We mentioned this problem to you a few days ago in this FAQ, but the topic is interesting and complex and we thought it might be worthwhile to devote more depth to it. IS the problem of the speed of light and how this cannot be measured in any way in one direction.
We are taught that the speed of light in a vacuum is c, to be precise 299,792,458 meters per second, but how did we measure this value, are we really sure that it is? Get ready to leave aside any certainty you think you have and follow us on this fascinating journey to discover one of the fundamental constants of physics.
Special relativity
To have a hope of answering these questions, the first tool we have to deal with is that of special relativity (or restricted) by Einstein.
In particular, reference is made to the first article of 1905, Einstein’s annus mirabilis (who won the Nobel Prize for physics in 1921), entitled “On the electrodynamics of moving bodies“A revolutionary article that demonstrates how time flows differently for different reference systems in motion with respect to each other. In other words, if I stand still, time passes more slowly for me than if I am in motion. This physical fact was then measured and demonstrated several times with experiments also carried out on our planet. In particular, the more my speed approaches that of constant c (the speed of light), the more time slows down, until it stops if I go exactly to c.
This gave us a practical demonstration of the possibility of time travel in the future (as we had already explained to you in this article), in fact it is enough to move fast enough. Imagine taking a plane that moves at speeds close to c for a week around the Earth. In such a system the time of the plane will move much slower than the time on our planet. On his return, not a week could have passed but, depending on the speed reached by the plane, perhaps a month, a year, a century: in fact the plane has traveled in time.
Unfortunately, to make the times so obvious you need a plane that goes at a speed beyond our technological reach (at the moment). However, in truth we travel in time every day, even just by taking the car to go to work, but we do such an infinitesimal fraction not to be aware of it in the slightest. In particular, the concept we need for our little experiment of measuring the speed of light is this: if I move at any speed my time runs differently compared to someone standing still. We will see later why this is important.
How do you measure the speed of an object?
Let’s take a baseball, how do we know what exact speed it is traveling? Speed is defined as space traveled after the time taken to travel it, for example, a car traveling at a speed of sixty kilometers per hour will be able to travel sixty kilometers in an hour. So to measure the speed of a baseball we need two measures: a measure of time and a measure of space.
We can then take a stopwatch and shoot a ball along a track where two bars are clearly visible. In this way we can do start the stopwatch when the ball passes the first bar and stop it when the ball passes the second bar. We can therefore easily measure the space between the two bars and divide it by the time measured with the stopwatch.
All of this may seem trivial to you but, as we will see, almost none of these concepts are applicable when it comes to measuring the speed of light. For starters, let’s think about the baseball itself, what a mistake I make in pressing the stopwatch?
For example, suppose that the button is pressed by a perfect machine, in order to eliminate any error due to a lack of reflexes (which cannot be eliminated in humans). The machine will press the button as soon as it sees the baseball pass the first bar. But this can only happen if a photon hits the ball and travels with a certain speed to the sensor of the car, which only then will press the button, that is late. This delay is infinitesimal compared to the speed we are trying to measure, but it may not be if we try to measure much higher speeds.
Moreover, the whole world we see with our eyes is nothing more than the result of a ray of light that hit an object and subsequently traveled to our eyes with some time. We therefore live in the past compared to the world around us, we see the world as it was a moment before, and not as it really is in the moment we are observing it. This is all the more true the further an object is. For example, the sunlight takes eight minutes to reach the Earth, so if we look at the Sun we don’t see it in real time, we see it as it was eight minutes ago. For example, if it were switched off at this moment, for eight minutes we would continue to believe that it was still switched on. Similarly, looking at a distant galaxy (even the most distant of all), for example six billion light years, means observing it not as it is now, but as it appeared six billion years ago.
How is the speed of light measured?
So let’s try to schematize an experiment to measure the speed of light similar to that used for the baseball. Let’s take a laser, a clock to time the time, a starting point A and an arrival point B one kilometer away from each other. Let’s start the clock at point A and when the laser reaches point B we stop it. But how do we know when the beam has reached point B? Even being at point B we should communicate instantly the clock at point A to stop, which obviously is impossible.
We need two clocks, one in point A and one in point B. We start them at the same time as the ray starts and we stop the second clock as soon as the ray reaches point B. But how can we be sure that they start at the same time? How do we do a synchronize them? Think about it: we can connect them with a wire and send a signal, but that too will move at the speed of light. So it will come with a delay, making the synchronization useless. You might think you can subtract this delay, but this delay is the time it takes for light to travel a kilometer which is exactly what it is. we do not know and we are trying to measure!
We can then think of synchronizing them first, when they are in the same point A, and then move one of the two clocks to point B, now they will be synchronized, right? Unfortunately no. As we said, moving at a certain speed changes the flow of time, the simple act of moving from A to B of one of the two clocks irreparably compromises synchronization. The watch at point B will have “traveled into the future” with respect to the clock at point A.
So how can we do? We can simply delete the second clock and insert in its place a mirror that reflects the light back to the starting point. In this way we can make the light travel the ABA tract and measure the time with the only clock placed at point A, which will obviously be synchronized with itself. We will have measured the distance of two kilometers and the time it took to travel it. If we carry out this experiment (which is easily reproducible even in a simple high school physics laboratory if you have an oscilloscope) we obtain that the value of the speed of light is just c, 299.792.458 meters per second. So what’s the problem?
The problem is that we can only measure the speed of anything in one direction (e.g. a baseball), except light, so we need to make a round trip from point A to point B and then back to point Loves who insures us that the speed to go from A to B is the same as to go from B to A?
Is the speed of light the same in all directions?
The problem, as described, is addressed by Einstein himself in the aforementioned 1905 article. He says verbatim: “up to now we have defined a time of A and a time of B. We have not defined a common time for A and for B, which cannot be defined unless it is stated by definition that the velocity to go from A to B is the same as to go from B to A.“This is known today as Einstein synchronization convention. At the basis of the fact that the speed of light is the same in any direction there is therefore no experimental fact, no objectivity, but a convention.
Things then get even more interesting when you consider that everything our physics works perfectly even considering a different speed in different directions for the speed of light. The light could be c in all directions, or it could be 5% faster in one direction and 5% slower in the opposite direction, or it could be half of c in one direction and snapshot in the opposite direction, provided that on average it is c: we would have no way of noticing it. To learn more, we invite you to watch an in-depth study a few years ago (but still current) made by Veritaserum with the collaboration of SmarterEveryDay on this topic, which has not just inspired the creation of this article.
The consequences
Okay, let’s say we convinced you and we accepted the fact that the speed of light it may not be c in all directions: so what? Well the truth is that in practice absolutely nothing changes, except the possibility that this uncertainty about the real directional speed of light does not leave room in the future for new physics, maybe able to explain what still does not come to us in the universe. But for us there’s more than that, of the simple practical aspect. The search for truth and the nature of the universe is the basis of everything that moves the efforts of a physicist and more generally of any scientist.
We want to know how things are, how do they work. In this sense, the speed of light, which for many is simply c, remains a fascinating mystery that we may one day solve, even if it were to prove once and for all that it is indeed c in all directions. Until then, we will continue to enjoy the deep sense of mystery that manages to give us the universe.
