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There is a glaring gap in our knowledge of the physical world: none of our well-established theories describe gravity’s quantum nature. Yet physicists expect that this quantum nature is essential for explaining extreme situations such as the very early universe and the deep interior of black holes. The need to understand it is called the problem of “quantum gravity.”
The established classical concept of gravity is Einstein’s general theory of relativity. This spectacularly successful theory has correctly predicted phenomena from the bending of light and the orbit of Mercury to black holes and gravitational waves. It teaches us that the geometry of space and time—spacetime—is determined by gravity. So when we talk about the quantum behavior of gravity, we’re really talking about the quantum behavior of spacetime.
We don’t currently have an established theory of quantum gravity, but we do have some tentative theories. Among them, loop quantum gravity (which one of us, Rovelli, helped to develop) and string theory are two leading contenders. The former predicts that the fabric of spacetime is woven from a network of tiny loops, whereas the latter posits that particles are fundamentally vibrating strings.
Testing these theories is difficult because we can’t study the early universe or black hole interiors in a laboratory. Physicists have mostly assumed that experiments that could directly tell us something about quantum gravity require technology that is many years away.
This situation might be changing. Recent developments suggest it may be possible to perform laboratory experiments that will reveal something about the quantum behavior of gravity. This potential is extremely exciting, and it has raised real enthusiasm among theoretical and experimental physicists, who are actively trying to develop the means to carry out the investigations. The proposed experiments could test the predictions of quantum gravity theories and provide support for the assumptions they’re based on.
The experiments all involve events happening at low energies, where the predictions of strings, loops, and the like agree, so they aren’t going to tell us which specific theory of quantum gravity is correct. Still, experimental evidence that gravity is actually quantized would be groundbreaking.
We already have plenty of observations about gravity’s effects on the quantum behavior of matter. Albert Einstein’s theory works fine in these situations, from stellar dynamics, to the cosmological formation of galaxy clusters, all the way to laboratory experiments on the effect of Earth’s gravity on quantum systems. But in all these scenarios, gravity itself behaves in a way that is consistent with classical physics; its quantum features are irrelevant. What’s much more difficult is to observe phenomena in which we expect gravity to behave quantum mechanically.
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Mark Ross
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Aug 30, 2024 @ 09:50:13
This article is very interesting.🙂🙏
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Aug 30, 2024 @ 10:22:40
It certainly is fruit for thought. Thanks for your comment!
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Aug 30, 2024 @ 12:11:20
A topic for much reflection! Thank you very much! 🙂🙏
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