The Universe-1.
The Universe is 13.7 billion years old.
It is 93 billion light years across.
It has more than 100 billion galaxies.
Each contains hundreds of billions of stars.
We still do not know how the Universe began.
We do have strong evidence that something interesting happened 13.75 billion years ago,
that can be interpreted as the beginning of the Universe.
We call it the Big Bang.
The interesting thing that happened corresponds to the horizon of everything in the skies.
All the ingredients required to build the hundreds of billions of galaxies,
and thousands of trillions of stars, were once contained in a volume,
far smaller than a single atom.
Unimaginably dense and hot beyond comprehension, this tiny seed has been expanding,
and cooling for the last 13.75 billion years.
This has been sufficient time for the laws of nature to assemble all the complexity,
and beauty we observe in the night skies.
When we say beginning of the Universe, we mean the events,
that happened during the Planck epoch.
This is the time period before 10 to the power of minus 42 of a second, after the Big Bang.
This is currently beyond our understanding.
This is because we lack a theory of space and time before this point.
Consequently we have very little to say about it.
Such a theory known as quantum gravity, is the holy grail of modern theoretical physics.
It is being energetically researched by hundreds of scientists across the world.
Einstein spent the last decades of his life searching in vain for it.
Conventional thinking holds that time and space began at time zero.
This is the beginning of the Planck era.
The Big Bang can therefore be regarded as the beginning of time itself,
and as such the beginning of the Universe.
There are alternatives.
In one theory, what we see as the Big Bang and the beginning of the Universe,
was caused by the collusion of two pieces of space and time, known as ‘branes’,
that has been floating forever in an infinite, pre-existing space.
The origin of the Universe is still being researched.
The Sun is 150 million kilometres from Earth.
When it disappears we can see about 10,000 stars with the naked eye.
They are all part of our own galaxy, the Milky Way.
A galaxy is a massive collection of stars, gas and dust bound together by gravity.
We think there are around 100 billion galaxies in the observable Universe.
Each of them contain millions of stars.
The smallest galaxies, known as dwarf galaxies have as few as 10 million stars.
The biggest, the giants, have been estimated to contain 100 trillion stars.
It is now widely accepted that galaxies also contain much more than the matter,
we can see using our telescopes.
They are thought to have giant halos of dark matter.
This is a new form of matter unlike anything we have discovered on Earth.
It interacts only weakly with normal matter.
Despite this, its gravitational effect dominates the behaviour of galaxies today.
It most likely dominated the formation of the galaxies, in the early Universe.
We now think that around 95% of the mass of galaxies, like our milky way,
is made up of dark matter.
The search for the nature of dark matter is one of the great challenges,
for 21st century physics.
The word galaxy came from the Greek word galaxias, meaning milky circle.
In Greek mythology the light from these galaxies came from spilt milk of Zeus’s wife Hera.
This story is the origin of the modern name of our galaxy, the Milky Way.
The name entered the English language not from a scientist,
but from a poet, Geoffrey Chaucer.
Our galaxy, the Milky Way contains 200 to 400 billion stars,
depending on the number of faint dwarf stars, which are difficult to detect.
The majority of stars lying in disc around 100 thousand light years in diameter.
On an average it is about 1000 light years thick.
These vast distances are difficult to visualise.
A distance of 100 thousand light years,
means that light travelling at 300 thousand kilometres per second,
would take 100 thousand years to make a journey across our galaxy.
At the centre of our galaxy, and possibly every galaxy in the Universe,
there is believed to be a super massive black hole.
Astronomers believe this because of precise measurements,
of the orbit of the star known as S2.
This star orbits around the intense source of radio waves, known as Sagittarius A*,
pronounced as Sagittarius A-star, that sits at the galactic centre.
S2’s orbital period is just 15 years.
This makes it the fastest orbiting object.
It reaches speeds of upto 2% of the speed of light.
If the precise orbital path of an object is known, the mass of the thing it is orbiting,
can be calculated.
The mass of Sagittarius A star is enormous at 4.1 million times the mass of our Sun.
Since the star S2 has a closest approach to the object of only 17 light hours,
it is known that Sagittarius A star must be smaller than this,
otherwise S2 would literally bump into it.
The only known way of cramming 4.1 million times the mass of the Sun,
into a space less than 17 light hours across is as a black hole.
This is why astronomers are so confident that a giant black hole,
sits at the centre of the Milky Way.
These observations have recently being confirmed and refined by studying a further 27 stars,
known as the S-stars.
All of these have orbits taking them close to Sagittarius A star.
Beyond the S stars the galactic centre is a melting pot of celestial activity.
It is filled with all sorts of different systems that interact and influence each other.
The Arches Cluster is the densest known star cluster in the galaxy.
Formed from 150 young, intensely hot stars that dwarf our Sun in size,
these stars burn brightly and are consequently very short lived.
They exhaust their supply of hydrogen in just a couple of million years.
The Quintuplet Custer contains one of the most luminous stars in our galaxy,
the Pistol Star.
It is thought to be near the end of its life, and on the verge of becoming a supernova.
It is in Central Clusters like the arches and the Quintuplet,
that the greatest density of stars in our galaxy can be found.
As we move out from the crowded galactic centre, the number of stars drops with distance,
until we reach the sparse cloud of gas in the outer reaches of the Milky Way,
known as the Galactic Halo.
In 2007, scientists using the Very Large Telescope, VLT, in Chile were able to observe a star,
in the Galactic Halo, that is thought to be the oldest object in the Milky Way.
H E 1523-0901 is a star in the last stages of its life.
It is a red giant.
It is a vast structure far bigger than our Sun, but much cooler at its surface.
It is interesting because astronomers have been able to measure the precise quantities,
of 5 radio active elements - uranium, thorium, europium, osmium, and iridium in the star.
Using techniques similar to carbon dating, astronomers have been able to get a precise age,
for the ancient star.
Radio active dating is an extremely precise and reliable technique,
when there are multiple radio active clocks ticking away at once.
This is why that the detection of 5 radio active elements,
in the light from H E 1523-0901 was so important.
This dying star turns out to be 13.2 billion years old.
That is almost as old as the Universe itself, which began 13.7 billion years ago.
The radio active elements in this star would have been created in the death throes,
of the first generation of stars, which ended their lives in supernova explosions,
in the first half a billion years of the life of the Universe.
As well as being vast and very very old, our galaxy is also beautifully structured.
There are three types of galaxies, ellipticals, barred spirals, and spirals.
Our galaxy is a barred spiral galaxy.
It consists of a bar shaped core surrounded by a disc of gas, dust, and stars,
that creates individual spiral arms, -
Perseus, Norma, Scutum-Centaurus and Carina-Sagittarius.
Our Sun is an offshoot of the latter the Orion spur.
It is now thought to be an additional arm called the Outer arm,
an extension to the Norma arm.
Close to the inner rim of the Orion spur is the most familiar star in our galaxy.
The Sun was once thought to be an average star.
But now we know that it shines brighter than 95% of all other stars in the Milky Way.
Its known as a main sequence star, because it gets all its energy and produces all its light,
through the fusion of hydrogen into helium.
Every second the Sun burns 600 million tonnes of hydrogen in its core,
producing 596 million tonnes of helium in the fusion reaction.
The missing 4 tonnes of mass emerges as energy,
which slowly travels to the Sun’s photosphere, where it is released into the galaxy,
and across the Universe as light.
Our Sun is in the middle of its life cycle.
If we look out into the Milky Way, we can see a whole cycle of stellar life playing out.
Roughly once a year somewhere in the Milky Way a new star is born.
The Lagoon Nebula is one such star nursery.
Within this giant interstellar cloud of gas and dust, new stars are created.
It was discovered by a French astronomer in 1747.
This is one of the handful of active star forming regions in our galaxy,
that are visible with the naked eye.
This huge cloud is slowly collapsing under its own gravity,
but slightly denser regions gradually accrete more and more matter,
and over time these clumps grow massive enough to turn into stars.
The centre of this vast stellar nursery, is illuminated by a intriguing object,
known as Herschel 36.
This star is thought to be a ZAMS star, or Zero Ago Main Sequence.
This is because it has just begun to produce the dominant part of its energy,
from hydrogen fusion in its core.
Recent measurements suggest that Herschel 36 may actually be 3 large young stars,
orbiting around each other.
The entire system has a combined mass of over 50 times that of our Sun.
This makes Herschel 36 a true system of giants.
Eventually Herschel 36 and all the stars in the Milky Way will die.
When they do many will go out in a blaze of glory.
Eta Carinae is a pair of billowing gas and dust clouds,
that are the remnants of a stellar explosion from an unstable star system.
The system consists of at least two giant stars.
It shines with a brightness 4 million times that of our Sun.
One of the stars is thought to be a Wolf-Rayet star.
These stars are immense, over 20 times the mass of our Sun.
They are engaged in a constant struggle to hang on to their outer layers.
They loose vast amount of mass every second in a powerful solar wind.
In 1843, Eta Carinae became one of the brightest stars in the Universe when it exploded.
The blast spat matter out at 2.5 million kilometres an hour.
It was so bright that it was thought to be a supernova explosion.
Eta Carinae survived intact and remains buried deep inside these clouds,
but its days are numbered.
Because of its immense mass, the Wolf-Rayet star is using up its hydrogen fuel,
at a ferocious rate.
Within a few hundred thousand years, or earlier it is expected that the star will explode,
in a supernova or even a hypernova.
The hypernova is the biggest explosion in the known Universe.
In 2004 an explosion thought to be similar to the 1843 Eta Carinae event,
was seen in a galaxy over 70 million light years away from the Milky Way.
Just two years later, the star exploded as a supernova.
Eta Carinae is very much closer, at a distance of only 7500 light years.
So as a supernova it might shine so brightly that it will be visible from Earth even in day light.
Seeing the light from these distant worlds and watching the life cycle of the Universe,
is a reminder that light is the ultimate messenger.
It carries information to us across interstellar and intergalactic distances.
Light allows us to journey back through time,
providing a direct and real connection with our past.
This is made possible not only because of the information carried by light,
but by the properties of light itself.
Light is the primary way by which we will be able to explore the Universe beyond our galaxy.
For now, even the stars are far beyond our reach.
We rely on their light alone for information about them.
By the seventeenth century many renowned scientists were studying,
the properties of light in detail.
The studies of Kepler, Galileo, Descartes, and some of the later great scientists,
Huygens, Hook, and Newton, were all fuelled by the desire to build better lenses,
for microscopes and telescopes, to enable them to explore the Universe.
By the end of the seventeenth century, two competing theories for light had emerged.
Both of them are correct.
On one side Newton, believed that light was composed of particles.
He published this in his ‘Hypothesis of Light’, in 1675.
On the other were Newton’s great scientific adversary, Robert Hooke,
and the dutch astronomer Christiaan Huygens, who proposed the wave theory.
The particle/wave debate went on till the turn of the 19th century.
Most scientists sided with Newton.
There were notable exceptions, including the great mathematician Leonhard Euler.
Euler felt that the phenomena of diffraction could be experienced by wave theory.
In 1801, the english doctor Thomas Young appeared to settle the matter once for all.
He reported the results from his famous double slit experiment,
which clearly showed that light diffracted, and therefore must travel in the form of a wave.
Diffracting is a fascinating and beautiful phenomena,
and very difficult to explain without waves.
If you shine light on to the screen through a barrier with a very thin slit,
you see a complex but regular pattern of light and dark areas.
The expectation for this is that when you mix lots of waves together,
they don’t only have to add up.
If we have two waves on top of each other with exactly the same wave length and amplitude,
but aligned so that the peak of one wave lies directly on the trough of the other,
the waves will cancel out each other.
In technical terms when the waves are 180 degrees out of phase, the waves will cancel out,
and there will be darkness.
This is exactly what is seen in diffraction experiments through small slits.
The slits act like lots of little sources of light, all slightly displaced from one and other.
This means that there will be places beyond the slits where the waves cancel each other out,
and places where they will add up.
This leads to light and dark areas seen by the scientists like Young.
This indicated that light was some kind of wave.
The correct explanation for the nature of light came from a unlikely source.
In the mid 19th century, the study of electricity and magnetism,
engaged many great scientific minds.
At the Royal Institution in London,
Michael Faraday was playing around with wires and magnets.
He discovered that if you push a magnet through a coil of wire,
an electric current flows through the wire, while the magnet is moving.
This is a generator.
It is used in power stations around the world to provide us with electricity.
Faraday wasn’t interested in inventing the foundation of the modern world.
He just wanted to learn about electricity and magnetism.
He encoded his experimental findings in mathematical form.
This is known today as Faraday’s law of electromagnetic induction.
Around the same time the French scientist Andre-Marie Ampere discovered,
that two parallel wires carrying electric currents experience a force between them.
This force is still used today to define the ampere, the unit of electric current.
Today the mathematical formula of this law is called Ampere’s law.
By 1860, a great deal was known about electricity and magnetism.
Magnets could be used to make electric currents flow.
Flowing electric currents could deflect compass needles,
in the same way that magnets could.
There was clearly a link between the two phenomena.
But nobody had come up with a unified description.
The breakthrough was made by Scottish scientist James Clerk Maxwell.
In 1862, he developed a single theory of electricity and magnetism.
It was able to explain all the experimental work of Faraday, Ampere and others.
Maxwell’s crowning glory came in 1864, when he published a paper,
that is one of the greatest achievements in the history of science.
Maxwell discovered that by unifying electrical and magnetic phenomena,
together into a single mathematical theory, a startling prediction emerges.
Electricity and magnetism can be unified by introducing two new concepts:
electric and magnetic fields.
The idea of a field is central to modern physics.
We can introduce the concept of a magnetic field by holding a compass,
at places around a wire carrying an electrical current,
and noting down how much the needle deflects, and in what direction.
The numbers and directions are the magnetic field.
Maxwell found that by introducing the electric and magnetic fields,
and placing them centre stage, he was able to write down a single set of equations,
that described all the known electrical and magnetic phenomena.
In writing down his laws of electricity and magnetism using fields,
Maxwell noticed that by using a bit of simple mathematics,
he could rearrange his equations, into a more compact and magically revealing form.
His new equations are known as wave equations.
In other words, they had exactly the same form as the equations,
that describe how sound waves move through air,
or how water waves move through the ocean.
The waves Maxwell discovered were waves in the electric and magnetic fields themselves.
His equations show that when a electric field changes, it creates a changing magnetic field.
In turn as the magnetic field changes, it creates a changing electric field,
which creates a changing magnetic field and so on.
In other words, once you have wiggled a few electric charges,
you can take the charges away, and the fields will continue sloshing around.
As one falls the other will rise.
This will continue to happen forever, as long as you do nothing to them.
This is profound in itself.
But there is an extra more profound conclusion.
Maxwell’s equations also predict exactly how fast these waves must fly away,
from the electric charges that create them.
The speed of the waves is the ratio of the strength of the electric and magnetic fields.
These quantities have been measured by Faraday, Ampere and others,
and well known to Maxwell.
Maxwell was astonished when he discovered that his equations predicted,
that the waves in the electric and magnetic fields travelled at the speed of light!
In other words, Maxwell had discovered,
that light is nothing more than oscillating electric and magnetic fields,
sloshing back and forth and propelling each other through space as they do so.
This led to the profound conclusion that light is an electromagnetic wave.
In order to have his epiphany, Maxwell needed to know exactly what the speed of light was.
The fact that light travels very fast, but not infinitely so,
and already being known for almost 200 years.
It was first measured by Ole Romer in 1676.
Aristotle and many philosophers and scientists believed light travelled without movement.
As scientist gave more thought to the nature of light,
a debate about the speed of light continued for more than thousand years.
On one side eminent scientists such as Euclid, Kepler and Descartes sided with Aristotle,
in believing that light traveled infinitely fast.
On the other side Empedocles and Galileo, separated by two millennia,
felt that light must travel at a finite extremely high velocity.
Galileo set out to measure the speed of light using two lamps.
He held one and sent an assistant a large distance away with another.
When they were in position, Galileo opened a shutter on his lamp, letting the light out.
When his assistant saw the flash, he opened his shutter,
and Galileo attempted to note down the time delay between the opening of his shutter,
and his observation of the flash from his assistant lamp.
He concluded that light must travel extremely fast,
because he was unable to determine its speed.
He could only conclude that light travels at least 10 times faster than the speed of sound.
The first experimental determination of the speed of light,
was made by Danish scientist Ole Romer.
In 1676, Romer was attempting to solve,
one of the great scientific and engineering challenges of that age :
telling the time at sea.
Finding an accurate clock was essential to enable sailors,
to navigate safely across the oceans.
Mechanical clocks using pendulums or springs were not good,
at being bounced around on the ocean waves.
In order to pinpoint your position on Earth, you need the latitude and longitude.
Latitude is easy.
In the northern hemisphere the angle of the North Star (Polaris),
above the horizon is your latitude.
In the southern hemisphere with some knowledge of astronomy and trigonometry,
it is possible to determine your latitude with sufficient accuracy for safe navigation.
Longitude is more difficult because you can’t just determine it by looking at the stars.
You have to know which time zone you are in.
Greenwich in London is defined as 0 degrees longitude.
Your precise time zone at any point on Earth’s surface is defined by the point,
at which Sun crosses an imaginary arc across the sky,
between the North and the South points on your horizon, passing through the celestial pole.
The celestial pole is the point marked by the North Star in the northern hemisphere.
Astronomers call this arc, the Meridian.
The point at which the Sun crosses the Meridian,
is also the point it reaches its highest position in the sky on any given day,
as a journey from sunrise in the east to sunset in the west.
We call this time noon, or midday.
Earth rotates fifteen degrees every hour, and once on its axis every 24 hours.
This means that the two points on Earth’s surface,
that are separated by 15degrees of longitude, will measure known exactly one hour apart.
So, to determine your longitude, set a clock to read 12’o clock,
when the Sun reaches the highest point in the sky at Greenwich.
If it reads 2pm when the Sun reaches the highest point in the sky where you are,
you are 30 degrees to the west of Greenwich.
To measure this we need a very accurate clock that keeps time for weeks or months on end.
The early seventeenth century, King Philip-3 of Spain offered a prize,
for devising a method for precisely calculating longitude at sea.
The technological challenge of building sufficiently accurate clocks was too great.
So scientists began to look for high precision natural clocks,
and it seemed sensible to look to the heavens.
Galileo, having discovered the moons of Jupiter,
was convinced he could use the orbits of these moons as a clock,
as they regularly passed in and out of the shadow of the giant planet.
The principle is beautifully simple.
Jupiter has 4 bright moons, that can be seen relatively easily from Earth.
The inner most moon, Io, goes around the planets every 1.769 days, precisely.
One can say that Io’s orbit is as regular as clockwork.
By watching Io’s daily disappearance and reemergence from behind Jupiter’s disc,
you have a very accurate and unchanging natural clock.
By using the Jovian system as a cosmic clock,
Galileo devised an accurate system for keeping time.
Observing the eclipses of these tiny pinpoints of light,
3 quarters of a billion kilometres from Earth, from a rolling ship was impractical,
but the logic was sound.
Galileo failed to win the king’s prize.
Despite this, the technique could be used to measure longitude accurately on land,
where stable conditions and high quality telescopes were available.
Cataloging the eclipses of Jupiter’s moons, particularly Io,
became a valuable astronomical endeavour.
By the mid seventeenth century, Giovanni Cassini was leading the study of Jupiter’s moons.
He pioneered the use of Io’s eclipses for the measurement of longitude .
He published tables on what dates the eclipses should be visible,
from many locations on Earth, together with high precision predictions of the times.
To refine his tables he sent one of his astronomers, Jean Picard, to Copenhagen.
Picard employed the help of a young Danish astronomer, Ole Romer.
In 1671, Romer and Picard observed over one hundred of Io’s elicipses .
Romer was invited to work as Cassini assistant at the Royal Observatory.
Here Romer made a crucial discovery.
Romer noticed that a celestial position of the Jovian clock wasn’t as accurate,
as everyone had thought.
Over the course of several months, the prediction for when Io would emerge,
from behind Jupiter drifted.
At some times of the year there was a significant discrepancy,
of over 22 minutes between the predicted and the actual observed timings of the eclipses.
This appeared to ruin the use of Io as a clock,
and end the idea of using it to calculate longitude.
However, Romer came up with an ingenious and correct explanation of what was happening.
Romer noticed that the observed time of the eclipses,
drifted later relative to the predicted time,
as the distance between Jupiter and Earth increased.
It then drifted back again when the distance between Jupiter and Earth began to decrease.
Romer’s genius was to realise that there was nothing wrong,
with the clockwork of Jupiter and Io.
The error depended on the distance between Earth and Jupiter.
His explanation, which is correct, was simple.
Light takes time to travel from Jupiter to Earth.
As the distance between the two planets increase, the light from Jupiter will take longer,
to travel from there.
This means that Io will emerge from Jupiter’s shadow later than predicted,
simply because it takes longer for the light to reach Earth.
Conversely, as the distance between Jupiter and Earth decreases,
it takes the light less time to reach Earth, and you see Io emergence sooner than predicted.
If we factor in the time it takes for light to travel between Jupiter and Earth, the theory works.
Romer did this by trial and error.
He was able to correctly account for the shifting times of the observed eclipses.
The number that Romer actually calculated was the light travel time,
across the diameter of the Earth’s orbit around the Sun.
He found this to be about 22 minutes.
Perhaps he felt the diameter of the Earth’s was not known with sufficient precision,
he never turned his number into the speed of light.
He simply stated that it takes 22 minutes to cross the diameter of the Earth’s orbit.
The first published number for the speed of light,
was that obtained by dutch astronomer Christiaan Huygens,
who had corresponded with Romer.
Huygens calculated the speed of light to be 220,000 kilometres per second.
This is close to the modern value of 299,792 kilometres per second.
The error was primarily in the determination of the diameter of Earth’s orbit around the Sun.
His measurement of the speed of light was the first determination,
of the value of what scientist call a constant of nature.
These numbers such as Newton’s gravitational constant, Planck’s constant,
have remained fixed since the Big Bang.
They are central to the properties of our Universe.
They are crucial in physics.
We would live in an Universe that was unrecognisable,
if their values were altered even by a tiny amount.
Everything in the Universe has a speed limit.
Sound travels at 1236 kilometres per hour in air at 20 degrees celsius.
Sound in a gas such as air is a moving disturbance of air molecules.
The speed of sound in air depends on the air’s temperature, which is a measure,
on how fast the molecules in the air are moving on average, the mass of the air molecules,
and the details of how air responds when it is compressed.
To a reasonable approximation the speed of a sound wave,
depends mainly on the average speed of the air molecules at a particular temperature.
The speed of sound is therefore not a speed limit at all.
It is simply the speed at which the wave of pressure moves through the air.
There is no reason why an object shouldn’t exceed the speed of sound.
In 1947, the first flight at supersonic speeds took place.
So the sound barrier is not a barrier at all.
It is a speed limit only for sound itself.
It is determined by the physics of movement of air molecules.
Is the light barrier the same?
We describe light as a electromagnetic wave.
Why shouldn’t a powerful spacecraft be able to fly faster,
than a wave in electric and magnetic fields?
The answer is that the light barrier is of a totally different character,
and cannot be smashed through, even in principle.
The reason for this is that light speed plays much deeper role in the Universe,
than just being the speed at which light travels .
A true understanding of the role of the speed of light, was achieved in 1905 by Einstein,
in his special theory of relativity.
Einstein, inspired by Maxwell’s work formulated a theory,
in which space and time are merged into a single entity, known as ‘space time’.
Einstein suggested we should not see the world as having only 3 dimensions.
He added a fourth dimension - past/future.
Hence space time is referred to as four dimensional, with time being in the fourth dimension.
Einstein was forced into this bold move primarily because Maxwell’s equations,
for electricity and magnetism were incompatible with Newton’s 200 year old laws of motion.
Einstein abandoned the Newtonian ideas of space and time as separate entities,
and merged them.
In Einstein’s theory there is a special speed built into the structure of space time itself,
that everyone must agree on.
This is irrespective of how we are moving relative to each other.
This special speed is a universal constant of nature,
that will always be measured as precisely 299,792.458 kilometres per second.
This is at all times and at all places in the Universe, no matter what they are doing.
This is critical in Einstein’s theory because it stops us doing something strange in spacetime.
If past/future is simply another direction like north/south,
why can’t we wander backwards and forwards in it?
Why can’t we travel into the future and not the past?
In Einstein’s theory of relativity, it is the existence of this special speed,
that makes time direction different to that of space, and prevents time travel.
In this sense, the special speed is built into the fabric of space and time itself,
and plays a deep role in the structure of our Universe.
What is this have to do with a speed of light?
Nothing much !
There is a reason why light goes at this speed, and it seems a complete coincidence.
In Einstein’s theory, anything that has no mass is compelled to travel,
at the special speed through space.
Conversely, anything that has mass is compelled to travel slower than this speed of light.
There is no deep reason why photons have to be massless particles.
So there is no deep reason why light travels at the speed of light.
We can only call the special speed ‘light speed’,
because it was discovered by measuring the speed of light.
The key point is that the speed of light is a fundamental property of the Universe,
because it is built into the fabric of spacetime itself.
Travelling faster than this speed is impossible.
Even travelling at this speed it is impossible if you have mass.
It is the property of the Universe that protects the past from the future,
and prevents time travel into the past.