• Question: Why can't anything travel faster than the speed of light?

    Asked by to Mark, Matthew, Mike, Paul, Sabina on 12 Mar 2014. This question was also asked by .
    • Photo: Matthew Malek

      Matthew Malek answered on 12 Mar 2014:

      This is a great question and there is no obvious answer! Basically, the speed of light seems to be “hard coded” into the fabric of spacetime as a speed limit for the universe.

      When you want to accelerate something, you have to put energy into it. If you want to make it go even faster, you have to put in even more energy. Bringing a car from rest to 60 miles per hour takes roughly the same amount of energy as going from 60 to 85 miles per hour, even though you are only adding 25 miles per hour to your speed.

      As you go faster and faster, it takes more and more energy to get each new bit of acceleration. And, as it turns out, it would take an infinite energy to make any particle with mass reach the speed of light. They just can’t do it! You can put it loads of energy and get it real close, like they do in particle accelerators like the Large Hadron Collider (LHC) at CERN. But you can’t make any massive particle move at the speed of light. (Not unless you’re hiding an infinite energy source somewhere!)

      For a massless particle, like light (photons), the speed of light is possible to reach. In fact, it is the only speed that a massless particle can travel at — no more, no less!

      As far as we can tell, there is no deep fundamental reason why the speed of light is what it is. I can easily imagine a universe where the speed of light is double, or triple, the value it is in ours. Physics would work a little differently, but it would still work.

      Again, good question! Hope this helps explain, and thanks for asking!

    • Photo: Paul Coxon

      Paul Coxon answered on 12 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 speed limit which can’t be beaten by anything which has mass.

      According to relativity, as an object moves faster, its mass increases, while its length contracts. As we get nearer to the speed of light, it takes more and more energy to make the object move faster, but the object gets heavier and heavier meaning it takes more and more energy increase the speed. At the speed of light, such an object would have an infinite mass, while its length is 0 — which is theoretically impossible.

      Now light can travel at light speed because the particles which make up light (called photons) have no mass, they are essentially pure energy. And because they have no mass they can *only* travel at this speed.

    • Photo: Mike Lee

      Mike Lee answered on 12 Mar 2014:

      Imagine light is a ball of light which is bouncing up and down on a moving train. If there is a person on the train looking at the ball, all they see is it moving up and down. If another person is on the platform, they can see the ball through the window. The ball is moving up and down but also along with the train. So, from the platform point of view, for each bounce, the ball has moved further compared to the point of view of person on the train. It’s up+down+along versus up+down.

      The next question is how fast is the light going, for each of the two view points. There are two options:

      1) Different
      2) Same

      If the light was going at different speeds, this would let the bounces to happen at the same time – As the light from the point of view of the platform has further to go, it must travel faster for it to do one bounce in the same time as the point of view as the person on the train.

      2) If the light was going at the same speed, this would mean that the time on the train would need to be going more slowly in order for the bounces to match up.

      According to Einstein, the second option is correct. And time really does slow down if you move very fast. This needs to happen to allow light to travel at the same speed whether you are moving or not. I’ve never understood why the second option is right and the first is wrong, maybe one day I shall learn.

      So to answer your question, if you tried to move at the speed of light, time would stop.

    • Photo: Mark Jackson

      Mark Jackson answered on 14 Mar 2014:

      The answer to this is sometimes given that “it would take an infinite amount of energy.” This is not quite true: while it would take an infinite amount of energy for a particle capable of being at rest to travel faster than the speed of light, it would be perfectly fine for a particle which is *always* traveling faster than light to do so. So the real question is, why can’t anything cross the ‘barrier’ separating less than the speed of light and faster than the speed of light?

      The answer is found in Einstein’s Special Theory of Relativity in which space and time are combined in a strange way. In particular, velocity addition is very non-intuitive: you probably take for granted that if you are at rest and see a train moving at 50 m/s, and a rabbit moving at 10 m/s, the rabbit would observe the train moving at 40 m/s. But it doesn’t: it is very close to 40 m/s but not quite this. The difference becomes very pronounced as one of them would approach the speed of light. And if you observed the train to *exceed* the speed of light, the rabbit could actually see it as moving back in time. And since we think there are many problems with backwards time-travel, we don’t think this is possible (and have never observed it actually happening).

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