The Universe-6.
The centrifuge at the Royal Netherlands airforce physiology department,
was one of the first devices built to spin humans around at high speed.
Its purpose is to subject fighter pilots to the high G forces they experience in combat,
both for research and to teach them not to blackout.
Acceleration is indistinguishable from gravity.
Spinning around is a good way to achieve high acceleration in a small space.
In the case of a human centrifuge, the acceleration is directed towards the centre,
of the spinning arm and is caused by the force, known as the centripetal force that acts on our body,
through the seat to keep us flying in a circle.
An experiment was conducted to stimulate the gravity conditions in other planets.
The gas giant Neptune is 17 times more massive than Earth.
Neptune’s radius is 3.89 times that of Earth at the equator.
Using Newton’s law of gravitation we can calculate the surface gravity on Neptune.
It is 14% more than Earth, or 1.14G.
Even with such a small change we can feel the difference, when we lift our arm,
because it is 14% heavier than normal.
Jupiter is 318 times more massive than Earth.
Its radius is 11.2 times greater.
It surface gravity would be 2.5 times that on Earth, or 2.5G
So our arms would feel 2.5 times heavier than normal, so more difficult to lift.
OGLE2 TR L9b is an exa planet in the constellation of Carina.
Its gravity is four times that on Earth, or 4G.
At 4G it would be not possible to lift your arm.
It will also be difficult to breathe, as our rib cage will be much heavier.
If we spend a minute or so at 5G, the blood begins to drain from the head,
because the heart finds it difficult to pump it into the brain.
This causes faintness, and a slight narrowing of vision.
At around 6G, our face will contort into a funny shape.
Fighter pilots are trained to deal with 9G, but only for brief periods.
Our bodies have not evolved to cope with the weak force of gravity,
at strengths much greater than those on Earth.
The body with the highest surface gravitational force in the solar system is the Sun.
It has a mass 330 thousand times that of Earth.
Its surface gravity is 28 times more powerful than Earth.
No Centrifuge can simulate this G force, since it can’t survive the load.
To find even stronger gravitational fields we have to travel beyond our Solar System,
and look for objects more exotic than mere stars.
In 1967, postgraduate student Jocelyn Bell and her supervisor Anthony Hewish,
were using a radio telescope at Cambridge, to search for quasars.
Quasars are the most luminous, powerful and energetic objects in the Universe.
Quasars are quasi-stellar radio sources.
They are believed to be the small, compact regions around supermassive black holes,
at the centre of very young galaxies.
A vast amount of radiation, in excess of the output of an entire galaxy of a trillion Suns,
is emitted, as gas and dust spiral into to the black hole.
As Bell and Hewish searched the data for this highly active, ancient galactic centres,
they stumbled upon a very strange signal.
It was a pulse that repeated every 1.3373 seconds precisely.
It seemed to the Cambridge team to be almost impossible to believe such a fast regular pulse,
could come from a natural source.
So they called it LGM-1, which stands for Little Green Men.
The source was entirely natural.
Astronomer Fred Hoyle realised this.
However, they made a new discovery for which Hewish and Fellow astronomer Martin Ryle,
received the Noble prize in physics in 1974.
Interestingly Bell and Hewish were not the first humans to see these wonders.
They were beaten by an ancient civilisation that witnessed a birth of one, a thousand years earlier.
Between AD 900 and 1150, a great civilisation built a series of vast stone structures,
known as the Great Houses, along the floor of the Chaco Canyon in New Mexico.
These buildings remained the largest man made structures in North America until the 19th century.
The largest contains more than 700 rooms, many of which are still intact.
These buildings were not used as permanent residences.
They were mostly ceremonial structures.
It has been suggested, that important moments in the yearly cycle of the Sun,
such as summer and winter solstices are indicated in these structures.
There is a painting made in AD 1054, which has a handprint, a crescent moon and a bright star.
This was the year of one of the most spectacular events in recorded history.
On 4th July AD 1054, a nearby star exploded.
Chinese astronomers recorded this precise date.
The Chacoans would certainly have seen it too, because the explosion was visible,
even in daylight for 3 weeks.
The fading new star remained visible to the naked eye at night for 2 years.
We do know precisely where the explosion happened in the sky,
because its remnant is one of the most famous and beautiful sights in the sky: The Crab Nebula.
Every 18.5 years, the moon and Earth will return to the same positions,
they were on the nights around 4th July AD 1054.
If we position ourselves beside the painting, the moon will pass by the position in the sky,
indicated by the hand print.
At that moment, to the left of the moon, exactly as depicted in the painting,
we will see the Crab Nebula.
The explosion of 4th July 1054 was a supernova, the violent death of a massive star.
It is expected that, on an average there should be around one supernova,
in our galaxy every century.
This one was almost uncomfortably close, and only 6000 light years away.
The Crab Nebula is the rapidly expanding remains of a star,
that was around 10 times the mass of the Sun.
After only a thousand years, the cloud of glowing gas is eleven light years across.
It is expanding at 1500 kilometres per second.
At the heart of the glowing cloud sits the exposed stellar core,
which is all that remains of a once massive star.
If we point a radio telescope at it, you will detect a radio signal,
pulsing at a rate of precisely 30.2 times a second.
It was an object like this that Jocelyn Bell and her colleagues observed in 1967.
The Cambridge team weren’t listening to little green men.
They were listening to the extraordinary signal of a rapidly rotating neutron star - called a pulsar.
Neutrons stars are truly amongst the strangest worlds of the Universe.
They are matter’s last stand against the relentless force of gravity.
For most of the star’s life , the inward pull of gravity is balanced by the outward pressure,
caused by the energy released from the nuclear fusion reactions within its core.
When the fuel runs out, the star explodes, leaving the core behind.
What prevents this stellar remnant from collapsing under its own weight?
The answer lies not in the physics of stars, but in the world of sub-atomic particles.
The answer to this was not proven until 1967.
Physicists Freeman Dysan and Andrew Lenard showed that the stability of matter,
is down to a quantum mechanical effect called the Pauli exclusion principle.
There are two types of particles in nature.
They are distinguished by a property called spin.
The fundamental matter particles, such as electrons and quarks, and composite particles,
such as protons and neutrons, have half integer spin.
They are collectively known as Fermions.
The fundamental force carrying particles such as photons have integer spin.
They are known as bosons.
Fermions have the important property that no two of them can occupy the same quantum state.
Put more simply, but slightly less accurately, this means you can’t pile lots of them,
into the same place.
This is the reason why atoms are stable and chemistry happens.
Electrons occupy distinct shells around the atomic nucleus.
As you add more and more electrons, they go into orbits further and further away from the nucleus.
It is only the behaviour of the outer most electrons that determine,
the chemical properties of an element.
Without the exclusion principle, all the electrons would crowd into the lowest possible orbit,
and there would be no chemical reactions, and therefore no people.
If you try to press atoms together, you force their electron clouds together,
until at some point you are asking all the electrons to occupy the same place,
or the same quantum state.
This is forbidden, and leads to an effective force,
that prevents you quashing the atoms together any further.
This force is called electron degeneracy pressure, and it is very powerful.
White dwarf stars are the fading embers of stars,
left to slowly cool after the nuclear fusion in their cores ceased.
How did they continue to defy the crushing force of gravity?
The answer is by electron degeneracy pressure,
the dogged reluctance of electrons to being forced too closely together.
What happens if you keep building more massive white dwarfs,
increasing the gravitational pull still further.
The Indian astrophysicist Chandrashekar found the answer,
in one of the landmark calculations of the early years of quantum theory.
In 1930, Chandrashekar showed that the electron degeneracy pressure,
can prevent to collapse of white dwarfs with masses up to 1.38 times the mass of our Sun.
For masses greater than this the electrons won’t give in to gravity and move closer together,
instead, they give up and disappear.
They don’t of course vanish into thin air, because they carry properties such as electric charge,
which cannot be created or destroyed.
Instead, the intense force of gravity makes it favourable for them to merge with protons,
in the nuclei of atoms to form neutrons.
This is possible through the action of the weak nuclear force,
in the process that turns protons into neutrons, in the heart of our Sun.
For dying stars with masses above the Chandrashekar limit, this is the only option.
The entire core turns into a dense ball of of neutrons.
Most of the matter that makes up the world around us is empty space.
Typical nucleus of a neutron star, which contains virtually all the mass,
is around a hundred thousand times smaller than the diameter of it’s atoms.
The rest is made up of the fuzzy cloud of electrons, kept well away from each other,
by the exclusion principle.
If the nucleus were the size of a pea, the atom would be a vast sphere,
around a hundred meters across, and this is all empty space.
With the electrons gone, matter collapses to the density of the nucleus itself.
All the space is quashed out of it by gravity, leaving an impossibly dense nuclear ball.
A star which is around 1.4 times as massive as the Sun, is just above the Chandrashekar limit.
It is crushed into a perfect sphere 20 Kilometres across.
Neutron star matter is so dense that one sugar cube of it, would weigh more than Mount Everest, here on Earth.
The anatomy of neutrons stars is still being intensely researched.
They are certainly far more complex than just a ball of neutrons.
The surface gravity is of the order of 10 to the power of 11 G.
The surface is probably made of thin crust of iron, and some lighter elements.
But the density of neutrons increases as you burrow inwards.
Deep in the core temperatures may be so great that more exotic forms of matter may exist.
Perhaps quark-gluon plasma.
This is the form of matter that existed in the Universe,
a few millionth of a second after the Big Bang.
The unimaginable density and exotic structure aren’t the only fantastic feature of neutron stars.
Many of these neutron stars including LGM1 and the neutron star at the heart of the Crab Nebula, have intense magnetic fields and spin very fast.
The magnetic field lines, which resemble those of a bar magnet,
get dragged around with the stars rotation.
If the magnetic axis is tilted with respect to the spin axis,
this results in two high energy beams of radiation sweeping around like lighthouse beams.
The details of this mechanism are the subject of intense theoretical and experimental study.
These are the pulses that Bell and Hewitt observed in 1967.
These stars are known as pulsars.
The fastest known pulsars - millisecond pulsars - rotate over a thousand times every second.
In 2004, astronomers announced the discovery of a double pulsar system.
It is surely one of the most incredible of all the wonders of the Universe.
The system is made up of two pulsars.
One with a rotational speed of 23 thousandths of a second.
The other with the period of 2.8 seconds.
They orbit with each other every 2.4 hours.
The diameter of the orbit is so small, that the whole system would comfortably fit inside our Sun.
Pulsars are incredibly accurate clocks, allowing astronomers to use the system,
to test Einstein’s theory of gravity in the most extreme conditions known.
We can imagine the intense warping and bending of space and time close to these two massive,
spinning neutron stars.
Remarkably the predictions of Einstein’s theory of General Relativity,
our best current theory of gravity, in the double pulsar’s system,
have been confirmed to an accuracy of better than 0.05 percent.
It is majestic, powerful and wonderful that the human intellect of a man living in the 20th century, could device a theory of gravity, inspired by thinking carefully about falling rocks and elevators,
is able to account precisely for the motion of the most alien objects in the Universe,
in the most extreme known conditions .
This is what is great about physics.
When Newton first published his law of Universal gravitation in 1687,
he transformed our understanding of the Universe.
His simple mathematical formula, is able to describe with unerring precision,
the motion of moons around the planets, planets around the Sun, solar system around galaxies,
and galaxies around galaxies.
Newton’s law is however only a model of gravity.
It has nothing at all to say about how gravity actually is.
It is certainly has nothing to say about a central mystery.
Why do all objects fall at the same rate in gravitational fields?
The question can be posed in a different way by looking again at Newtons famous equation.
F=G into m1 into m2 divided by r squared.
This states that the gravitational force between two objects,
is proportional to the product of their masses.
Let us say m1 is the mass of the Earth, and m2 is the mass of the stone falling towards Earth.
Another of Newton’s equations is F=m into a.
Or we can say a = F divided by m.
This is the second law of motion, which describes how this stone accelerates if a force is applied to it.
Acceleration of the stone is equal to the force you apply to it, F divided by its mass, m.
The reason why things fall at the same rate in a gravitational field, irrespective of their mass,
is that the mass of the stone, m and m2 in both the equations is equal to each other.
When we work out the acceleration, the mass of the stone cancels out,
and we get an answer which only depends on the mass of the Earth.
This is how we get the famous value of 9.81 meters per second squared.
If you double the mass of something falling towards Earth, the gravitational force on it doubles,
but so does the force needed to accelerate it.
There is a very important assumption here that had no justification at all,
other than the fact it works.
Why should these two masses be the same?
Why should the so called inertial mass - which appears in F=ma and tells you,
how difficult it is to accelerate something - have anything to do with the gravitational mass,
which tells how gravity acts on something?
This is a very important question and Newton had no answers to it.
Newton then provided a beautiful model for calculating how things move around,
under the action of the force of gravity, without actually saying what gravity is.
He knew this of course.
He famously said that gravity is the work of God.
If a theory is able to account for every piece of observational evidence, however,
it is very difficult how to replace it with the better one.
This did not stop Einstein, who thought very deeply about the equivalence,
of gravitational and inertial mass and the related equivalence,
between acceleration and the force of gravity.
In 1905, he had great success with the special theory of relativity,
which included the famous equation E=mc squared.
He then began to search for a new theory of gravitation, that might offer a deeper explanation,
for these profoundly interesting assumptions.
Although not specifically motivated by it, Einstein would certainly have known,
that there were problems with Newton’s theory, beyond the philosophical.
The most unsettling of these was the distinctly problematic behaviour of a ball of rock,
that was located over 77 million kilometres from Earth.
The planet Mercury has been a source of fascination for thousands of years.
It is the nearest planet to the Sun.
It is tortured by the most extreme temperature variations in the solar system.
Due to its proximity to the Sun, Mercury is a difficult planet to observe from Earth.
Occasionally the planets align such that Mercury passes directly across the face of the Sun,
as seen from Earth.
These transits of Mercury are one of the great astronomical spectacles,
occurring 13 or 14 times every century.
Mercury has the most eccentric orbit of any planet in the solar system.
At its closest, Mercury passes just 46 million kilometres from the Sun.
At its most distant it is over 69 million kilometres from the Sun.
This highly elliptical orbit means that the speed of the movement of this planet,
varies a lot during its orbit.
This means that very high precision measurements were necessary to map its orbit,
and make predictions of its future transits.
Through out the 17th and 18th centuries, scientists would gather across the globe,
to watch the rare transits of Mercury.
These scientists used Newton’s law of gravity to predict exactly when and where,
they could view the spectacle.
It became a source of scientific fascination, and no little embarrassment when,
time after time, Mercury did not appear on cue.
The planet regularly crossed the Sun’s disc later than expected,
sometimes by as much as several hours.
Mercury’s unusual orbit was a real problem.
Because of the observational uncertainties it wasn’t until 1859,
that the astronomers Urbain Le Verrier proved that the details of Mercury’s orbit,
couldn’t be completely explained by Newtonian gravity.
To solve the problem, many astronomers reasoned that there must be another planet,
between the Sun and Mercury.
This planet had to be invisible to our telescopes, but it must also exert a gravitational force,
large enough to disturb Mercury’s orbit.
Encouraged by the recent discovery of the planet Neptune,
based on a similar anomaly in the orbit of Uranus, they named the ghost planet Vulcan.
For decades astronomers searched and searched for Vulcan, but they never found it.
The reason for this is that Vulcan doesn’t exist.
The errors in the predictions in fact signalled something far more profound.
Newton’s theory of Universal gravitational is not correct.
Einstein would have loved the Vomit Comet.
The fact that the effects of gravity can be completely removed by falling freely,
in a gravitational field was, for him,
the thought experiment that led to his theory of General Relativity.
Einstein was very interested in free fall.
Inside the plane, falling towards Earth, is absolutely impossible to tell that you are moving.
It is impossible to tell that you are near a planet.
It is impossible to tell that, according to someone on the ground,
you are accelerating at 9.81 meters per second towards the ground.
You are simply floating, along with everything in the plane.
If you let some drops of water out of the bottle, they float in front of your face.
The camera man and the director float next to the water droplets.
There was self evidently no force acting on anything at all,
otherwise things would have moved around.
And yet, from the point of view of someone on the ground, we were flying in a parabolic arc,
moving forwards through the air at hundreds of kilometres an hour,
and accelerating violently towards the ground.
The force of gravity is very much present in this description.
Einstein’s theory takes the view that the two ways of looking at the Vomit Comet -
from the inside and outside - should be treated as equivalent.
No one inside the plane or outside the plane, has the right to claim,
that they are right, and the other is wrong!
If, inside the plane, there is no experiment you can do to prove,
that you are accelerating towards the ground.
You are well within your rights to claim that you are not.
Acceleration has cancelled out gravity.
Of course, you could look out of the windows.
But even then you could claim that the Earth is accelerating towards you,
and that you are simply floating.
From this perspective, everyone on Earth feels a gravitational force pulling them on to the ground, because they are being accelerated upward at a rate of 9.81 meters per second squared.
Acceleration is therefore equivalent to gravity.
This is known as the equivalence principle, and it was very important to Einstein.
In technical language, Einstein would have defined the Vomit Comet, during its time in free fall,
has an inertial frame of reference.
This is to say that it can be legitimately considered to be at rest, with no forces acting on it.
The assertion that sitting in a falling aircraft should be considered,
as being absolutely equivalently floating around in space,
far beyond the gravitational pull of any planet or moon,
can be used to explain why all objects fall at the same rate.
Why?
Simply because there are two equally valid ways of looking at what is happening.
From the point of view inside the plane, nothing at all is happening.
Everything is simply floating, untouched by any forces of any kind.
If no forces are acting, then everything naturally stays were it is put.
Shift outside the plane, however, and things appear different.
Everything is falling towards Earth, accelerating under the action of the force of gravity.
But, very importantly, the reality of this situation cannot change depending on which view we adopt.
Everything has to behave in the same way in reality, irrespective of how you look at it.
If the drops of water float in front of your face when viewed from the vantage point,
inside the plane then the water drops better float in front of your face,
when viewed from a vantage point outside the plane.
In other words, everything in the plane is accelerating towards the ground at exactly the same rate!
We have made no assumptions about the equivalence,
between gravitational and inertial masses here.
We have just said that a freely falling box in Earth’s gravitational field,
is indistinguishable from a freely falling box in space,
or indeed any freely falling box anywhere in the Universe, around any planet, any star or any moon.
So what, then, is gravity?
The explanation in Einstein’s theory is beautifully simple.
Gravity is the curvature of spacetime.
What is spacetime?
Spacetime is the fabric of the Universe itself.
A good way to picture spacetime, and what it means to curve it, is to think about a simpler surface:
This surface of Earth.
Our planet has a two dimensional surface.
This is to say you need only two numbers to identify any point on it:
Latitude and longitude.
Earth’s surface is curved into a sphere, but you don’t need to know that to move around on it,
and navigate from place to place.
The reason we can picture the curvature, is that we are happy to think in three dimensions.
So we can actually see that Earth’s surface is curved.
But imagine we were two-dimensional beings, confined to move on the surface of Earth,
with absolutely no concept of a third dimension.
We would know nothing about up and down, only about latitude and longitude.
It would be very difficult indeed for us to picture in our mind’s eye,
the curvature of our planet’s surface.
Now let’s extend our analogy to see how the curvature of something can give rise to a force.
Imagine that a pair of two-dimensional friends standing on the equator,
and decide to take a journey to the north.
They decide to walk parallel to each other, with a intension of never bumping into each other.
If they both keep walking, they will walk up parallel lines of longitude,
and they will find that as they get closer and closer to the North Pole,
they will get closer and closer together.
Eventually, when they reach the north pole they will bump into each other!
As three-dimensional beings, we can see what happened.
Earth’s surface is curved, so all the lines of longitude meet at the poles.
However, from the perspective of the two-dimensional friends,
even though they kept assiduously to the parallel lines, they still were mysteriously drawn together.
They may well conclude from this that a force was acting between them,
attracting them towards one and another.
In Einstein’s theory, that force is gravity.
The complicated bit about Einstein’s theory of General Relativity,
is that the surface we need to think about, is not two-dimension but four-dimensional.
It is the mixture of the familiar 3 dimensions of space, plus an addition dimension of time mixed in.
Spacetime was found to be necessary by Einstein and others, to explain, in particular,
the behaviour of light, and the form of Maxwell’s equations.
Suffice to say that the surface of our Universe, is four-dimensional.
What Einstein showed is, that the presence of matter and energy -
in the form of stars, planets and moons - curves the surface of spacetime,
distorting it into hills and valleys.
His equations describe exactly what shape spacetime should be around any particular object,
such as the Sun, for example.
They also describe how things move over the curved surface.
And here is the key point: just like our two-dimensional friends, things move in a straight line:
but just like our two dimensional friends, this isn’t what it looks like,
if you don’t know the spacetime is curved.
When your moving across the curved surface, it appears that a force is acting on you,
distorting your path.
One of the first things Einstein did with his new, geometric theory of gravity,
was to calculate what Mercury’s straight line path,
through the curved spacetime around the Sun, would look like to us.
To his delight he found that Mercury would orbit the Sun,
in precisely the way that had been observed over the centuries of transit observations.
Where Newton had failed, Einstein succeeded.
Einstein had found a completely geometrical way of describing the force of gravity,
and it is quite wonderfully elegant.
Not only does it predict the orbit of Mercury, but it also provides a very appealing explanation,
for the equivalence principle.
Why do all objects fall at the same rate in a gravitational field,
irrespective of their mass or composition ?
Because the path they take has nothing to do with them at all - they are simply following straight line paths through the curved spacetime.
Perhaps the most startling demonstration of this, is the bending of light by gravity.
Light has no mass, and so according to Newton’s theory it shouldn’t be affected by gravity at all.
However, according to Einstein’s theory it does not matter that it has no mass.
It will still be following a straight line through the curved spacetime.
So, it will appear to follow exactly the same path as everything else.
Let’s do a thought experiment to see how strange this is.
Stand on the ground on a very, very big planet, with the rock on one hand,
and a laser beam in the other.
Point the laser beam horizontally, and drop the rock, and fire the laser beam.
Which one hits the ground first?
The answer is both hit the ground at the same time,
because they both move through the same curved space.
Light falls at the same rate in a gravitational field as everything else.
Now there is a caveat here.
Why did we say a very, very big planet?
Light travels at almost 300,000 kilometres per second,
so if the rock takes one second to hit the ground, so will the light.
But it would have flown 300,000 kilometres in the horizontal direction,
by the time it reaches the ground.
On Earth this would mean that the surface of the planet had long since curved away!
However the principle still holds.
What would happen if you fired the laser beam directly at the ground.
Light must always travel at the same speed, it cannot speed up.
It will travel to the ground at exactly 299,792,458 metres per second.
But shouldn’t it accelerate at 9.81 meters per second squared, as it drops?.
No, it cannot, because it always travels at exactly 299,792,458 metres per second.
So, what happens?
Well, the energy of the light can change, although the speed cannot.
So, the light gets shifted towards the blue end of the spectrum, as it flies towards the ground,
and gains energy from its fall.
That is to say the wavelength gets shorter, and its frequency increases.
This is very interesting because the second is defined as the length of time,
it takes a fixed number of wavelengths, of a particular colour of light to pass by an observer.
Let’s say you use the frequency of the laser beam, held in your hand, to synchronise the clock.
This means that the peaks and troughs of the laser light beam are arriving more frequently,
then they set off.
So from the point of view of someone on the ground, the clock above the ground will be slightly fast.
The effect is known as gravitational time dilation.
Gravity slows down time, so clocks on the ground run slower than those in orbit.
In the language of General Relativity, we might say that the presence of Earth,
bends spacetime near it, such that time passes more slowly than it does far away.
This is a very real effect and one that has to be taken into account,
in the GPS satellite navigation system, which needs precise timekeeping to measure distances.
The GPS satellites orbit, at an altitude of 20,000 kilometres.
This means that there clocks run faster than they do on the ground by 45 microseconds per day,
because they are in a weaker gravitational field.
The fact that they are moving relative to the ground also affects the rate of their clocks.
When everything is taken into account the time shift reduces to 38 microseconds per day.
This will be equivalent to a distance error of over 10 kilometres,
which would make the system useless.
So every time we get into our cars and use satellite navigations,
we are using Einstein’s theory of gravity in order to correctly ascertain our position,
in the surface of Earth.
To summarise, if Einstein experienced the Vomit Comet, he would have described it,
during its time in freefall, as following a straight line path through spacetime.
As long as it continues in the path, the plane and its passengers,
will not feel the force of gravity at all.
It is only when something stops the plane following its straight line path through spacetime,
that a force is felt.
If the plane did not stop itself falling, this obstacle would be the ground!.
The experimental fact that triggered all this discussions,
is that gravitational and inertial masses of objects are the same.
Einstein provides a natural explanation for this.
Gravity is simply result of the fact there is such a thing as spacetime, and that it is curved,
and that things move in straight lines through this curved spacetime.
It is also possible to take a different view.
There could be some deep reason why the gravitational and inertial masses of things are equal -
a reason that we have yet to discover.
The fact that they are equal allows us to build a geometric theory of gravity.
In that case, Einstein theory might more properly be considered to be a model,
in the same way that Newton’s theory is a model.
At the moment we have no way of deciding between these two possibilities,
but it is worth being aware that they both are valid ways of looking at the situation.
Einstein’s theory of General Relativity is rightly considered,
to be one of the great intellectual achievements of all time.
It is conceptually elegant, and what most physicists call beautiful.
Ultimately, though, it doesn’t matter how beautiful a theory is.
The only thing that matters is that its predictions are in accord with our observations,
of the natural world.
The orbit of mercury is one such observation.
The slowing down of time in gravitational fields is another.
To really test Einstein’s theory to the limit, we have to journey far out into space,
and visit the most exotic and massive objects in the known Universe -
places where the force of gravity becomes exceptionally strong.
The success of Einstein’s theory of General Relativity is one of the greatest human achievements.
But there is a final twist to the story of gravitation,
because Einstein’s remarkable theory predicts its own demise.
The collapse of a neutron star is prevented by neutron degeneracy pressure.
Neutrons are fermions, as are electrons.
Because they are more massive than electrons, they can be packed much more tightly together, before the Pauli exclusion principle steps in once more, and forbids further contraction.
Another stable staging post against gravity should be provided by quark degeneracy pressure,
because quarks too are fermions.
But ultimately, if the star is too massive, gravity will overwhelm,
even these fantastically dense objects.
It is believed that the limit above which no known law of physics can intervene to stop gravity,
is around 3 times the mass of the Sun.
This is known as the Tolman-Oppenheimer-Volkoff limit.
For the remnants of stars beyond this limit, gravity will win.