The speed of light is kind of slow.

Deep Breadth
Feb 26, 2018 · 5 min read

And also the fastest anything can move.

Anything that can affect or be affected by the observer exists within these past and future light cones.

How can the ultimate speed limit of the universe be slow?

Light is the fastest thing there is, but the universe is also big.

Really big.

Modern communication technologies sending information at the speed of light enable us to have seamless conversations with people on the other side of the planet, but if you wanted to talk to a friend of yours on the moon, there would be almost a 2.5 second delay.

The distance between the earth and moon, and the speed of light (to scale).

If you want to talk to someone on Mars, you’d have to wait between 6 and 46 minutes (depending on where earth and mars are in their respective orbits).

The star closest to the sun (Alpha Centauri) is 4.3 light-years away, making for a conversational delay of 8.6 years.

To talk to someone on the other side of our galaxy, you’d have to wait about 100,000 years for a response*.

Our nearest major galactic neighbor, Andromeda, is 2.5 million light-years away *. [s]

Honestly these numbers just start to become too difficult to think about and we haven’t even left our galactic neighborhood…

So what does it mean?

The blue dot on this image is not the Earth, though the earth is at its center.

It is a sphere 200 light-years in diameter* that represents how far our radio signals have travelled out into space since we discovered and started transmitting radio signals about 100 years ago.

This is the furthest away from earth that any man-made signals have reached.

If there are aliens trying to listen for us, they’d have to be within that sphere to have heard us.

In fact the sheer vastness of space is one of the possible solutions to the Fermi Paradox*.

Exploring space…

That’s 976 km every second (482 mi/s).

At that speed, it would cross the length of the US in five minutes, but it still took nine years to get to Pluto.

At that speed, travelling to our closest stellar neighbor, Alpha Centauri (4.3 light-years away) would take 78,000 years.

Could we build faster ships? Certainly…

None of our current space probes were optimized for speed. There is a lot of room for improvement…in fact there are many hypothesized designs and propulsion methods that could easily go faster.

Could we build a ship that goes the speed of light?


Not unless it was massless…

Why can’t anything go faster than light?

There are two ways to think about this…

1. Accelerating objects makes them have more energy.

As by described by relativity, energy and mass are interchangeable.*

When you accelerate an object, you give it more energy, in effect making it more massive, which makes it harder to push.

In the LHC, we accelerate protons, neutrons, and other hadrons to 99.9999991% the speed of light, this gives them a relativistic mass 7,460 times greater than their rest mass*[s].

But to accelerate an object all the way to the speed of light, even if it’s just a proton, requires an infinite amount of energy, and at the end the object would have infinite relativistic mass.

…this is not possible because there is no source of infinite energy that we could put to use towards acceleration.

Nothing with mass can go the speed of light…nothing without mass can go slower than the speed of light.

Mass is an impediment to motion. No mass, no impediment.

2. The speed of light is the speed of causality.

Light (aka electromagnetism) is fundamental. In fact, it’s one of the four fundamental forces (alongside the strong nuclear force, weak nuclear force, and gravity).

At quantum scales, every interaction takes place though force carrying particles called bosons.* All bosons are massless and therefore, move at the speed of light.

That means that the forces governing all interaction at the smallest scales operate at the speed of light.

No faster, no slower.

Another way to think about this is that light speed is the fastest that information can be transmitted from one place to another in the universe*, even within the particles inside the nucleus of an atom.

It’s the fastest way that any two parts of the universe can talk to each other.

Naturally, all action and motion on large scales is limited by the speed at which forces can be transferred at small ones.

If you had a rod of some sort that was a light-year in length, and you moved if forward one meter, it would take a year for the end of the rod to move that same one meter*.

Why is the speed of light as fast as it is?

Some interpretations of quantum mechanics account for the values of the constants, however.

For example, the infinite universes created in the many worlds interpretation have essentially random values for the constants. It could be that the vast majority of universes don’t have the constants arranged in a way that gives rise to complexity. Some universes could flash in and out of existence in an instant. Others could persist for trillions of years, but with no structure ever emerging.

If the gravitational constant were a bit weaker, galaxies may have never formed. If electromagnetism were stronger, it could be that most of the chemistry we know simply couldn’t work…

The odds of a universe coming into existence with values for the constants that gives rise to our kind of life could be exceedingly small…but the chances of finding ourselves in a universe with constants that give rise to our kind of life, are exactly one*.

In any case it could be that if the speed of light were faster or slower, we wouldn’t notice anyway…the speed that our neurons fire, giving rise to our thoughts and perceptions, is governed by the speed of light. So while the ‘absolute speed’ could be slower or faster, the relative speed of our perception to this ‘absolute speed’ would remain the same.

It’s probably a good thing that it’s not infinite, though. Afterall, a finite speed of light is what keeps everything from happening all at once.

This article was originally published at Check it out there to see the astericked* details and other additional tidbits.

Deep Breadth

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Big pictures, thresholds, and cross sections.