To get a better sense, for instance, of the true distances to exoplanets — planets around other stars — we might start with the theater in which we find them, the Milky Way galaxy. Our galaxy is a gravitationally bound collection of stars, swirling in a spiral through space. Groups of them are bound into clusters of galaxies, and these into superclusters; the superclusters are arranged in immense sheets stretching across the universe, interspersed with dark voids and lending the whole a kind of spiderweb structure.
Our galaxy probably contains to billion stars, and is about , light-years across. That sounds huge, and it is, at least until we start comparing it to other galaxies.
Our neighboring Andromeda galaxy, for example, is some , light-years wide. Another galaxy, IC , spans as much as 4 million light-years. In our galaxy of hundreds of billions of stars, this pushes the number of planets potentially into the trillions. If you are wondering, there are just about 31,, seconds in a year, and if you multiply this by , the distance that light travels each second , you get 5. In short, on Earth, we talk about things in relation to feet or meters, but in the cosmos, we talk about things in relation to light.
For example, the Milky Way galaxy is some , light-years across, and our closest galactic neighbor, Andromeda, is some 2. In other words, it takes light 2. Remember that the next time that you see a Hubble image that shows a host of galaxies dancing across the cosmos—what you are looking at is amazingly far away.
The time that it takes us to travel one light-year is unsurprisingly considerably longer than a year. In his theory of electromagnetism, the speed of light was represented as c. And then in , Albert Einstein proposed his theory of Special Relativity , which postulated that the speed of light c was constant, regardless of the inertial reference frame of the observer or the motion of the light source.
By , after centuries of refined measurements, the speed of light in a vacuum was calculated at ,, meters per second. Ongoing research also revealed that light travels at different wavelengths and is made up of subatomic particles known as photons, which have no mass and behave as both particles and waves.
As already noted, the speed of light expressed in meters per second means that light travels a distance of 9,,,, km or 5,,,, miles in a single year. For example, the nearest star to Earth Proxima Centauri is roughly 4. The center of the Milky Way Galaxy is 26, light-years away, while the nearest large galaxy Andromeda is 2. And the Cosmic Microwave Background , the relic radiation which is believed to be leftover from the Big Bang, is located some The discovery of this radiation not only bolstered the Big Bang Theor y, but also gave astronomers an accurate assessment of the age of the Universe.
This brings up another important point about measuring cosmic distances in light years, which is the fact that space and time are intertwined.
As a result, looking at objects billions of light-years from Earth is to see billions of light-years back in time. And just as 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" also known as "ether". Though Michelson and Morley built a sophisticated interferometer a very basic version of the instrument used today in LIGO facilities , Michelson could not find evidence of any kind of luminiferous aether whatsoever.
Light, he determined, can and does travel through a vacuum. The equation describes the relationship between mass and energy — small amounts of mass m contain, or are made up of, an inherently enormous amount of energy E. That's what makes nuclear bombs so powerful: They're converting mass into blasts of energy. Because energy is equal to mass times the speed of light 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, even small amounts of mass must equate to vast quantities of energy. In order to accurately describe the universe, Einstein's elegant equation requires the speed of light to be an immutable constant. Einstein asserted that light moved through a vacuum, not any kind of luminiferous aether, and in such a way that it moved at the same speed no matter the speed of the observer.
Think of it like this: Observers sitting on a train could look at a train moving along a parallel track and think of its relative movement to themselves as zero.
But observers moving nearly the speed of light would still perceive light as moving away from themselves at more than million mph. That's because moving really, really 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 light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. According to the theory, objects with mass cannot ever reach the speed of light.
If an object ever did reach the speed of light, its mass would become infinite. And as a result, the energy required to move the object would also become infinite. That means if we base our understanding of physics on special relativity, the speed of light is the immutable speed limit of our universe — the fastest that anything can travel. Although the speed of light is often referred to as the universe's speed limit, the universe actually expands even faster. The universe expands at a little 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 Space.
A megaparsec is 3. Special relativity provides an absolute speed limit within the universe, according to Sutter, but Einstein's theory regarding general relativity allows different behavior when the physics you're examining are no longer "local. That's the domain of general relativity, and general relativity says: Who cares!
That galaxy can have any speed it wants, as long as it stays way far away, and not up next to your face," Sutter wrote. And neither should you.
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