The origin-the debate between particle theory and wave theory of light has been controversial in the scientific community since Newton's time. At that time, there were two completely different views in the scientific community. One view is that Newton thinks that light is a particle, and the other view is that Hooke, Huygens and others think that light is a wave. Because of Newton's dominant position and prestige in physics, his theory of light particles overtook the theory of light fluctuation and became the mainstream view in the scientific community in 100.
It was not until the publication of Newton Optics 180 1 year that the British scientist Thomas Young reversed this situation with an experiment, which revived the theory of light fluctuation and even became the mainstream. This experiment is the double-slit interference experiment!
A powerful proof of the light fluctuation theory-the double-slit interference experiment is derived from the single-slit diffraction experiment, which makes a beam of light pass through two closely spaced slits and project onto the screen behind the slit. Generally speaking, when light passes through a vertical slit, it will be diffracted-the phenomenon that light bends and propagates around obstacles. What you see on the screen is not vertical bright stripes, but horizontal bright bands. As shown in the figure below, this phenomenon was discovered by Italian Jesuit priest grimaldi grimaldi as early as17th century, and named it diffraction.
When the bright bands of two seams overlap behind the screen, the brighter band should be displayed in theory, but something magical happened, and finally the bright and dark stripes are displayed on the screen ... (lower part of the picture below)
This is a strange thing, because there should be light on the dark lines of these stripes, but when the light of two seams overlaps, there will be no light at those positions. This phenomenon cannot be explained by Newton's particle theory. Inspired by the interference of water waves, Thomas Young proposed that light is a kind of wave and dark lines are produced by the interference of light waves.
The theory of light particle revival-Einstein's photoelectric effect The revolution of human cognition of the universe is always driven by a few geniuses. The experiment of distance double-slit interference proves that the wave theory of light has passed one hundred years, and a genius has refreshed people's cognition of light again. This genius is Einstein, the greatest physicist of the twentieth century.
1905, Einstein, who just graduated with a doctor's degree in physics and worked as a clerk in the patent office, published a paper entitled "Heuristic Views on Light Emission and Transformation", which explained the photoelectric effect in the form of light as an energy quantum. This groundbreaking theory won him the 192 1 year Nobel Prize in Physics.
The Exploration of Particles and Waves —— The proposal of light quantum in the single-photon double-slit experiment revived the theory of light particles, but the question followed: Is light a particle or a wave? Both forms have experimental evidence, and it seems that both are correct. But the problem is coming again. Wave and particle are two completely different characteristics. Under what circumstances will light show the characteristics of waves? Under what circumstances will it show the characteristics of particles?
Four years after the light quantum hypothesis was put forward, Jeffery Taylor, a British physics graduate student, designed a weak light double-slit experiment. He used blackened glass to greatly reduce the brightness of the light source until theoretically only one photon passes between the light source and the screen at any time. Then he exposed the camera's photographic film for a long time and recorded the light passing through the double seam.
After the photos were developed, a magical scene appeared. Even if the photons pass through the double slits one by one, clear light-dark interference fringes still appear in the final exposed photos.
This means that even if only one photon passes through the double slit, it will still interfere, so who is the photon interfering? There seems to be only one explanation: photons interfere with themselves.
Strange physics-the problem of wave-particle duality of light was not solved by French scientist de Broglie until 10 years later. He proposed wave-particle duality in his doctoral thesis in physics. He pointed out that not only photons, but also all microscopic particles have the characteristics of waves, which he called matter waves. Matter wave theory predicts that a single electron can also produce interference fringes through double slits.
When do light waves become particles? After determining the wave-particle duality of light, a question is put before scientists: when did light waves become particles?
An obvious fact is that in all experiments, light is finally presented in the form of particles! For example, in the photoelectric effect, electrons are ejected, and some sporadic light spots will appear in underexposed photos, so the answer is obvious: light is particles when observed. That is, observation turns waves into particles? What if the measurement happened earlier? Scientists decided to upgrade the double-slit interference experiment-measuring which slit particles pass through.
The myth in popular science books-interference fringes disappear at a glance ... Many people see that the descriptions of double-slit interference observation experiments in popular science books are almost fairy tales. Some stories are taken with cameras, while others are simpler and can be seen directly with eyes. These descriptions are obviously not facts, but some imaginary thought experiments. Now it seems that these thought experiments are extremely misleading.
Direct observation will not cause interference fringes to disappear! Whether you look at it with your eyes or shoot it with your camera, you won't!
Revealing the secret-it is not surprising to observe the interference fringes, which may be different from many people's imagination. Scientists are actually not scared by the experimental results of observing the disappearance of interference, but the results are created in their expectation. What scientists really want to do is not to observe, but to measure!
They want to measure which gap the particles pass through and how they pass through it. Taking photons as an example, there is no way to see photons from the side without disturbing them, so the measurement method is actually to let photons pass through one of the slits ... for example, a vertical polarizer is placed on one of the slits, so that all horizontally polarized photons cannot pass through this slit. ...
Seeing the cleverness here, you must have found something fishy. What's so special about this? Light can only pass through one of the gaps. Isn't this equivalent to a single seam? As we said before, photons will interfere with themselves. Now photons can only pass through one of the cracks. Who else can they interfere with? ……
What's really weird? To sum up, it is not surprising that observation leads to the disappearance of interference fringes, but this does not mean that quantum mechanics is not strange! What is really weird about quantum mechanics is the "fluctuating" state of microscopic particles before they are observed! The state of this wave is called superposition state, and the really weird thing about quantum mechanics is this superposition state! But this is beyond the scope of this paper, and we will talk about superposition state later.
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