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Bottom Left: [Nuclear fission bomb explosion - credit wikipedia]
The Sun does a little better - extracting somewhere around 1% efficiency from the reactions - but then it
does have size on its side. Black holes can do somewhat better - getting up to maybe 40% of the possible
energy from stuff falling into it.
Another aspect of this equation is that energy has mass, and mass causes gravity. So even a photon of
light, which has no real mass in the conventional sense, has an effective mass. This is why light can be
bent by large masses caused by gravity. However this is very simplistic, as light actually bends a little more
than you might expect just treating it as a mass. This is where general relativity comes in, and lets agree
to sweep that under the carpet, as the maths is epically horrendous.
But… why the speed of light, how has that got involved? This looks a little incongruous, why have it in
there? Well its a little complicated - but then it did take Einstein to figure it out.
It comes down to the speed of light being the universal speed limit. Nothing can go faster. Also that
Einstein turned space and time into a single space-time. We travel through the universe in space-time at
the speed of light. If we’re standing still we shoot forward in time only. If we move in our regular 3 dimensions we go through time a little more slowly. For any normal speed we don’t notice the change in time.
So - that’s a not very convincing justification for why we have the c. You need to follow through the maths
to see in more detail.
Words: Julian Onions
ICY SCIENCE | WINTER 2013- 2014