Thursday, April 6, 2023

3946 - DOUBLE – SLIT EXPERIMENT - light is wave-particle

 

-   3946 -   DOUBLE – SLIT  EXPERIMENT  -  light is wave-particle.   Few science experiments are as strange as the double-slit experiment.  Few experiments in modern physics are capable of conveying such a simple idea,  “that light and matter can act as both waves and discrete particles depending on whether they are being observed”.


----  3946  -  DOUBLE – SLIT  EXPERIMENT  -  light is wave-particle

-    Not only has the double-slit experiment been repeated countless times in physics labs around the world, but it has also even spawned many derivative experiments that further reinforce its result, that particles can be waves or discrete objects and that it is as if they "know" when you are watching them.

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-    In 1925, Werner Heisenberg presented his mentor, the eminent German physicist Max Born, with a paper to review that showed how the properties of subatomic particles, like position, momentum, and energy, could be measured.

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-   Max Born saw that these properties could be represented through mathematical matrices, with definite figures and descriptions of individual particles, and this laid the foundation for the matrix description of quantum mechanics.

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-    In 1926, Edwin Schrödinger published his wave theory of quantum mechanics which showed that particles could be described by an equation that defined their waveform; that is, it determined that particles were actually waves.

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-    This gave rise to the concept of wave-particle duality, which is one of the defining features of quantum mechanics. According to this concept, subatomic entities can be described as both waves and particles, and it is up to the observer to decide how to measure them.

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-    That last part is important since it will determine how quantum entities will manifest. If you try to measure a particle's position, you will measure a particle's position, and it will cease to be a wave at all.

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-   If you try to define its momentum, you will find that behaves like a wave and you can't know anything definitive about its position beyond the probability that it exists at any given point within that wave.

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-   Essentially, you will measure it as a particle or a wave, and doing so decides what form it will take.

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-   The easiest way to describe the double-slit experiment is by using light. First, take a source of coherent light, such as a laser beam, that shines in a single wavelength, like purely blue visible light at 460nanometers, and aim it at a wall with two slits in it. The distance between the slits should be roughly the same as the light's wavelength so that they will both sit inside that beam of light.

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-    Behind that wall, place a screen that can detect and record the light that impacts it. If you fire the laser beam at the two slits, on the recording screen behind the wall you will see a stripey pattern.

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-    This is probably not what you might have been expecting, and that's perfectly rational if you treat light as if it were a wave. If the light was a wave, then when the single wave of light from the laser hit both slits, each slit would become a new "source" of light on the other side of the wall, and so you would have a new wave originating from each slit producing two waves.

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-    Where those two waves intersect causes something known as interference, and it can be either constructive or destructive. When the amplitude of the waves overlaps at either a peak or a trough, it acts to boost the wavelength in either direction by adding its energy together.

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-    This is constructive interference, and it produces these brighter bars in this pattern.  When the waves cancel each other out, as when a peak hits a trough, the effect neutralizes the wavelength and diminishes or even eliminates the light, producing the blacked-out spaces in between the blue bars.

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-   But in the case of quantum entities like photons of light or electrons, they are also individual particles. So what happens when you shoot a single photon through the double slits?

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-   One photon alone reacting to the screen might leave a tiny dot behind, which might not mean much in isolation, but if you shoot many single photons at the double slits, those tiny dots that the photon leaves behind on our screen actually show up in that same stripey interference pattern produced by the laser beam hitting the double slits.

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-    In other words, the individual photon behaves as if it passed through both slits like it was a wave.

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-    We can set up a detector in front of one of the slits that can watch for photons and light up whenever it detects one passing through. When we do this, the detector will light up 50% of the time, and the pattern left behind on the screen changes.

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-    And to make things even wilder, we can set up a detector behind the wall that only detects a photon after it has passed through the slit and we get the same result. That means that even if the photon passes through both slits as a wave, the moment it is detected, it is no longer a wave but a particle.

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-    And,  not just that, that second wave emerging from the other slit also collapses back into the particle that was detected passing through the other slit.

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-    In practice, this means that somehow the universe "knows" that someone is watching and flips the metaphorical quantum coin to see which slit the particle passed through. The more individual photons you shoot through the double slit, the closer that photon detector comes to detecting photons 50% of the time.  

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-   Just as flipping a coin 10 times might give you heads 70% of the time while flipping it 100 times might give you tails 55% of the time, and flipping it 1 billion times gives you heads 50.0003% of the time.

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-    This seems to show that not only is the universe watching the observer as well, but that the quantum states of entities passing through the double slits are governed by the laws of “probability”, making it impossible to ever predict with certainty what the quantum state of an entity will be.

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-   The nature of light was a particularly contentious topic, with many, like Isaac Newton himself, arguing in favor of a corpuscular theory of light that held that light was transmitted through particles.

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-   Others believed that light was a wave that was transmitted through "aether" or some other medium, the way sound travels through air and water, but Newton's reputation and a lack of an effective means to demonstrate the wave theory of light solidified the corpuscular view for just shy of a century after Newton published his “Opticks” in 1704.

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-    The definitive demonstration came from the British polymath Thomas Young, who presented a paper to the Royal Society of London in 1803 that described a pair of simple experiments that anyone could perform to see for themselves that light was in fact a wave.

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-   First, Young established that a pair of waves were subject to interference when they overlapped, producing a distinctive interference pattern.   He initially demonstrated this interference pattern using a ripple tank of water, showing that such a pattern is characteristic of wave propagation.

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-   Young then introduced the precursor to the modern double-slit experiment, though instead of using a laser beam to produce the required light source, Young used reflected sunlight striking two slits in a card as its target.

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-   The resulting light diffraction showed the expected interference pattern, and the wave theory of light gained considerable support. It would take another decade and a half before further experimentation conclusively refuted corpuscles in favor of waves, but the double-slit experiment that Young developed proved to be a fatal blow to Newton's theory.

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-    To replicate Young's experiment, you only need as large a box as is practical with a hole cut in it a little smaller than an index card. Then, take an Exacto knife or similar blade for fine cutting work and cut two slits into a piece of cardboard larger than the hole in your box. The slits should be between 0.1mm and 0.4mm apart, as the closer together they are, the more distinct the interference pattern will be.

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                   April 4, 2023               Doubleslit  Experiment                               3946                                                                                                                         

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