The Speed of Light is Not Constant

“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it.” —Terry Pratchett

In a Nutshell

Most believe that the speed of light is constant and unchanging, but it simply isn’t the case. The speed depends entirely on what exactly the light is traveling through.

The Whole Bushel

It is often quoted that the speed of light is constant, and the misconception is understandable. The speed of light IS constant, but only in a vacuum. The speed of light in a vacuum is 299,792,458 meters per second and this number remaining constant is important in because it is the speed used to correctly define the length of the meter.

But what happens to the speed of light when not traveling through a vacuum? It changes, a lot. A vacuum contains no matter, but if the area the light is traveling through DOES contain matter such as dust or moisture, the light will bend and decrease in speed. When light travels through a diamond, it slows to less than half it’s vacuum speed.

But it can be slowed even further when passed through a type of matter most haven’t heard of called a Bose-Einstein condensate or ‘bec’. This ‘new’ matter was first proposed in 1924 by Albert Einstein and Satyendra Nath Bose. Light passing through a bec can be slowed to under fifty miles an hour. In 2000, a team at Harvard university managed to bring a beam of light to a complete standstill by passing it through a bec version of Rubidium.

So the speed of light isn’t constant and in the right conditions, you could probably even run faster.

Show Me The Proof

Harvard Gazette: Physicists Slow Speed of Light The Speed of Light
Huffington Post: Speed Of Light May Not Be Constant After All, Physicists Say

  • rhijulbec

    Love it!

  • Phil_42

    Nice Nut!


    The speed of light is constant in vacuum… but technically speaking, the speed of light is still constant when travelling through any medium.

    It appears to slow down because there is more to travel in a medium like water compared to vacuum… since it’s denser and light bumps into more particles. The denser the medium, the more particles light bump into and the longer it takes to pass through. The speed however, between bumps… remains constant.

    It doesn’t mean that the speed of light changes… but there is more distance to travel.

    Go watch minutephysics’ video on this.

    • anonymouse72

      Minute physics!

      Awesome but their videos aren’t always in one minute D:

      • FILOSOFY

        And besides… is this new to anybody…? I mean I learned this basic stuff just in highschool- the fact that light appears to slow down when travelling in a medium that has a higher refractive index (density).

        Shouldn’t this be… I dunno, obvious to most adults?


        Simple analogy for the above: If I place a bunch of metal pillars in the middle of a field and have you run straight across it, it would take longer time than if it just was an empty field. Doesn’t necessarily mean you’re a slower runner just because of pillars… but because you have to walk AROUND it, and it takes more time (unless you can phase through matter).

        • anonymouse72

          Light goes in straight lines, doesn’t it? But then I think I studied in very basic section of waves that waves can turn or bend?

          • FILOSOFY

            Yes light travels in straight lines, and yes, it can turn or bend when approaching an interface (line of division between two mediums of different refractive index, or density).

  • Wonderful Nut!
    My email sig includes “299, 792, 458 mps., it’s not just a good idea, it’s the law”.
    Do I now have to change that? 😉

  • anonymouse72

    This list is nice info. I used to think people never discovered a way to slow light or even make it stop.

    But stop to humans. Maybe. I mean the intensity of light can decrease, can’t it. Dim light wont fill a room but bright ones will. But both lights went the same way. Maybe we can’t see light after low intensity like we cant hear very high/low frequencies. Is this true?

  • deepak

    my guess is in the existing universe we cant say any thing or light is fastest let me give example if boy through a ball in train speed is different for a viwer in train and flat form

  • So the speed of “unfettered” light is 299,792,458 meters per second. Interesting!

  • The Speed of Light is Not Constant – KnowledgeNuts

  • The Speed of Light is Not Constant – KnowledgeNuts

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