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Gravity is of course something that we can all observe. Stuff falls towards the ground. But not everything: some things like steam or smoke defy this force and instead float up.

During Ancient Greek and Roman times (when the two shared a lot of culture and scientific knowledge), how was this natural force explained? What was believed to be the cause of gravity? And why was it that some things obeyed it while steam and smoke defied it?

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    $\begingroup$ Can you clarify what you think is missing from the Wikipedia article? $\endgroup$ – sempaiscuba Nov 26 '18 at 3:42
  • $\begingroup$ @sempaiscuba I was unaware that there was a Wikipedia article on this (although I shouldn't be surprised). I guess you could say that my Google/Wikipedia-foo failed me on that. An answer that includes the relevant information from there would address my question. $\endgroup$ – Thunderforge Nov 26 '18 at 15:42
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    $\begingroup$ Thunderforge, you need to be a little cautious of Wikipedia articles. They're usually very good, but not always. $\endgroup$ – John Duffield Nov 30 '18 at 13:48
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See Aristotle's Natural Philosophy.

According to Aristotle, change in the natural world can be :

[either] in accordance with the nature of the object — in which case the change is natural (phusei) or according to nature, or can happen in the face of a contrary disposition on the part of the nature of the entity — in which case the change is forced or contrary to nature.

A mover can effect a motion which is contrary to its own nature. Aristotle’s example of such an unnatural mover is the lever, an object heavy by nature, with which loads can be lifted.

[The] large class of natural motions [is formed by] the natural motions of the elements. [...] the elements move to their natural places — the light bodies up and the heavy ones down — by an appeal to their respective natures as causes (“that it is simply their nature to move somewhere, and this is what it is to be light and to be heavy,” Physics Book VIII, 255b13–17).

See Aristotle's text :

[Phys, 255b] The activity of lightness consists in the light thing being in a certain place, namely high up: when it is in the contrary place, it is being prevented. The case is similar also in regard to quantity and quality. But, be it noted, this is the question we are trying to answer — how can we account for the motion of light things and heavy things to their proper places? The reason for it is that they have a natural tendency towards a certain position; and this is what it is to be light or heavy, the former being determined by an upward, the latter by a downward, tendency.

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  • $\begingroup$ Interestingly (at least to me :-) ), this seems as much an explanation of density and buoyancy as it does the underlying force of gravity. $\endgroup$ – Carl Witthoft Nov 28 '18 at 12:59
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The aristotelian theory affirmed that there was a natural 'affinity' between substances that were alike. Thus, a stone fell towards the ground because of a natural, telluric affinity, and fire/smoke rose to the sky because of a natural affinity, too...

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  • $\begingroup$ Hi xxavier. This post could be much improved if you would back it up with some sources: Please include them in your post (and keep doing so in the future :) ). $\endgroup$ – Danu♦ Nov 28 '18 at 8:54
  • $\begingroup$ True, thanks, but Mauro has already given good references. I wrote my answer just 'off the top'... :) $\endgroup$ – xxavier Nov 28 '18 at 9:23
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How was gravity explained in Ancient Greek and Roman times?

It wasn't.

As xxavier and Mauro say, people like Aristotle claimed that things had a natural affinity or tendency towards a certain location. That's no explanation at all. That's what's missing from the Wikipedia article sempaiscuba was referring to.

The Wikipedia article does include a section on general relativity which purports to explain gravity. However it doesn't. Note this: "Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime". He didn't, because:

Spacetime curvature is associated with the tidal force, not the force of gravity.

Spacetime models space at all times, so there's no motion through spacetime.

A concentration of energy causes spacetime curvature not just matter.

What Einstein did say is this: “As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”. Unfortunately you tend not to hear about this, which means gravity is generally not explained in modern times either.

Gravity is of course something that we can all observe. Stuff falls towards the ground. But not everything: some things like steam or smoke defy this force and instead float up.

However they don't do this in a vacuum. Note that the Wikipedia vacuum article says this: "Vacuum has been a frequent topic of philosophical debate since ancient Greek times, but was not studied empirically until the 17th century". The ancient Greeks weren't big on experiment.

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  • $\begingroup$ I was more looking for "why is this a natural phenomenon", which I think that Aristotle's explanation is. Although it is indeed an incomplete explanation, it would probably have satisfied many of his day. Still, I'll upvote your answer since it does add some useful information. $\endgroup$ – Thunderforge Nov 27 '18 at 20:48
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    $\begingroup$ So if Aristotle's theory, Einstein's theory and Newton's theory (I assume) did not explain gravity what would qualify as an explanation? Is there an example of a theory (of anything) that does explain what it describes? $\endgroup$ – Conifold Nov 27 '18 at 20:55
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    $\begingroup$ Hi John Duffield. In this post, you don't seem to be adding anything to the previously existing answers, when it comes to the explanation of gravity in Antiquity. Thus, you post is essentially non-historical in nature. History of Science and Mathematics is not the right place for such posts. Please improve your post to include some relevant historical information (not mentioned before by others), and remove the non-historical content. $\endgroup$ – Danu♦ Nov 28 '18 at 8:52
  • $\begingroup$ @Danu: what I added was a crucial point - gravity was not explained in antiquity. My further point was relevant because it concerned the Wikipedia article on the history of gravitational theory. Sempaiscuba referred to this in a comment with 5 upvotes, but he was directing the OP to an article that was factually incorrect. My last point concerned vacuum and the way the Greeks didn't experiment much. That's relevant historical information which nobody else covered. $\endgroup$ – John Duffield Nov 29 '18 at 21:41
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    $\begingroup$ In fact, your assertion that "gravity was not explained in antiquity" is nothing but your assessment of the physical claims made in some ancient sources that had already been pointed out in other answers. Thus, this is not any historical contribution. Similarly, your assessment of the wikipedia page on the history of topic X is not relevant insofar it does not directly pertain to the historical content, but rather to the physical content. I stand by my earlier comment: Please remove the non-historical parts of your post and improve it so that it adds something to the already existing posts. $\endgroup$ – Danu♦ Nov 29 '18 at 23:06

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