There is a strange force in the Universe.  A force so familiar that we forget how strange it is.  That force is called Gravity.

Objects in space seem to be attracted to one another; there seems to be a pulling force that draws them together.  On the face of it, this is self evident; if you drop a glass, it is immediately drawn towards the Earth and shatters when it hits the ground.   Objects also orbit the Earth (and other celestial bodies); the outward force of their momentum is trying to make them fly off into space (Centrifugal force); this is perfectly balanced by the Earth’s gravity trying to pull them back.  The net result is they travel in curved paths around the Earth.  This is exactly how Isaac Newton thought it worked.

Newton worked out (with the aid of earlier work by Galileo and an idea stolen from Robert Hooke) that the ‘pull’ of any object is directly related to its ‘mass’ and ‘the distance we are away from the object’.  This is his Inverse Square Law; part of his Laws of Motion which he set down in a grandly named book called Philosophiae Naturalis Principia Mathematica.  Even today Newtonian Physics is used to calculate orbits and trajectories quite accurately.

Image via Wikipedia

Mass is simply a way of expressing how much matter the object contains; it’s expressed as a weight measurement.  The objects mass is the same as its weight on Earth.  A cannon ball with a mass of 20kg, of course, weighs 20kg on Earth.  It would be lighter on the moon and weigh nothing at all in a free-fall orbit, but, even in those places, it’s mass is still 20kg.

Enter Einstein

There was a problem with Newton’s Laws of Motion; they didn’t always quite work properly.  This is bad – especially for a mathematical theory because, if it isn’t spot on every time, it cannot really be describing ‘reality‘.  From his early teens, Einstein suspected that there was something very wrong with the Universe as described by Newton, and not just with the laws of motion.

A professor by the name of James Clark Maxwell had worked out the mathematical rules for electricity and magnetism.  He’d also proved an idea put forward by Michael Faraday, that light, radio and many other forms of radiation are a ‘vibrating electro-magnetic flux’.  I don’t want to stray too far from the point so I’ll just tell you that the upshot of these ideas are that the speed of light never varies from 299,792,458 metres per second through a vacuum (an airless void like outer space); thats roughly about 300,000 kilometres per second!.

Nobody really believed that the speed of light was constant because it just didn’t seem to make any sense.  After all; if you travel in a starship towards a star at 10,000 kilometres per second, it’s logical to assume that the light coming from the star is streaming through your craft’s front viewport at 310,000 kilometres per second; 300,000 + 10,000 = 310,000.  People knew that this part of Maxwell’s calculations couldn’t be right, but because all the other predictions about electricity and magnetism were spot-on, they just put it down to an anomaly; i.e. a weird thing.

Later, two guys called Albert Michelson and Edward Morley built an apparatus to try to measure the Earth’s movement through the ‘aether’ (space was thought, at the time, to contain a substance called ‘Luminiferous aether’ that light and other radiations travelled through – in the same way that sound waves travel through the air).

They measured the speed of light in several directions expecting to find differences showing the Earth’s movement through space.  They found no difference no matter what direction they took their measurements, in spite of the fact that the earth is moving at a fair speed around the sun and the sun is moving even faster around the centre of the galaxy – not to mention how fast the Galaxy itself is moving.

Newton’s ideas about motion were beginning to look inadequate.  To him, time and space were separate.  Things happened in space, and time ticked along unaffected.  It ticked along at same speed throughout the Universe, and gravity effected everything instantly in a universal ‘now’.

A bit about Special Relativity.

Einstein knew that, if the speed of light is constant wherever you measure it, no matter how fast you’re moving, then something else must be changing.  Maybe space itself was changing.

Perhaps, if you were travelling toward a light source, the light seemed to be travelling at its normal speed back at you because the spacecraft was somehow stretched and a metre is somehow longer onboard than it is outside the craft – or the spacecraft was the same but the outside was somehow shrunk and it was ‘truncated light’ that shone into the front view port.  Once inside the comparatively longer spacecraft (the only place it could be measured by the crew), it seemed to be travelling at it‘s normal speed again.

If the light is truncated, its frequency must be higher than normal.  The light waves would be squashed closer together and it would make the star look bluer than it should.  This truncated light would also show the star ahead aging faster than it should; like a speeded up video.  The star could be hundreds of years older by the time the craft arrives if it’s traveling at a sizable percentage of the speed of light.  Just think of how much the earth will age on the return journey.

Newton’s rules insist that time is separate from space and cannot be altered by any event that happens in space.  Einstein came to the conclusion that this was fundamentally wrong.

He started to think of time as being part of space, much like width height and length; a fourth dimension – and if that was true then space and time could be compressed or stretched in the same way.  Because clocks (and organic body clocks), would speed up and slow down together, and rules, tape measures and other measuring devices (as well as everything else) would shrink or expand together, the observer wouldn’t notice any difference.

As soon as Einstein got around to that way of thinking, things started to add up.  Instead of having two separate things; space and time, it became apparent they were both part of one entity – the four dimensional space-time continuum.

With a great deal of number crunching he formulated his Special Theory of Relativity; the consequences of which are mind boggling, but mostly beyond the scope of this article.

Special Relativity Has Two Postulates (Assumptions)

   1. The laws of physics are the same for all observers in uniform motion relative to one another.

In other words, for a group of people all moving together, for instance, all standing on the surface of the same planet, everything is as it should be.

The point is, though: if one of our group blasts off on a spaceship and ends up travelling at a hugely different speed from the rest, perhaps he rockets away at sizable chunk of the speed of light.  The people on the planet will see the laws of physics behaving very differently for him.  For the guy on the ship everything will be normal including the speed of light, but to him, physics will be behaving differently for the people on the planet.

   2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.

Everyone sees the speed of light as travelling 300,000 kilometres per second, no matter how fast they are travelling themselves, or how fast the light source is travelling.  The only thing that changes is the frequency (red-shifted if the frequency is slower or blue-shifted if faster).

Because the speed of light looks normal to everyone no matter how fast they’re moving, there is no way of knowing if anything is completely at rest (completely still), and no way of knowing anything’s ‘absolute’ speed.  In fact, everything in the universe appears to be moving, so absolute rest is pretty much a meaningless concept.  So is absolute time and absolute position.  This is why it’s called Relativity,  Speed, position, and even time can only be measured ’relative’ to other things.

So what is true depends entirely on where you are or how fast you’re going relative to your surroundings.  There are no absolutes except the speed of light in a vacuum

I could continue about the wonderful weird world of Special Relativity, but that would take up the whole article.  We’re really here to talk about Einstein’s description of gravity – General Relativity.

General Relativity.

I’ve kept equations and algebraic expressions out of this article because they tend to be off-putting to all except those who love maths, but I will make an exception with one of Einsteins equations.  It must be the most famous equation of all time.

E=mc^2

E represents energy, m is mass and c is the speed of light.  The equation reads ‘energy is equal to ‘mass’ multiplied by ‘the speed of light squared’ (squared means multiplied by itself).

The equation is telling us that energy and matter (mass) are equivalent; basically they‘re the same thing.  Matter is sort of ‘condensed’ energy – and when you consider that, in this equation, mass is measured in grams and is multiplied by the speed of light (measured in metres per second) squared, that’s a huge amount of energy (in Joules) for a very small amount of mass.

One of the consequences of this mass/energy equivalence is that adding huge amounts energy to something – like, for instance, accelerating a space craft to a super-huge speed, is going to add mass to it.  As the craft approaches the speed of light it’s going to get super massive (as well as totally mashed by any tiny particles it will strike).  The upshot of this is that it’s never going to reach the speed of light; nothing can except mass-less sub atomic particles like photons (light particles).

Energy distorts space-time.  Mass distorts it more.  A good analogy (although not a perfect one because space is three dimensional) of this distortion is a heavy weight sitting on a trampoline.  If you think of the weight as a planet the dimple it causes represents the distortion on space-time.  If ball bearings are rolled over the trampoline their paths will be deflected toward the weight as if pulled by it.  It’s plain that the ball bearings are not being directly effected by the weight, but are reacting to the slope of the fabric.

By the same token, objects falling towards the Earth are not being ‘pulled’ by the Earth itself; they’re reacting to the distortion in their local space-time.

Because these distortions in space-time cannot travel out from an object faster than the speed of light, we begin to see why Newton’s theory of gravity started to become inaccurate; especially for the outer planetary orbits.  It takes from several minutes to hours for the distortions from the sun to reach the outer bodies of the solar system.

Remember that Newton thought gravity acted instantly in an absolute present.  The fact is, there is no absolute present, and from Earth’s point of view, the outer objects are orbiting a point where the sun ‘was’ hours ago, causing a marked deviation from the course predicted by Newton.  So the problems with Newton’s theory of gravitation were solved by replacing it with a totally different theory.

How do we know that space-time is distorted around mass?

Photons (light particles) are mass-less, they shouldn’t be effected by gravity as described by Newton, because they have no weight.  But if gravity is caused by distorted space-time, light should then be deflected by it.

In 1922 an experiment was carried out to find out for certain if light would bend as it passed massive objects.  If you look at the stars beyond the object, they should appear slightly out of position, because the light from them is being deflected through the distorted space-time.  The moon isn’t massive enough for the effect to be noticeable.  The gas giant planets might be ‘just’ massive enough, but they’re too far away – and the earth’s atmosphere meant that our view of them was pretty poor at that time.  The sun was definitely massive enough, but so bright it obliterated any view of the stars close to it (by line of sight); except during a total eclipse.  There was such an eclipse in Australia at that time.  The experiment was carried out and Gravitational Lensing, as the effect is called, was seen for the first time.  The sun definitely had a field of distorted space-time around it; Einstein was right.

Since that time, Gravitational Lensing has been seen time and time again out in the cosmos around massive galaxies.

You have to feel sorry for Newton.  He devised a mathematical theory that ‘almost’ described gravity, and is accurate enough to be useful tool today – especially since the calculations for General Relativity are so complex.  It just goes to prove that you can be almost right in science and still be completely wrong.

But before we start patting Einstein on the back too much.  We should consider that, even General Relativity, is giving results that don’t tally with observation.  Were having to invent stuff like Dark Energy to account for the fact that the universe is expanding faster than it should be, and Dark Matter to account for the fact that galaxies seem to have too much mass in them.