Light Explained: The Speed of Light

In Part 1 of our Light Series, we discuss how light is a form of energy. In Part 2, we will discuss the speed it travels as it radiates through space.

Gemini North Observatory, Maunakea, Hawai‘i : image credit Gemini Observatories Laser Guide Star


We now know that light is energy and energy needs time to travel from point A to point B. The amount of time that any object takes to travel between two points depends on its speed. How fast can energy and light travel?


Prepare for Light Speed
Light always travels at a constant speed in the emptiness of space. In a vacuum (an area of empty space) light travels at the constant and blistering rate of:

  • 299,792,458 meters per second which is
  • 1,079,252,848.8 kilometers per hour
  • for my US visitors that is 670,616,629 miles per hour


WOW – over one BILLION kilometers per hour! It’s really difficult to imagine how ridiculously fast light travels. If you had a plane that was able to travel at the same speed as light, you would be able to fly around the Earth seven and a half times – in one second!

It’s not just visible light that travels that fast. All energy within the electromagnetic radiation spectrum (X-rays, microwaves, etc) all travel at this same speed when in a vacuum.


Massively Mass-less
You may have heard the phrase “Nothing can go faster than the speed of light” – but why is this true? It is simply because visible light and other forms of energy have no mass.

Mass vs Weight
A boulder on Earth might weigh a few tons on Earth. However if you move that boulder into the low-gravity of space, its weight would be reduced to almost zero. But its mass or density would remain the same. Light has neither mass or weight.

Most people think about weight when they think about mass. A boulder on Earth might weigh a few tons on Earth. However if you move that boulder into the low-gravity of space, its weight would be reduced to almost zero.

But its mass or density would remain the same. If that boulder was slowly floating towards you in space, you would best not to get between it and another object like your spaceship. Since the boulder has the same mass in space as it does on Earth, it would crush your body against your ship with the same force as rolling on top of you on Earth.

In physics we define mass as the amount of matter (or atoms) in an object. The more atoms an object has, the more massive it becomes. Light has no atoms and therefore it has no mass.

Without any mass, light is free to travel as fast as possible, at all times possible without any pesky force like gravity slowing it down. It is like driving with no speed limits – you can go as fast as the car’s engine can propel you, and light is driving the fastest car on the road.


Even Space has a Speed Limit
But why not faster – why is there a speed limit that even light can’t go past? If light can travel 299,792,458 meters per second, why can’t it be sped up to 299,792,459 meters per second?

The short answer is – light just can’t go faster in our Universe. You see, our Universe has a set of laws when it comes to energy and forces. We are still trying to figure out why or where these laws originate form. Not only are we trying to find a reason for laws like the speed of light, we are trying our best to break them!

Unfortunately mankind doesn’t have the knowledge or technology yet; likewise we have not been able to observe anything going faster than the speed of light to prove that it is possible even if we don’t know how to do it.

Now you know that light has a constant speed. When you look up to the dark skies above you and see the light of hundreds or even thousands of stars shining down, you should remember that:

  • this visible light is massless energy emitted by the stars
  • the light you are seeing took time to travel from the stars to your eyes
  • this visible light moved at a constant speed as it traveled through the cosmos


In our final article about Light, we will bring all that we have learned about light together to discuss time and distances in space.

Read Light Explained – Part 3: Seeing Back into the Past

Share this:

Leave a Reply

Your email address will not be published.