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Physicist Jeff Steinhauer and his colleagues at the Technion - Israel Institute of Technology in Haifa used an extremely cold gas called a Bose-Einstein condensate to model the event horizon of a black hole, the invisible boundary beyond which nothing can escape. In a flowing stream of this gas, they placed a cliff, creating a "waterfall" of gas; when the gas flowed over the waterfall, it turned enough potential energy into kinetic energy to flow faster than the speed of sound.

Source: Live Science

Can you see what "cliff" mean here? Dictionaries offer me little clue and when I typed "what does "cliff" mean in physics?" into Microsoft Bing and it replies with "Tectonic activity Cliffs are steep formations of rock that occur frequently in nature along coasts, riverbeds and in mountainous regions...", which does not look like a correct answer.

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    I think it's being used figuratively, as in like a cliff, a sudden drop in height, creating a "waterfall"
    – Billy Kerr
    Jun 8, 2023 at 16:29
  • This is a scale model in which a cliff was placed. How can you not get that?
    – Lambie
    Jun 8, 2023 at 18:04
  • Normal word order would be: they placed a cliff In a flowing stream of this gas,
    – Lambie
    Jun 21, 2023 at 14:24

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"Cliff" does not have a special meaning in physics. The passage is talking about creating a waterfall, which is what you get when water flows over the edge of a cliff.

In the experiment, the cliff appears to be a solid edge over which the fluid was to pour, though it is not necessarily on the scale we usually mean by a "cliff".

It is possible that the meaning is more metaphorical, and it is something else whose function is to turn the potential energy into kinetic, but I think they would have put "cliff" in quotes or otherwise explained it if they didn't mean something physically resembling a cliff.

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  • This is a physics experiment and there are not metaphors here.
    – Lambie
    Jun 8, 2023 at 18:05
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    @Lambie: perhaps I should have said "analogue" rather than "metaphor". Some branches of physics use common words in very analogical ways (eg the nomenclature for quarks). But I don't think that is the case here.
    – Colin Fine
    Jun 8, 2023 at 18:55
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    No, it is a physical barrier in the experiment.
    – Lambie
    Jun 21, 2023 at 14:24
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This is as much a physics question as a grammar question. A cliff, in the normal sense of the term, is a location where gravitational potential energy changes rapidly over a small (horizontal) distance. The researchers have inserted into their experimental setup an electrical potential which changes rapidly over a short distance. They refer to this in their scientific papers as a "waterfall" potential. In a 2014 paper in Nature Physics, they put "waterfall" in quotation marks. In a 2019 paper in Nature, the quotation marks are gone. I searched three papers from this research group for the word "cliff" and never found it; I suspect that "cliff" was chosen by the writers at Live Science.

The important point is that the cliff in these experiments is not what you would think of as a physical cliff, it is an electrical cliff. It is not something you would think of as a solid edge. They manipulate lasers in such a way that one part of apparatus is at high electrical potential energy and another part has low potential energy with a relatively sharp threshold between these two regions. The Bose-Einstein condensate is initially in the region of high potential energy. They then further manipulate the lasers so that the threshold moves across the Bose-Einstein condensate, causing the atoms to lose electrical potential energy and gain kinetic energy.

This is in no way a standard use of the word "cliff." It is the word chosen by the authors who were explaining the research to a general audience because they thought it explained the concept best. We can disagree on whether that was a good choice.

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