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---------------------- 2229 - Photons - The mysteries of light
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- Seeing is the most amazing thing. It is not easy. It takes over one forth of the brain ad your calorie to make it work. Everyone knows that our eyes see with light photons entering the eye and striking the retina in the back of the eye ball. Our brain creates an image from the pattern of chemical excitations created by these light photon energy swaps. This Review delves into the photons themselves. What are these little devils. And, how can they create light?
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- Here are previous Reviews that are available for more background learning about photons:
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- 2181 - The photon is the force carrier for all electric charge repulsive and attractive forces. These forces exit between particles because photons travel between them. One eye blink contains as many erg-seconds as there are centimeters across the Observable Universe.
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- 2113 - The transfer of energy from a photon is dependent on its frequency. The higher the frequency the higher the energy. Ultraviolet light has higher energy than red light. It can give you a sunburn. The human eye has 18 powers of 10 dynamic range. The number of photons entering the eye in daylight is 79,000,000,000,000,000 photons per second.
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- 1863 - From starlight to star dust. How the stars create the light?
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- 1810 - How many photons enter your eye? How many photons exist in the Universe?
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- 1074 - Physics the way I learned it.
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- 2229 - Photons, what have we learned about light?
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- What are these photons really? While trying to understand this simple question Albert Einstein discovered the Theory of Relativity. He imagined himself riding on a photon like a witch riding on a broom. What would he see? Of course he could not see anything because no photons were entering his eyes. But, imagination does not need photons. The brain can create pictures out of the darkness.
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- His thoughts that created the Theory of Relativity is still one of the most puzzling and perplexing discoveries about the nature of the Universe we live in. Science has been studying this theory for the last 100 years. Here is some of what we have learned:
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- The laws of physics that Isaac Newton gave us are what we are used to here on Earth and they remain valid under almost all conditions. The physics and math we were taught in college were all that were used to send astronauts to the Moon and back. But , everything changes as we start moving faster than their rocket and get closer to the speed of light.
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- When we approach the speed of light, 186,000 miles per second, Clocks run at different rates, distances appear to shrink, and objects themselves change color depending on their speed relative to your speed. Yet, at the same time, relativity declares that the laws of physics are the same and invariant for all observers, regardless of their motion. So what does this mean for a photon, which itself moves at the speed of light?
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- Relativity says all inertial frames of reference are equally valid and true. From a photon’s point of view the entire cosmos is flattened into a two-dimensional timeless plane. No space , No time.
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- Let’s try what Einstein did on his broom stick and let’s imagine what happens in three cases: for someone at rest, for someone moving close to the speed of light, and then for a photon that always travels at this speed.
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- For an observer at rest clock ticks by at the same rate it always does: one second per second. You look out at your environment, and the clocks you see there are all running at the same speed as yours: one second per second. Objects appear to be the colors that they actually are, the sizes that they actually are, and nothing behaves counter intuitively. Everything seems exactly as it should. That is the way we know it.
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- This is our conventional experience with the world. Here on Earth, typical human speeds are minuscule compared to the speed of light. Even aboard an airplane moving at nearly the speed of sound, you’re only traveling at 0.0001% the speed of light. From a position at rest relative to your surroundings, you see the three-dimensional Universe consistent with everyone else.
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- A light-clock can be formed by a photon bouncing between two mirrors which can be used to define time for an observer. Every round trip between the mirrors is one unit of time. We will use this for our clock.
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- Even the theory of special relativity, with all the experimental evidence for it, can never be proven, but it can be tested and either validated or falsified. These rules only work for two observers at the same ‘event’ in space and time.
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- An observer moving close to the speed of light in one particular direction, relative to your otherwise stationary surroundings. The first difference you would notice is in terms of time. The clock traveling with you would still travel at the same rate you were used to, one second per second. But the clocks in the outside environment all appear to run slower. One second takes longer.
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- The reason for this is simply that space and time are not independent entities, but inextricably interrelated ones. Every object in the Universe moves through spacetime so that it’s total motion adds up to a certain value. When you are stationary with respect to space, your motion is 100% through time, and time passes for everyone at one second per second.
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- But, when you increase your motion through space, you decrease your motion through time. Relative to you, the environment’s clocks appear to run slow, because the entire environment appears to be moving.
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- Because the speed of light must be invariant for all observers in all reference frames, if time appears to pass more slowly (there is less time), then distances need to contract (there needs to be less distance) in order for the speed of light to remain constant.
Speed = distance / time , like: velocity = miles / hour
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- In addition to length contraction and time dilation, there’s yet another effect that comes into play: redshift and blueshift. In the direction that you’re moving. The light’s wavelength appears compressed, or shorter and bluer. In the opposite direction, any light you receive will appear stretched, with longer wavelengths and redder colors.
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- An object moving close to the speed of light will see the Universe external to it as either redshifted or blue shifted, depending on its apparent motion relative to the observer. The light waves are compressed (blue shifted) in the direction of motion, and stretched (redshifted) opposite to the direction of motion.
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- The faster you move, the worse these effects get. Distances of physical objects contract more and more severely, and even the electric fields produced by charged particles contract along their direction of motion.
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- Time dilates more severely for Muons which are unstable particles produced in our upper atmosphere can travel the full 100 kilometers down to Earth’s surface, even though their lifetime of 2.2 microseconds indicates that they shouldn’t make it even 1 kilometer if they were moving at the speed of light. Time shortens and distances shrink for these high speed particles. Muons are high energy electrons. Taos are even higher energy electrons. Both decay with very short lifetimes.
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- Redshifts and blueshift are so severe at ultra-high speeds that even photons left over from the Big Bang which have an energy corresponding to just 3 degrees Kevin can spontaneously produce new particles when they collide with protons in the atmosphere. E = mc2 produces enough energy at high enough blueshift to cause these collisions.
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- These effects of time dilation, length contraction, and redshift/blueshift get more severe the closer to the speed of light you get. But there is a limit.
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- Time dilation and length contraction show how time appears to run slower and distances appear to get smaller the closer you move to the speed of light. As you approach the speed of light, clocks dilate towards time not passing at all, while distances contract down to infinitesimal amounts of length.
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- An observer moving at the speed of light experiences more severe amounts of time dilation, length contraction, and redshifts and blueshift relative to himself. Apples would appear yellow, blue, and then ultraviolet as you moved towards them; bananas would appear orange, red, and then infrared as you moved away from them.
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- But if you actually reached the speed of light as photons do, then time and space would no longer behave as you were used to them behaving. If you moved at the speed of light relative to your surroundings, then your surroundings would appear to have no time pass at all relative to you. Because its motion would appear to be at the speed of light, there could be no additional motion allowing a photon to move relative to your surroundings. A clock would be impossible to create.
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- All photons, and in fact all massless particles, move at the speed of light. If you saw something moving at the speed of light relative to you, its clock would appear frozen, as no time could pass for it at all. Another photon, traveling with it, could never move relative to it in a way that either photon could experience.
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- All of the equations of special relativity break down at the speed of light. Time doesn’t pass for your surroundings. All distances along your direction of motion contract down to zero. Redshifts and blueshift occur in infinite amounts of frequency change.
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- It might seem that since the distances along your direction of motion contract down to zero, the Universe becomes two-dimensional to you. That time doesn’t pass, it’s timeless, and space would appear just as a plane with infinite length contraction.
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- But , what happens in reality is, perhaps, is even more surprising.
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- The production of matter/antimatter pairs from pure energy is a completely reversible reaction , with matter/antimatter annihilating back to pure energy. Whenever a photon exists, it has an interaction that creates it and an interaction that destroys it, often ,but not always, resulting in yet another photon. Yet, to the photon itself, its creation and destruction happen instantaneously; it cannot experience anything else.
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- A photon cannot see or experience anything. Time doesn’t pass for a photon in relativity, it represents what we call a null geodesic. It travels from its point-of-origin to its point-of-termination: from where an interaction creates (or emits) it to where another interaction destroys (or absorbs) it. This is exactly what happens whether it’s emission/absorption, emission/reflection, a scattering interaction, or any type of interplay with another particle.
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- When you ask what a photon would “see,” you are assuming that it’s possible for something to interact with a photon and for the photon to experience that interaction somehow. Yet all it experiences are two “things” during its existence: the interaction that creates it and the interaction that destroys it. Whether there is a photon that persists after the destruction, such as via scattering or reflection, is immaterial. All that a photon experiences are those two events at the endpoints of the photon’s journey.
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- Distant sources of light, even that microwave light from the cosmic microwave background, must pass through clouds of gas. While we could calculate redshifts and blueshift, absorption and emission, and other properties like light-travel time from an inertial frame of reference, we couldn’t do any of those things from the photon’s point of view.
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- This is why we demand that we do our relativity calculations in an inertial reference frame. We can calculate how a photon redshifts or blueshift if we use a reference frame moving slower than the speed of light, but not from the photon’s reference frame.
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- From an inertial frame of reference, we can calculate the distance between its emission and absorption point, but not from the photon’s reference frame. We can calculate its light-travel time, from any inertial reference frame, but not from the photon’s reference frame.
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- The problem is that the photon’s reference frame isn’t an inertial reference frame: In an inertial reference frame, there are physical laws which don’t depend on the motion of anything external to the system. Yet for a photon, the physical rules it obeys depend exclusively on everything going on external to it. You cannot calculate anything meaningful for it from the photon’s reference frame alone.
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- The farther a galaxy is, the faster it expands away from us, and the more its light appears redshifted. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years multiplied by the speed of light that it took the light emitted from it to reach us.
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- But we can only calculate redshifts and blueshift from an inertial reference frame. If you try to do this from the frame of reference of the photon, you quickly realize that your calculations yield only nonsense.
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- This is because photons lack a rest mass. That rest mass is what is required to live in an inertial reference frame. Mass and how that mass is distributed provide us with our definition of inertia! A photon cannot see the Universe at all, because seeing requires interacting with other particles, antiparticles, or photons, and once such an interaction occurs, that photon’s journey is over.
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- According to any photon, its existence is instantaneous. It comes into existence with an interaction and it goes out of existence with another interaction. This could be emission from a distant star or galaxy and its arrival at the back of your eye. It doesn’t matter whether it is from our own Sun or an object tens of billions of light years away. When you move at the speed of light, time ceases to pass, and your lifetime only lasts an instant.
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- Physicists often joke that time is what we have to keep everything from happening all at once. But the real joke is on any object that’s so unfortunate to experience the Universe at light speed. If you were so unlucky, you wouldn’t see, hear, or feel anything. You wouldn’t be able to experience existence at all. Photons are unique and hard to figure out. They do ,however, make good food for thought.
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- (Reviews on this subject listed in the beginning and are available if you are interested)
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- January 8, 2019
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