How Far Can Light Travel
What is the speed of light?
The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per 2d. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed.
According to physicist Albert Einstein'due south theory of special relativity, on which much of mod physics is based, zippo in the universe can travel faster than light. The theory states that as affair approaches the speed of light, the matter's mass becomes infinite. That means the speed of low-cal functions as a speed limit on the whole universe. The speed of light is and then immutable that, co-ordinate to the U.South. National Plant of Standards and Technology (opens in new tab), it is used to define international standard measurements like the meter (and past extension, the mile, the pes and the inch). Through some crafty equations, it likewise helps define the kilogram and the temperature unit of measurement Kelvin.
Simply despite the speed of light's reputation as a universal constant, scientists and science fiction writers alike spend fourth dimension contemplating faster-than-low-cal travel. So far no one's been able to demonstrate a real warp drive, merely that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.
Related: Special relativity holds upwards to a high-energy test
What is a light-twelvemonth?
A l ight-year is the distance that light tin travel in one twelvemonth — well-nigh half dozen trillion miles (10 trillion kilometers). Information technology'south one way that astronomers and physicists mensurate immense distances across our universe.
Light travels from the moon to our eyes in about 1 2nd, which ways the moon is about i low-cal-second away. Sunlight takes about 8 minutes to reach our eyes, so the sun is about 8 light-minutes away. Light from Alpha Centauri, which is the nearest star system to our ain, requires roughly 4.3 years to get hither, so Blastoff Centauri is 4.three low-cal-years away.
"To obtain an idea of the size of a calorie-free-year, accept the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line past 7.5 (the respective distance is one light-second), then place 31.half-dozen meg similar lines end to finish," NASA's Glenn Research Center says on its website (opens in new tab). "The resulting distance is about 6 trillion (six,000,000,000,000) miles!"
Stars and other objects across our solar system lie anywhere from a few calorie-free-years to a few billion light-years abroad. And everything astronomers "see" in the distant universe is literally history. When astronomers study objects that are far away, they are seeing lite that shows the objects every bit they existed at the fourth dimension that low-cal left them.
This principle allows astronomers to see the universe as it looked later on the Large Blindside, which took place near 13.8 billion years ago. Objects that are 10 billion light-years abroad from us announced to astronomers as they looked 10 billion years ago — relatively soon after the kickoff of the universe — rather than how they appear today.
Related: Why the universe is all history
How did nosotros learn the speed of light?
As early as the fifth century, Greek philosophers like Empedocles and Aristotle disagreed on the nature of low-cal speed. Empedocles proposed that light, any information technology was made of, must travel and therefore, must have a rate of travel. Aristotle wrote a rebuttal of Empedocles' view in his own treatise, On Sense and the Sensible (opens in new tab), arguing that calorie-free, dissimilar sound and smell, must be instantaneous. Aristotle was wrong, of grade, but it would have hundreds of years for anyone to evidence it.
In the mid 1600s, the Italian astronomer Galileo Galilei stood two people on hills less than a mile autonomously. Each person held a shielded lantern. One uncovered his lantern; when the other person saw the wink, he uncovered his too. But Galileo's experimental distance wasn't far enough for his participants to record the speed of low-cal. He could only conclude that light traveled at least 10 times faster than audio.
In the 1670s, Danish astronomer Ole Rømer tried to create a reliable timetable for sailors at sea, and according to NASA (opens in new tab), accidentally came up with a new all-time estimate for the speed of light. To create an astronomical clock, he recorded the precise timing of the eclipses of Jupiter'southward moon, Io, from Earth. Over fourth dimension, Rømer observed that Io's eclipses oftentimes differed from his calculations. He noticed that the eclipses appeared to lag the most when Jupiter and Earth were moving away from 1 another, showed up ahead of time when the planets were budgeted and occurred on schedule when the planets were at their closest or farthest points. This observation demonstrated what we today know as the Doppler effect, the change in frequency of light or audio emitted by a moving object that in the astronomical world manifests as the so-called redshift, the shift towards "redder", longer wavelengths in objects speeding abroad from us. In a jump of intuition, Rømer adamant that light was taking measurable fourth dimension to travel from Io to World.
Rømer used his observations to approximate the speed of light. Since the size of the solar system and Earth'southward orbit wasn't yet accurately known, argued a 1998 paper in the American Journal of Physics (opens in new tab), he was a bit off. Just at terminal, scientists had a number to work with. Rømer'south calculation put the speed of light at about 124,000 miles per second (200,000 km/s).
In 1728, English physicist James Bradley based a new fix of calculations on the change in the apparent position of stars caused past Earth's travels around the sun. He estimated the speed of light at 185,000 miles per second (301,000 km/s) — accurate to within about ane% of the real value, according to the American Physical Club (opens in new tab).
Two new attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set up a beam of low-cal on a rapidly rotating toothed wheel, with a mirror gear up five miles (8 km) away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the calorie-free to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a bicycle to perform essentially the same experiment. The two contained methods each came within nearly 1,000 miles per second (i,609 km/s) of the speed of low-cal.
Another scientist who tackled the speed of low-cal mystery was Poland-built-in Albert A. Michelson, who grew upwards in California during the country'due south aureate rush period, and honed his interest in physics while attending the U.S. Naval Academy, according to the University of Virginia (opens in new tab). In 1879, he attempted to replicate Foucault'due south method of determining the speed of light, but Michelson increased the distance between mirrors and used extremely loftier-quality mirrors and lenses. Michelson'due south upshot of 186,355 miles per second (299,910 km/s) was accepted as the almost authentic measurement of the speed of light for 40 years, until Michelson re-measured it himself. In his 2d round of experiments, Michelson flashed lights between 2 mountain tops with carefully measured distances to get a more than precise estimate. And in his third attempt just before his death in 1931, according to the Smithsonian's Air and Space (opens in new tab) magazine, he built a mile-long depressurized tube of corrugated steel piping. The pipe simulated a nearly-vacuum that would remove any issue of air on light speed for an even finer measurement, which in the stop was but slightly lower than the accepted value of the speed of lite today.
Michelson also studied the nature of lite itself, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang (opens in new tab). The best minds in physics at the fourth dimension of Michelson's experiments were divided: Was light a wave or a particle?
Michelson, along with his colleague Edward Morley, worked under the assumption that low-cal moved as a wave, just similar sound. And only every bit sound needs particles to move, Michelson and Morley and other physicists of the time reasoned, light must have some kind of medium to move through. This invisible, undetectable stuff was called the "luminiferous aether" (besides known as "ether").
Though Michelson and Morley congenital a sophisticated interferometer (a very bones version of the instrument used today in LIGO facilities), Michelson could non find evidence of any kind of luminiferous aether whatsoever. Light, he determined, can and does travel through a vacuum.
"The experiment — and Michelson'due south body of work — was and so revolutionary that he became the only person in history to have won a Nobel Prize for a very precise not-discovery of anything," Siegal wrote. "The experiment itself may accept been a consummate failure, only what nosotros learned from it was a greater boon to humanity and our understanding of the universe than any success would accept been!"
Special relativity and the speed of calorie-free
Einstein'south theory of special relativity unified free energy, thing and the speed of light in a famous equation: Due east = mc^2. The equation describes the human relationship between mass and energy — small amounts of mass (thousand) contain, or are made upwards of, an inherently enormous amount of energy (Due east). (That'due south what makes nuclear bombs so powerful: They're converting mass into blasts of free energy.) Considering energy is equal to mass times the speed of lite squared, the speed of light serves as a conversion factor, explaining exactly how much energy must be within matter. And because the speed of light is such a huge number, fifty-fifty small amounts of mass must equate to vast quantities of energy.
In order to accurately describe the universe, Einstein'southward elegant equation requires the speed of calorie-free to exist an immutable constant. Einstein asserted that calorie-free moved through a vacuum, not any kind of luminiferous aether, and in such a style that it moved at the same speed no matter the speed of the observer.
Call back of it like this: Observers sitting on a train could look at a train moving along a parallel track and think of its relative motion to themselves equally zero. But observers moving nearly the speed of lite would even so perceive light as moving away from them at more 670 meg mph. (That's because moving really, really fast is one of the only confirmed methods of fourth dimension travel — fourth dimension actually slows down for those observers, who will age slower and perceive fewer moments than an observer moving slowly.)
In other words, Einstein proposed that the speed of calorie-free doesn't vary with the time or identify that yous measure it, or how fast you yourself are moving.
Therefore, objects with mass cannot ever attain the speed of light. If an object ever did accomplish the speed of lite, its mass would become space. And as a result, the energy required to move the object would also get infinite: an impossibility.
That ways if we base of operations our agreement of physics on special relativity (which most modern physicists do), the speed of light is the immutable speed limit of our universe — the fastest that annihilation can travel.
What goes faster than the speed of light?
Although the speed of low-cal is often referred to every bit the universe's speed limit, the universe actually expands even faster. The universe expands at a niggling more than 42 miles (68 kilometers) per second for each megaparsec of distance from the observer, wrote astrophysicist Paul Sutter in a previous article for Infinite.com. (A megaparsec is 3.26 million light-years — a actually long mode.)
In other words, a milky way one megaparsec abroad appears to be traveling away from the Milky Style at a speed of 42 miles per 2d (68 km/southward), while a milky way two megaparsecs abroad recedes at nearly 86 miles per second (136 km/south), and then on.
"At some bespeak, at some obscene distance, the speed tips over the scales and exceeds the speed of light, all from the natural, regular expansion of space," Sutter explained. "It seems similar it should be illegal, doesn't it?"
Special relativity provides an absolute speed limit inside the universe, according to Sutter, simply Einstein's 1915 theory regarding full general relativity allows dissimilar behavior when the physics you're examining are no longer "local."
"A galaxy on the far side of the universe? That'south the domain of full general relativity, and general relativity says: Who cares! That galaxy can take any speed it wants, as long every bit it stays manner far abroad, and not up next to your face," Sutter wrote. "Special relativity doesn't intendance about the speed — superluminal or otherwise — of a distant galaxy. And neither should you lot."
Does light ever ho-hum down?
Light in a vacuum is generally held to travel at an absolute speed, but light traveling through whatever material can be slowed down. The corporeality that a material slows down light is chosen its refractive index. Light bends when coming into contact with particles, which results in a decrease in speed.
For example, light traveling through World's atmosphere moves almost as fast as calorie-free in a vacuum, slowing down by just iii ten-thousandths of the speed of light. Simply light passing through a diamond slows to less than half its typical speed, PBS NOVA (opens in new tab) reported. Nevertheless, it travels through the gem at over 277 million mph (almost 124,000 km/s) — enough to brand a deviation, simply notwithstanding incredibly fast.
Calorie-free can exist trapped — and even stopped — inside ultra-common cold clouds of atoms, co-ordinate to a 2001 written report published in the journal Nature (opens in new tab). More recently, a 2018 report published in the journal Physical Review Messages (opens in new tab) proposed a new way to stop calorie-free in its tracks at "infrequent points," or places where 2 separate lite emissions intersect and merge into i.
Researchers have besides tried to slow down light even when information technology'southward traveling through a vacuum. A team of Scottish scientists successfully slowed down a single photon, or particle of light, fifty-fifty as information technology moved through a vacuum, as described in their 2015 study published in the journal Science (opens in new tab). In their measurements, the difference betwixt the slowed photon and a "regular" photon was just a few millionths of a meter, but it demonstrated that light in a vacuum tin be slower than the official speed of light.
Can we travel faster than light?
Scientific discipline fiction loves the idea of "warp speed." Faster-than-light travel makes countless sci-fi franchises possible, condensing the vast expanses of space and letting characters pop back and along betwixt star systems with ease.
But while faster-than-light travel isn't guaranteed incommunicable, we'd need to harness some pretty exotic physics to make information technology piece of work. Luckily for sci-fi enthusiasts and theoretical physicists alike, there are lots of avenues to explore.
All we have to do is effigy out how to not motility ourselves — since special relativity would ensure nosotros'd be long destroyed before we reached high enough speed — but instead, move the space around us. Like shooting fish in a barrel, right?
Ane proposed idea involves a spaceship that could fold a space-time bubble around itself. Sounds great, both in theory and in fiction.
"If Captain Kirk were constrained to move at the speed of our fastest rockets, it would accept him a hundred thousand years just to get to the next star organisation," said Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Constitute in Mountain View, California, in a 2010 interview with Space.com's sister site LiveScience. "So science fiction has long postulated a manner to beat the speed of light bulwark so the story tin can move a trivial more than chop-chop."
Without faster-than-lite travel, any "Star Trek" (or "Star State of war," for that matter) would be impossible. If humanity is ever to achieve the uttermost — and constantly expanding — corners of our universe, information technology will be upwards to future physicists to boldly go where no one has gone before.
Additional resource
For more on the speed of light, check out this fun tool from Academo (opens in new tab) that lets y'all visualize how fast light can travel from any place on Earth to any other. If y'all're more than interested in other of import numbers, go familiar with the universal constants that ascertain standard systems of measurement effectually the earth with the National Institute of Standards and Technology (opens in new tab). And if y'all'd like more on the history of the speed of calorie-free, cheque out the book "Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light (opens in new tab)" (Oxford, 2019) past John C. H. Spence.
Previous research for this article provided by Space.com contributor Nola Taylor Redd.
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How Far Can Light Travel,
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