Inspired by a question from CuttlefishPi on our Head Squeezers Google+ community, I thought
I'd take you through one of my favourite bits of Physics where energy comes from.
Now in my mind I think there are three different interpretations to this question and therefore
three different answers:
1.
Where did the idea of energy come from historically?
2.
What is the energy in the Universe and where did it physically come from?
3.
Why does the quite frankly abstract concept of energy exist at all?
And trust me, the answer to that last one I think is amazing!
First up... history.
The word energy derives from the Ancient Greek word energeia which means activity or operation
and was probably first used by Aristotle in the 4th Century, but the concept of energy
didn't really emerged until vis viva or living force of Gottfried Leibniz in the 1670s & 80s,
this is one of the first known applications of what's now call kinetic energy -- the energy
associated with motion.
But the idea of energy and all it's forms didn't really catch on until the 19th century
with the advent of thermodynamics and reformulating of Newton's laws of mechanics.
This leads us onto the different forms of energy... which there are loads but they can
generally be lumped into 2 categories.
As I said before, kinetic energy is all about motion be it Sonic the Hedgehog wizzing past
Green Hill or a spinning top that tells you whether you're in a dream or not... apparently
I'm not.
In fact heat is just kinetic energy, it's the average random motion of a load of particles
whizzing about and bumping into one another.
The other major category is potential energy, which you can think of as stored energy - be
that gravitational potential energy, the energy stored in electric and magnetic fields or
even just in elastic bands
Categorising all forms of energy into either kinetic or potential isn't always easy though,
as Nobel prize winning and bongo playing Physicist Richard Feynman pointed out it depends on
the level of zoom you're looking at -- something that might be considered potential energy
at the everyday macroscopic scale can be a mixture of both potential and kinetic at the
microscopic scale.
One of the amazing realisations of the 20th Century was that matter itself, what everything's
made of, is simply highly condensed energy.
That's what Einstein's famous E=mc2 formula from his Special Theory of Relativity says.
Mass and energy are equivalent.
You can release the energy stored in mass, that's exactly what happens in a nuclear bomb
by very minuet changes in mass.
Conversely by giving something energy you actually increase its mass... usually by an
immeasurably small amount.
So energy comes in lots of different forms, and it can change between them.
Fundamental to this though is energy can't be created or destroyed, it can only change
form.
Or as we say in Physics say, energy is conserved.
That's got nothing to do with making sure you have to leave your lights off when you
leave a room, it more to do with that when you add up all the different energy associated
within an object that always has to come to the same number.
Of course, some forms of energy are far more useful than others, so you shouldn't waste
it... but that's got more to do with entropy which is a whole other topic.
So if energy can't be created or destroyed, then where did it come from?
It must have originated at beginning of time itself - the Big Bang.
Now all the things I have told you are correct but they doesn't really answer the question
of what energy actually is and that's because at its heart energy is an abstract concept.
But the reason why the concept exists in my mind is quite frankly amazing.
It boils down to such a simple idea -- symmetry!
Now you're most used to mirror symmetry, because I'm guessing you check our hair every morning.
The human body roughly has mirror symmetric, it looks the same when you flip left and right.
Another symmetry is Rotational symmetry you can rotate a circle by any number of angles
and it will look exactly the same, but if you take a square it will only be multiple
of 90 degrees and it will look the same, any other angle and it look different.
But what we care about here isn't the symmetries of shapes of people it's the symmetries of
the laws of Physics.
In particular we care about time symmetry, the laws of physics don't change with time,
they're the same now as they were when you were born, or when the dinosaurs roamed the
Earth, and even at the Big Bang.
Even though all the stuff does change in time, the Universe is very different now from when
it was a single point at the Big Bang, got a bit more breathing room.
But time is a symmetry of nature.
Now in 1915 German Mathematician and Theoretical Physicist Emmy Noether proved that if the
laws of Nature have a symmetry then there must be a physical quantity which is conserved.
She showed that energy is a consequence of the fundamental time symmetry of Nature, without
this symmetry energy would not exist and nothing would exist.
Noether's Theorem in my opinion and in the mind's of many others is one of the most elegant
and beautiful bits of science to date.
It tells us so much about why the Universe is the way it is and it's all down to symmetry.
Because the laws of physics don't change from place to place, we get momentum.
Because we have rotational symmetry, every direction I look the laws of Physics are the
same, we get angular momentum.
Another sort of rotational physics gives us the electric charge and so on and so on.
In fact almost but not quite perfect symmetries is where a lot of physics is going right now,
it's already given us the Higgs boson and therefore the origin of mass and it might
tell us why there is so much matter in the Universe, and very little anti-matter when
we think there should have been equal amounts of the stuff at the Big Bang.
Essentially the way the Universe is is intimately tied to the symmetries of Nature and to me
that's just amazing.