You live 200 kilometers away from me. You just arrived, and you drove an average 100 km/h. So you left 2 hours ago. Therefore, you existed 2 hours ago. Simple.
There are many stars farther than 6,000 light years. Light travels… at the speed of light. So it left at least 6,000 years ago. Therefore, the universe existed at least 6,000 years ago. Simple.
6,000 light-years, in cosmological terms, it’s really next door. If the observable universe were as big as France from north to south (about 1,000 km), 6,000 light-years would literally be the tip of my nose. 6 centimeters. How can we explain that I can see farther than the tip of my nose, if light appeared only 6,000 years ago? This is, for young earth creationists, the “starlight problem”.
A solution often proposed is that light did not always go the same speed. C, the letter that stands for the speed of light, would have changed in the past. That’s what I want to talk about here.
The blue and the red sections can be read separately.
When possible and/or needed, I link the (free) arXiv version of the (usually behind a paywall) peer-reviewed papers.
What do observations say?
The speed of light is not a minor parameter that can be changed without touching the rest of physics. This is, for example, the only parameter involved in Maxwell’s equations, which describe the propagation of light. This is, again for example, one of the 3 parameters that appear in Einstein’s equations that govern gravitation (General Relativity). Changing c means changing the laws of nature.
The constancy of the laws of nature in time is not a dogma. Indeed, as physicist Paul Dirac wrote,
It is usually assumed that the laws of nature have always been the same as they are now. There is no justification for this. The laws may be changing, and in particular quantities which are considered to be constants of nature may be varying with cosmological time. Such variations would completely upset the model makers.
Paul Dirac, On Methods in Theoretical Physics, June 1968, Trieste
It is precisely because many are well aware of this, that the search for clues pointing to a variation of fundamental constants, c included, has been the subject of many studies.
Recently, in 2010, a 145-pages article reviewed the state of the art in this matter. Here are some of the main points which indicate that no variation of c has been detected, including on cosmological times (that is, billion years time scale):
- Each atom nucleus, each atom, each molecule, emits light on a series of wavelengths of its own. This is called its “spectrum”. A bit like its barcode. And whether we observe the tip of my nose (6,000 light-years), the end of my desk (100,000) or even farther, we see the same bar codes. The same nuclei, the same atoms, the same molecules, governed by the same laws which depend, among others, on c.
The number of observations is mind blowing. The SIMBAD database, for example, contains spectral measurements for more than 9 million celestial objects in our galaxy (less than 100,000 light-years around – the end of my desk).
The NASA/IPAC Extragalactic Database is a database of objects outside our galaxy, that is, more than 100,000 light-years away. It contains more than 200 million entries.
The Sloan Digital Sky Survey is another extragalactic database, with about 3 million spectra.
- The laws of nuclear physics also depend on the speed of light. They determine how nuclei split (as in our nuclear power plants), merge (as in the center of the sun, or in laboratories), or disintegrate when radioactive. Nuclear astrophysics is a growing discipline that studies astrophysical nuclear events.
Just an example. When a “Type 1a supernova” explodes, we observe the radioactive decay of Nickel to Cobalt, then of Cobalt to Iron. Observations allow to measure the half-life of these elements on site. It is the same we observe on earth . A search of this type of supernova on this database returns more than 10,000 entries.
- The equations of General Relativity also depend on c. When two very dense stars turn around each other, they lose energy because they emit gravitational waves. As a result, the period of rotation decreases by an amount that depends on the speed of light.
This kind of duo is sometimes called “binary pulsar”. A census conducted in 2008 numbered 160. The most famous one is the first that was discovered, in 1975. Located 21,000 light years away, it is under scrutiny since then. You will find below 40 years of comparison between the predictions of General Relativity concerning the period of rotation (the continuous line), and the observations (the black points). The horizontal line at the top of the graph is the prediction of Newtonian gravitation (nothing changes).
Weisberg & Huang, ApJ, 2016
So, what do observations say? We’re not talking about a few here and there, suggesting that perhaps, the speed of the light did not vary too much in the past. We’re talking about literally millions of observations showing the same.
Let’s finish this section with a rather remarkable and very emblematic observation.
Mass bends light. The heavier it is, the more it bends. That’s how one can observe gravitational lensing in a situation like this,
Now, imagine a star explodes. A supernova. Suppose also that something heavy (a few galaxies, for example) is between me and the star. The supernova sends me light both by the blue path and the red path. And as light is bent, I see double (the red and blue mirages).
If you look closely, you will see that the blue path is shorter than the red. So I should see the blue glow before the red. If, at the time I detect the blue mirage, I have enough information on the whole system, I can predict the moment the red image should appear.
That’s exactly what happened in 2014-2015 with the Refsdal supernova. In 2014, astronomers observed a supernova 10 billion light-years away. Its light had been curved by a cluster of galaxies, so that four mirages were visible. The study of the observations allowed to predict that another mirage would appear a year later, around November 2015. A year later, the forecast mirage appeared. Needless to say, the speed of light plays a leading role in the timing: a variation of just 1%, along a path of only 1 million light years, would result in a nearly 10,000 years shift on arrival. Being able to pin down the appearance of the 5th mirage with a few weeks precision, does not leave much room for speed of light variation.
The question is thus unambiguously resolved by observations: the speed of light has not changed for more than ten billion years, at least  . Perhaps one day a tiny variation will be detected  , “tiny”, that is, tinier than the error margins of current measurements. At any rate, a multiplication of c by 10, 100 or 1,000, necessary for the light emitted 60,000, 600,000 or 6,000,000 light-years away, to come to us in less than 6,000 years, is completely excluded by observations.
At this point, those who have enough can leave. Others can make a break and/or keep reading. Even if the deal is done, I would like to address in addition some conceptual problems implied by a variation of c. Of course, if observations were telling that c has changed, we would have to face these issues. But this is not the case, while it seems to me that the proponents of a variable c are usually not aware of them.
What about the fine tuning?
The fine tuning of the laws of nature is a popular argument for the existence of God. And it definitely implies that the speed of light has not changed. One could indeed list the many reasons why c could not have varied much, simply following the rationals of the fine-tuning argument.
Again, if observations had showed that c had changed, the fine-tuning argument would just have to come back where it came from. But this is not the case, and it is therefore completely incoherent to hold the fine tuning in one hand, while playing with the speed of light with the other.
And yet, they spin (the skaters)
Here on earth, experience shows that the laws of nature do not change when we change places, times, or directions.
- Will your hair dryer behave differently at home and at a friend’s house? No. Nothing happens when you change places.
- Will your hair dryer behave differently when you stand in front of the bathroom mirror, or when you turn your back on it? No. Nothing happens when you change direction.
- Will your hair dryer behave differently in the morning and in the afternoon? No. Nothing happens when you change the moment.
It all seems obvious, and yet it has amazing consequences. For example, imagine that tomorrow the gravitational constant G goes up. I put a weight on a ladder today. It costs me a certain amount of energy proportional to G. Tomorrow, when G is bigger, I let it down… recovering more energy than I spent to lift it. Conclusion: if G changes, energy is not conserved. Now, a few questions:
- Question 1: If something else than G changes, like c for example, is energy not conserved either? No, it is not conserved either. Emmy Noether actually proved in 1918 that energy is conserved if and only if the laws of nature do not change over time.
- Question 2: The fact that the laws of physics do not change over time gives the conservation of energy. OK. And the fact that these laws do not change when we change places, or when we change direction, also gives the conservation of something? Yes. According to Noether’s theorem, the independence of the place gives the conservation of momentum. And the independence of the orientation gives the conservation of the angular momentum (which makes skaters spin faster when they close their arms, hence the title of the paragraph).
What does it have to do with our problem? If light goes faster when it comes to earth than when it leaves it, it implies Maxwell’s equations are not invariant by a change of orientation. If light went faster in the past, it implies the same equations are not invariant in time. And if these equations can do whatever they want beyond the tip of my nose (6,000 light years), while here on earth they keep quiet, it means they are not invariant either when we change places. And we could say exactly the same with the laws of General Relativity, or nuclear physics.
So if the speed of the light changes, then energy is not conserved, momentum either, and skaters do not spin faster when they tighten their arms. In addition, the duos energy/time, momentum/position and angular momentum/orientation are found in quantum mechanics as conjugate quantities via the Heisenberg uncertainty principle. It is not a mere coincidence.
Bottom line: Many consequences should encourage some to think twice before naively proclaiming, “well, the speed of light must have changed after all”.
Let’s conclude by commenting on some experiments often mentioned in relation to our topic.
- In her Harvard lab, Lene Hau has fun putting photons in bizarre substances they take forever to come through. Journalistic version: “Lene Hau slows down the light! Alert! Einstein was wrong! The speed of light changes!”.
It is “just” that the medium involved absorbs photons, re-emits them, re-absorbs them, re-re-emits them, re-re-absorbs them, etc., so that the poor guys cannot move forward. Quite like a photon emitted at the center of the sun can take hundreds of thousand years to get out. But Maxwell’s equations are the same at Harvard as in your living room. That’s why Lene Hau’s smartphone keeps working in her lab.
- Same thing with this experiment, in which researchers have “slowed down light”. Here again, Maxwell’s equations are the same in their laboratory than at home. It is “just” that by skillfully tailoring several lasers, they have succeeded in slowing down the speed at which the beams carry the energy. This is called “group velocity”. But no, c has not changed . And their smartphones work fine in their lab.
- Other experiments got superluminal velocities, that is, light going faster than good old physics says… in a medium. It’s always in a medium. But the c of Maxwell’s equations hasn’t changed.
The speed of light in vacuum remained the same in all these labs. More on these experiments here.
So observations are clear: light in vacuum has been going the same speed for ten billion years, at least. And the direction of propagation does not change anything.
May the young earth creationists admit it as soon as possible.
 More in The World Is Not Six Thousand Years Old So What?
 We can actually go back to a few milliseconds after the Big Bang, as known physics successfully describes the observational consequences of primordial nucleosynthesis. Physics before that is unknown territory, in which can flourish theories with varying speed of light, that observations cannot refute or confirm so far.
 Some would have detected tiny variations of the “fine structure constant”, in which the speed of light enters, of the order of 0.001%, 10 billion years ago. But the observations are very debated.
 Which is why this article was published in “Scientific Reports” (the less demanding of the “Nature” journals), and why no one will get a Nobel Prize out of it.