Its behaviour has gone from periodic to chaotic and that chaos is stable. The only way to stop it is to stop the heart with a large electric shock and hope it starts pumping regularly again.
As the patient lies, recovering, in his hospital bed in London, he may not be aware that the storm system causing the rain to lash against the window of his room was caused by the fluttering of the wings of a butterfly above the trees in Beijing. He may have heard of the Butterfly Effect but is less likely to see any link between the storm outside and the heart attack he has just suffered. If so, he could do worse than spend some of his enforced rest reading about the theory of Chaos.
The Butterfly Effect, which is synonymous with Chaos Theory, was discovered by Edward Lorenz, an American mathematician and meteorologist, who used a very early computer to try and forecast the weather. On day, he typed the same initial conditions into his machine, expecting it to come up with the same forecast. What it produced was an outcome that was wildly different. The reason, he later discovered, was that he had rounded the figures slightly. Numbers which were originally given to six decimal places, he typed in to only three. This small, seemingly insignificant, change in the initial conditions had produced an enormous change in the outcome. Just as a butterfly flapping its wings over one city could produce the tiny changes in initial conditions that would lead to storms on another continent.
The study of chaotic systems could not begin in earnest until the development of more powerful computers in the seventies. It is a science that involves physicists, mathematicians, chemists and biologists. It is a science of the global nature of systems. Some even say that, alongside Relativity and Quantum Mechanics, Chaos is the greatest scientific discovery of the twentieth century.
Chaotic behaviour turns out to be widespread in nature. It even affects the orbits of planets: Neptune has twelve satellites but instead of following the same orbits year after year, chaos causes these moons to move around in unstable orbits which are constantly changing. The Lorentz Attractor diagram shown opposite is really a three-dimensional graph of displacement with time for chaotic motion. The object never follows the same path.
In a normally beating heart, the muscle tissue contracts and relaxes in a repetitive, periodic way. Electrical signals from the brain travel as a coordinated wave through the three-dimensional structure of the heart. When the signals arrive, each cell contracts then relaxes for a critical time. In heart attack victims, the wave breaks up and the heart's muscle tissue writhes in an uncoordinated way, like a bag of worms. Its behaviour has gone from periodic to chaotic and that chaos is stable. The only way to stop it is to stop the heart with a large electric shock and hope it starts pumping regularly again.
As the patient lies, recovering, in his hospital bed in London, he may not be aware that the storm system causing the rain to lash against the window of his room was caused by the fluttering of the wings of a butterfly above the trees in Beijing. He may have heard of the Butterfly Effect but is less likely to see any link between the storm outside and the heart attack he has just suffered. If so, he could do worse than spend some of his enforced rest reading about the theory of Chaos.
The Butterfly Effect, which is synonymous with Chaos Theory, was discovered by Edward Lorenz, an American mathematician and meteorologist, who used a very early computer to try and forecast the weather. On day, he typed the same initial conditions into his machine, expecting it to come up with the same forecast. What it produced was an outcome that was wildly different. The reason, he later discovered, was that he had rounded the figures slightly. Numbers which were originally given to six decimal places, he typed in to only three. This small, seemingly insignificant, change in the initial conditions had produced an enormous change in the outcome. Just as a butterfly flapping its wings over one city could produce the tiny changes in initial conditions that would lead to storms on another continent.
The study of chaotic systems could not begin in earnest until the development of more powerful computers in the seventies. It is a science that involves physicists, mathematicians, chemists and biologists. It is a science of the global nature of systems. Some even say that, alongside Relativity and Quantum Mechanics, Chaos is the greatest scientific discovery of the twentieth century.
Chaotic behaviour turns out to be widespread in nature. It even affects the orbits of planets: Neptune has twelve satellites but instead of following the same orbits year after year, chaos causes these moons to move around in unstable orbits which are constantly changing. The Lorentz Attractor diagram shown opposite is really a three-dimensional graph of displacement with time for chaotic motion. The object never follows the same path.
The Lorentz Attractor - a picture of chaos
Black Holes
The first thing to know about black holes is that they are not black and they aren't holes. Everyone has heard of them and references to them pepper science fiction novels but they may not attract so much attention if we simply referred to them as what they are: dead stars.
If a ball is thrown into the air, it reaches a certain height then falls back down, pulled by gravity. To make it rise higher, you would have to throw it faster. To make the ball rise so high that it escapes the gravity pull of the Earth altogether and never falls back down, you would need to throw it very fast indeed. This "escape speed" is about 11 kilometres per second for the Earth. On more massive planets, the escape speed is greater because the gravitational pull is stronger.
Black holes are the collapsed remains of massive stars, more than three times the mass of our Sun. The collapse cases them to shrink to a very small size. The black hole at the centre of our galaxy, for example, has a mass of a million Suns squashed into a diameter of only 10 million kilometres. The escape speed of a black hole is greater than the speed of light and since his is the Universal speed limit, no ball or rocket or anything else could ever be thrown upwards fast enough to escape. This is why is was called "black": it cannot be seen.
But black holes are not completely black. In the 1970s, Stephen Hawking showed that if particle-antiparticle pairs are created out of the space near the edge of a black hole, one may be pulled into the black hole while the other, which would be travelling in the opposite direction, would escape and appear as though it had been given out by the black hole. This is now known as Hawking Radiation but, as yet, none has been detected.
The first thing to know about black holes is that they are not black and they aren't holes. Everyone has heard of them and references to them pepper science fiction novels but they may not attract so much attention if we simply referred to them as what they are: dead stars.
If a ball is thrown into the air, it reaches a certain height then falls back down, pulled by gravity. To make it rise higher, you would have to throw it faster. To make the ball rise so high that it escapes the gravity pull of the Earth altogether and never falls back down, you would need to throw it very fast indeed. This "escape speed" is about 11 kilometres per second for the Earth. On more massive planets, the escape speed is greater because the gravitational pull is stronger.
Black holes are the collapsed remains of massive stars, more than three times the mass of our Sun. The collapse cases them to shrink to a very small size. The black hole at the centre of our galaxy, for example, has a mass of a million Suns squashed into a diameter of only 10 million kilometres. The escape speed of a black hole is greater than the speed of light and since his is the Universal speed limit, no ball or rocket or anything else could ever be thrown upwards fast enough to escape. This is why is was called "black": it cannot be seen.
But black holes are not completely black. In the 1970s, Stephen Hawking showed that if particle-antiparticle pairs are created out of the space near the edge of a black hole, one may be pulled into the black hole while the other, which would be travelling in the opposite direction, would escape and appear as though it had been given out by the black hole. This is now known as Hawking Radiation but, as yet, none has been detected.
Stephen Hawking - showed that black holes are not black
Are you going for interview? What will you say when they ask you which particular parts of physics especially grab your interest? Below are a dozen of the biggest ideas and discoveries in Physics of the last century (or two).
Just click on the box to go to the idea. Roll your mouse over the pictures to see larger versions.
Just click on the box to go to the idea. Roll your mouse over the pictures to see larger versions.
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