About 4 billion years ago, our young planet got hit by another one, the size of Mars, which tore a huge chunk of it off into space. During the following millennia, all the debris from that collision settled into a single ball in orbit around our world. When that was done, the Moon you are now standing upon was born.
Were it to happen today, such a collision would be more than enough to wipe out all life forms on Earth. At the time, though, our world was bare, and it is funny to think that without such a catastrophic bang, we would have no Moon to illuminate our nights, no significant tides, and life as we know it probably wouldn’t exist on our planet.
If mankind could, one way or another, harvest all the energy the Sun radiates in one second, it would be enough to sustain the entire world’s energy-needs for about half a billion years.
the heart of the Sun, more than 310,000 miles below its surface. The Earth, by comparison, is about 4,000 miles from surface to core.
all the matter we breathe or see or touch or feel or detect, even the matter your real body contains, is made of atoms. Atoms are the building blocks of everything. They are the Lego bricks of your environment, if you will.
The tiniest of them all has been called hydrogen. The second smallest has been named helium. Take these two atoms together, and you have about 98 per cent of all the known matter in the known universe.
The Sun, being a star, is not a big ball of fire – fire needs oxygen, and although the Sun creates bits of it along with other heavy elements, there’s not enough free oxygen in outer space to sustain any fire whatsoever. A struck match would never catch fire in space. The Sun, like all the stars in the sky, is simply a bright ball of shining plasma, a hot mixture of electrons, of atoms stripped of some of their electrons (they are called ions), and of atoms stripped naked of all their electrons – the bare atomic nuclei.
Proxima Centauri belongs to a family of stars called red dwarves. It is much smaller than the Sun (about one seventh its size and mass) and has a rather red hue. Hence the name. Red dwarves are very common, indeed scientists believe they account for most of the stars in the sky, even though they are too faint for our eyes to see.
It so happens that the Earth’s axis is tilted in such a way that the southern hemisphere always faces the galactic centre while the northern hemisphere always looks away from it, where there are far fewer stars. Accordingly, nights in the north are rather dull compared to southern ones.
Humanity has known about the existence of planets like Jupiter or Venus for ages, that is true. But the first time in history someone actually saw a planet orbiting a star that is not the Sun was only about twenty years ago, when, in 1995, two Swiss astronomers, Michel Mayor and Didier Queloz, spotted a giant world, which they called 51 Pegasi b, orbiting a star located about 60 light years away from us.
Gravity suddenly wasn’t a mystery any more. Gravity was a bending of the fabric of the universe caused by the objects it contains. Newton had not seen this. No one had seen this before, and we are still trying to figure out all the consequences of this vision today.
Except us. And we’ve become quite good at it. From the least to the most energetic, the lights that surround us are: radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma rays.
haven’t we all heard someone, somewhere, at a dinner party or elsewhere, trot out the line that the stars we see in the sky are actually all dead? Is it true? Are all the stars dead? Well no, they’re not. Not all of them, at least.
In outer space, its unrivalled speed reaches a staggering 186,282.397 miles per second. Light can travel about twenty-six times around the Earth while you read this sentence. It is fast, the fastest thing there is, but amazingly slow considering the intergalactic distances involved out there.
As was announced on February 11, 2016, ripples of the very fabric of our universe have been detected, measured, and analyzed. These ripples are not made of light. As you will soon see, they are made of space and time, which they stretch and compress as they flow through everything at the speed of light. These very special wave detectors are a new window for us to probe our reality through: We are now able to detect what cannot be seen using light.
And all the light-gathering telescopes mankind has used so far tell us that all the stars in the universe are made of the same stuff as the Sun, and as the Earth, and as ourselves. All the cosmic objects of the night sky are made out of the same atoms as we are. If it were not the case, our telescopes would tell us so. The laws governing nature can thus be presumed to be the same everywhere. This is why the first cosmological principle is considered correct by everyone.
To be clear, it is not the galaxies that are moving away. It is the distance that separates us from the already faraway galaxies that is growing. It is the very emptiness of space that is being stretched. Scientists have given that phenomenon a name. They call it the expansion of the universe.
The distances between faraway galaxies do stretch all the time. It doesn’t apply to close-by objects, though, because gravity is locally stronger. Galaxies create a gravitational attraction that cancels out such expansion, both within their boundaries (the distance between the Sun and nearby stars is not expanding) and around (neighbouring galaxies are actually getting closer and closer, all the time). Over large distances, however, expansion rules.
as long as it is denser than air, any object dropped on Earth will fall, just as Newton (and everyone sane) has thought since for ever.
To make it to space from the surface of Earth, an object has to be thrown vertically at more than 25,000 miles per hour. Shoot it slower and it will fall back down.3 Always.
Everything creates a slope in every direction, an invisible slope that we call gravity, and the denser the object that creates it, the steeper the slope.
In truth, whatever gravity you are feeling right now is actually the sum of all the slopes created by everything that surrounds you, including the walls and the ceiling and even a bird or a plane that might fly high above your head. But everything that is below you right now is far more significant than anything that is above. The Earth beneath your feet contains more matter and stored energy than the sky above your head. So it creates the steeper slope. Hence you are inclined to slide down that one first, and feel it most. That’s Earth’s gravity.
the fabric of our universe (the mixture of space and time we just called spacetime) is not a band, nor is it flat. It is everywhere. A planet or a star, in outer space, is therefore better represented by a ball not lying on a flat surface, but immersed in an ocean filling the whole universe. No surface above, no ground below. Just water, everywhere.
Any theory that asserts that everything our visible universe contains once had zero (or very close to zero) size at some stage in its past is called a (hot) Big Bang theory.
The visible universe that we can see from Earth is a sphere 13.8 billion light years in radius. But that doesn’t mean there is no beyond. Someone else, on another planet, would be surrounded by his or her own cosmic horizon, which also would be 13.8 billion light years in radius, because there is no reason for the universe to be younger or older there compared to here.
By now, you should be used to the fact that when you gather the light coming from stars shining billions of light years away, you are seeing what they looked like many billions of years ago. You are watching the past. You can therefore check if the universe was smaller back then, or look for hints to this effect in the way light reaches you.
Penzias and Wilson received the Nobel Prize in Physics in 1978. They had discovered the heat that remains from the furnace our universe was a long time ago, the heat that radiates from the surface of last scattering, the surface that marks the end of the visible universe.2 This radiation, one of the (hot) Big Bang’s smoking guns, is called the cosmic microwave background.
Gravity, as Einstein told us, is the result of a bending of spacetime. It is not a force.
Speed changes everything. Even space and time. A clock that is moving through space at a very fast speed does not tick at the same rate as a slow-moving watch gently attached to your wrist as you stroll on a tropical beach. The idea of a universal time – a godlike clock that could somehow sit outside our universe and measure, in one go, the movement of everything in it, how its evolution unfolds, how old it is and all that – does not exist.
This theory is called the theory of special relativity. * * * Let us imagine twins. Two of them, since they usually come in pairs. A couple of years after Einstein’s publication, French physicist Paul Langevin calculated, using special relativity, that if one of the twins was sent in a rocket for a six-month round trip away from Earth at 99.995 per cent of the speed of light, the one who stayed on Earth would have to wait fifty years to see his sibling come back. So according to Einstein, six months lived by the one who left in the rocket ship should equate to fifty years for the one who stayed on Earth, and for the whole of humanity too:
When you heat up a metal bar, it expands and becomes longer. It is said to dilate. If you aim the heat carefully, it is possible to have only the bar dilate, rather than, say, the anvil it sits on – that is, its surroundings are unaffected. According to Einstein’s special relativity, a similar phenomenon happens with time. With a rocket shooting at 99.995 per cent the speed of light or a plane flying at 99.999,999,999 per cent of it, it is the rocket and the plane and everything they contain that move fast. Not their surroundings. So it is their times, and their times only, that are affected by their extreme speed relative to the world around them.
But Einstein’s special relativity also suggested something even harder to accept: it said that as your time undergoes a dilation, the very length of things should contract
Once their two planes had landed back at the airport, Hafele and Keating found that their three atomic clocks were not synchronized any more. The eastward-flying plane’s clock was 59 billionths of a second late compared to the one that stayed in the airport. The westward one was 273 billionths of a second ahead. It would have taken more than 300 million years for such a mismatch to happen naturally, had the three clocks remained next to each other.
Within our universe, clock-ticking rates depend on who is looking at them: it depends upon where you are and what is next to you (that’s the gravity bit) and on your speed.
In this universe of ours, time and distances are not universal concepts. They depend on the observer, on who is experiencing them and on who is looking at them. They are both relative.
There are clocks up there within the satellites that communicate with your GPS device to localize it. If no correction was made to take into account the time-ticking difference between the ground and the satellite, your position would quickly come out wrong. It would drift by about 6 miles per day. GPS would be useless. It is thanks to Einstein’s special and general theories of relativity that GPS works.
You experienced it yourself: everything that is massive becomes more massive when accelerated too much. To reach the speed of light, one must therefore have no mass to begin with. Still, what would happen if you were able to transform yourself into a massless being of some sort? How, then, would your time flow? As shocking as it may sound, the answer is that it would not flow. The ticking of any clock you had with you (also turned massless) would just stop. At the speed of light, time freezes. Completely. And that is the reason why the light that has travelled throughout the universe to reach us today is exactly the same as it was when it was emitted.
Fast-forwarding your present to quickly reach another person’s future is theoretically possible (and may even also be in practice some day),2 but living longer by travelling fast isn’t.
Einstein found out, though, about a century ago. Gravity is not a force, he told us, but a fall. A fall down spacetime curves.