"Light and matter are both single entities, and the apparent duality arises in the limitations of our language." - Heisenberg
Paradox of wave-particle duality: How can light show properties of a particle and a wave?
The concept of light being both a wave and particle at the same time (a paradox) is the foundation quantum mechanics was built on. English scientist Thomas Young challenged the existing theory (Isaac Newton's) that light is a particle. In an effort to verify the properties of light, Thomas Young created the double-slit light experiment during the early 1800s.
"But what is light really? Is it a wave or a shower of photons? There seems no likelihood for forming a consistent description of the phenomena of light by a choice of only one of the two languages. It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do".
-- Albert Einstein and Leopold Infeld, The Evolution of Physics, pg 262-263
One 2 inch by 2 inch square is cut in the middle of a piece of thick paper (opaque, construction paper). A second piece of thick paper (opaque, construction paper) has a 2 inch by 2 inch square cut out. Two large squares of aluminum foil (large enough to cover the squares) in the thick paper are cut out. The foil is taped to each paper, covering the holes. A razor blade cuts a 1 inch slit in the center of the aluminum foil on the first. On the second, 2 razor blades together to cut 2 parallel slits (1 inch in length) in the center.
Laser light shines through the slit in the first paper. The reflective side of aluminum foil faces away from the laser.
Light shines through the slits and interference patterns are observed.
Interference patterns are visible. Light is showing properties of a wave. Interference patterns are the distinct bands of color separated by dark regions. The interference patterns happen when light travels as a wave.
Atoms, electrons, photons exhibit a wave-particle duality. It starts to get weird. Is the electron or photon a particle or a wave? Or is it BOTH - a wave-particle duality?
If photons behave as waves, one photon at a time can be sent through the double-slit to confirm. Wrong. When a single photons are fired through the double-slit, interference patterns continue to be observed! The single photon creates interference patterns with itself. This behavior uncovers a fundamental truth of quantum mechanics. In a wave-form state, quantum objects exist in a probability state.
The photon will "probably" travel through either slit at any given time (Schrodinger's Probability Equation). The wave-form ends up as a particle (waveform collapsing) when it hits the wall.
Fundamental elements of matter pop in and out of existence through probability states.
During the "which-way" experiment, scientists started to measure probability states as photons travel through each slit. The "which-way" detector attempts to measure (or observe) the trajectory of each particle to determine which slit the particle will pass through.
The result shocked the science community because the "act of measuring/observing" causes the collapse of the waveform and results in 2 lines. By measuring or observing the particles with a detector, no interference patterns were observed! The act of measuring causes the waveform to collapse. The particle exists as a "probability wave" but becomes a distinct particle as soon as it is measured. What constitutes observation or measurement and why?
How does conscious measurement or observation change the nature of matter?
John Archibald Wheeler created the "delayed-choice" experiment that could change a barrier from one slit to two slits while the particle is in transit. By changing the structure of the second slit, it altered the behavior of the particle at the first slit. An observation gathered from the delayed-choice experiment:
Future events change the present.
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