Gravity explained in 761 words

People seem to be harbouring the impression that there is no good theory of Gravity yet. I asked a few friends – most thought Newton had explained it, but couldn’t explain it themselves. This is rather sad, 80-odd years after a darn good theory was proposed.

Of course there is still some controvery and the odd contradiction with other beloved theories, but the heart of the General Theory of Relativity really does a great job of explaining gravity and it is really wonderfully beautiful, and can be roughly explained without recourse to jargon and equations.

This is a theory that’s just so darn elegant, it looks, smells and tastes right – once you get it. Of course, the ‘taste’ of a theory doesn’t hold much water; for a theory to survive it needs to make testable predictions (this one does) and needs to survive all manner of logical challenges (so-far-so-good for this one too).

This is not a theory that needs to remain the exclusive domain of physicists, so for my own personal development as a scientist and writer, I thought I might try an exercise in explaining what gravity is – according to the general theory of relativity.

For some reason, my wife thinks this is strange behaviour!


The story really got started when Einstien realised that someone in an accelerating  spaceship would experience forces indistinguishable from the gravity felt back on Earth. 

He or she could drop things and they would fall to the floor (assuming the spaceship is accellerating upwards)  just as they would fall on earth.

So perhaps that’s all gravity is… some sort of accelleration? Let’s see.

In the spaceship, it’s clear to us that the objects would appear to fall to the floor, but in reality, it is the floor of the spaceship that is rushing up towards the objects – this explains why things fall at the same speed whether heavy or light, matching Galileo’s own test results when he dropped various things, supposedly from the leaning tower of Pisa. It further implies that things will ‘fall’ even if they have no mass at all… such as light beams.

The thought experiment goes thus: Consider if you had a laser-beam shining across the spaceship control room; it would curve slightly downwards, because the light hitting the opposite wall would have been emitted a little time ago, when the spaceship was a little way back, and going a bit slower (remember, its accellerating).

We know the light is not bending, it is just that the source is accellerating, resulting in a curved beam. Imagine a machine-gun spraying bullets across a field – as you swing the gun back and forth the bullets may form curved streams of bullets, but each individual bullet still goes straight.

So Einstein suggested that perhaps light beams will bend in this same way here on earth under a gravitational field. Now Newton’s theory of gravity says light beams may also bend if they have ‘mass’, but the mass of light is a dodgy concept at best (it has inertia but no rest mass, but that’s a whole different blog posting). Anyway, even it it does have mass, it would bend differently from what Einstien predicted. So the race was on to see how much gravity could bend light…

This bending of light prediction was proven by a fellow called Eddington who showed that during a solar eclipse, light from distant stars was indeed bent as it passed near the sun, and by exactly the predicted angle.

Einstein went further though, suggested that light beams on Earth are, just like on the spaceship, really travelling straight, and only appear to bend, and that this can be so if space-time itself is curved. They are going straight, but in curved space.

We know that the shortest distance between two points is a straight line, but if that line is on a curved surface, supposedly straight lines can do strange things – like looping back on themselves. Think of the equator. This model therefore allows things like planets to travel in straight lines around the sun (yes, you read right).

The model has been tested and shown to work, and gives good predictions for planetary motion.

So what can we take home from all this?

Well mainly, if this model is right, we need to let it sink in that gravity may not be a force at all, but an illusion, like the centrifugal ‘force’ you experience when you drive around a corner.

Secondly, it is an open invitation to think about curved space and its marvellous implications!

7 thoughts on “Gravity explained in 761 words

  1. If I understand you correctly, you’re saying that what keeps us to the ground is that the earth is moving upward en pushing our feet?
    if that’s so, than this works for only halve (living on the top) the people on the planet. But I guess you’re more clever than me so I don’t really understand your theory.

    Best Regards,
    Ron Tesselaar

  2. Hi Ron,
    Understanding that gravity is essentially the same as acceleration is the first step to understand why we are sucked down – the second step is to think of space and time as a warped 4-d object.
    Let’s first try to simplify. If we discard one of the space dimensions for a minute and think of a 3-d ‘box’ with space laid out like a map on the bottom and time as the vertical axis. You can see that points on the map (you and I for example) become threads going upward through the box.
    If we walk across the 2-d map, time passes and we track a diagonal line up through the box. It turns out the only way to get ‘curves’ in one’s path is with acceleration. If we hop in a car and accellerate up to 100mph, we will have a curved path for a while then a straight (though slanted) line when we cruise at top speed.
    Now it turns out (thanks again to Einstein for guessing it) that space time is actually warped (by mass), there can be more ‘space time’ in one part of the box than in another for example – just like there can be more smoke in one part of a burning room than another.
    So even as we try to travel in a vertical line in the box by sitting still in our La-Z-Boy, we land up taking a curved path through the warpage in spacetime caused by the mass of the earth. We feel this curvature as acceleration (upwards from our recliner) – aka gravity!
    Hope that helps! Dang, now technically I’ve used over a 1000 words 🙂

  3. A missing step explains everyone
    1). Imagine a swimming pool with a specific water level, never changing. The level of the water is constant. The evaporation rate is matched by the automatic filler.
    Mass and energy are the pool water, space is the evaporating water and the sun is the automatic filler.
    Time and space are measurable actions of this process of MASS and ENERGY decay into space itself, via the gravitational wave. Gravity is the result of the back reaction to wavefront formation.
    C. Michael Turner copyright2003,@ALLrights reserved

  4. Hi C. Micheal Turner, I’m interested in why you put ‘copyright’ in your signature, what does it protect?
    As for the relationship between energy and space, it’s been shown that the flux of the gravity field through any closed membrane is proportional the mass enclosed by that membrane, so sure, space and time are clearly closely entwined with mass and energy; however the key nexy step is not to explain ‘that’ they are, but ‘how’. If you’re saying gravity is the symptom of a back-reaction to a wavefront as a form of explaining ‘how’, I’m afraid I don’t understand what you mean… sorry!

  5. there in no theory of gravity – well .. we have a mathematical models to predict its activity – but what is the mechanism?? we are no where close to identify this mechanism except for a book called Gravity Explained via amazon books. If you want to remain stuck with the maths model that is fine – if however you would like to learn of a new theory on the block then you should check out this book. Its all to do with information…….

  6. Dear Edward (ewj9), I took a look at a few of your discussion threads (toequest) and your TONS paper, plus a few YouTube videos. First let me say that this is a subject close to my heart and I love to discuss with others who have a similar passion. However, I have to confess I still don’t quite get your line of reason. I do agree that ‘flow’ or ‘flux’ models for space have potential, and also think Sorli and Fiscaletti are onto something in challenging the idea of time as a continuous dimension. I know that, for example we can think of standing on the earth as being similar to accelerating upwards by the force on the soles of our feet, and one way that works is if space is constantly flowing down through us. However, in order to avoid being labelled a crackpot it is essential that any formal foray into the physics community takes the form of a purely logical assertion, i.e. rigorous maths, not hand drawings and descriptive language. I will be very glad to join discussions, but I am a bit of a stickler for good maths! Does your book present any maths showing the conservation laws are upheld? Your discussion of information flow is pretty bold too…

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