# Can we test QFT on a curved spacetime?

It is possible to extend a quantum field theory to a curved spacetime. But does this lead to predictions that can be tested and measured? Had it been confirmed?

The underlying reason I am asking this is: curved spacetime means emergence of gravity and therefore General Relativity regime. And we know that GR and QFT are incompatible. I realise that in order to include gravity, one should put its Lagrangian in from the very beginning and this, I guess, does not work. But does the current mathematical framework for extending the known field theories to a curved spacetime work?

• We are always testing QFT on curved spacetime, even experimentally. There is no such thing as completely flat space. More importantly, QFT works just fine inside neutron stars, even in a rather strongly curved regime, so it has to be compatible. Just because there is strong gravity doesn't mean that nature turns quantum mechanics off. Does our math work well for these case? I don't think so. – CuriousOne Jan 5 '15 at 11:26
• I meant does QFT on a curved spacetime make predictions that can be actually observed / measured? – SuperCiocia Jan 5 '15 at 16:17
• I got that. But you see the problem... it's with the math, not with the world. The way I see it is that with gravity coordinates bend, with gravity and QFT they break... and nobody seems to know how to pick up the pieces. – CuriousOne Jan 5 '15 at 16:34
• This is exactly how Stephen Hawking came up wiht the prediction of his famous Hawking radiation. Robert Wald did a lot of famous work in the field. – Jerry Schirmer Jan 5 '15 at 23:51

The biggest prediction of QFT on curved (not dynamical!) space-time is the Hawking radiation . This radiation can in principle be measured experimentally, even though it's an effect so small that with current technology there's probably no hope of a measurement. It's still possible that with some clever way of maximizing the experimental signal we could achieve such goal (for instance with something like the measurement of the proton life time, in which there's no hope to follow a single proton for $10^{33} yr$, but it's "easy" to do that with $10^{33}$ protons.)

Moreover, in the solar system gravity is weak, the space is only slightly curved. Therefore, while QFT on flat space-time can be routinely tested in a laboratory on earth, for a significant curved space-time effect you have to usually look at astrophysical or cosmological experiments, with all the related uncertainties.

The math: Is known that a theory with interacting gravitons (spin 2 massless particles) is not renormalizable. QFT with gravitons as an effective field theory could work, in the sense that you can make predictions like in the Fermi's theory of weak interactions. For instance see the beautiful treatment of Schwartz (Quantum Field Theory and The Standard Model, p.404) in which he finds the quantum gravity predictions to the Mercury's perihelion shift.

Cosmology and inflation provide a hugely important test of quantum field theory in curved spacetime. During inflation, there is a scalar field (the inflaton) that is providing the vacuum energy that drives inflation. This scalar field is subject to the rules of quantum field theory in curved spacetime. The quantum fluctuation of this scalar field lead to the temperature fluctuations we see in the Cosmic Microwave Background today, and as these fluctuations grow due to gravitational collapse, they produce the structure we see today in galaxies and clusters. Measuring properties of the CMB and large scale structure is thus a test of quantum field theory in curved spacetime from the inflationary epoch.