Our two best theories of the physics of the universe – quantum mechanics and general relativity – often fail to agree. Physicists have been trying to unite them for over a century, and now, researchers have found one place where they don’t seem to clash. Working out from this one example may open the doors for building a more general, universe-wide theory.
It all started with an intuition that at the Federal University of Goiás in Brazil had about how a quantum object accumulates disorder, or how its entropy changes. He guessed that the observer measuring an object’s entropy, and how they move through space and time, should matter.
This is already true in both quantum physics and the general theory of relativity, which is Einstein’s theory of gravity. For quantum objects, it is impossible to describe their state with absolute certainty until an observer interacts with them. In general relativity, different observers see different times on their respective clocks, depending on where in space-time they are, because space-time is curved and this affects time. Céleri and his colleagues have now added entropy into the mix and found it can be observer-dependent too.
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The researchers mathematically examined a quantum oscillator – the quantum version of a pendulum or a spring – travelling through four-dimensional space-time and interacting with an observer who is doing the same. The paths of the observer and the quantum oscillator, called their worldlines, were different: they moved through different parts of curved space-time.
If the observer measured the oscillator’s entropy twice and then calculated the change between those two numbers, the result was related to how much the experimenter’s worldline differed from the oscillator’s. Adding another observer who has moved through space-time differently to the first would produce a new number for the change in entropy, because the second observer would also have a distinct worldline.
“Suppose we’re in your office with the system and we both make measurements [of entropy], then you stay there, but I take a plane and fly around the world, then come back to your office. Then we make measurements again. We will see different things because my worldline will be different than yours,” says Céleri.
at the University of Queensland in Australia says that this result combines general relativity and an expression of the second law of thermodynamics – which says that entropy should always increase – in a new and interesting way.
The idea isn’t as surprising as it may sound, but adds to our understanding of all the ways the entropy of an object can change, says at the Max Planck Institute of Quantum Optics in Germany. It is also an example of quantum theory and general relativity not clashing – even though the two theories are incompatible at more extreme points in space-time, such as black holes, he says.
The new result is also a part of a bigger picture of how quantum objects exist in curved space-time, says at the University of Southampton in the UK. Such questions are incredibly interesting, but they may have more rigorous and far-reaching answers when applied to quantum fields, instead of an oscillator, she says. This is because fields extend everywhere in space, so any statement about how they change because of space-time curvature would add to our universe-wide understanding of when gravity and quantum physics can – or cannot – be united. The researchers are already working on extending their arguments in this direction, but such work is likely to be a lot more mathematically challenging.
at Virginia Tech says that the new work could lead to some big implications, especially if it leads to new concrete experimental tests of quantum physics. Empirically probing observer dependence of a quantum object’s properties could be a way to find a new theory capable of describing all objects in curved space-time. And that could be the “tip of the iceberg” of another theory even more fundamental than quantum theory, he says.
Physical Review Letters
