I am a beam of light traveling from a distant star

Like a human with two sides to their personality, the classic Dr. Jekyll and Mr. Hyde, I behave like a wave in some cases, for example, when I am cruising across the universe, and like a solid object in others, literally a tiny, massless ball of light known as a photon. 

I am a beam of light traveling from a distant star.  If you can call me a thing, I am the fastest in the known universe, racing along at 300,000 kilometers per second across the void, but still I will travel for eons before I reach Earth.  When I left the star you call Earendel, some 12.9 billion years ago, the sun and the Earth weren’t even formed yet, wouldn’t exist for another seven billion years or so, and the heavens themselves looked much different than they do now, much younger and much less populated by mature stars.  Physicists will debate what that means for decades, whether the universe was born in a Big Bang or something else, but it doesn’t matter to me as I travel. Though beams of light from Earendel like myself have been bathing the night sky since before there was a night sky, humans only detected this particular star’s presence very recently, first using the Hubble Space Telescope in 2013 and more recently, with the James Webb Space Telescope.  Earendel is the most distant start known to humanity, besting the previous record by almost four billion light years. Even armed with powerful viewing devices orbiting in space, scientists can only see me through a gravitational lens, where the massive center of a galaxy, in this case the Sunrise Arc which I call home, lying well beyond the reaches of your Milky Way, magnifies the light passing through.  Earendel itself is also much bigger and brighter than any star you are familiar with, almost a million times more luminous than the sun, and yet I am so far away generations of your astronomers peered into the darkness of space and still could not see me.  Though I have traveled for billions of years to reach you whether you can see me or not, it is a marvelous quirk of physics that I do not change, I do not age, and time does not pass for me at all.  When I am seen through a telescope, you are actually witnessing the universe as it was almost thirteen billion years ago, exactly as I was when the fusion reaction that powers Earendel produced me as a byproduct of light and heat, as though the intervening eons didn’t happen and the moment was frozen in time.  If you had a telescope sharp enough, you could in principle, study the events that have occurred on Earandel’s surface as you do on a nearby planet or a birdwatcher might set up on a beach.  I might be a 12.9 billion year old relic of an older universe, but I am light nonetheless, the same as that which fills your eyes, either produced by the sun, the reflection of the sun on the moon, or some artificial source,  allowing you to see in the first place.  Normally, I travel so fast that you don’t notice it, but technically speaking, everything you see around you is at least slightly in the past, as it was when light left the surface you are observing.  On Earth itself, the time difference is barely measurable, but from the sun to Earth it takes eight minutes to travel, meaning the sun itself could explode right now and you would be blissfully unaware of the time it takes to smoke a last cigarette.

While some enterprising science fiction authors have proposed that we could directly observe the past by traveling faster than light, for example flying from Earth so far and fast we could look back at the dinosaurs, that doesn’t appear to be possible in this universe at least.  Since 1905, Albert Einstein’s Special Theory of Relativity has fixed my velocity as the absolute maximum, that by which everything else is measured, and meaning you could never catch me if you tried.  This, he achieved by making one of physic’s most clever and insightful assumptions.  The idea that there was a principle of relativity was first proposed by Galileo hundreds of years earlier, who observed that if you are traveling inside the cabin of a ship on a smooth body of water, you cannot detect whether or not you are moving unless you can see a landmark outside the vessel.  If you jump up and down inside the cabin, you land in the same place on the floor, even though you technically would have traveled some distance forward with the motion of the ship itself.  Likewise, if you drop a ball, it seems to fall straight to the floor, though it is actually traveling on an arc as the ship moves forward, or if you pour yourself a pint of beer as I would prefer if I could drink, it’s the same as if you were on solid ground, even though it too is bending in mid-air.  Of course, there is no such thing as solid ground in the first place.  The Earth itself is rotating under your feet, revolving around the sun, the sun revolving around the center of the Milky Way, the Milky Way revolving around other galaxies as the entire universe expands.  This motion is undetectable to us as we stand, sit, walk, talk, eat, sleep, and more , however, because it is occurring at a constant speed.  Albert Einstein took this general principle and turned it into a physical law that held two things to be true.  First, the laws of physics are the same in every non-inertial frame of reference – meaning when you are traveling at a constant speed – and second, the speed of light is the maximum speed in the universe.  While this sounds complicated and indeed some of the consequences, for example nuclear energy and time dilation, are complicated, it is beautifully straightforward at its heart.  The laws of physics being the same simply means that there is no test you can perform to determine whether you are moving or standing still, unless you consider it relative to something else.  As Galileo proposed with the analogy of the ship, if our hypothetical cabin was sealed to the outside world and the motion was perfectly smooth and constant, any experiment you performed inside the ship, no matter how many times you jumped or dropped a ball, would have the same result as it it was performed outside the ship, assuming the outside location was also moving at a constant speed, meaning there is no absolute test of motion, everything is measured relative to everything else.

Further, because the speed of light is dictated by a law of physics concerning electromagnetic waves discovered by James Clerk Maxwell, the speed of light is the same in all frames of reference in addition to being the maximum speed in the universe.  From this, an incredible symmetry and subtlety emerges, one that struck Einstein years before he developed the theory, when he imagined what it would be like to travel alongside a beam of light.  If the speed of light is the same no matter how fast you are moving, that means if you are traveling close to the speed of light and turn on a flashlight, you do not leave slow moving light beams behind you or catch up with the ones ahead.  Instead, the room lights up the same as it would if you weren’t moving at all, but if that is the case, what does someone not moving that fast see, when light is traveling just as fast from their perspective?  To resolve that, Einstein found that time itself must slow down relative to a stationary observer as you increase velocity.  They will always see light moving at the same speed, but for that observation to work, a person traveling alongside a beam of light, going almost the same speed, would appear to be moving incredibly slowly.  Thus, time rather than light is transformed and as a direct consequence mass needs to be as well.  In order for the speed of light to be the universe’s maximum, Einstein proposed that mass increases with velocity and becomes infinite if you are traveling at the speed of light itself, but perhaps needless to say, there isn’t enough energy in the entire universe to do so.  As if this wasn’t enough, the combination of the two, that is the connection between mass and the energy required to move it, leads directly to the most famous equation in history, energy equals mass times the speed of light squared, which underlies the nuclear world, promising both untold destruction and endless energy, the source of all light in the entire universe.  My connection to this world is more than merely theoretical, however.  Maxwell’s famous wave equations only described a part of my behavior.  Like a human with two sides to their personality, the classic Dr. Jekyll and Mr. Hyde, I behave like a wave in some cases, for example, when I am cruising across the universe, and like a solid object in others, literally a tiny, massless ball of light known as a photon.  Interestingly, this aspect of my being was also proposed by Einstein in the same year as special relativity, what he called the photo-electric effect, where an incoming photon can affect the motion of subatomic particles, knocking them around or more accurately exciting them to higher energies.

This is the same principle that underlies your ability to see, a photograph’s ability to capture an image, the ability of plants to convert carbon dioxide into energy, solar power, the color of the sky, and much, much more.  Without it, there would be no life in the universe. The combination of my two personalities quickly became known as “wave particle duality,” but how each can manifest in different contexts, indeed seeming to change contexts even backwards in time remains one of the universe’s ongoing mysteries.  A simple experiment, carried out thousands of times in hundreds of different ways to confirm its validity shows why.  Imagine a flashlight with a beam so low it emits only a single photon at a time like a pitching machine in a batting cage.  The flashlight is pointed at a screen with a small slit, large enough for a single photon to pass through.  Beyond the slit, there is a photographic plate to track where the photons land.  If the experimenter uses just a single slit, the pattern on the photographic plate is similar to what you would expect if a baseball pitcher were tasked with throwing fast balls through a small hole.  The photons are scattered, some coming straight through, some bouncing off the slit and changing direction slightly, resulting in a series of random, unpredictable hits on the plate, but if there are two slits, something strange happens.  The photons can still travel through one or the other even if neither is directly in front of the flashlight, but the pattern on the other side goes from random to highly structured.  Somehow, the photons no longer travel as if they were individual balls of light, but instead as if they were part of a wave.  The pattern on the photographic plate reveals a series of unique hits as they did before, meaning the photons continue to pass through one by one, except those hits are arranged like two waves have collided together, where the crest of one can meet the trough of another cancelling them out, creating a marked series of lighter and darker strips.  Somehow, the individual photon “knows” that it’s a part of a wave and though it’s still an individual photon as measured by the plate, it behaves as a part of a group.  Further, this “knowledge” appears to be based on how the photon is measured.  If you keep the two slits but install a detector at each slit to determine which one the photon traveled through in real time, it reverts to the one slit pattern, as though light itself were trying to trick you.  

No one has ever been able to explain his behavior.  You have seen it.  You can study it.  You can even use it in your electronics and your telescopes, but the underlying reality – forget why I behave differently, how does a photon appear to know anything and why does it care – remains an essential mystery.  As the great physicist Richard Feynman put it, it’s “a phenomenon which is impossible…to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics].”  This mystery remains unsolved for more than a century, and it might never be solved, but what do I care?  I am a beam of light.  Me and my kind are everywhere, bathing the entire universe, providing the fundamental energy needed to run the entire universe including life on Earth, but we still have our secrets.  You might well be able to observe me and observe the past, as you do when you look at a star like Earendel that is 12.9 billion years old.  I might well be the source of your entire existence and without me you might all wither and die, but truly understand me?  That’s another matter entirely, but can’t you say the same about life itself?