Never Mind the Bullocks, Here's the Science (16 page)

However, there is continuing neurogenesis (growth of new nerve cells) in large areas of the brains of many non-humans, such as fish, zebra finches, canaries, frogs and reptiles.

Why has this ability been lost in human beings? We don’t know, but it might be to ensure the preservation of memories.

So perhaps the permanent loss of neurons due to stroke, infection, trauma, ageing and degeneration is part of the price that we have to pay for being human.

The Many Ages of Man

The research of other scientists tells us that cells in different parts of the body become renewed at different rates.

Your gut runs approximately 10 m from your mouth to your anus. The cells that line the gut are replaced every five days. On the other hand, the cells in the gut that are not part of the lining get replaced every 15.9 years.

The turnover time for skin cells is about two weeks. Red blood cells are recycled every 120 days, while muscle cells hang around for 15.1 years. Bone is rebuilt every ten years. The liver is about two years old.

The enamel in your teeth is laid down at certain and well-defined times in your childhood. The enamel consists of 0.4% carbon. This carbon is never replaced, making teeth a ‘clock’ which is accurate to 1.6 years.

On average, most of your body is less than ten years old—probably around 5.6 years. But your rate of growing new neurons drops if you suffer from depression or dementia—these are other factors that change your ‘body age’.

Your brain is older than the rest of your body. So you can definitely say that ‘you’ (i.e. the body you are wearing today) have not been around since ‘you’ (the consciousness) were born—it all depends on your definition of ‘you’.

So back to the question of how old you are. First, let’s say that the person that is ‘you’ is the consciousness that makes you different from every other person on the planet. Now ‘you’ have a body. That body is made up of several hundred different types of cells (e.g. in your liver, lung and heart). The vast majority of those cells have a life span considerably shorter than the life span of your consciousness.

We still don’t know how ‘old’ you are, and how many ‘bodies’ you use up in your lifetime—but we do know that the bland claim that your cells are replaced every seven years is as wrong as the claim that a cat has nine lives.

C-14 Not Constant? Inaccurate?

Unfortunately, the C-14 levels are not the same everywhere in the atmosphere and, even trickier, not the same at all times.

C-14 is not the same everywhere in the atmosphere? Nope. First, the Earth’s magnetic field is most effective at blocking incoming radiation at the Equator, and weakest at the North and South Poles. So cosmic rays are weakest at the Equator, and strongest at the poles. As a result, most of the C-14 is made in the atmosphere near the poles, at altitudes of 9-15 km (30,000-50,000 ft). However, given enough time, the heat of the Sun stirs the atmosphere quite well, and spreads the C-14 quite evenly across the planet.

C-14 is not the same at all times? The cosmic ray activity varies with time due to, for example, changes in the Earth’s magnetic fields.

In addition, old carbon (such as coal, natural gas and oil, resources that have been underground for millions of years) has lost practically all of its C-14, due to radioactive decay. When we take this old carbon (which is deficient in C-14) out of the ground and burn it, this carbon (now in carbon dioxide) enters the atmosphere and lowers the levels of C-14. Volcanoes can erupt lots of carbon low in C-14 into the environment, thus lowering the local concentration of C-14. If it’s a small eruption it’s just a local effect, but if it’s an enormous eruption it can affect the atmosphere of the entire Earth.

But working in the other direction, the atmospheric testing of nuclear weapons raised the C-14 levels in the atmosphere.

Marine uptake of carbon can change with time, either up or down.

However, by making C-14 measurements in many different situations, and comparing the C-14 dates to the dates taken by (say) measuring tree rings, we can apply various corrections. (This field of study is called ‘dendrochronology’, and we have continuous records to about 11,000 years ago that can be linked directly with radiocarbon dating.) We can also cross-link radiocarbon dating with deposits from caves, going back 45,000 years.

Radiocarbon dating requires skill and knowledge. When these are applied, it can give us dates back to 60,000 years or so, with an accuracy of a few per cent. After about 60,000 years, there are not enough C-14 atoms left to be useful for radiocarbon dating.

References

Nowakowski, Richard S., ‘Stable neuron numbers from cradle to grave’,
Proceedings of the National Academy of Sciences
(PNAS), 15 August 2006, Vol 103, No 33, pp 12219-12220.

Rakic, Pasko, ‘No more cortical neurons for you’,
Science
, 18 August 2006, Vol 313, No 5789, pp 928, 929.

Spalding, Kirsty L., et al., ‘Forensics: age written in teeth by nuclear tests’,
Nature
, 15 September 2005, pp 333, 334.

Spalding, Kirsty L., et al., ‘Retrospective birth dating of cells in humans’,
Cell
, 15 July 2005, Vol 122, Issue 1, pp 133-143.

Vince, Gaia, ‘The many ages of man’,
New Scientist
, 17 June 2006, pp 50-53.

Zero Gravity in Space?
(Don’t Fall for It!)

The first human being to fly in Space was the Russian cosmonaut Yuri Gagarin, on 12 April 1961. Since then, we have all seen TV and movie footage of astronauts floating both inside, and outside, their spacecraft—seemingly unaffected by gravity. But despite what the journalists tell us, these astronauts are not in zero gravity.

There Ain’t No Zero

The astronauts in orbit are
not
in ‘zero gravity’.

Nope, the gravity of every object (no matter how little mass it has) reaches to the very edges of the Universe.

The gravitational field becomes weaker as it gets further away, but it never drops to zero.

As a result, every location in the Universe is filled with squillions of individual gravitational fields, each generated by the squillions of individual objects in the Universe, such as giant stars, small asteroids, and even the pen in my shirt pocket. At any location, these gravitational fields can combine to make a bigger gravitational field, or cancel to make a smaller one. It would be almost impossible to find a location in the Universe where the gravitational fields all cancel out to be exactly zero.

Astronaut? In Space?

Space travellers from Western countries are called ‘astronauts’. Those from Russia are called ‘cosmonauts’, while those from China are called ‘taikonauts’. In all of these words, the ‘naut’ part comes from the Greek word
nautes
meaning ‘sailor’, while both ‘astro’ and ‘cosmo’ refer to ‘star’. The ‘taik’ in ‘taikonaut’ comes from the Chinese word
taikong
meaning ‘Space’.

Originally, all Space travellers were sent up into Space by government agencies. But commercial Space flight began in 2004 with the launch of the privately funded spacecraft called
SpaceShipOne.

The definition of ‘Space’ varies. The FAI (Federation Aéronautique Internationale) Sporting Code for astronautics recognises Space flights only as those that exceed the arbitrary altitude of 100 km above the surface of the Earth. But the USA awards Astronaut Wings to those who have reached above 80 km.

Sergei K. Krikalev holds the male record for time spent in Space (2.2 years, or 803 days, 9 hours and 39 minutes), while Peggy A. Whitson holds the female record (1.03 years, or 377 days).

However, in the spaces between the galaxies, millions of light years from each other, the gravity would be pretty close to zero.

But that’s not the case with astronauts orbiting the Earth.

Construction of the International Space Station (ISS) began in 1998. It has been orbiting the Earth, with astronauts continually on board, since 2 November 2000. Its height varies between 330 and 410 km above the surface of the Earth. At this altitude, the ‘atmosphere’ is very thin, but the ISS is large and moves at around 25,000 kph. Atmospheric drag makes it lose about 200 m of altitude every day, so it has to be reboosted every 10-45 days.

The ISS is not far from Earth—you could cover this distance on a German autobahn in a few hours.

Gravity Not Constant Off Earth

The acceleration that gravity gives to a falling body weakens with altitude above the ground.

90% of Normal ‘Weight’

The Earth has a radius of about 6,400 km. (It’s actually 6,378.1 km at the Equator, and 6,356.8 km at the North and South Poles.) So the ISS is actually very close to the Earth—it’s only about 5% of the radius away.

Isaac Newton told us, and he was right, that there is a gravitational attraction between any two bodies. The mathematicians tell us that you can think of all of the mass of the Earth as being concentrated at the centre. So when you stand on the surface, about 6,400 km away from the centre, your weight is (let’s say) 80 kg.

Now, let’s put you on the very top of a Giant Pin—about 350 km high. This is a great height, but it’s only a pimple on the Earth, measuring only about 5% of its radius. I have chosen 350 km because this is the height at which the ISS and the space shuttles orbit. Gravity gets weaker as you move further away from its source.
Plug this extra distance into Newton’s Equation of Gravity, and you find that you now weigh only 72 kg, about 10% less.

Please note—that’s not 100% less, only 10% less.

So if the astronauts were standing on a weighing scale, on top of a Giant Pin 350 km high, they would weigh 90% of their normal weight.

Gravity Not Constant On Earth

There are many ways to measure the strength of the Earth’s gravity. One very easy method is to see what acceleration our gravity gives to a freely falling object. This acceleration, called ‘g’, is often rounded off to 9.8 m/sec
²
, i.e. with each passing second, the velocity of the falling object will increase by 9.8 m/sec.

It’s higher at the North and South Poles, and lower at the Equator, for two different reasons.

First, it’s higher at the poles because they are closer to the centre of the Earth than the Equator. This happens because the Earth spins, making the planet bulge outwards at the Equator. The closer distance makes the gravity stronger at the poles.

Second, it’s weaker at the Equator, because the Earth spins on its own axis, generating an outward ‘centrifugal force’. This ‘outwards force’ slightly weakens the inward ‘suck’ of gravity. So, again, gravity is stronger at the poles.

Adding these two factors together, ‘g’ is 9.832 m/sec
²
at the poles, and 9.782 m/sec
²
at the Equator.

How Come They Float?

So how come the astronauts float, if they still weigh 90% of their normal weight?

Simple: (1) they are falling because they have weight; but (2) they are moving forward really really quickly; and (3) the Earth is curved, not flat.

Let me explain this to you.

First, the astronauts are so close to the Earth that the Earth’s gravitational field ‘sucks’ on them with 90% of its normal ‘suck’. So they do actually fall towards the Earth. At ground level, if you jumped off a diving board into a pool, you would fall 5 m in the first second. But the Earth’s gravity is a little weaker at their altitude of about 350 km, so they fall 4.5 m in the first second. And yes, they really do fall.

Second, at their altitude, they are also moving forward at about 25,000 kph—in other words, 7 km in each second.

Third, the Earth is not flat—it has the shape of a ball, with a curved surface. At the astronauts’ altitude, the curve of the Earth drops away by roughly 4.5 m for every 7 km that they fly forward.

Therefore in the exact second that they move forward 7 km, gravity pulls them 4.5 m vertically downwards towards the Earth. But (as I said earlier) in the 7 km that they cover in that second, the curve of the Earth is such that the surface of our planet has ‘fallen’ away by 4.5 m.

Free Fall

For our astronauts, the net result is simple. After one second of falling, they are still the same distance from the surface of the Earth.

And so it goes for every single second that they are in orbit at that particular speed and altitude. After 90 minutes, they have made a complete loop of the Earth and are still the same distance from the surface.

In one second, the shuttle moves forward 7 kilometres, while at the same time, gravity pulls it 4.5 metres vertically downwards towards the Earth.

But in the 7 kilometres it covers in that second, the curve of the Earth is such that the surface of our planet has ‘fallen’ away by 4.5 metres.