The speed of light represents one of the basic physical constants and, at the same time, a very strict limitation that is currently burying our hopes for journeys to other stars, or even galaxies. When people discovered that the Earth was surrounded by a whole universe full of stars, star islands and also a huge number of other planets, their imaginations worked at full speed. What kind of worlds are we talking about, what kind of environment do they offer, would it be possible to live in them? It would be very exciting to see with our own eyes what exoplanets and their parent stars look like in diverse systems. Or what it looks like in an alien galaxy.
The problem is that all the mentioned objects are incredibly far away. And if we wanted to fly to them in the classic way, we would encounter a fundamental limit given by the insurmountable speed of light. It is a physical limit in the universe, which for now fatally limits our possibilities.
Three hundred thousand kilometers per second
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The speed of light in a vacuum is 299,792,458 m/s and is often given as a rounded 300,000 km/s. It represents the universal constant c that occurs in many physical equations. According to Einstein’s special theory of relativity, which forms the basis of a substantial part of physics today, nothing in the known cosmos can move as fast as light. As in the case of similar physical extremes, the concept of infinity is related to the speed of light. It follows from the theory that as matter approaches the speed of light, its mass increases at the same time. If it reached that limit, its mass would reach infinity, which is not possible within standard physics. Therefore, the speed of light under the current state of knowledge creates an absolute speed limit for the entire universe.
The speed of light in a vacuum is so constant that it is used to define basic quantities such as the unit of length, the meter. It also contributes to the definition of the kilogram for mass or the kelvin for temperature. As a constant it is very useful. At the same time, however, many scientists and science fiction creators are irritated by its insurmountability, combined with the fact that because of it, we realistically cannot even get out of the Solar System in a time that would not significantly exceed the length of a human life.
So there are constantly ideas that we could bypass the given limit in some way, so that we would actually travel faster than light, but at the same time we would not physically exceed its speed. However, whether it is a warp drive from the world Star Trekmovement through hyperspace Star Warsfly through wormholes like in the movie Interstellarpassages through stargates, or a fascinating improbable drive from the world The Hitchhiker’s Guide to the Galaxytravel at superluminal speeds remains the domain of science fiction.
A year as a distance
Scientific fields dealing with space sometimes have a tendency to use rather confusing terms, and one of them is the light year, or ly, from the English “light year”. It is not a unit of time, as it might seem at first glance: A light year expresses the distance that light travels in a vacuum in one year, or roughly ten trillion kilometers. This is a practical way to describe the vast distances separating stars and galaxies.
Light from the Moon reaches us in approximately one second. So we can say that our companion is one light second away from us – which shows how staggering a distance a light year is. Light from the Sun reaches Earth in about eight minutes, so the star is about eight light minutes away from us. But once we’re outside the Solar System, light years come into play. We currently observe the nearest star, the red dwarf Proxima Centauri in the constellation Centauri, at a distance of 4.22 ly. If it did explode, we would know in 4.22 years.
How to imagine 1 ly? An Earth year is roughly 31.5 million seconds long. Thus, a light year encompasses about 31.5 million times the distance between the Earth and the Moon. The manned spacecraft in the Apollo program traveled at around 39,400 km/h, yet it would take them approximately 27,000 years to travel 1 ly. An airliner flying at a speed of 965 km/h would need a million years, and a car traveling at a speed of 90 km/h even 12 million years.
Space objects fare somewhat better. The Earth orbits the Sun at a speed of about 107,000 km/h, so it completes 1 ly in 10,000 years. Even the movement of our planet pales in comparison to the entire Solar System, which rushes through the Milky Way at a speed of around 720,000 km/h. It thus crosses a distance of 1 ly once every 1,500 years, and since the end of the youngest ice age roughly 10,000 years ago, it has traveled less than 7 ly in the Galaxy.
Denied discovery
The influence of the phenomenon of the speed of light on the observation of the cosmos is so significant that people noticed it already 350 years ago. Danish mathematician and astronomer Ole Rømer sought to create a reliable astronomical clock for sailors. Meanwhile, in 1676, from observations of Jupiter and eclipses of the moon Io, he deduced that the speed of light was finite. However, he ran afoul of the scientific community with his discovery, as it contradicted the ideas of the universe at the time.
Rømer’s conclusions were only confirmed in 1851 by a French physicist and astronomer Hippolyte Fizeauwho also estimated the speed of light to be 315,000 km/s. A very precise measurement for its time was already carried out in 1862 by his colleague Léon Foucaultwho arrived at a figure of 298,000 km/s. The speed of light and its physical nature were also intensively dealt with Albert Abraham Michelson: In 1879, he applied Foucault’s method for his purpose, but improved it by using extremely high-quality mirrors and lenses. He thus achieved a value of 299,910 km/s, which then remained the most accurate determination of the speed of light for the next four decades, before the same American physicist refined it again.
Rippling of the ether
At that time, physicists wrestled with the nature of light. It was not clear whether they were waves or particles. Michelson and his colleague Edward Morley assumed that light had a wave nature, just like sound – which presented a problem, however, because sound needs a medium to travel. Therefore, many other researchers were also convinced that light actually represents mechanical waves of an as yet unknown medium, which the devices of the time could not detect. A similar approach was applied by scientists much later in the case of dark matter and dark energy.
The hypothetical omnipresent and invisible substance of extremely low density through which light was supposed to propagate came to be called aether. Experts then tried hard to discover the said wonderful substance and to prove that light is its mechanical waves. The efforts at that time culminated in the famous Michelson-Morley experiment, which took place in 1887 in Cleveland. However, it turned out completely differently than the couple expected.
The Nobel Prize for Failure
Michelson and Morley built an ingenious interferometer, actually a very simple version of the instruments that today detect gravitational waves in the LIGO observatory’s facilities. In the mentioned instrument, the light rays traveled along different paths. At that time, researchers assumed the existence of the so-called ether wind, created by the movement of bodies through the ether – that is, primarily the Earth around the Sun and our system around the center of the Galaxy. The pair expected the wind to delay the light depending on which direction the light beam was moving.
The experiment was a continuation of the initial attempts in Potsdam from 1881, but it was a complete failure. This was a “tectonic break” that caused most scientists to reject the ether hypothesis once and for all. He later followed up on his results Albert Einsteinwhen he published his special theory of relativity in 1905. In the end, the failed experiment was so influential and important for the further development of physics that, apparently, as the only such failure, it became the main motivation for awarding the Nobel Prize in Physics: Michelson received it in 1907.
How to slow him down?
In a vacuum, light usually travels at absolute speed. However, if it passes through a material, the absolute index of refraction of the material in question will be reflected: The resulting speed of the light rays then corresponds to the ratio of the speed of light in a vacuum and this index. In a variety of transparent or translucent materials, the speed of light can vary greatly. For example, Earth’s atmosphere slows it down to about three ten-thousandths of its speed in a vacuum, while it travels at about 225,000 km/s in water and only about 200,000 km/s in glass, so it’s about a third slower than in a vacuum. It then flies through the diamond at a speed of about 124,000 km/stherefore compared to its typical movement it will slow down to less than half – although to our perception such a value still seems completely unreal.
Experiments conducted about a quarter of a century ago showed that light can be trapped and even stopped inside ultracold clouds of atoms. Scientists tried to slow it down even during a vacuum flight, and in 2015 a team led by Danielem Giovanninim from the University of Glasgow, of which he was also a member Václav Potoček from CTU in Prague. The researchers slowed down the flight of light through a vacuum using specifically spatially structured photons and confirmed that under certain circumstances it can also move slower than its official speed directly in a vacuum.
Faster than light
The speed of light is often referred to as the maximum allowed speed of the cosmos. Can anything in the universe exceed it? The somewhat surprising answer is “under special circumstances, yes.” It can be exceeded, for example, by the universe itself by its expansion. It is expanding at a speed of over 68 km/s for every megaparsec (MPc), or about 3.26 million light-years: A galaxy located 1 MPc away therefore moves away from us by 68 km every second, a galaxy located 2 MPc away by 136 km, etc.
At a certain distance, the speed of cosmic expansion from a given point of view will exceed the speed of light. Einstein’s general theory of relativity allows for such a situation. Of course, this does not mean that something at the stated distance would move at superluminal speed relative to its immediate surroundings.
