tohave any metal objects or instruments present because of the magnetic fields involvedâthese objects could be wrapped in metamaterial to block their interaction with the magnetism. And, as with any breakthrough area of scientific research, there will be umpteen other applications too, many of which no one will have seen coming.
CHAPTER 12
How to be everywhere at once
⢠Youngâs experiment
⢠Why bands?
⢠Coherent light
⢠Monochrome vision
⢠Lasers
⢠Quantum double slits
⢠Schrödingerâs equation
In the busy 21st-century world, many of us might wish we could be in more than one place at the same time. For subatomic particles this isnât a problem. According to the abstruse laws of quantum mechanics, particles of matter also behave like wavesâenabling them to be not just in two places at once, but everywhere. One simple experiment gave physicists the insight they needed to unravel the laws of quantum theory.
Youngâs experiment
The double-slit experiment is probably the most startling demonstration of quantum physics. It was first carried out before quantum theory was even a twinklein the eye of Max Planck, Einstein and colleagues. Nobel prize-winning US quantum physicist Richard Feynman would later remark that pretty much every aspect of quantum physics is encapsulated by this astounding experiment. So what is it?
British physicist Thomas Young was the first person to perform this experiment in 1801. Young was trying to figure out whether light is made of particles or waves. To do this he shone a beam of light onto a screen with a pair of narrow slits cut into it. The light passed through the slits to illuminate a second screen, this time with no slits. Young postulated that if light was made of particles then the second screen would be evenly illuminated by the light from the two slits. But if it was made of waves there should be a pattern of bright and dark bands, known as âinterference fringes.â And this second possibility is exactly what Young observed.
The fringes form as the light waves from each slit collide with one another at the second screen and overlap, rather like the way ripples on water can overlap. When this happens on water, the two waveforms add together. So where the peaks of two waves coincide, a large peak is formed (called constructive interference); where the dips, or troughs, of two waves meet, a deep trough results (constructive interference again); and where a peak and a trough of equal sizemeet, the two simply cancel one another out (known as destructive interference). This principle, in which waveforms add together, is known as superposition.
Exactly the same thing happens with light rays at the screen in Youngâs experiment. A ray of light is a wave, like a wave on a string, with peaks and troughs. Where a wave peak in the light ray from one slit coincides at the screen with a peak in the light ray from the other, a bright fringe is formed. And the same thing happens where two troughs meet. But where a peak and a trough coincide the two light waves cancel one another out to form a dark band.
Why bands?
Bands are formed because the light from one slit falling on the second screen will generally have traveled a different distance from the light from the other slit. Right in the middle of the second screen both light rays have traveled exactly the same distance, so their peaks and troughs both overlap exactly. There is thus constructive interference, resulting in an overlapping bright image of both slitsâi.e. a bright band. Look to either side of the bright central band and you reach a point where the ray from one slit has traveled half a wavelength less than the other. These rays then interfere destructively, resulting in overlapping dark images of each slitâi.e. a dark band.
Coherent light
Of course, this all assumes that the light waves were all in lockstep with one another when they passed through
CHAPTER 12
How to be everywhere at once
⢠Youngâs experiment
⢠Why bands?
⢠Coherent light
⢠Monochrome vision
⢠Lasers
⢠Quantum double slits
⢠Schrödingerâs equation
In the busy 21st-century world, many of us might wish we could be in more than one place at the same time. For subatomic particles this isnât a problem. According to the abstruse laws of quantum mechanics, particles of matter also behave like wavesâenabling them to be not just in two places at once, but everywhere. One simple experiment gave physicists the insight they needed to unravel the laws of quantum theory.
Youngâs experiment
The double-slit experiment is probably the most startling demonstration of quantum physics. It was first carried out before quantum theory was even a twinklein the eye of Max Planck, Einstein and colleagues. Nobel prize-winning US quantum physicist Richard Feynman would later remark that pretty much every aspect of quantum physics is encapsulated by this astounding experiment. So what is it?
British physicist Thomas Young was the first person to perform this experiment in 1801. Young was trying to figure out whether light is made of particles or waves. To do this he shone a beam of light onto a screen with a pair of narrow slits cut into it. The light passed through the slits to illuminate a second screen, this time with no slits. Young postulated that if light was made of particles then the second screen would be evenly illuminated by the light from the two slits. But if it was made of waves there should be a pattern of bright and dark bands, known as âinterference fringes.â And this second possibility is exactly what Young observed.
The fringes form as the light waves from each slit collide with one another at the second screen and overlap, rather like the way ripples on water can overlap. When this happens on water, the two waveforms add together. So where the peaks of two waves coincide, a large peak is formed (called constructive interference); where the dips, or troughs, of two waves meet, a deep trough results (constructive interference again); and where a peak and a trough of equal sizemeet, the two simply cancel one another out (known as destructive interference). This principle, in which waveforms add together, is known as superposition.
Exactly the same thing happens with light rays at the screen in Youngâs experiment. A ray of light is a wave, like a wave on a string, with peaks and troughs. Where a wave peak in the light ray from one slit coincides at the screen with a peak in the light ray from the other, a bright fringe is formed. And the same thing happens where two troughs meet. But where a peak and a trough coincide the two light waves cancel one another out to form a dark band.
Why bands?
Bands are formed because the light from one slit falling on the second screen will generally have traveled a different distance from the light from the other slit. Right in the middle of the second screen both light rays have traveled exactly the same distance, so their peaks and troughs both overlap exactly. There is thus constructive interference, resulting in an overlapping bright image of both slitsâi.e. a bright band. Look to either side of the bright central band and you reach a point where the ray from one slit has traveled half a wavelength less than the other. These rays then interfere destructively, resulting in overlapping dark images of each slitâi.e. a dark band.
Coherent light
Of course, this all assumes that the light waves were all in lockstep with one another when they passed through
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