• Question: How can light travel so fast?

    Asked by lumiereclair to Mark, Matthew, Mike, Paul, Sabina on 10 Mar 2014.
    • Photo: Sabina Hatch

      Sabina Hatch answered on 10 Mar 2014:

      Mainly because it does not have any mass, it is pure energy. If you have no mass then no energy is used up to move it. This is not my field of expertise (Matthew or Mark know a lot more so my answer will be quite brief) but most particles that have virtually no mass can travel near to the speed of light.

    • Photo: Mike Lee

      Mike Lee answered on 10 Mar 2014:

      If you drop a stone in a pond, a ripple will travel outwards at a certain speed, which depends on various things (depth of the pond for instance). A light bulb is a bit like that, sending ripples of light outwards at a certain speed which depends on some things, such as the “refractive index”. Most refractive indexes are small, meaning light travels really fast. But there are some materials like ruby which can have a refractive index of a million so light goes slower than Usain Bolt!

    • Photo: Matthew Malek

      Matthew Malek answered on 10 Mar 2014:

      Great question! Light isn’t the only thing that moves so quickly — any particle without mass moves through the vacuum at the “speed of light”. This includes gluons, which carry the strong force (the same way that particles of light, called “photons” carry the electromagnetic force) and hold together the nuclei of atoms.

      For a long long time, it was thought that neutrinos had no mass, in which case they would also move at “lightspeed”. In 1998, it was shown that neutrinos do has mass, it is just very very tiny (the smallest we know that isn’t actually zero!). So although the neutrinos move at close to the speed of light, they are ever so slightly slower.

      The speed of light (or gluons) is a built-in constant of the universe. It walls naturally out of the electromagnetic wave equations, and was known before Einstein came along and invented special relativity (in which the finite speed of light plays a crucial role in causing time dilation and messing up our concepts of simultaneity). Any massless particle MUST move through the vacuum at this speed; for them, it isn’t just a speed limit, it’s also a minimum speed. For any massive particle, the speed of light is forever out of reach. Even for those with tiny masses, like neutrinos. They can come very very close… but it would require infinite energy to bring them up to lightspeed.

      Why is this particular speed (exactly 299,792,458 meters per second, by definition) so special? No one knows. There is no a priori reason that this shouls have been the universal speed limit, and we can easily imagine a universe forming where the top speed is twice this value. That said, in our universe, it is what it is and makes up one of the fundamental constants of the universe.

      Another great question! Thanks for asking — pardon the long answer, but a fundamental query like this deserves a detailed explanation.

    • Photo: Mark Jackson

      Mark Jackson answered on 11 Mar 2014:

      It’s not that light travels so fast, it’s that you travel so slow.

      Einstein was the first to understand that time is actually another dimension like the three spatial ones we are familiar with, but gets combined with them in a funny way (I explain this more at http://goo.gl/oUbSKc). This might seem strange – how can we combine time, which is measured in seconds, with distance, which is measured in meters? (or something else in antiquated countries like Burma, Liberia and the USA). To convert one to the other we need a speed, and it turns out that Nature demands we use the speed of light. It’s neither fast nor slow, since these require a comparison against something else. The real question is then, why are we usually so much slower than the speed of light? This is related to Einstein’s famous equation E=mc^2.

      An object’s energy is comprised of both its kinetic energy (how fast it is moving) and its rest mass energy (just from existing). If the kinetic energy is much less than the rest mass energy, the particle will be traveling slowly compared to the speed of light. If the kinetic energy is much more than the rest mass energy, the particle will be traveling at almost the speed of light. So the fact that we travel so much slower than light is just because we usually have a kinetic energy far less than our rest mass. If you were a proton in the CERN collider, you would be traveling at 99.999999% the speed of light. This makes it the awesome roller coaster ever.

    • Photo: Paul Coxon

      Paul Coxon answered on 11 Mar 2014:

      The speed of light in a vacuum is 186,282 miles per second, and in theory nothing can travel faster than it.

      Early scientists were unable to think of light as ‘moving’ – they originally thought it shot out instantaneously from our eyes – a bit like laser beams. But, over time, and with more understanding the measurements of the motion of these wave-like particles of light became more and more precise. Thanks to the work of Einstein on relativity, we now understand light speed to be a theoretical limit which can’t be beaten by anything with mass.

      According to relativity, as an object moves faster, its mass increases, while its length contracts. At the speed of light, such an object has an infinite mass, while its length is 0 — which is theoretically impossible. Light can travel at light speed because it has no mass and no definable size.

      Hope this answers your question, lumiereclair 🙂

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