The Universe-5.
It is possible to demonstrate another strange but equally important and related aspect of gravity.
Newton wrote down this in his theory of gravity in 1687, as did Galileo many decades before him.
The strange thing is this : all objects fall at the same rate under the force of gravity,
even though gravity acts on objects in proportion to their mass.
Newton and Galileo knew this to be a case because they did experiments,
and noticed that it was true, but they had no idea why.
Newton found that the gravitational force between the two objects, such as the Earth and you,
is proportional to the products of their masses.
The force you feel due to the pull of Earth’s gravity, is proportional to the mass of the Earth,
multiplied by the mass of you.
If you were to double your mass, the force between you and Earth would double.
But, the rate at which you accelerate towards Earth because of its gravitational pull,
is also proportional to your mass.
When you work everything out it turns out that your mass completely cancels out.
So, therefore all things fall at the same rate under gravity.
This looks very strange and famously demonstrated by Apollo 15 commander Dave Scott,
on the surface of the moon in 1971.
Scott dropped a feather and a hammer to the ground.
Of course both hit the ground at the same time.
The reason you can’t do this on Earth, is because air resistance slows the feather down.
But in the high vacuum of the Lunar surface the only force acting on the falling objects is gravity.
This is interesting because it is not in accord with common sense.
Surely a cannon ball should fall to the ground faster than an atom?
The answer is no, it doesn’t.
Even a beam of light falls to the ground at the same rate as a cannon ball.
Understanding this concept is key to understanding gravity.
All objects, no matter what there mass, fall at the same rate in a gravitational field.
This simple fact inspired Einstein to construct his geometric theory of gravitation,
called General Relativity.
It is to this day the most accurate theoretical description of gravity that we possess.
Across the Universe, from the smallest speck of dust to the most massive star,
gravity is the great sculpture that created order out of chaos.
Everything in the cosmos is subject to the force of gravity.
From manmade satellites that rotate around our planet, to the orbit of the moon,
which journeys around the Earth every 27.3 days, it is gravity that provides the invisible string,
to guide them on their path.
The journey of every planet, moon, ball of rock and mote of dust in our solar system,
is guided by gravity.
All the planets, and their 166 known moons, are guided by gravity.
Beyond our solar system, gravity continues to conduct the flow of the Universe.
Everything is affected by the gravitational pull of something else,
no matter how tiny or how massive.
Our galaxy is part of the collection of galaxies, call the Local Group.
This is the cluster of over 30 galaxies named by the astronomer Edwin Hubble, in 1936.
It is over 10 million light years across.
This vast dumbbell shaped structure contains billions and billions of stars,
including the trillion stars that make up our giant galactic neighbour, Andromeda.
The Sun orbits the Milky Way.
The local group orbits its common centre of gravity,
located somewhere in the 2.5 million light years between the two most massive galaxies,
in the group : the Milky Way and Andromeda.
Even this giant community of galaxies isn’t the largest known gravitationally bound structure.
As we read this, we are spinning around as Earth rotates once a day on its axis.
We are orbiting at over 100,000 kilometres per hour around the Sun.
We are rotating around the centre of our galaxy at 220 kilometres per second.
The entire Milky Way is tearing around the centre of gravity of the Local Group,
at 600 kilometres per second.
We are also part of a grander gravitationally driven cycle.
The local group is part of a much larger, gravitationally bound family called the Virgo Super cluster.
It is a collection of over 100 galaxies clusters.
Nobody is sure how long it takes our Local Group to journey around the Virgo Super cluster.
Vast beyond words, stretching over 110 million light years, it is, even so,
only one of the millions of super clusters in the observable Universe.
It is now thought that even super clusters are part of the largest structures bound together,
by gravity, known as galaxy filaments or great walls.
We are part of the Pisces-Cetus super cluster complex.
Gravity’s scope is unlimited.
Its influence is all pervasive at all distance scales through out the entire history of the Universe.
Yet perhaps surprisingly, given its colossal reach and universal importance,
it is the first force that we humans understood in any detail.
The history of science is littered with examples of circumstance and serendipity,
leading to the greatest discoveries.
This is why curiosity driven science is the foundation of our civilisation.
Among the most celebrated is the convoluted story of Newton’s journey to his theory of gravity -
the first great universal law of physics.
The great plague of 1665 was the last major outbreak of bubonic plague in England,
but also the most deadly.
Over 100,000 people are thought to have died in this plague.
Extreme and often useless measures were taken to preventing its spread.
Infected villages were quarantined and schools and colleges closed.
One place affected was Trinity college, Cambridge.
One of the students to take a leave of absence in the summer of 1665 was Issac Newton.
Newton was 22 years old and newly graduated, when he left plague ridden Cambridge,
to return to his home in Lincolnshire.
He took with him a series of books on Mathematics and the geometry of Euclid and Descartes.
Although by all accounts he was an unremarkable student,
his enforced absence allowed him to think.
His interest in the physical world and the loss underpinning it began to coalesce.
Over the next two years his private studies laid the foundations for much of his later work,
in subjects as diverse as calculus, optics and of course gravity.
He returned to Cambridge in 1667, and was elected as a fellow.
He became the Lucasian professor of mathematics in 1670.
This post was held by Stephen Hawking, and string theorist Michael Green.
Both of them worked on the nature of gravity.
Newton spent the next twenty years lecturing and working,
in a diverse range of scientific and pseudoscientific endeavours,
including alchemy and predictions of the date of the apocalypse.
Newton’s greatest contribution was the publication in 1687 of Principia.
This book contains an equation that describes the action of gravity,
so precisely that it was used to guide the Apollo astronauts on their journey to the moon.
It is beautiful in its simplicity and profound in its application and consequences,
for scientific thought.
The equation is F=G into m1 into m2 divided by r squared.
This is the mathematical expression of Newton’s law of universal gravitation.
It says that the force F between two objects, is equal to the product of their masses m1 and m2,
divided by the square of the distance between them.
G is a constant of proportionality known as the gravitational constant.
Its value encodes the strength of the gravitational force.
The force between two, one kilogram masses, one metre apart,
is 6.67428 into 10 to the power of minus 11 Newtons.
This is not much.
For comparison the force exerted on your hand by a 1kg bag of sugar,
is approximately 10 Newtons.
In other words the gravitational constant,
is 6.67428 into 10 to the power of minus 11 Newton metre per kg squared.
The reason why G is so tiny is unknown, and one of the greatest questions in physics.
The electromagnetic force is 10 to the power of 36 times stronger.
There are many reasons why Newton’s law of universal gravitation is beautiful.
It is universal, which means it applies everywhere in the Universe.
Exceptions are in the vicinity of black holes, too close to massive stars,
or moving close to the speed of light.
In these special cases Einstein’s more accurate theory of general relativity is required.
For planetary orbits around stars, orbits of stars around galaxies,
and the movement of galaxies themselves, Newton’s equation is accurate enough.
It has also applied to all times in the universe’s history, beyond the first instants after the Big Bang.
This is not to be taken for granted, because the law was derived based on the work of Kepler,
and the observations of Tycho Brahe, who were concerned only with the motion of planets,
around the Sun.
The fact that the law that governs, the clock work of our solar system is the same law,
that governs the motion of galaxies is interesting and important.
It is the statement that the same laws of physics govern our whole Universe,
and Newton’s law of gravitation was the first example, of such a universal law.
It is also profoundly simple.
That the complex motion of everything in the cosmos can be summed up,
in a single mathematical formula, is elegant and beautiful,
and lies at the heart of modern fundamental science.
You can work out where the planets and moons of the solar system will be at any point in the future, using Newton’s simple equation.
This applies not just to our solar system, but also to every solar system in the Universe.
Such is the power of mathematics and physics.
Newton found that gravity is a force of attraction that exists between all objects,
from the tiny immeasurable force of attraction between two rocks on the ground,
to the rather larger force, we experience between our bodies and Earth.
With the mass of 6 to the power of 24 kilograms, the force between us and our planet,
is strong enough to keep our feet on the ground.
On the scale of planets, gravity can do much more than simply keeping planets in orbit,
and hold things on the ground.
It can sculpt and shape their surfaces in profound and unexpected ways.
The strength of Earth’s gravitational field has a powerful influence on its surface features.
Every droplet of water raised high by the heat of the sun has energy,
due to its position in Earth’s gravitational fields.
Some of this energy is available deep into the Earth’s surface to form Canyons.
The strength of Earth’s gravitational field has a powerful influence on its surface features.
This is not only visible in the action of falling, tumbling water, but in the size of its mountains.
On Earth, the tallest mountain above sea level is Mount Everest.
At almost 9 kilometres, it towers above the rest of the mountains.
But Everest is dwarfed by the tallest mountain in the solar system.
Surprisingly its sits on a much smaller planet, Mars.
Mars is 78 million kilometres from Earth.
Mars is similar to our planet in many ways.
Its surface is scarred by the action of water that once tumbled from the highlands to the seas,
dissipating its gravitational potential energy as it fell.
Today the water has left Mars.
The planet is only 10% as massive as Earth.
So its gravitational pull is significantly weaker.
This is one of the reasons why Mars was unable to hang on to its atmosphere,
despite being further away from the Sun.
The possibility of liquid water flowing on the Martian surface vanished with its atmosphere,
leaving the red planet to an arid and geological dead future.
Mar’s lower surface gravity has a surprising consequences for its mountains.
Towering over every other mountain in the solar system is the extinct volcano,
Olympus Mons, in Mars
It rises to an altitude of 24 kilometres, 3 times the height of Mount Everest.
The fact that a smaller planet has higher mountains is not a coincidence.
It is partly down to environmental factors such as the rate of erosion,
and the details of the planets geological past.
There is also a fundamental limit to the height of mountains on any given planet:
The strength of its surface gravity.
Mars has a radius half that of Earth.
It is only 10% as massive.
Calculating using Newton’s equation will tell you that the strength of the gravitational pull,
at its surface is 40% of that on our planet.
This changes everything’s weight.
Here on Earth we don’t often think about the difference between mass and weight.
But the distinction is very real.
The mass of something is an intrinsic property of that thing.
It is a measure of how much stuff the thing is made of.
This doesn’t change, no matter where in the Universe the thing is placed.
In Einstein’s theory of special relativity, the rest mass of an object is an invariant quantity.
This means every one in the Universe, no matter where they are, or how they are moving,
would measure the same value for the rest mass.
Weight is different.
For one thing, it is not measured in kilograms.
It is measured in the units of force - newtons.
When you stand on a weighing scale, they measure the force being exerted on them by you.
The force you are exerting is dependent on the strength of Earth’s gravity.
The weight of something on Earth is defined as W=m into g.
W is weight.
m is the thing’s mass.
g is the Earth’s gravitational field strength, which is 9.81 meters per second squared.
For absolute accuracy, the correct definition of weight is the force,
that is applied on you by the scales to give you an acceleration,
equal to the local acceleration due to gravity.
That is the force the scales exert on you to stop falling through them.
So here on Earth, a human being with a mass of 80kgs weighs 785 newtons.
On Mars the same 80kg person would weigh 295 newtons.
Weight depends on a few things.
One is your mass.
Another is the mass of the planet you are on.
Your weight would also change if you are accelerating when you measure it.
This is another manifestation of the equivalence principle.
So if you took Olympus Mons, from Mars, and stuck it on Earth,
then as well as dwarfing every other mountain on the planet, it will weigh two and a half times,
as it does on Mars.
This enormous force would put it’s base under such intense pressure,
that it would be unable to support the mountain, so it would sink in the ground.
A planet the size of our Earth cannot sustain the size of Olympus Mons, - it would weigh too much.
The highest mountain on Earth, as measured from its base, is Mauna Kea,
the vast dormant volcano in Hawaii.
It is over 1km higher than Everest, and its gradually sinking.
So Mauna Kea is as high as a mountain can be on Earth.
This the absolute limit set by the strength of our gravity.
The definition of weight can get a little convoluted.
There are caveats to the rule of thumb that your weight is 9.81 times your mass.
One problem is that the strength of Earth’s gravity varies slightly at every point on its surface.
The most obvious effect is altitude.
On the floor of a canyon, your weight would be slightly less than on top.
That’s because at the top of the canyon, you are further from the centre of the Earth,
then at the bottom.
So the gravitational pull is weaker at the top.
Earth is not uniformly dense - some areas of Earth’s surface and subsurface,
are made of more massive stuff than others, which also affects the local gravitational field.
To complicate matters further, Earth is spinning.
This means that you are accelerating when you stand on its surface.
This means that the strength of gravity you feel changes in accord with the equivalence principle.
This acceleration increases as you go towards the equator.
This reduces the gravitational acceleration you feel, near the equator.
Earth bulges out near the equator because it is spinning,
which weakens the gravitational pull there still further.
The upshot of all this is that you weigh about .5% less at the north and south poles,
than at the equator.
The effects of this varying density of Earth subsurface,
and the presence of surface features on Earth’s gravitational field,
have been measured to extremely high precision, and presented as a map known as the geoid.
A picture of the Earth’s gravitational field was taken by a European space agency satellite,
GOCE, in 2009.
GOCE is equipped with three ultra sensitive accelerometers,
arranged so that they respond to very tiny changes in the strength of Earth’s gravitational field,
has the satellite’s orbits.
Skimming the edge of the Earth’s atmosphere at an altitude of 250 kilometres,
GOCE took two months to complete this extraordinary image.
It is the first time the strength of gravity across the globe has been mapped this accurately.
The blue patches in the image indicate areas that have a weak gravitational field.
The green areas are average, and the red areas are places where it is stronger.
The reason for these fluctuations is the density of the rocks below Earth’s surface,
and the presence of features such as mountains or ocean trenches.
From the map it is clear that Ireland has a higher gravitational field than that of India.
The unparalleled level of detail will enable a deeper understanding of how a planet works.
One particular benefit will be for oceanographers:
because the map defines the baseline water surface, in the absence of tides, wind, and currents.
It is critical to understanding the factors that determine the movement of water,
across the oceans of our planets.
This is a very important part of understanding and predicting the way,
energy is transferred around our planet.
This in turn is an important factor in generating accurate climate models.
The geoid therefore reveals a vast amount of detailed information about the structure of our planet,
just for measuring the strength of gravity.
As far as the actual height of the ocean surface is concerned, however,
the most influential factor is not shown, which is the moon.
Many of the planets in our solar system have families of moons.
Jupiter has 63 moons, Neptune has 13 moons, and Mars has 2 tiny moons.
Our planet has only a single moon.
It is our constant companion, with which we have travelled through space,
for four and half billion years.
No other planet in our solar system has a moon as large as ours, in relation to its parent planet.
The moon orbits 380,000 kilometres from Earth.
It is a quarter of the Earth’s diameter.
It is fifth largest moon in the solar system after Titan, Ganymede, Callisto and Io.
Of course their parent planets Jupiter and Saturn, are significantly larger than Earth.
This makes the Earth and moon close to being a double planet system.
The current best theory of our moon is that it was created 4.5 billion years ago,
when a Mars sized planet named Theia, crashed into the newly formed Earth,
blasting rock into orbit which slowly condensed into the lunar surface we see today.
The evidence for this theory is partly that the moon has a very similar composition,
to that of Earth’s outer crust, although it is much less dense,
because it has a significantly less iron core.
This would be expected if the Theia-Earth collusion was a glancing blow.
This would leave the Earth’s iron core intact, reducing the relative amount of iron in the moon.
This in turn means the moon’s gravitational field is much weaker than ours.
When Neil Armstrong took his first small step on the moon, he weighed just 26kgs.
This is despite the fact that he was wearing a space suit which weighed 81kgs on Earth.
This is because the moon’s gravitational field is 1/6th of Earth’s.
Despite this relatively weak gravitational pull, the moon has a profound impact on Earth.
Because of the moon’s proximity to our planet, its gravitational pull varies significantly,
from one side of the Earth to the other.
There is a net gravitational force pulling the side of the Earth, that is facing the moon.
There is also a net force pulling the opposite side of Earth away from the moon.
At right angles to the position of the moon,
the lunar gravity actually adds to the Earth’s gravitational pull and squashes everything.
This is the origin of the tides: because water is easier to stretch,
than the rock that forms the ocean floor.
The water in the oceans bulges outwards relatively to the ground beneath the moon,
and on the opposite side of the Earth to the moon.
The difference in water heights is only a few meters,
but can be much higher depending on the shoreline.
There are also tides in the rocks of Earth’s surface.
But rocks are very rigid, and don’t stretch much.
The surface of the Earth does, however, rise and fall by few centimetres, due to tidal effects.
As Earth rotates beneath the tidal bulge raised in the oceans,
the distorted water sweeps past the shore lines, and we experience two high and low tides per day.
The relationship between the Earth and the Moon is two way.
Just as the Moon’s gravity has transformed our planet, so in turn Earth has transformed the Moon.
Throughout human history, half of the Moon’s surface remain hidden from view.
It was in 1959, the Soviet Luna 3 probe photographed the far side of the Moon, for the first time.
Nine years later astronauts on board Apollo-8 became the first humans to leave Earth’s orbit,
and the first humans to observe the far side of the Moon.
The reason only one side of the Moon faces Earth, is due to the tidal effects.
Billions of years ago, the view of our Moon from Earth would have been very different.
In its childhood, the Moon rotated much faster, and both sides of the surface,
would have been visible on Earth.
From the moment of its Birth, the Moon felt the tug of Earth’s gravity.
This force would have been even greater than it is today, because the Moon was also closer to Earth.
Newton’s law of Universal Gravitation tells you that gravity is a two way street.
Just as the Moon raises tides on Earth, so Earth must cause tides to sweep across,
the surface of the Moon.
These tides are in the solid rock of the Lunar surface.
In an amazing piece of planetary heavy lifting,
the Moon’s crust would have been distorted upto seven metres.
This giant tidal bulge sweeping across the Moon had an interesting effect.
As the Moon turned beneath the giant parent Earth, the rock tide was dragged across its surface.
But the rising of the tide isn’t instantaneous.
It takes time for the Moon to respond to the pull of the Earth.
During the time, the Moon would have rotated a bit, carrying the peak of the rock tide with it.
The tidal bulge will therefore not be in perfect alignment with Earth, but slightly ahead of it.
Earth’s gravity acts on the Moon’s in such a way it tries to pull it back into sync.
In other words it acts like a giant brake.
Over time, this effect known as tidal lock in, gradually synchronises the rotation rate of the Moon,
with its orbital period.
This effectively means that the tidal bulge can remain in exactly the same place,
on the Moon’s surface, and doesn’t have to be swept around.
The Moon is now almost, but not quite, tidally locked to Earth.
This means that it takes 1 month to rotate around its axis, and one month to orbit Earth.
So there is no dark side of the Moon.
The side we can’t see gets plenty of sunlight.
Its just a side that perpetually faces away from Earth.
The Earth Moon system is in fact still evolving towards being perfectly tidally locked.
One interesting consequences of this is that Moon is gradually drifting,
further and further away from Earth at a rate of 4 centimetres per year
The power of gravity is able to reach across empty wastes of space,
and shape the surface of the planets and moons.
Gravity also has the power to create whole new worlds, and we can see the process of that creation,
frozen in time in the sky, everyday and night.
A strange light appears in our night sky.
A light that for centuries has puzzled those who have witnessed its glow,
fooling them into thinking that a new day was arriving.
The Prophet Muhammed called it the false dawn.
This magical glow that appears on the horizon just before sunrise and just after sunset,
has nothing to do with the arrival or departure of the Sun.
Instead it is a ghostly reminder of our world’s origin’s and the power of gravity.
It is the zodiacal light: a wispy whitish glow that appears to form,
a rough triangular shape rising from the horizon.
The astronomer Giovanni Cassini first investigated this strange phenomena in 1683.
The ethereal light perplexed many scientists of the age.
A common explanation was that the light came from the atmosphere of the Sun,
as it rose above the horizon, before the Sun itself.
It was Nicholas, one of Cassini’s students, who finally explained its origin.
In doing so he provided a first glimpse of the origin of the planets and moons in our Solar System.
The story of the zodiacal light can be traced back 5 billion years to the origins of our Solar System.
Back then, there was no Sun, nor any planets or moons.
There was only a cloud of gas and dust, the building block of our Solar System.
It is thought that the explosion of a nearby star sent a shock wave through the cloud,
creating small fluctuations in density.
It also imparted rotation.
The denser regions, had slightly more gravitational pull then the less dense regions.
The denser regions began to grow, and the largest one became the Sun.
The earliest days there would have been no planets surrounding the young Sun.
The Sun was a spinning disc of matter, a proto-planetary disc.
Over time, the minute particles of dust in the disc collided and clumped together,
and large objects of the size of small asteroids, known as planetesimals,
would have formed by chance.
Once the larger planetesimals were big enough to have significant gravity,
they began to sweep up the matter close to them, and their growth accelerated.
Roughly one hundred million years later, the largest planetesimals evolved,
into the planets and moons we see today.
However, not all this matter from the primordial cloud became a planet or moon.
Out in the Solar System beyond mars there should be another planet.
But a gravitational tug of war between Jupiter and the Sun stops it forming.
Now instead of a ninth planet, there is a band of dust and debris- the asteroid belt.
Normally there is no way of seeing the asteroid belt from Earth with a naked eye -
it is just too far away and the asteroids are too small.
But collusions with in the asteroid belt produce dust, that is the secret behind the false dawn.
The faint glow of the zodiacal light after sunset and before sunrise,
is caused by sunlight reflecting off the debris of a failed planet.
It is a remnant of the early Solar System, and a beautiful, glimmering reminder of our origins.
In 1972, astronauts on board Apollo 17, during its journey to the moon, took a picture of Earth.
This photo was widely circulated, and was known as the ‘Blue marble”.
It raises a question - why is Earth a sphere?
Actually, why are all planets and all stars spherical?
We know that stars and planets are formed by the gravitational collapse of clouds of dust.
You could say that the force of gravity pulls everything together, which is one way of looking at it.
Another way of saying the same thing is that all the little particles in the primordial cloud of dust,
had gravitational potential energy.
This is because they were all floating around in each other’s tiny gravitational fields.
Just like the water droplets that fall as rain, these particles would all try to fall ‘downhill’,
to minimise their gravitational potential energy.
This leads us to a very general and very deep principle in physics.
We can pretty much explain everything that happens in the Universe by applying it.
Things will minimise their potential energy, if they can find a way of doing so.
We could answer the question, ‘why does a ball roll down a hill?’,
by saying that the ball would have lower potential energy at the bottom of the hill than the top,
so it rolls down.
We could also say that there is a force pulling the ball down the hill.
Physicists often work with energies, rather than forces, and the two languages are interchangeable.
With collapsing cloud of dust, the shape that ultimately forms will therefore be the shape,
that minimises the gravitational potential energy.
The shape must be one that allows everything within the cloud to get as close to the centre of it,
as it possibly can, because anything that is located further away from the centre,
will have more gravitational potential energy.
So the shape that ensures everything is as close to the centre as possible is naturally, a sphere.
This is why stars and planets are spherical.
In the U.S. state of New Mexico,
is located one of the most spectacular and iconic observatories of Earth.
The Very Large Array (VLA) is a radio astronomy observatory consisting of 27 identical dishes,
each 25 metres in diameter.
It is arranged in gigantic Y shape across the landscape.
Although each dish works independently, they can be combined together,
to create a single antenna with an effective diameter of 36 kilometres.
This allows this vast virtual telescope to achieve very high resolution images of the sky,
at radio wavelength.
Radio astronomy has the history dating back to the 1930’s.
The astronomer Karl Jansky discovered that the Universe could be explored,
not just through the visible part of the the electromagnetic spectrum,
but also through the detection of radio waves.
Over a period of several months, Jansky used an antenna that looked more like a Meccano set,
to record the radio waves from the sky.
He initially observed two types of signals.
Radio waves generated by nearby thunderstorms,
and radio waves generated by distant thunderstorms.
He also found a third type, a form of what he thought was static.
The interesting thing about the static was that it seemed to rise and fall once a day.
This suggested to Jansky, that it consisted of radio waves generated by the sun.
Over a period of weeks the rise and fall of the static deviated from a 24 hour cycle.
Jansky could rotate his antenna on a set of Ford model T tyres to follow the mysterious signal.
He soon realised that the brightest point was not coming from the direction of the Sun.
It was coming from the centre of the Milky Way galaxy,
in the direction of the constellation of Sagittarius.
Jansky’s pioneering work did not immediately lead to a expansion,
in the new science of radio astronomy.
But ultimately exploring the radio sky has become one of the most powerful techniques,
in understanding the Universe beyond our Solar System.
Of the 6000 stars we can see from Earth with the naked eye,
only one object lies beyond the gravitational pull of our galaxy.
The Andromeda galaxy is the nearest spiral galaxy to the Milky Way,
and the most distant object visible to the naked eye.
It may appear as nothing more than a smudge in the heavens,
but observations by NASA’s Spitzer Space Telescope, suggest that it is home to a trillion stars.
Andromeda is just one of a hundred billion galaxies in the observable Universe.
There is one thing that singles it out, other than its proximity.
Most galaxies are moving away from each other as the Universe expands.
Andromeda is in fact moving directly towards us.
It is getting closer at the rate of half a million kilometres every hour.
It seems the two galaxies are destined to meet, guided by the force of gravity.
A galactic collusion sounds like a rare and catastrophic event.
But such collisions and the resultant mergers of galaxies are not unusual occurrences,
in the history of the Universe.
Both the galaxies of Andromeda and the Milky Way, have absorbed other galaxies.
into their structure’s over the billions of years of their existence.
A computer was used to simulate what would happen during a galactic collision,
between our neighbour Andromeda and our MilkyWay.
The images are one million light years across.
The timescale between each frame of the sequence is 90 million years.
After the initial collision, an open spiral pattern is excited in both the galaxies.
Long tidal tails and a connecting bridge of stars are formed.
Initially the galaxies move apart from one another,
but then they fall back together to meet in a second collision.
As more stars are thrown off in complex ripple patterns, they settle into one huge elliptical galaxy.
Spiral galaxies such as Andromeda and the Milky Way,
are the pinnacle of complexity, order, and beauty.
If we humans are still there in 3 billion years, this collision will be a spectacular event.
When the two galaxies collide, there will be so much energy pumped into the system,
that vast amounts of stars will form.
Gravity certainly feels like a powerful force.
It built our planet, our Solar System, and all the billions of stars in the Universe.
It diligently assembled clouds of dust and gas into to neatly ordered spheres.
Matter curves the fabric of the Universe.
In doing so the spheres are bound together and marshalled into orbits,
generating the cyclical cosmos we witness from Earth.
Gravity reaches far across this place between the star systems, forming galaxies,
clusters and super clusters, which all beat out orbital rhythms on longer and longer time scales.
Gravity is the creator of order and rhythm in our dynamic and turbulent Universe.
Despite its reach and influence, there is a mystery surrounding nature’s great organisational force.
Although it is an all pervasive influence, it is in fact an incredibly weak force.
It is by far the weakest force in the Universe.
It is so weak that we overcome it everyday, in the most mundane of actions.
When we lift a tea cup, you are resisting the force of gravity exerted on the cup,
by the entire planet Earth, is trying to stop you, but is no match to the power of our arm.
The reason for this weakness is not known.
The puzzle is brought into stark relief by considering what happens when we lift the cup.
The force that operates our muscles and holds the atom of our body together is electromagnetism.
It is 10 to the power 36 times stronger than gravity, which is why we will always win the battle,
with Earth’s gravity.
Even so, we have evolved to live on the surface of a planet,
with a particular gravitational field strength.
Evolution does not produce animals with muscles and skeletons,
that are stronger than they need to be.
Biology rarely wastes precious resources!.