Is spacetime quantum, or should we 'gravitize the quantum'? PBS SpaceTime explores experiments that could test these two alternatives: youtube.com/watch?v=eAtb...youtube.com/watch?v=yfzo....

So classically, higher derivative actions are fine, but in quantum theory, higher derivatives in the action spell trouble (Ostragradsky's instability which implies negative norm "ghosts"). The preprint is available at arxiv.org/abs/2402.17844. 5/5

Renormalisation of postquantum-classical gravity

One of the obstacles to reconciling quantum theory with general relativity, is constructing a theory which is both consistent with observation, and and gives finite answers at high energy, so that...

Here is the path integral for Brownian motion straight out of Feynman and Hibbs. It has higher derivatives, but just describes a free particle being hit by random forces. On average the acceleration (q double dot) is zero, but there are contributions away from this. 4/

The PQCG action has higher derivative terms (hence it is renormalisable) but because it is computing a probability and not an amplitude, it doesn't suffer from negative norm ghosts or tachyons (which is what plagues higher derivative theories of quantum gravity). 3/

The main message is that while perturbative quantum gravity (PQCG) in 3+1 dimensions is not renormalisable (meaning the theory is not valid at high energies), if spacetime is classical, then the pure gravity path integral is formally renormalisable! 2/

The talks from the Quantum Information and Quantum Field Theory workshop @ictstifr are up! youtube.com/@ICTStalks/v...youtu.be/dE-_EF0aEK4?... . 1/

International Centre for Theoretical Sciences

The International Centre for Theoretical Sciences (ICTS) of the Tata Institute of Fundamental Research was founded in 2007. Its aim is to foster excellence in the basic sciences through its programs, ...

Thanks to Curt for inviting me once again on his show, and check out the other talks in the series at youtube.com/playlist?lis...youtube.com/watch?v=yfzo... 9/

Rethinking the Foundations - YouTube

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The alternative approach (the postquantum theory), requires the simplest modification to make GR consistent with quantum mechanics. It's also experimentally testable and the pure gravity theory is formally renormalisable, while quantum gravity in 3+1 dimensions is not. 8/

The Wheeler-deWitt eqn is a thorn in the side of quantum gravity programs. In string theory, one approach to resolving the paradoxes that stem from it, is the idea that certain measurements are too complex to perform. That seems more radical than anything I've proposed! 7/

A consequence of diff invariance is that the Hamiltonian density is zero. This is different to what we encounter in gauge theories. The quantum version of this, is the Wheeler-deWitt eqn, which implies that the wavefunction doesn't evolve. 6/