Mar 25, 2026 02:11 AM
(This post was last modified: Mar 25, 2026 04:58 PM by C C.)
Gravity and quantum physics are fundamentally incompatible
https://bigthink.com/starts-with-a-bang/...m-physics/
KEY POINTS: In 1915, Einstein put forth our current theory of gravity in its final form: general relativity. It’s passed every observational and experimental test it has ever faced. Quantum physics took a little longer to develop, with the Standard Model describing the particles and the other three fundamental forces in the Universe perfectly well: agreeing with all measurables. But at a fundamental level, these two descriptions of the Universe are fundamentally inconsistent. Here’s why that’s an important problem, and possibly an important clue for what’s next.
EXCERPT: This works both ways: because we don’t understand gravitation at a quantum level, that means we don’t quite understand the quantum vacuum itself. The quantum vacuum, or the properties of empty space, can be measured in various ways. The Casimir effect, for instance, lets us measure the effect of the electromagnetic interaction through empty space under a variety of setups, simply by changing the configuration of conductors. The expansion of the Universe, if we measure it over all of our cosmic history, reveals to us the cumulative contributions of all of the forces to the zero-point energy of space: the quantum vacuum.
But can we quantify the quantum contributions of gravitation to the quantum vacuum in any way?
Not a chance. We don’t understand how to calculate gravity’s behavior in a variety of realms:
at high energies,
on small scales,
near singularities,
or when quantum particles exhibit their inherently quantum, indeterminate nature.
Similarly, we don’t understand how the quantum field that underpins gravity — assuming there is one — behaves at all under any circumstances. This is why attempts to understand gravity at a more fundamental level must not be abandoned, even if every pathway we’re actively exploring now ultimately turns out to be a dead-end. We’ve actually managed to identify the key problem that needs to be solved to push physics forward beyond its current limitations: that itself is a huge achievement that should never be underestimated. The only options are to keep trying, which is what conducting science is, or to give up. Even if all of our attempts turn out to ultimately be in vain, it’s better than ensuring a failed outcome before even making an earnest, sustained attempt at success. (MORE - missing details)
Is string theory still our best hope for a theory of everything?
https://www.quantamagazine.org/are-strin...-20260323/
INTRO: Fifty-eight years after it first appeared, string theory remains the most popular candidate for the “theory of everything,” the unified mathematical framework for all matter and forces in the universe. This is much to the chagrin of its rather vocal critics. “String theory is not dead; it’s undead and now walks around like a zombie eating people’s brains,” the former physicist Sabine Hossenfelder said on her popular YouTube channel in 2024.
String theory is a “failure,” the mathematical physicist and blogger Peter Woit often says. His complaint is not that string theory is wrong — it’s that it’s “not even wrong,” as he titled a 2006 book. The theory says that, on scales of billionths of trillionths of trillionths of a centimeter, extra curled-up spatial dimensions reveal themselves and particles resolve into extended objects — strands and loops of energy — rather than points. But this alleged substructure is too small to detect, probably ever. The prediction is untestable.
A further problem is that uncountably many different configurations of dimensions and strings are permitted at those tiny scales; the theory can give rise to a limitless variety of universes. Amid this vast landscape of solutions, no one can hope to find a precise microscopic configuration that undergirds our particular macroscopic world.
These issues are profound indeed. Yet in my experience, the typical high-energy theorist in a prestigious university physics department still thinks string theory has a good chance of being correct, at least in part. The field has become siloed between those who deem it worth studying and those who don’t.
Recently, a new angle of attack has opened up. An approach called bootstrapping has allowed physicists to calculate that, under various starting assumptions about the universe, a key equation from string theory naturally follows. For some experts, these findings support the notion of “string uniqueness,” the idea that it is the only mathematically consistent quantum description of gravity and everything else.
Responding to one bootstrap paper on her YouTube channel, mere weeks after the “undead” comment, Hossenfelder said it was “string theorists do[ing] something sensible for once.” She added, “I’d say this paper strengthens the argument for string theory.”
Not everyone agrees, but the findings are reviving an important question... (MORE - details)
https://bigthink.com/starts-with-a-bang/...m-physics/
KEY POINTS: In 1915, Einstein put forth our current theory of gravity in its final form: general relativity. It’s passed every observational and experimental test it has ever faced. Quantum physics took a little longer to develop, with the Standard Model describing the particles and the other three fundamental forces in the Universe perfectly well: agreeing with all measurables. But at a fundamental level, these two descriptions of the Universe are fundamentally inconsistent. Here’s why that’s an important problem, and possibly an important clue for what’s next.
EXCERPT: This works both ways: because we don’t understand gravitation at a quantum level, that means we don’t quite understand the quantum vacuum itself. The quantum vacuum, or the properties of empty space, can be measured in various ways. The Casimir effect, for instance, lets us measure the effect of the electromagnetic interaction through empty space under a variety of setups, simply by changing the configuration of conductors. The expansion of the Universe, if we measure it over all of our cosmic history, reveals to us the cumulative contributions of all of the forces to the zero-point energy of space: the quantum vacuum.
But can we quantify the quantum contributions of gravitation to the quantum vacuum in any way?
Not a chance. We don’t understand how to calculate gravity’s behavior in a variety of realms:
at high energies,
on small scales,
near singularities,
or when quantum particles exhibit their inherently quantum, indeterminate nature.
Similarly, we don’t understand how the quantum field that underpins gravity — assuming there is one — behaves at all under any circumstances. This is why attempts to understand gravity at a more fundamental level must not be abandoned, even if every pathway we’re actively exploring now ultimately turns out to be a dead-end. We’ve actually managed to identify the key problem that needs to be solved to push physics forward beyond its current limitations: that itself is a huge achievement that should never be underestimated. The only options are to keep trying, which is what conducting science is, or to give up. Even if all of our attempts turn out to ultimately be in vain, it’s better than ensuring a failed outcome before even making an earnest, sustained attempt at success. (MORE - missing details)
Is string theory still our best hope for a theory of everything?
https://www.quantamagazine.org/are-strin...-20260323/
INTRO: Fifty-eight years after it first appeared, string theory remains the most popular candidate for the “theory of everything,” the unified mathematical framework for all matter and forces in the universe. This is much to the chagrin of its rather vocal critics. “String theory is not dead; it’s undead and now walks around like a zombie eating people’s brains,” the former physicist Sabine Hossenfelder said on her popular YouTube channel in 2024.
String theory is a “failure,” the mathematical physicist and blogger Peter Woit often says. His complaint is not that string theory is wrong — it’s that it’s “not even wrong,” as he titled a 2006 book. The theory says that, on scales of billionths of trillionths of trillionths of a centimeter, extra curled-up spatial dimensions reveal themselves and particles resolve into extended objects — strands and loops of energy — rather than points. But this alleged substructure is too small to detect, probably ever. The prediction is untestable.
A further problem is that uncountably many different configurations of dimensions and strings are permitted at those tiny scales; the theory can give rise to a limitless variety of universes. Amid this vast landscape of solutions, no one can hope to find a precise microscopic configuration that undergirds our particular macroscopic world.
These issues are profound indeed. Yet in my experience, the typical high-energy theorist in a prestigious university physics department still thinks string theory has a good chance of being correct, at least in part. The field has become siloed between those who deem it worth studying and those who don’t.
Recently, a new angle of attack has opened up. An approach called bootstrapping has allowed physicists to calculate that, under various starting assumptions about the universe, a key equation from string theory naturally follows. For some experts, these findings support the notion of “string uniqueness,” the idea that it is the only mathematically consistent quantum description of gravity and everything else.
Responding to one bootstrap paper on her YouTube channel, mere weeks after the “undead” comment, Hossenfelder said it was “string theorists do[ing] something sensible for once.” She added, “I’d say this paper strengthens the argument for string theory.”
Not everyone agrees, but the findings are reviving an important question... (MORE - details)