**Key Takeaways:**

- The speed of light is an assumption, not a certainty.
- The best we have been able to do, even with our most sophisticated modern technology, is measure the roundtrip speed of light and assume that the speed is the same in both directions.
- Since measurements of the roundtrip speed of light are always consistent, Einstein suggested simply assuming that the speed of light towards one point was the same as the speed of light returning from that point.
- This time they begin in the middle and both are moved an equal distance and at equal speeds to the start and finish lines so that time dilation is the same for both clocks.
- It could also be a clue to something even more spectacular — the relationship between general relativity and quantum mechanics.
- It is often the case that logic does not apply at the quantum level.

The speed of light is an assumption, not a certainty. It’s an assumption in that we have never experimentally measured the one-way speed of light. The best we have been able to do, even with our most sophisticated modern technology, is measure the roundtrip speed of light and assume that the speed is the same in both directions. This is what’s known as being isotropic — a value is uniform no matter the direction. To be anisotropic, then, means that a value varies depending on the direction. Einstein’s theory of special relativity assumes that the one-way speed of light is constant and isotropic. So what happens if the one-way speed of light is anisotropic? What does it mean for our universe if one of our most successful theories of physics is based on an assumption that turns out to be false?

The current accepted value for the speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second). Light is able to travel at this incredible speed not because of what it is, but because of what it isn’t.

Fundamental particles of light — photons — are not very massive at all. In fact, they have no mass whatsoever. A particle’s mass comes from the emergence of the Higgs field. The Higgs field is a property of spacetime, one that arose as the universe cooled and and its temperature was able to fall below a critical value. Once this happened the field expanded and any particle that interacted with the field acquired mass.

Mass reduces down to this: how much a particle interacts with a spacetime field known as the Higgs field. The more a particle interacts with it, the more mass it acquires. The photon is able to have no mass because it doesn’t interact with this field at all. Consequentially the speed of light is really the speed of all massless particles. Gluons, the carriers of the strong nuclear force, are also massless and can move at lightspeed. Gravitational waves are another example of a phenomenon which propagates at lightspeed.

Once traveling at the speed of light, there is no such thing as space or time. The relationship between time and the speed of light is an especially interesting one. There are three main outcomes: moving at near the speed of light allows you to travel into the future because of time dilation (time will progress slower for you than for someone on Earth), moving at exactly the speed of light halts time altogether, and moving faster than the speed of light will allow you to go back in time. Of course it is impossible for any object with mass to travel at lightspeed. But there are loopholes, such as the one exploited by the Alcubierre warp drive.

One of the first attempts to measure lightspeed (a value known in physics as “c”) came in the 17th century when Galileo devised a simple experiment. He had two people with covered lanterns stand a set distance apart. The first person would uncover their lantern. Once the second person saw this light, they would proceed to uncover their own lantern. Galileo acted as an observer and recorded the time between the uncovering of the first and second lanterns, but there simply wasn’t enough distance between the two people for Galileo to draw any certain conclusions. Light simply moves too fast.

As time went on increasingly complex experiments were devised to try to measure lightspeed. These included such instruments as rotating mirrors, cogs, lasers, and cesium clocks. But again, these always measured the roundtrip travel time of light. No experiment conducted has ever measured the one-direction speed of light. Einstein himself was somewhat complacent on this point. Since measurements of the roundtrip speed of light are always consistent, Einstein suggested simply assuming that the speed of light towards one point was the same as the speed of light returning from that point. Assume that light is isotropic.

But why has this never been experimentally confirmed? It all comes down to one paradoxical set of terms: to measure the one-way speed of light we need two synchronized clocks. But to have two synchronized clocks we need to first know the speed of light.

At first it seems easy enough to do. Finding the one-way speed of light simply involves measuring the start and end time for a traveling photon. We will use two clocks to measure these start and end times.

Let’s say, for example, that you begin with the two clocks side-by-side (picture below). The clocks at this point are synchronized. However, one clock must be moved to the finish line. This is where the problem arises: once the clock is moving it is subject to time dilation which means the two clocks will no longer be synchronized.

Let’s begin again. The two clocks are side-by-side and synchronized. This time they begin in the middle and both are moved an equal distance and at equal speeds to the start and finish lines so that time dilation is the same for both clocks. But there’s a problem with this. Time dilation will only be the same for both clocks if the speed of light is the same in both directions. If it’s not, then the clocks will no longer be synchronized.

Various arrangements have been suggested, but none ever results with two undoubtedly synchronized clocks with which to measure the speed of photons. Neither have GPS satellites or gamma rays been able to give us conclusive results during experimentation. In fact, since the 1970’s various experiments (including this one and this other) have even suggested that the one-way speed of light may be anisotropic.

But why should it be that light behaves differently depending on direction? It would be a mystery akin to the arrow of time. There is no specific reason that time must flow forward in our universe, and in fact all the major equations of physics work just as well whether time moves forwards or backwards. Relativity, too, also continues to apply whether or not the one-way speed of light is anisotropic. What matters in relativity is that the roundtrip average of light stays the same.

These same laws of relativity that assume that the speed of light is constant also tell us that the speed can never be measured. There is no way, in so far as we know, to verify once and for all that the one-way value of ‘c’ is isotropic.

And what does any of that really mean in the big picture of things? Well, finding an inconsistency in the speed of light is a prediction of string theory. It could also be a clue to something even more spectacular — the relationship between general relativity and quantum mechanics. A stepping stone towards a theory of everything.

It is often the case that logic does not apply at the quantum level. To understand the very nature of reality requires something beyond reason alone. We may never know how the speed of light truly works and, as a consequence, we cannot be certain of what we’re seeing in the world around us. There are some uncertainties which we must accept as part of our lives. This may be one of them.