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World apple day | Dariusz Kajewski PhD, DSc

28.09.2021 - 07:00 update 28.09.2021 - 08:40
Editors: jp

28 September


Save the date with our scientists

„Save the date” is a series of articles that have been written to celebrate various unusual holidays. The authors of the presented materials are students, doctoral students and employees of the Faculty of Science and Technology of the University of Silesia.

28 September is a World Apple Day. Undoubtedly, an apple is one of our national fruits – fragrant, juicy and crunchy. Apples are associated with the end of summer holidays and beautiful autumn. It is said that it was the apple that contributed to the discovery of one of the fundamental laws governing the universe. You probably guess which law – the law of universal gravitation as defined by Sir Isaac Newton

Dariusz Kajewski, PhD, DSc
fot. Press Section


Institute of Physics of the University of Silesia

Newton is indispensably associated with the apple that was believed to fall on his head and “enlighten” him. This story, however, is very often passed down with a mistake – and it is not about the apple, because it actually did play a key role in the story. But let’s start from the beginning. It’s 1665 – four years after Newton was enrolled in Cambridge. A great plague begins in England. As in present times, universities are closed due to the pandemics. Young Newton, 22, decides to return to his family home. While sipping his tea under an apple tree, he was supposed to experience an epiphany – either by observing the falling apple or by direct contact with it. This is where it gets a little wild – most believe that Newton discovered that bodies fall down to Earth as a result of some force – the force of gravity. In fact, the story often ends here. However, even an inexperienced squirrel would be able to tell the fact that objects are falling down – not sideways, not up, but straight down. However, do you think that such a genius as Isaac Newton – the same one who, during his one-year “vacation”, worked out the calculus, split light on a prism and came to the conclusion that the color spectrum observed as a “rainbow” is not a property of the medium but the fact that white light is simply a mixture of colors (which was not so obvious as it is today), he simply stated that lifted objects must fall to Earth? It would be too banal and totally stripped of the beauty of discoveries that are always accompanied by a mental “trigger” – an impulse pushing us to combine apparently unrelated facts.

Newton discovered something definitely more important! Not only does the apple fall to the Earth, but the Earth also falls onto the apple – more precisely, the apple also acts with the force of gravity on the Earth, because the gravitational forces are mutual. What does it mean? That all bodies with mass are the source of gravitational interactions and their mutual gravity pulls them together – this is what holds together not only the entire Solar System, but the entire Universe! And thus, thanks to this conclusion, he could easily explain where the tides of the sea come from, he could describe mathematically the motion of planets and comets and even calculate their orbits (which Kepler had only described). He literally “dotted the i” in the revolution initiated by Copernicus. Suddenly it turned out that there is no such a direction as up or down! Objects stopped falling “down” and began to move in direction of the other body.

The little apple was therefore a “trigger” for the mind of a great scientist on a compulsory “vacation”. As you can see, sometimes you don’t need much to start an avalanche of thoughts leading to great discoveries that can literally shake the Universe

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