The Speed Limit of the Universe.

671,080,887.616 mph. That is nearly 700 million miles per hour for those of you who are horrified by the number of digits. Nothing in the universe can travel faster than this speed. Don’t misunderstand me, this isn’t that nothing has travelled faster than this but with improved technology we could go faster, it is impossible to travel faster than this. Full stop. Ever. (Well probably science has been wrong before). In fact the only way you can even travel this fast is if you have no mass. This is of course the speed of light.

Light travels at this speed. Because light is so fast it would be reasonable to assume that light is instantaneous but this is in fact not the case. It takes light 8 minutes to reach us from the Sun for example. Light can only travel at this speed because it has no mass, we can never reach this speed but theoretically at least we could get to 99.99999999999999999999999999999% of this speed. However, before we get bogged down in why the Universe has a speed limit lets put this speed into context.

A car travels at about 70 mph on a motor way, a plane travels at about 500 mph and the fastest man made objects ever (the voyager space crafts) travel at 35,000 mph. Light travels 10 million times faster than your car and 20 thousand times faster than the fastest thing we (as a species) have ever made. Light is pretty quick then, it would have to be else we would notice. Turning the light on and seeing the light spread out from the bulb would be pretty strange to say the least.

So why can’t we travel at the speed of light? As we approach the speed of light two things happen:

  1. Time slows down
  2. Mass increases

Now I know what you’re thinking, hang on a minute isn’t that the opposite of what is meant to happen. Time goes more slowly when I’m stuck in traffic not when I’m cruising the open road and quite frankly I started jogging to lose weight so I’m not happy about you telling me I’ve gained mass.

The problem is that we aren’t moving fast enough for these effects to be noticeable. To gain a gram more you’d need to travelling at just over 3 million mph. However as we get closer to the speed of light this effect increases exponentially. So as we get faster, our mass increases, which means it takes more energy to accelerate any faster and so it would take infinite energy to reach the speed of light. When you reached, 671,013,778 mph, your mass would be that of an elephant, at 671,080,081 mph a small family dwelling. So that is one reason we can’t reach the speed of light.

The other reason is time slowing down. As with above, time slows down when you move fast but you have to be travelling incredibly fast to notice it. Much like the energy we talked about before, at the point you were about to reach the speed of light (supposing you some how found infinite energy) time would slow to a stop and so you wouldn’t reach it. These things happen because we have mass, light doesn’t so it’s immune. Light is also always travelling at the speed of light – I mean it’s in the name – so it doesn’t have to accelerate up to it and so avoids the problems above.

Let’s say you managed to start at a speed faster than the speed of light, therefore by passing all the problems of accelerating to the speed of light we had above. What would happen then? Well this is difficult to say because it’s impossible so maybe the Universe, much like a toddler throwing a temper tantrum when they doesn’t get their way, would implode. What the equations in Relativity suggest however is that time would run backwards. So wouldn’t you trust travel back in time to a point where you weren’t travelling at the speed light and so you’d no longer be travelling that fast and so you’d move forwards in time again to the point when you decided to travel faster than the speed of light and start going backwards in time… oh dear we appear to be stuck in a time loop and Bill Murray isn’t here to save us. Confused? Yeah, that’s because it doesn’t make any sense. Why doesn’t it make sense? BECAUSE IT’S IMPOSSIBLE!

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