 Fact

In theory, the speed of light, in a perfect vacuum, measured from an inertial frame, is constant.

## Is the Speed of Light Constant?

In theory, the speed of light, in a perfect vacuum, measured from an inertial frame, is constant with an exact value of 299,792,458 m/s.

In other words, the best current theory out there essentially says: “the maximum speed of light (and thus the speed light travels unimpeded) is constant.”

With that said, this theory isn’t immune to being proven wrong (and as we’ll show below recent studies into “the variable speed of light” have tried to do just that).

Furthermore, fully understanding what the idea that “the speed of light is constant” does and doesn’t mean requires understanding some key points.

The following points should considered for a correct and full answer to the question “is the speed of light constant”:

• The value of the light speed constant, and the idea that light speed is constant, is THEORY. It is a fact that scientists have constantly found the speed of light (in a perfect vacuum) to be 299,792,458 m/s using mathematics and tests (one can’t create a perfect vacuum to run tests, instead tests with atomic clocks and lasers are used; see that link for the long discussion on how the speed of light is measured today)… but that doesn’t mean a better theory won’t come along. In physics, one doesn’t seek to prove theories to be right. Instead, mathematical proofs are given and then tests are constructed to seek to prove a theory wrong. Every time a theory isn’t proven wrong, and every time observations back up a theory, it becomes stronger. As long as a theory of science has lots of scientific facts pointing at it, and as long as it works to predict phenomena without fail, it is considered true for the time being and is called “a scientific theory.” In this way, here in 2017, “it is a fact that the speed of light, in a vacuum, measured from an inertial frame, is constant.” Learn more about “what is a theory?
• In simple terms, light is another name for electromagnetic energy particle-waves, photons (one of four fundamental forces in the universe). See more about “what is light.”
• When we say “the speed of light is constant” we mean “in a perfect vacuum, measured from an inertial (resting) frame.” We do not mean “it always goes the same speed no matter what.”
• The speed of light is like a universal speed limit; nothing can travel faster than the speed of light (in a perfect vacuum); although “non-things” like shadows can. Despite this, everything (including light) can move slower than the speed of light’s max speed!
• In real life there is no empty space, so light is never actually traveling through a perfect vacuum. A perfect vacuum is a theoretical concept.
• Light is slowed down when traveling through a medium like air or water due to refraction. The ratio by which it is slowed is called the refractive index of the medium. In any refractive medium, light is slowed down when photons (the particles of light) refract (bounce off) the particles of the medium.
• Light can be slowed down to very slow speeds, for example using the right type of photonic crystal. Light has be slowed to a little under 40mph according to recent studies.
• The speed at which light travels appears to be the same for all observers regardless of what speed they are traveling. Time and space are relative to one’s frame of reference, but the light speed constant isn’t.

The Speed of Light is NOT About Light | Space Time | PBS Digital Studios.

TIP: For simplicity one can denote the speed of light as “c” (the light speed constant; a universal physical constant) and can round the speed to “about 186,000 miles per second” (or “about 300 million meters per second” or “about 3.00×108 m/s”).

## The Theory that the Speed of Light is Not Constant

Some studies, including two recent (2014) studies, theorize that even when in a perfect vacuum, light speed can vary slightly depending on a few slightly complex factors.

1. One study postulated that light speed could differ slightly depending on the energy of the particles capturing and emitting photons as they move through non-empty space.
2. Another study postulated that light speed could differ depending on the number of species of elementary particle that exist in the universe.
3. Another study sent photons through a special mask and changed the shape of the photon and this resulted in them traveling slower than light speed through “free space” (the lab version of a vacuum).

All of these studies only present hypotheses/theories, they don’t tell us that the speed of light isn’t constant for sure. They are interesting, and are worth noting, but they don’t go as far as actually proving “the speed of light is not constant.”

TIP: A variable speed of light (VSL) is a feature of a family of hypotheses stating that the speed of light in vacuum may in some way not be constant (such as by varying in space or time, or depending on frequency). In other words, all the “new study finds speed of light isn’t constant” studies are of a class of studies called VSL for short. We aren’t yet at the point where these VSL hypotheses have supplanted the general theory of a constant light speed. Thus, we (our website) have ultimately rated the idea that the speed of light is constant as fact, while also noting this family of skeptical hypotheses.

### Thoughts on What it Could Mean that the Speed of Light Might Not be Constant

With that said, if you think about the above studies, they are saying things like: “we have to factor in the source emitting the light,” or “if there are more elementary particles that could change things,” or  “if we change the shape of the photons it changes their speed.” In other words, none are really saying the speed of light isn’t constant (assuming we factor out these things), they are just saying the speed of light in a vacuum can change depending on “factors.”

In other words, despite all the studies, it is still correct to say that the light speed constant is 299,792,458 m/s, or at least very close (perhaps the average max speed of light is 299,792,458 m/s and the law of averages means that using that number for a calculation works? We don’t know, that is just me offering another VSL speculative hypothesis).

WOW! Scientists Slow the Speed of Light.

## The Takeaway on the Light Speed Constant

Given all the above, based on our research, I would conclude that:

Until there is proof otherwise, we have to assume the speed of light is constant and that, despite these studies, two exactly-the-same photons (same spin, same shape, same energy content), emitted in exactly the same way (from a particle with exact the same spin, shape, charge, etc), measured from an inertial frame, in a theoretical perfect vacuum, would travel at exactly the same speed, the speed of light.

The speed of light constant works to predict phenomena, and a better theory is yet to come along, therefore we can consider the theory of a constant speed of light as a fact.

SPEED OF LIGHT EXPLAINED | Physics documentary | Simulated reality docs.

Conclusion

In theory, the speed of light, in a perfect vacuum, measured from an inertial frame, is constant.

This isn’t to say that studies haven’t cast doubt on this longstanding theory, and this isn’t to say that light speed has ever been directly observed from an internal frame in a perfect vacuum… it is only to say, the best evidence we have today suggests that the speed of light is constant.

"The Speed of Light is Constant (in a Perfect Vacuum)" is tagged with: Light, Quantum Mechanics Mark on

It’s great that you take the time to talk about real science rather than dumbed down soundbites, refreshing to read 🙂

You’ve let a contradiction slip through though:

“It is a fact that scientists have constantly found the speed of light (in a perfect vacuum) to be 299,792,458 m/s using mathematics and tests”
and…
“In real life there is no empty space, so light is never actually traveling through a perfect vacuum. A perfect vacuum is a theoretical concept.”

If scientists can’t make a perfect vacuum, they can’t have measured the speed of light in a perfect vacuum.

BIt nitpicky I know, but as you’d taken so much care to try to do it right I thought you wouldn’t mind having your attention drawn to this. Thomas DeMichele on

Really solid point. I clarified the meaning and added in a link that shows how the speed of light is measured (HINT: it doesn’t involve creating a perfect vacuum, as that would be impossible; it does however involve lasers and atomic clocks).

Thanks for the comment and the kind words!