How Much Time Passes When You Travel at the Speed of Light
What is the speed of calorie-free?
The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's nearly 186,282 miles per 2nd — a universal constant known in equations as "c," or light speed.
According to physicist Albert Einstein's theory of special relativity, on which much of modern physics is based, nothing in the universe tin can travel faster than calorie-free. The theory states that as matter approaches the speed of light, the thing'south mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe. The speed of light is so immutable that, according to the U.South. National Institute of Standards and Technology, it is used to ascertain international standard measurements like the meter (and past extension, the mile, the pes and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit of measurement Kelvin.
But despite the speed of light's reputation every bit a universal constant, scientists and scientific discipline fiction writers alike spend fourth dimension contemplating faster-than-calorie-free travel. So far no one's been able to demonstrate a real warp drive, but that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.
Related: Special relativity holds up to a high-energy test
What is a calorie-free-year?
A l ight-yr is the distance that light can travel in i year — most 6 trillion miles (x trillion kilometers). It's one way that astronomers and physicists measure immense distances across our universe.
Calorie-free travels from the moon to our eyes in about 1 2nd, which means the moon is about ane light-second away. Sunlight takes near 8 minutes to accomplish our eyes, and then the sun is about eight light-minutes away. Low-cal from Alpha Centauri, which is the nearest star organisation to our own, requires roughly 4.3 years to get hither, so Alpha Centauri is 4.3 calorie-free-years away.
"To obtain an thought of the size of a light-year, accept the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line by seven.5 (the corresponding distance is one calorie-free-2nd), then place 31.half-dozen million similar lines end to end," NASA's Glenn Inquiry Center says on its website. "The resulting distance is almost half-dozen trillion (6,000,000,000,000) miles!"
Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. And everything astronomers "come across" in the afar universe is literally history. When astronomers report objects that are far away, they are seeing light that shows the objects as they existed at the time that light left them.
This principle allows astronomers to see the universe as it looked subsequently the Big Bang, which took place about 13.viii billion years ago. Objects that are x billion light-years away from us appear to astronomers as they looked x 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 we learn the speed of calorie-free?
As early every bit the 5th century, Greek philosophers like Empedocles and Aristotle disagreed on the nature of lite speed. Empedocles proposed that light, whatever it 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, arguing that light, unlike sound and smell, must be instantaneous. Aristotle was wrong, of form, but information technology would accept hundreds of years for anyone to prove it.
In the mid 1600s, the Italian astronomer Galileo Galilei stood 2 people on hills less than a mile apart. 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 tape the speed of lite. He could just conclude that light traveled at least 10 times faster than sound.
In the 1670s, Danish astronomer Ole Rømer tried to create a reliable timetable for sailors at sea, and according to NASA, accidentally came up with a new best estimate for the speed of light. To create an astronomical clock, he recorded the precise timing of the eclipses of Jupiter's moon, Io, from Earth. Over time, Rømer observed that Io's eclipses often differed from his calculations. He noticed that the eclipses appeared to lag the most when Jupiter and Earth were moving away from one another, showed upwardly alee of time when the planets were approaching and occurred on schedule when the planets were at their closest or farthest points. This observation demonstrated what we today know equally the Doppler effect, the alter in frequency of light or sound emitted by a moving object that in the astronomical world manifests equally the and then-chosen redshift, the shift towards "redder", longer wavelengths in objects speeding away from usa. In a jump of intuition, Rømer determined that light was taking measurable fourth dimension to travel from Io to Earth.
Rømer used his observations to estimate the speed of low-cal. Since the size of the solar system and Globe'southward orbit wasn't yet accurately known, argued a 1998 newspaper in the American Journal of Physics, he was a bit off. But at last, scientists had a number to work with. Rømer'due 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 set of calculations on the change in the apparent position of stars caused by Globe's travels around the sun. He estimated the speed of light at 185,000 miles per second (301,000 km/s) — authentic to within almost 1% of the existent value, according to the American Physical Lodge.
Two new attempts in the mid-1800s brought the problem dorsum to Globe. French physicist Hippolyte Fizeau set a beam of low-cal on a rapidly rotating toothed wheel, with a mirror set up upward five miles (8 km) away to reflect it back to its source. Varying the speed of the bicycle allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the next mirror, and dorsum through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a bike to perform essentially the same experiment. The two independent methods each came within about 1,000 miles per second (ane,609 km/due south) of the speed of light.
Another scientist who tackled the speed of light mystery was Poland-born Albert A. Michelson, who grew up in California during the country'due south gold blitz menstruation, and honed his interest in physics while attention the U.S. Naval Academy, according to the University of Virginia. In 1879, he attempted to replicate Foucault's method of determining the speed of light, but Michelson increased the altitude between mirrors and used extremely high-quality mirrors and lenses. Michelson's result of 186,355 miles per second (299,910 km/s) was accustomed equally the about accurate measurement of the speed of light for xl years, until Michelson re-measured it himself. In his second round of experiments, Michelson flashed lights between two mountain tops with carefully measured distances to go a more than precise estimate. And in his 3rd try but before his death in 1931, co-ordinate to the Smithsonian's Air and Space magazine, he built a mile-long depressurized tube of corrugated steel pipe. The pipe false a nearly-vacuum that would remove any effect of air on light speed for an fifty-fifty effectively measurement, which in the finish was just slightly lower than the accepted value of the speed of lite today.
Michelson also studied the nature of light itself, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang. The best minds in physics at the time of Michelson's experiments were divided: Was light a wave or a particle?
Michelson, along with his colleague Edward Morley, worked under the supposition that lite moved equally a moving ridge, simply similar audio. And merely every bit audio needs particles to move, Michelson and Morley and other physicists of the fourth dimension reasoned, light must have some kind of medium to move through. This invisible, undetectable stuff was called the "luminiferous aether" (besides known equally "ether").
Though Michelson and Morley built a sophisticated interferometer (a very basic version of the instrument used today in LIGO facilities), Michelson could not observe prove of any kind of luminiferous aether whatever. Lite, he adamant, tin and does travel through a vacuum.
"The experiment — and Michelson's body of work — was so revolutionary that he became the only person in history to take won a Nobel Prize for a very precise non-discovery of anything," Siegal wrote. "The experiment itself may have been a complete failure, but what we learned from it was a greater benefaction to humanity and our understanding of the universe than whatsoever success would accept been!"
Special relativity and the speed of low-cal
Einstein's theory of special relativity unified energy, affair and the speed of light in a famous equation: E = mc^two. The equation describes the human relationship between mass and energy — small amounts of mass (m) contain, or are fabricated up of, an inherently enormous amount of energy (E). (That's what makes nuclear bombs then powerful: They're converting mass into blasts of free energy.) Considering free energy is equal to mass times the speed of light squared, the speed of low-cal serves as a conversion factor, explaining exactly how much energy must be within thing. And because the speed of lite is such a huge number, fifty-fifty minor amounts of mass must equate to vast quantities of energy.
In guild to accurately describe the universe, Einstein'south elegant equation requires the speed of lite to be an immutable constant. Einstein asserted that light moved through a vacuum, not whatsoever kind of luminiferous aether, and in such a mode that information technology moved at the same speed no matter the speed of the observer.
Recall of it like this: Observers sitting on a train could wait at a train moving along a parallel track and think of its relative motion to themselves as zero. But observers moving most the speed of light would still perceive light as moving away from them at more than 670 million mph. (That'southward because moving really, actually fast is one of the only confirmed methods of time travel — time 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 lite doesn't vary with the time or place that you measure it, or how fast yous yourself are moving.
Therefore, objects with mass cannot ever achieve the speed of calorie-free. If an object ever did reach the speed of lite, its mass would become space. And equally a result, the energy required to move the object would also get space: an impossibility.
That ways if we base our understanding of physics on special relativity (which about modern physicists do), the speed of lite is the immutable speed limit of our universe — the fastest that anything can travel.
What goes faster than the speed of light?
Although the speed of light is frequently referred to equally the universe's speed limit, the universe really expands even faster. The universe expands at a little more than 42 miles (68 kilometers) per 2nd for each megaparsec of distance from the observer, wrote astrophysicist Paul Sutter in a previous commodity for Space.com. (A megaparsec is iii.26 million calorie-free-years — a really long manner.)
In other words, a milky way 1 megaparsec away appears to exist traveling away from the Milky way at a speed of 42 miles per second (68 km/s), while a galaxy two megaparsecs abroad recedes at nearly 86 miles per second (136 km/due south), and so on.
"At some point, at some obscene altitude, the speed tips over the scales and exceeds the speed of calorie-free, all from the natural, regular expansion of space," Sutter explained. "It seems like information technology should be illegal, doesn't it?"
Special relativity provides an accented speed limit within the universe, according to Sutter, only Einstein's 1915 theory regarding general relativity allows different behavior when the physics you're examining are no longer "local."
"A galaxy on the far side of the universe? That's the domain of general relativity, and general relativity says: Who cares! That galaxy tin have any speed it wants, as long as it stays style far away, and not upwards adjacent to your face up," Sutter wrote. "Special relativity doesn't care about the speed — superluminal or otherwise — of a distant galaxy. And neither should you."
Does light always ho-hum down?
Light in a vacuum is mostly held to travel at an absolute speed, merely lite traveling through any fabric can be slowed down. The amount that a material slows downwardly calorie-free is called its refractive alphabetize. Light bends when coming into contact with particles, which results in a decrease in speed.
For example, calorie-free traveling through Earth's atmosphere moves about every bit fast equally light in a vacuum, slowing down by just three 10-thousandths of the speed of lite. But light passing through a diamond slows to less than half its typical speed, PBS NOVA reported. Fifty-fifty so, it travels through the gem at over 277 meg mph (almost 124,000 km/s) — enough to make a difference, merely nevertheless incredibly fast.
Light can be trapped — and even stopped — inside ultra-cold clouds of atoms, co-ordinate to a 2001 written report published in the journal Nature. More recently, a 2018 study published in the journal Physical Review Messages proposed a new way to stop calorie-free in its tracks at "infrequent points," or places where two carve up light emissions intersect and merge into i.
Researchers have too tried to tedious down low-cal even when it's traveling through a vacuum. A team of Scottish scientists successfully slowed down a single photon, or particle of light, even as it moved through a vacuum, as described in their 2015 study published in the journal Scientific discipline. In their measurements, the difference between the slowed photon and a "regular" photon was just a few millionths of a meter, merely it demonstrated that low-cal in a vacuum can be slower than the official speed of calorie-free.
Can nosotros travel faster than light?
Science fiction loves the idea of "warp speed." Faster-than-light travel makes endless sci-fi franchises possible, condensing the vast expanses of space and letting characters pop dorsum and forth between star systems with ease.
But while faster-than-light travel isn't guaranteed impossible, we'd need to harness some pretty exotic physics to get in 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 non move ourselves — since special relativity would ensure we'd exist long destroyed earlier nosotros reached high enough speed — but instead, motion the space around united states. Piece of cake, right?
One proposed idea involves a spaceship that could fold a space-fourth dimension bubble around itself. Sounds bang-up, 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 but to get to the next star organization," said Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, in a 2010 interview with Infinite.com's sister site LiveScience. "And then science fiction has long postulated a way to shell the speed of light barrier so the story tin motility a picayune more than speedily."
Without faster-than-light travel, any "Star Trek" (or "Star State of war," for that thing) would be impossible. If humanity is ever to reach the farthest — and constantly expanding — corners of our universe, it will be up to future physicists to boldly get where no one has gone before.
Additional resources
For more on the speed of low-cal, cheque out this fun tool from Academo that lets you visualize how fast light can travel from any place on Globe to any other. If you're more interested in other important numbers, get familiar with the universal constants that define standard systems of measurement around the world with the National Institute of Standards and Technology. And if you'd like more on the history of the speed of light, bank check out the book "Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light" (Oxford, 2019) by John C. H. Spence.
Previous enquiry for this article provided past Space.com contributor Nola Taylor Redd.
Bibliography
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D'Alto, Nick. "The Pipeline That Measured the Speed of Light." Smithsonian Mag, January 2017. https://www.smithsonianmag.com/air-infinite-mag/18_fm2017-oo-180961669/.
Fowler, Michael. "Speed of Low-cal." Mod Physics. University of Virginia. Accessed January 13, 2022. https://galileo.phys.virginia.edu/classes/252/spedlite.html#Albert%20Abraham%20Michelson.
Giovannini, Daniel, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen K. Barnett, Daniele Faccio, and Miles J. Padgett. "Spatially Structured Photons That Travel in Costless Space Slower than the Speed of Low-cal." Science, February 20, 2015. https://www.science.org/doi/abs/x.1126/science.aaa3035.
Goldzak, Tamar, Alexei A. Mailybaev, and Nimrod Moiseyev. "Light Stops at Infrequent Points." Physical Review Letters 120, no. i (January 3, 2018): 013901. https://doi.org/10.1103/PhysRevLett.120.013901.
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"How Long Is a Calorie-free-Year?" Glenn Learning Technologies Project, May 13, 2021. https://www.grc.nasa.gov/www/one thousand-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm.
American Concrete Society News. "July 1849: Fizeau Publishes Results of Speed of Calorie-free Experiment," July 2010. http://www.aps.org/publications/apsnews/201007/physicshistory.cfm.
Liu, Chien, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau. "Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Low-cal Pulses." Nature 409, no. 6819 (January 2001): 490–93. https://doi.org/10.1038/35054017.
NIST. "Run across the Constants." October 12, 2018. https://www.nist.gov/si-redefinition/meet-constants.
Ouellette, Jennifer. "A Brief History of the Speed of Light." PBS NOVA, Feb 27, 2015. https://www.pbs.org/wgbh/nova/article/brief-history-speed-light/.
Shea, James H. "Ole Ro/Mer, the Speed of Light, the Apparent Menstruation of Io, the Doppler Upshot, and the Dynamics of Earth and Jupiter." American Journal of Physics 66, no. 7 (July i, 1998): 561–69. https://doi.org/10.1119/i.19020.
Siegel, Ethan. "The Failed Experiment That Inverse The World." Forbes, April 21, 2017. https://www.forbes.com/sites/startswithabang/2017/04/21/the-failed-experiment-that-changed-the-world/.
Stern, David. "Rømer and the Speed of Light," October 17, 2016. https://pwg.gsfc.nasa.gov/stargaze/Sun4Adop1.htm.
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