Weekend Post - Theories

Does anyone deny gravity? Is there anyone who believes that gravity doesn’t exist, that things aren’t somehow attracted to massive objects and, if possible, move towards them? Are there gravity-deniers out there prepared to jump off a high building to prove their point?

Of course not. No sane person denies the facts of gravity. But that doesn’t mean that there hasn’t been debate about HOW gravity works. The first great description of gravity came from Isaac Newton who described how objects attract each other and was the first to describe the mathematics of it. His “Inverse Square Law” described how gravity’s strength diminishes in proportion to the square of the distance between two objects. Double your distance from a large object and the gravitation attraction will only be one quarter of what it was. At ten times the distance the gravity will be a mere hundredth of what it was. All of this is true, certainly true enough for everyday purposes. True enough for getting spacecraft to the moon and back. True enough for almost all circumstances.

It explains things like the tides. Few people who’ve travelled to the coast have thought much about tides, why the sea level rises and falls twice a day and I suspect most people find it surprising that the water is being pulled away from the center of the Earth by the gravitational attraction of the moon. They would find it even more perplexing that the tide rises on both the side closest to the Moon AND the side furthest away. How can that be? (The first person to email me the correct explanation will get a prize.)

The problem is that although Newton described the mathematics of gravity he didn’t explain how it actually happened. He referred to objects attracting each other but didn’t say how they do this. How can a star like our Sun exert an instantaneous force on a planet like Earth from such an enormous distance? Newton didn’t know.

C/o Wikipedia http://en.wikipedia.org/wiki/General_theory_of_relativity

It took a couple of centuries for a convincing explanation to come along. Einstein’s General Theory of Relativity contained a lengthy list of new ideas but perhaps the most revolutionary was that space, the three-dimensional framework within which we operate, is indistinguishable from a fourth dimension, time. Einstein and his followers talk a lot about “space-time”, a combination of the three dimensions of space that we know and another dimension that reflects time. The less well-known thing is this was how Einstein was able to explain what gravity actually was. He suggested that space-time is curved. The reason that satellites move in a curve around the earth is because the mass of the Earth has warped the space-time through which the satellite moves. Imagine water circulating around a bath plughole and you get an idea of what it would look like if we could see in 4 dimensions. The satellite is actually taking the easiest route. This also explains how gravitational attraction appears to happen faster than the speed of light. Gravity is no longer an action that happens over a distance, it’s an object just following the simplest path.

Here’s the key point. Einstein’s “Theory” of General Relativity is a theory. It’s a way of explaining thing, including gravity. There is no “theory of gravity” because there’s no need. Gravity is like radiation, reproduction and rain, we don’t need proof that these things exist, the evidence is overwhelming. Theories are ways of explaining WHY and HOW known things happen, not that they DO happen.

The controversy isn’t with gravity, it’s with the other great known fact. Evolution.

Evolution happens, it’s as simple as that. It’s been observed in a wide variety of creatures, their characteristics adapting gradually as a result of changes in their environment. That isn’t denied by anyone who’s seen the evidence. Fossils show that creatures in the past were different to similar creatures today and the further back you go in the fossil record, the bigger the differences are.

There is no “theory of evolution” just like there’s no “theory of gravity”. There ARE however theories of HOW and WHY evolution happens and what makes species gradually change. So far, just as Einstein’s Theory of General Relativity explains gravity well, the best explanation we have for evolution is Darwin’s Theory of Natural Selection. Despite what you might think, and unlike Einstein’s theory, Darwin’s suggestion is remarkably simple to understand. It has only two basic ideas. When species reproduce they combine their genes randomly and this occasionally leads to offspring with particular strengths or weaknesses. The second element is just as simple to understand. Those random variations that give the offspring a better chance of reproducing and passing on his or her genes to the next generation are most likely to stick around. That’s all there is to it. Over time and thousands of generations these slight changes bring about a much bigger overall change to the species as a whole.

So far, just like Einstein’s theory, Darwin’s has shown considerable strength. Both make predictions that can be tested and so far no test of either theory has failed. Of course it might fail tomorrow and then we’ll need to come up with a new, better theory but so far there’s no need.

For now, despite what certain belief-based groups will tell you, we can stick with both General Relativity and Natural Selection as the best theories we have to explain the FACTS of gravity and evolution.

Weekend post - The physics of road safety

I’m a little obsessed by road safety at the moment. This is partially because a very good friend of mine, a loving and vibrant young wife and mother, was killed a few weeks ago in a car accident. It’s also because I’m a driver. I spend up to 2 hours each weekday behind the wheel of a car, driving mainly around Gaborone. You can’t do that without spending much of the day considering matters of mental health. How did that combi driver not see me in my great big car or perhaps he just didn’t care? Why did that BX driver think it was acceptable to overtake in the face of oncoming traffic, crossing a solid white line and exceeding the speed limit? How did that person ever get a driving licence?

As well as questioning the mental health of drivers I often find myself wondering whether drivers studied any science at school. Don’t they know anything at all about physics?

Newton’s First Law of Motion says that the velocity of an object will remain constant unless a force acts upon it. The cup of tea on your desk isn’t going to move unless you pick it up. The ball you throw in the air will only change direction and speed because of air resistance and gravity. A passenger in a car will continue moving in a straight line within the car when the car hits a brick wall. If a car travelling at 60 km/h hits a solid object and suddenly stops the occupants will continue moving at 60km/h until something stops them as well. If they’re lucky it’ll be their seatbelt or an airbag that will slow them down as gradually as possible. However if they’re not stopped by something soft it’s likely to be something very hard indeed that does it. Something like the steering wheel, the windscreen or the wall the car hit after they’ve burst through the windscreen in a spray of blood and bone.

Let’s take a simple, not too scary example. Imagine a child is in the front seat of a car that’s just left your house. Let’s say it’s a perfectly average 9-year old girl who weighs 30kg. She’s not strapped in. The car is travelling slowly at only 30 km/h but collides with the back of a massive truck and stops almost instantly. Let’s say it takes a meter to come to a halt. In the 8th of a second the car takes to stop the girl becomes a flying object within the car. She’ll hit the windscreen at something close to 30 km/h with a force equivalent to a man weighing 117kg jumping on her. She’ll probably live.

The problem is that this isn’t typical. Who drives at 30 km/h? The other problem is that the amount of energy a moving object possesses is not directly related to it’s speed. The kinetic energy of an object is proportional to the square of its speed. Double the speed and you quadruple the energy. To make matters worse if you double your speed you don’t necessarily double your stopping distance. If the car driving that unrestrained little 9-year old girl hits the same truck while travelling twice as fast, at 60 km/h, it’ll probably still stop in a metre. This time she’ll hit the windscreen with a force of almost half a ton. She’ll probably die.

Let’s be completely depressing and double the speed again to 120 km/h. The same collision will hit her with a force of almost 2 tons. She won’t just die. Someone will be forced to reassemble her body so a relative can formally identify her. I know a very brave uncle who volunteered to identify the pieces of his niece’s body so his brother, the girl’s father, didn’t have to. He couldn’t bring himself to drive his car for weeks afterwards. Every time he got behind the wheel of his car he was plagued with the images from the morgue.

That’s why I lose my temper when I see loving parents driving their beautiful children to and from school each day and they allow their kids to stand up on the front passenger’s seat. I get even madder when I see an unbelted adult in that seat with a kid on their lap. You know what? It might not just be the windscreen that kills her, it’ll be your body hitting her from behind, crushing her from front and back. Could you live with the knowledge that your body, perhaps the body that brought that child into the world, is the thing that crushed her to death?

Maybe that’s what we need to teach in physics classes in schools. Instead of balls hitting each other or weights going up and down inclines we should use the idea of a little girl inside a metal box being driven by a psychopath. Maybe that’s the sort of science we need to cut down the slaughter on our roads.

Sources

For a quick summary of Newton's Laws of Motion see the Wikipedia page here. There's also a good explanation of the physics of car crashes here.

I'd like to say that I did all the maths in this piece myself but that would be lying. I did SOME of it myself but then was lazy and used the online car crash impact calculator here.