Among all the rules that govern the universe, one of the most iconic and at the same time difficult to understand is the universal speed limit. The speed of light is not only an unwavering constant: it is the link between matter and energy, as Albert Einstein described with the most famous formula of science: E = Mc². We can look at the foundations of our own existence, but not travel to more than “C”. Only light can travel a light year in a year.
Let’s define constants: the speed of light
The speed of light is the key piece in Einstein’s equation. That “C” is not only a number, but the conversion factor that unites the concepts of mass (m) and energy (e). It is a constant that represents the speed of light in a vacuum, but also the speed limit for the spread of any type of information, signal or material particle in the universe. If you think very strong, it is the limit of causality itself: an effect cannot occur before its cause, spreading at the maximum “C” speed, can reach it.
This speed is the same for any observer in the universe, regardless of their own state of movement. If you travel in a hypothetical 99% spacecraft of the speed of light and light a flashlight, the light of that flashlight will move away from you exactly at the speed of light, not to a fraction of it. It is one of the universal constants of physics. And the observations of the cosmic microwave fund, the remaining light of the Big Bang, confirm that it has not changed measurable in more than 13.8 billion years.
What speed is light, then? Although it sounds strange, the speed of light in vacuum has an exact and defined value: 299,792,458 meters per second. To put it in more earthly figures, it is equivalent to almost one billion kilometers per hour. A photon of light would go around the earth’s Ecuador about 7.5 times in a single second. It is, according to Albert Einstein’s special relativity theory, the definitive and unwavering speed limit of the universe.
An epic about measuring the above
Calculating the speed of light has been one of the great sagas of science. After the philosophical debates of ancient Greece and an ingenious but failed attempt by Galileo using lamps between distant hills, the first estimate came in 1676. Observing the eclipses of ío, one of Jupiter’s moons, the Danish astronomer Ole Rømer noticed that they had a different duration according to the time of the year. He deduced that it was due to the additional time that the light took to cross the orbit of the earth when our planet moved away from Jupiter. Rømer estimated the speed of light in 220,000 km/s, a surprisingly close figure for the time.
Half a century later, in 1728, the English physicist James Bradley refined this measure using a different method: the aberration of stellar light. He noted that the apparent position of the stars changed slightly due to the speed of the earth in their orbit. Something similar to how rain seems to fall at angle when we run. From this effect, it calculated a speed of 301,000 km/s, a value with an error of just 1%.
Michelson’s experiment. Image | Popular Science (1930)
It was not until 1887 that scientists discovered the most surprising aspect of the speed of light. Albert Michelson and Edward Morley tried to detect the “luminous ether”, an invisible half assumption that, according to the belief of the time, filled the space to allow the propagation of light. With their experiment they hoped to measure a difference in the speed of light depending on whether it moved in favor or against the “ether wind” created by the movement of the earth. However, they found no variation at all.
Sometimes, scientific progress does not come from finding what is sought, but of accepting the evidence that hits old certainty. This was how this failure It became one of the most important results in the history of physics. He showed that the speed of light was constant regardless of the observer’s movement, knocking down the ether theory and laying the empirical bases for the revolution that Einstein would unleash later.
What is a light year and what is used for
Since 1983, the speed of light is no longer something that scientists try to measure with increasing precision. Its value was fixed with such accuracy that it is now the Metro itself that is defined according to light. One meter is “the length of the path traveled by light in a vacuum during a time interval of 1/299792458 seconds.”
This change hides a deep truth: the constancy of the speed of light is a more fundamental property of our universe than our own units of measure. We no longer use meters to measure the speed of light, we use the speed of light to define the subway. And this is how one of the largest units of measure we use is born, and that has been crucial to understand the immense scales of the universe.
Although your name includes the word “year”, a light year is not a measure of time, but of distance. In a nutshell, a light year is the distance that a ray of light travels in a vacuum during the course of a terrestrial year. That is, in 365 days. Given the incredible speed at which the light travels, it is an astronomical distance, of approximately 9.5 billion kilometers.
We use the light years because the distances in space are so huge that measuring them in kilometers would be totally impractical. For example, the exoplanet closest to Earth, next Centauri B, is about 4.2 light years away. In kilometers, that figure would be almost 40 billion, a much more difficult number to handle and contextualize.
How a light year is calculated in kilometers
A laser indicates the center of the galaxy from the VLT telescope. Image | THAT
We go in parts. If the speed of light is a universal constant, why should it clarify that “C” is the speed of light in a vacuum? Because, in reality, one thing does not take away the other. The light goes more slowly as materials such as water (225,000 km/s) or glass (200,000 km/s). This is not a contradiction, but the result of the interaction of light with matter.
The light is made up of particles without mass called photons. Individually, photons always travel at 299,792 km/s. However, when a beam of light crosses a material medium, its photons are continuously absorbed and reigned by the atoms of said material. Each of these interactions introduces a tiny delay. The sum of billions of delays makes the effective speed of the wave of light as a whole is less than c.
Light is also an electromagnetic wave. Upon entering a medium, its electric field causes the electrons of the atoms to oscillate. These oscillating electrons generate, in turn, their own electromagnetic waves. The original wave and the waves generated by the electrons interfere with each other, forming a combined wave that spreads more slowly. But the light travels at a constant speed: its slowdown is an effect of crossing a field of atoms.
That said, the emptiness of space is not a perfect emptiness. There are free electrons and protons in the form of plasma; There are dispersed atoms and molecules, mainly hydrogen and helium; There is interstellar dust, and there are also background radiation and magnetic fields. But its density is very low, which means that light travels through space at a very, very close speed to c. So that the light year is calculated by taking the ideal vacuum as a reference.
A light year is the distance that runs the light in a year. If something was clear in high school is that distance = speed × time. Therefore, the distance equivalent to a light year is calculated by multiplying the speed of light by the time that lasts a terrestrial year:
- In round numbers, the light moves at 300,000 km/Sy a year is 365 days. 365 days × 24 hours × 3600 seconds are 31.6 million seconds. 300,000 km/s per 31,600,000 seconds results in a distance of about 9.5 billion kilometers.
- Taking the exact speed of the light (299,792,458 km/s) and taking into account the leap years (365.25 days), the result would be 9,460,730,472,581 km.
How much is a light year in earthly terms
The light year measures such great distances that escape from our imaginary. The light takes about eight minutes to cover the distance from the sun to the earth. If in eight minutes it travels the 150 million kilometers that separate us from the sun, in one hour it would travel the same distance 11 times; In one day the daily distance would travel 24 times; And in 365 days, accumulating all those journeys, 9.5 billion kilometers would reach the aforementioned.
This gigantic tour is what we call a light year. It does not indicate time, but how far things are in the cosmos. To measure astronomy times we continue to use years, days, seconds, etc., while for very large distances we use, for mere convenience, light years or parsecs, another astronomical unit.
You just have to look at the night sky to understand the immensity of the cosmos. The brightest stars are dozens of light years. With little light pollution we can also see with the naked eye the Andromeda Galaxy, the closest to our Milky Way, which is 2.5 million light years.
The light that our eyes captured departed from Andromeda when the Australopithecus populated the earth, then suffering multiple ice ages. In a sense, looking at the night sky is looking at the past. The farther we look at, the more in time we travel. In this way we have seen, with our most powerful telescopes, what happened after the Big Bang.
It is impossible to travel at the speed of light
Can you travel at the speed of light? The answer to this question is one of the most famous theories of physics: Einstein’s special relativity. And to explain it you have to return to the iconic formula E = mc², which connects the speed of light with two very different concepts.
Moving an object with mass requires energy. As the mass of an object increases, so does the energy needed to continue pushing it. Einstein’s famous equivalence between mass and energy tells us that energy and mass are intrinsically linked.
According to relativity, as an object with mass accelerates and approaches the speed of light, its relativistic mass increases. To accelerate an object with infinite mass, an infinite amount of energy would be needed, which is simply impossible. The speed of light works as the definitive cosmic speed limit.
Because? Because only dough -free particles, such as photons, can travel at this speed. Having no mass, they don’t face this infinite energy and mass barrier. For the rest of us, and for any spacecraft we can build, the speed of light will continue to be an unattainable horizon. Nothing with dough can really reach it. It is the speed limit of the universe.
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