Newton never mentioned an apple falling until 40 years after his discovery of the law of gravity<\/strong><\/h3>The falling apple is now a symbol of Newton’s ideas about gravity. Credit: Andreas Coerper Mainz \/ Getty Images<\/figcaption><\/figure>Everyone knows the story of Newton and the falling apple. His key insight was that the apple and the Moon both fall under gravity (the Moon, because of its sideways velocity, never gets any closer).<\/p>
By comparing their rates of fall, he deduced the inverse-square law \u2013 that the force between bodies is four times as weak when they are twice as far apart, and so on.<\/p>
But the first time Newton told the story of the apple was to his biographer, William Stukeley, four decades after discovering the law of gravity.<\/p>
Perhaps, with his creative days behind him, he was concocting his own legend.<\/p>
Certainly, the story depicts Newton as the lone genius struck by a flash of inspiration.<\/p>
Gravity keeps your feet on the ground yet no such \u2018force\u2019 actually exists<\/strong><\/h3>![\"Gravity](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/02\/GettyImages-1407222973-1024x683.jpg?fit=800%2C534\")
Gravity keeps your feet on the ground, but it’s not a force that pulls you down to Earth. Credit: Catherine McQueen<\/figcaption><\/figure>Although Newton imagined the force of gravity between the Sun and Earth as like a piece of invisible elastic connecting the bodies, Einstein discovered the truth is different.<\/p>
A mass like the Sun warps spacetime<\/a> around it, creating a valley.<\/p>We can\u2019t see the valley because spacetime is a four-dimensional thing and we are three-dimensional beings.<\/p>
But Earth circles around the valley like a ball around a roulette wheel. It experiences no \u2018force\u2019 but travels along the shortest path through warped spacetime.<\/p>
Similarly, there is a tendency for you to fall to the bottom of the valley of spacetime around Earth.<\/p>
The surface obstructs, however. In pushing back, it creates the \u2018feel\u2019 of gravity.<\/p>
Gravity on the International Space Station is about 90% of gravity on the ground<\/strong><\/h3>![\"Astronaut](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/06\/04-Scott-Kelly-fruit-982caf2.jpg\" "\\"\\"\/")
Astronaut Scott Kelly on the International Space Station, where gravity is about 90% that of gravity on Earth. Credit: NASA<\/figcaption><\/figure>Orbiting at an altitude of 400km above Earth, the International Space Station<\/a> (ISS) is so close to our planet that gravity barely differs from what is experienced on the ground.<\/p>Astronauts are weightless for the same reason as someone unfortunate enough to be in a lift after its cable has snapped \u2013 because, as fast as they fall towards the lift floor, the lift floor falls away from them.<\/p>
It is not obvious that the ISS is falling. That is because it has sideways motion.<\/p>
This means that, as fast as gravity curves its path down towards the Earth\u2019s surface, the Earth\u2019s surface curves away from it.<\/p>
ISS therefore falls forever in a circle, never reaching the ground.<\/p>
When the tide at sea rises, the water in wells falls, and vice versa<\/strong><\/h3>![\"When](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/02\/GettyImages-692276418-1024x820.jpg?fit=800%2C641\")
When the tide at sea rises, the water in wells falls, and vice versa. Credit: Arsenio Marrero \/ Getty<\/figcaption><\/figure>This is something known since around 100 BC and explained only in 1940.<\/p>
The Greek philosopher Poseidonios, who lived between 135 and 51 BC, discovered this effect.<\/p>
He noticed that, when a spring at the temple of Heracleium in modern-day Cadiz in Spain was low, the tide in the nearby Atlantic was high, and vice versa.<\/p>
The explanation is that the gravity of the Moon, the principal cause of the tides, not only makes the ocean bulge upwards but it does so to the rock as well.<\/p>
The waterlogged rock in which a well sits is like a wet sponge.<\/p>
It sucks water out of the well when the sponge is stretched upwards (high tide) and squeezes water back into the well when the sponge is released (low tide).<\/p>
Everyone thinks gravity sucks, but in most of the Universe it blows<\/strong><\/h2>![\"Dark](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/11\/GettyImages-1427997799-1024x576.jpg?fit=800%2C450\")
Dark energy is accelerating the expansion of the Universe. Credit: Ploystock \/ Getty<\/figcaption><\/figure>Children at school are still taught that gravity is a force of attraction.<\/p>
However, in 1998, astronomers discovered to their immense surprise that the expansion of the Universe<\/a> is speeding up, and they have attributed this to the repulsive gravity of mysterious \u2018dark energy\u2019, invisible stuff filling all of space.<\/p>Dark energy<\/a> accounts for more than two-thirds of the mass-energy of the Universe.<\/p>Nobody knows what it is. In fact our very best theory (quantum theory) predicts an energy for the vacuum of space (dark energy) \u2013 that is one followed by 120 zeroes bigger than what is observed.<\/p>
This is the biggest discrepancy between a prediction and an observation in the history of science. Something is wrong!<\/p>
Squeezed and stretched by gravity, the body in the Solar System that generates heat most efficiently is Jupiter\u2019s moon Io<\/strong><\/h3>![\"Jupiter's](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/07\/io-eruption-262f4b7-e1663228053822.jpg\")
Jupiter’s moon Io is the most active volcanic body in the Solar System. Credit: NASA\/JPL\/USGS<\/figcaption><\/figure>On 8 March 1979, NASA\u2019s Voyager 1 space probe, having streaked through the Jupiter system faster than a speeding bullet, turned its camera back the way it had come.<\/p>
Navigation engineer Linda Morabito<\/a> became the first person to see, silhouetted against the black of space, the super-volcanoes (strictly speaking, geysers) of the moon Io<\/a>.<\/p>The most geologically active body in the Solar System is heated by nearby moons Europa and Ganymede, and Jupiter.<\/p>
The moons tug its orbit into an elongated ellipse. As it moves close to and far away from Jupiter, the gravitational stretching effect of the giant planet changes.<\/p>
Io is heated for exactly the reason a squash ball gets hot when squeezed repeatedly in your hand.<\/p>
If gravity had not \u2018switched on\u2019 after the Big Bang, time might not flow forwards<\/strong><\/h2>![\"If](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/05\/GettyImages-1395723783-90efb71.jpg\")
Credit: Tetra Images \/ Getty<\/figcaption><\/figure>The direction of time is associated with order changing to disorder \u2013 the direction in which castles crumble, eggs break and so on.<\/p>
Ultimately, therefore, the \u2018arrow of time\u2019 must be due to the Big Bang being in a highly ordered state, which is very unlikely.<\/p>
Actually, early on, when matter from the Big Bang fireball was smeared smoothly across space, the Universe was disordered.<\/p>
However, when the Universe was 380,000 years old, gravity was able to gather matter together for the first time.<\/p>
In a gravitating system, clumped is the most likely state.<\/p>
The smeared-out Universe was therefore suddenly thrown into a very unlikely state \u2013 precisely the requirement to create our forward direction of time.<\/p>
A gravitational wave detected on 15 September 2015 came from merging black holes that emitted 50 times the power of all the stars in the Universe<\/strong><\/h3>![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2022\/03\/black-holes-colliding-gravitational-waves-7f7c970.jpg\")
The collision of two black holes produces ripples in spacetime known as ‘gravitational waves’. Credit: Mark Garlick \/ Science Photo Library \/ Getty Images<\/figcaption><\/figure>In a galaxy far, far away, at a time when Earth boasted nothing more complex than a bacterium, two monster black holes were locked in a death spiral.<\/p>
One was 29 times the mass of the Sun, the other 36 times the mass of the Sun.<\/p>
Each travelling at half the speed of light, they whirled about each other one last time.<\/p>
As they coalesced, three whole solar masses were destroyed and converted into gravitational waves<\/a>.<\/p>Bear in mind that an atomic bomb converts only about a kilogram of mass-energy into other forms.<\/p>
Had the tsunami of tortured space-time that surged outwards been in the form of light, it would have shone tens of times brighter than a Universe of stars.<\/p>
Without Einstein\u2019s theories your mobile phone could not pinpoint your location<\/strong><\/h2>![\"Without](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/02\/GettyImages-1306359673-1024x683.jpg?fit=800%2C534\")
Without Einstein\u2019s theories your mobile phone could not pinpoint your location. Credit: Oscar Wong \/ Getty Images<\/figcaption><\/figure>
Everyone knows the story of Newton and the falling apple. His key insight was that the apple and the Moon both fall under gravity (the Moon, because of its sideways velocity, never gets any closer).<\/p>
By comparing their rates of fall, he deduced the inverse-square law \u2013 that the force between bodies is four times as weak when they are twice as far apart, and so on.<\/p>
But the first time Newton told the story of the apple was to his biographer, William Stukeley, four decades after discovering the law of gravity.<\/p>
Perhaps, with his creative days behind him, he was concocting his own legend.<\/p>
Certainly, the story depicts Newton as the lone genius struck by a flash of inspiration.<\/p>
Gravity keeps your feet on the ground yet no such \u2018force\u2019 actually exists<\/strong><\/h3>Gravity keeps your feet on the ground, but it’s not a force that pulls you down to Earth. Credit: Catherine McQueen<\/figcaption><\/figure>Although Newton imagined the force of gravity between the Sun and Earth as like a piece of invisible elastic connecting the bodies, Einstein discovered the truth is different.<\/p>
A mass like the Sun warps spacetime<\/a> around it, creating a valley.<\/p>We can\u2019t see the valley because spacetime is a four-dimensional thing and we are three-dimensional beings.<\/p>
But Earth circles around the valley like a ball around a roulette wheel. It experiences no \u2018force\u2019 but travels along the shortest path through warped spacetime.<\/p>
Similarly, there is a tendency for you to fall to the bottom of the valley of spacetime around Earth.<\/p>
The surface obstructs, however. In pushing back, it creates the \u2018feel\u2019 of gravity.<\/p>
Gravity on the International Space Station is about 90% of gravity on the ground<\/strong><\/h3>Astronaut Scott Kelly on the International Space Station, where gravity is about 90% that of gravity on Earth. Credit: NASA<\/figcaption><\/figure>Orbiting at an altitude of 400km above Earth, the International Space Station<\/a> (ISS) is so close to our planet that gravity barely differs from what is experienced on the ground.<\/p>Astronauts are weightless for the same reason as someone unfortunate enough to be in a lift after its cable has snapped \u2013 because, as fast as they fall towards the lift floor, the lift floor falls away from them.<\/p>
It is not obvious that the ISS is falling. That is because it has sideways motion.<\/p>
This means that, as fast as gravity curves its path down towards the Earth\u2019s surface, the Earth\u2019s surface curves away from it.<\/p>
ISS therefore falls forever in a circle, never reaching the ground.<\/p>
When the tide at sea rises, the water in wells falls, and vice versa<\/strong><\/h3>When the tide at sea rises, the water in wells falls, and vice versa. Credit: Arsenio Marrero \/ Getty<\/figcaption><\/figure>This is something known since around 100 BC and explained only in 1940.<\/p>
The Greek philosopher Poseidonios, who lived between 135 and 51 BC, discovered this effect.<\/p>
He noticed that, when a spring at the temple of Heracleium in modern-day Cadiz in Spain was low, the tide in the nearby Atlantic was high, and vice versa.<\/p>
The explanation is that the gravity of the Moon, the principal cause of the tides, not only makes the ocean bulge upwards but it does so to the rock as well.<\/p>
The waterlogged rock in which a well sits is like a wet sponge.<\/p>
It sucks water out of the well when the sponge is stretched upwards (high tide) and squeezes water back into the well when the sponge is released (low tide).<\/p>
Everyone thinks gravity sucks, but in most of the Universe it blows<\/strong><\/h2>Dark energy is accelerating the expansion of the Universe. Credit: Ploystock \/ Getty<\/figcaption><\/figure>Children at school are still taught that gravity is a force of attraction.<\/p>
However, in 1998, astronomers discovered to their immense surprise that the expansion of the Universe<\/a> is speeding up, and they have attributed this to the repulsive gravity of mysterious \u2018dark energy\u2019, invisible stuff filling all of space.<\/p>Dark energy<\/a> accounts for more than two-thirds of the mass-energy of the Universe.<\/p>Nobody knows what it is. In fact our very best theory (quantum theory) predicts an energy for the vacuum of space (dark energy) \u2013 that is one followed by 120 zeroes bigger than what is observed.<\/p>
This is the biggest discrepancy between a prediction and an observation in the history of science. Something is wrong!<\/p>
Squeezed and stretched by gravity, the body in the Solar System that generates heat most efficiently is Jupiter\u2019s moon Io<\/strong><\/h3>Jupiter’s moon Io is the most active volcanic body in the Solar System. Credit: NASA\/JPL\/USGS<\/figcaption><\/figure>On 8 March 1979, NASA\u2019s Voyager 1 space probe, having streaked through the Jupiter system faster than a speeding bullet, turned its camera back the way it had come.<\/p>
Navigation engineer Linda Morabito<\/a> became the first person to see, silhouetted against the black of space, the super-volcanoes (strictly speaking, geysers) of the moon Io<\/a>.<\/p>The most geologically active body in the Solar System is heated by nearby moons Europa and Ganymede, and Jupiter.<\/p>
The moons tug its orbit into an elongated ellipse. As it moves close to and far away from Jupiter, the gravitational stretching effect of the giant planet changes.<\/p>
Io is heated for exactly the reason a squash ball gets hot when squeezed repeatedly in your hand.<\/p>
If gravity had not \u2018switched on\u2019 after the Big Bang, time might not flow forwards<\/strong><\/h2>Credit: Tetra Images \/ Getty<\/figcaption><\/figure>The direction of time is associated with order changing to disorder \u2013 the direction in which castles crumble, eggs break and so on.<\/p>
Ultimately, therefore, the \u2018arrow of time\u2019 must be due to the Big Bang being in a highly ordered state, which is very unlikely.<\/p>
Actually, early on, when matter from the Big Bang fireball was smeared smoothly across space, the Universe was disordered.<\/p>
However, when the Universe was 380,000 years old, gravity was able to gather matter together for the first time.<\/p>
In a gravitating system, clumped is the most likely state.<\/p>
The smeared-out Universe was therefore suddenly thrown into a very unlikely state \u2013 precisely the requirement to create our forward direction of time.<\/p>
A gravitational wave detected on 15 September 2015 came from merging black holes that emitted 50 times the power of all the stars in the Universe<\/strong><\/h3>The collision of two black holes produces ripples in spacetime known as ‘gravitational waves’. Credit: Mark Garlick \/ Science Photo Library \/ Getty Images<\/figcaption><\/figure>In a galaxy far, far away, at a time when Earth boasted nothing more complex than a bacterium, two monster black holes were locked in a death spiral.<\/p>
One was 29 times the mass of the Sun, the other 36 times the mass of the Sun.<\/p>
Each travelling at half the speed of light, they whirled about each other one last time.<\/p>
As they coalesced, three whole solar masses were destroyed and converted into gravitational waves<\/a>.<\/p>Bear in mind that an atomic bomb converts only about a kilogram of mass-energy into other forms.<\/p>
Had the tsunami of tortured space-time that surged outwards been in the form of light, it would have shone tens of times brighter than a Universe of stars.<\/p>
Without Einstein\u2019s theories your mobile phone could not pinpoint your location<\/strong><\/h2>Without Einstein\u2019s theories your mobile phone could not pinpoint your location. Credit: Oscar Wong \/ Getty Images<\/figcaption><\/figure>
Although Newton imagined the force of gravity between the Sun and Earth as like a piece of invisible elastic connecting the bodies, Einstein discovered the truth is different.<\/p>
A mass like the Sun warps spacetime<\/a> around it, creating a valley.<\/p> We can\u2019t see the valley because spacetime is a four-dimensional thing and we are three-dimensional beings.<\/p> But Earth circles around the valley like a ball around a roulette wheel. It experiences no \u2018force\u2019 but travels along the shortest path through warped spacetime.<\/p> Similarly, there is a tendency for you to fall to the bottom of the valley of spacetime around Earth.<\/p> The surface obstructs, however. In pushing back, it creates the \u2018feel\u2019 of gravity.<\/p> Orbiting at an altitude of 400km above Earth, the International Space Station<\/a> (ISS) is so close to our planet that gravity barely differs from what is experienced on the ground.<\/p> Astronauts are weightless for the same reason as someone unfortunate enough to be in a lift after its cable has snapped \u2013 because, as fast as they fall towards the lift floor, the lift floor falls away from them.<\/p> It is not obvious that the ISS is falling. That is because it has sideways motion.<\/p> This means that, as fast as gravity curves its path down towards the Earth\u2019s surface, the Earth\u2019s surface curves away from it.<\/p> ISS therefore falls forever in a circle, never reaching the ground.<\/p> This is something known since around 100 BC and explained only in 1940.<\/p> The Greek philosopher Poseidonios, who lived between 135 and 51 BC, discovered this effect.<\/p> He noticed that, when a spring at the temple of Heracleium in modern-day Cadiz in Spain was low, the tide in the nearby Atlantic was high, and vice versa.<\/p> The explanation is that the gravity of the Moon, the principal cause of the tides, not only makes the ocean bulge upwards but it does so to the rock as well.<\/p> The waterlogged rock in which a well sits is like a wet sponge.<\/p> It sucks water out of the well when the sponge is stretched upwards (high tide) and squeezes water back into the well when the sponge is released (low tide).<\/p> Children at school are still taught that gravity is a force of attraction.<\/p> However, in 1998, astronomers discovered to their immense surprise that the expansion of the Universe<\/a> is speeding up, and they have attributed this to the repulsive gravity of mysterious \u2018dark energy\u2019, invisible stuff filling all of space.<\/p> Dark energy<\/a> accounts for more than two-thirds of the mass-energy of the Universe.<\/p> Nobody knows what it is. In fact our very best theory (quantum theory) predicts an energy for the vacuum of space (dark energy) \u2013 that is one followed by 120 zeroes bigger than what is observed.<\/p> This is the biggest discrepancy between a prediction and an observation in the history of science. Something is wrong!<\/p> On 8 March 1979, NASA\u2019s Voyager 1 space probe, having streaked through the Jupiter system faster than a speeding bullet, turned its camera back the way it had come.<\/p> Navigation engineer Linda Morabito<\/a> became the first person to see, silhouetted against the black of space, the super-volcanoes (strictly speaking, geysers) of the moon Io<\/a>.<\/p> The most geologically active body in the Solar System is heated by nearby moons Europa and Ganymede, and Jupiter.<\/p> The moons tug its orbit into an elongated ellipse. As it moves close to and far away from Jupiter, the gravitational stretching effect of the giant planet changes.<\/p> Io is heated for exactly the reason a squash ball gets hot when squeezed repeatedly in your hand.<\/p> The direction of time is associated with order changing to disorder \u2013 the direction in which castles crumble, eggs break and so on.<\/p> Ultimately, therefore, the \u2018arrow of time\u2019 must be due to the Big Bang being in a highly ordered state, which is very unlikely.<\/p> Actually, early on, when matter from the Big Bang fireball was smeared smoothly across space, the Universe was disordered.<\/p> However, when the Universe was 380,000 years old, gravity was able to gather matter together for the first time.<\/p> In a gravitating system, clumped is the most likely state.<\/p> The smeared-out Universe was therefore suddenly thrown into a very unlikely state \u2013 precisely the requirement to create our forward direction of time.<\/p> In a galaxy far, far away, at a time when Earth boasted nothing more complex than a bacterium, two monster black holes were locked in a death spiral.<\/p> One was 29 times the mass of the Sun, the other 36 times the mass of the Sun.<\/p> Each travelling at half the speed of light, they whirled about each other one last time.<\/p> As they coalesced, three whole solar masses were destroyed and converted into gravitational waves<\/a>.<\/p> Bear in mind that an atomic bomb converts only about a kilogram of mass-energy into other forms.<\/p> Had the tsunami of tortured space-time that surged outwards been in the form of light, it would have shone tens of times brighter than a Universe of stars.<\/p>Gravity on the International Space Station is about 90% of gravity on the ground<\/strong><\/h3>
When the tide at sea rises, the water in wells falls, and vice versa<\/strong><\/h3>
Everyone thinks gravity sucks, but in most of the Universe it blows<\/strong><\/h2>
Squeezed and stretched by gravity, the body in the Solar System that generates heat most efficiently is Jupiter\u2019s moon Io<\/strong><\/h3>
If gravity had not \u2018switched on\u2019 after the Big Bang, time might not flow forwards<\/strong><\/h2>
A gravitational wave detected on 15 September 2015 came from merging black holes that emitted 50 times the power of all the stars in the Universe<\/strong><\/h3>
Without Einstein\u2019s theories your mobile phone could not pinpoint your location<\/strong><\/h2>
![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2017\/06\/CERN_LHC_Tunnel1-152c4d0.jpg\")
![\"You](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/02\/GettyImages-1329284965-1024x691.jpg?fit=800%2C540\")
![\"mysteries](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2022\/04\/mysteries-universe-4e3dc50.jpg\")