8 Strangest Properties Of Light (work in progress)
Summary
(0:00 - 2:05) A photon has zero mass, which contradicts classical physics like Newton's law where force equals mass times acceleration. Einstein's energy-momentum relation, E² = M²C⁴ + P²C², shows that for a mass-less particle, this simplifies to E = PC. Photons carry momentum, which allows for practical technologies like solar sails used by space agencies to navigate the solar system without fuel. (2:05 - 4:18) While the speed of light in a vacuum is a hard limit, light can be significantly slowed or even stopped in other mediums. By firing a laser through a Bose-Einstein condensate, scientists have reduced light's speed to a jogging pace and even frozen light pulses into atomic excitations, allowing the information to be stored and later released. (4:18 - 5:58) Special relativity dictates that time dilation becomes infinite at the speed of light. Because photons have no rest frame and travel at exactly light speed, they experience zero proper time between emission and absorption. A photon from the Big Bang experiences no duration; for light, the universe has no age. (5:58 - 7:59) The double-slit experiment demonstrates light's dual nature. When unobserved, photons behave as waves, creating an interference pattern. When observed or measured, they collapse into particle behavior, showing a single definite path. The act of observation fundamentally changes the outcome, a core mystery of quantum mechanics. (7:59 - 9:57) In quantum electrodynamics, virtual photons can be mathematically interpreted as traveling in either direction in time. This time symmetry is a fundamental feature of electromagnetic equations. Additionally, delayed-choice quantum eraser experiments suggest that measurements made after the fact can influence the historical behavior of a photon. (9:57 - 11:59) Quantum entanglement allows two photons to share a correlated state across any distance. Measuring one photon instantly determines the state of its partner, violating Bell's inequalities and proving that this correlation is not due to pre-existing properties. This non-locality is a verified feature of the universe, though it cannot be used for faster-than-light communication. (11:59 - 13:47) General relativity explains gravity as the curvature of spacetime. Because light follows this curvature, it bends around massive objects. Astronomers use this phenomenon, known as gravitational lensing, to magnify and observe extremely distant, faint galaxies that would otherwise be invisible to our telescopes. (13:47 - 17:18) The speed of light is not just a velocity, but a fundamental constant of reality that defines the geometry of cause and effect. It establishes the boundaries of light cones for every event. Since the speed of light is invariant for all observers, it serves as the foundation for special relativity, dictating how time and space behave throughout the universe.
—- All right, let's go.
Number eight. Light has no mass but can push you.
The contradiction that holds the universe together. A photon has zero mass. This is not a rounding error, not an approximation. Zero. Exactly zero. And yet, light pushes things. This is not intuition. This is physics. And for most of the history of science, nobody could explain how both things were simultaneously true. Newton's second law force equals mass time acceleration. If mass is zero, force should be zero. Simple. Except that is wrong because the equation F= ma is not the complete picture. Einstein's full energy momentum relation, the one that usually gets reduced to E= MC^2 for convenience, tells a different story. The complete equation is E^2 = M^2 C 4 + P ^2 C^2. For a particle with mass, this simplifies down to the famous version. For a particle with zero mass, a photon, the mass term vanishes entirely. What remains is E= P C. Energy equals momentum time the speed of light. The photon has zero mass, but it has energy. and energy gives it momentum. Momentum without mass. The classical framework simply cannot handle this. Newton could not have predicted it. It required Einstein and it required quantum mechanics working together. The practical consequence, solar sales, real ones. The European Space Agency and Japan's JAXA have both launched spacecraft propelled entirely by photon pressure. The sun fires photons. The photons carry momentum. The momentum transfers to the sail. No fuel, no engine, just light pushing metal through space. The same contradiction that breaks high school physics is also how we navigate the solar system for free.
Number seven, light can be completely stopped and then restarted.
The speed of light in a vacuum, 299,792,458 m/s, is a hard limit. Nothing with mass can reach it. Nothing with mass can exceed it. But the speed of light in a vacuum and the speed of light in general are two different things. In 1999, Lenvestard How, a physicist at Harvard University, slowed light down to 17 m/s. 17 a jogging pace. She did this by firing a laser through a Bose Einstein condensate, a cloud of sodium atoms cooled to within a fraction of a degree of absolute zero, where all the atoms collapse into a single quantum state and behave as one object. The condensate's refractive index becomes so extreme, a 100 trillion times higher than optical fiberglass, that photons effectively spend almost all their time being absorbed and remitted by atoms rather than propagating freely. The result is group velocity, the effective speed at which information and energy travel, reduced by a factor of 30 million. Then he stopped it completely. The light pulse entered the condensate and was converted into a coherent atomic excitation. Its information encoded in the quantum state of the atoms rather than traveling as photons. It sat there stored, not traveling, not oscillating, just held frozen in the quantum structure of ultra cold matter. Then the researchers released it. The atomic excitation reconverted back into photons and the light pulse continued on its way carrying the same information it had when it entered. Light can be frozen, stored, released. The photons that exit are not technically the same photons that entered, but the information they carry is identical. It is the closest thing to a physical pause button on light that has ever been demonstrated in a laboratory.
Number six, light experiences no time.
Absolutely none. From the perspective of a photon, if a photon could have a perspective, no time passes between the moment it is created and the moment it is absorbed. This is not poetic. This is what special relativity predicts. As an object accelerates towards the speed of light, time dilation increases without limit. At the speed of light, if it were possible for a massive object to reach it, time dilation would be infinite. Time stops. Photons travel at the speed of light. Always, exactly. They have no rest frame. You cannot accelerate to the speed of a photon and look at it from a long side. There is no reference frame in which a photon is stationary. The concept of a lapsed time for a photon is mathematically undefined. But if you extrapolate the relativistic equations to the limiting case, the answer is clear. Zero proper time elapsed. This means that light crossing the observable universe a 13.8 billion lightyears at our reference point takes 0 seconds from the photon's perspective. The photon from the big bang that we detect in the cosmic microwave background, the oldest light in a universe, experienced no duration between emission and detection. It did not travel for 13.8 billion years. From its own frame, it was created and absorbed in the same instant. The universe is old, but light does not know that. For light, the universe has no age at all.
Number five, light is both a wave and particle simultaneously, and it chooses based on whether you look.
The double slit experiment. One of the most replicated experiments in the history of physics. Also, one of the most disturbing. Fire photons through two slits toward a detector screen. What do you expect? Two bright lines, one behind each slit. That is what you get with any classical particle. Bullets through two holes make two bands. Simple. That is not what happens. You get an interference pattern. Dozens of alternating bright and dark bands spread across the screen. The kind of pattern that only waves make. The photons are interfering with themselves. Each individual photon apparently goes through both slits simultaneously and interferes with its own wave function on the other side. This only makes sense if the photon is a wave while in transit. Now add a detector at the slits. Try to observe which slit each photon goes through. The interference pattern disappears instantly, completely. Two bands. Classic particle behavior. The act of measurement of observation changes the physical outcome. This is not a disturbance problem. It is not that measuring equipment nudges the photon. The Copenhagen interpretation holds that the photon does not have a definite path until it is observed. Before measurement, it exists as a wave of probabilities distributed across both paths. Observation forces it to collapse into a single definite location. Richard Fineman said, "The double slit experiment contains the only mystery of quantum mechanics. Everything else is a variation of it. Light does not have a fixed nature. Its nature depends on whether you are looking."
Number four, light can travel backward in time in a specific mathematical sense.
This one requires precision. It is real physics, not science fiction. In 1949, physicist Richard Feineman introduced a mathematical framework for calculating quantum electronamics, the theory of how light and matter interact. In his framework, virtual photons, the carriers of the electromagnetic force between charged particles, can be interpreted as traveling in either direction in time. An electron emitting a photon that travels backward in time is mathematically equivalent to an electron absorbing a photon traveling forward in time. This is not a trick. This is a genuine symmetry of the equations. The laws of electromagnetism are time symmetric. They work identically whether you run time forward or backward. The photon propagator and quantum field theory, the mathematical object that describes how photons travel between interactions, includes terms corresponding to both forward and backward time propagation. There is a more concrete version. Experiments in delayed choice quantum eraser, first proposed by John Wheeler and later conducted by multiple experimental groups, show that what a photon did in the past can be influenced by measurements made after the fact. In the Kim delayed choice experiment in 2000, a decision made after a photon had already been detected appeared to change whether that photon had previously behaved as a wave or particle. The interpretation is disputed. the results are not. Fineman's own words on this in his formulation of quantum electronamics were that the distinction between particles traveling forward and backward in time is not fundamental. The universe does not enforce a single direction for light the way we intuitively expect.
Number three, two photons can become entangled across any distance and stay that way forever.
Quantum entanglement is real, confirmed by experiment, and deeply strange. Two photons generated together under specific conditions share a quantum state. The polarization of one is correlated with the polarization of the other. Before measurement, neither has a definite polarization. Both exist in superp position. The moment you measure one, the other instantly resolves into the correlated state. No signal passes between them. No information is exchanged. The correlation simply exists non-locally across whatever distance separates them. In 1964, John Bell derived a set of inequalities. Bell's inequalities that any local hidden variable theory must satisfy. If the correlations between particles were due to pre-existing properties established at creation rather than genuine quantum non-locality, the measurements would fall within Bell's limits. Every experiment runs since. Most definitively, the 2022 Nobel Prize-winning experiments by Elaine Aspect, Koser Clauser, and Anton Zylinger have violated Bell's inequalities. The correlations are real. They cannot be explained by pre-existing properties. The particles genuinely do not have definite states before measurement and they genuinely do influence each other's outcomes instantly regardless of distance. This does not allow faster than light communication. You can't use entanglement to transmit a message because you cannot control what value you measure, only predict its correlation with the other particle. But the non-locality is real. Two photons separated by light years can share a quantum state that resolves in correlated ways simultaneously. Distance is not a factor. The universe is not locally real for photons.
Number two, light bends around mass and we use it to see the early universe.
Gravity is the curvature of spaceime. Mass curves spacetime. Light follows the curvature of spaceime. Therefore, mass bends light. Einstein predicted this in 1915. In 1919, Arthur Edington traveled to the island of Principe off the west coast of Africa to photograph a solar eclipse. By measuring the apparent positions of stars near the eclipse sun and comparing them to their known positions, he confirmed that starlight passing close to the sun was deflected by exactly the amount Einstein's general relativity predicted. The result made Einstein famous overnight. But the practical application goes far beyond confirming theory. When a massive object, a galaxy cluster for example, sits between Earth and something behind it, its gravitational field acts as a lens. It bends and magnifies the light from whatever is behind it. This is gravitational lensing and astronomers use it deliberately. The Hubble Space Telescope has used galaxy clusters as gravitational lenses to see objects 13 billion lightyears away that would otherwise be too faint to detect. The James Web Space Telescope has pushed this further, using lensing clusters to observe galaxies from the first 400 million years of the universe's existence. Light that would be invisible is being redirected and magnified by the mass of entire galaxy clusters sitting between us and the source. Gravity bends light and we use the universe itself as a telescope because of it.
Number one, light is the speed of causality itself.
Not just a fast thing, a fundamental structure of reality. Nothing travels faster than light. This is taught as a fact about speed. It is actually a fact of causality. Special relativity does not just say light is fast. It says that the structure of spaceime, the geometry of cause and effect, is built around the speed of light. Every event in the universe divides all other events into three categories. Events inside your past light cone, things that could have caused you. events inside your future light cone, things you could cause, and events outside your light cone, things that are completely causally disconnected from you. Space like separated, neither able to cause nor be caused by your current moment, no matter what. The speed of light is the boundary of that cone. It is not the speed limit of a race. It is the edge of the causal universe available to you at this moment. Here is the part that makes it genuinely strange. Every observer in the universe, regardless of their own emotion, measures the speed of light exactly 299,792,458 m/s. Not approximately, exactly. An observer moving toward a light source at half the speed of light does not measure the photons approaching at 1.5 times the speed of light. They measure them at exactly C. An observer moving away at 99% the speed of light measures the same incoming photons at exactly C. The speed of light is invariant across all reference frames. This is not trivially true. It is the foundation of all of special relativity. Einstein built the entire theory on two postulates. One, the laws of physics are the same in all inertial reference frames. Two, the speed of light in a vacuum is constant for all observers regardless of the motion of the source or the observer. Every consequence of special relativity, time dilation, length contraction, E= MC², and the impossibility of faster than light travel falls out of that second postulate. Light does not just travel fast. The constancy of its speed is the reason time works the way it does, the reason mass and energy are equivalent, and the reason the past cannot be changed and the future cannot be reached instantly. It is the structural constant of reality itself. Eight properties. Zero mass with real momentum. A speed that can be frozen and released. No elapsed time across any distance. A nature at changes based on whether you look. Time symmetric propagation. Non-local entanglement across any distance. Gravity bending it into a telescope. and a constant speed that defines the structure of cause and effect in the entire universe. Light is not just fast. It is the most fundamental thing we can observe and we still do not fully understand it.
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