#### CEA News, January 2020

# A limit on Planck-scale froth with ESPRESSO

##
Models of quantum gravity predict that the very structure of
spacetime on length scales comparable to the Planck length,
l_{P} = 1.6 x 10^{-35} m, is `frothy' or `foamy' - spacetime should
contain `bubbles' of uncertainty. The basic idea is that
distance measurements come with some intrinsic uncertainty, and
this uncertainty becomes magnified over very large distances.
Consider a laser that is very far away, and emits light of
precisely a single wavelength (i.e. colour). As this light
travels through spacetime, it accumulates uncertainty, and the
laser light would exhibit a larger spread in energy as it
travels further.

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Astronomers from the CEA have used ESPRESSO* Science Verification
observations to provide a novel limit on the existence of
spacetime foam. ESPRESSO is a spectrograph on the European
Southern Observatory's Very Large Telescope at Cerro Paranal in
Chile. This study was made possible because ESPRESSO is a very
well-calibrated and stable instrument, allowing this team to
directly measure the energy width of an intrinsically narrow
Fe II absorption line. The width of this line places an upper
limit on the amount of spacetime foam-induced broadening. This
team concludes, if photons take discrete steps through spacetime,
and accumulate a Planck-scale uncertainty at every step due to
quantum foam, then the ESPRESSO observations require that the
photon step size must be at least 10^{13.2} l_{P}. They have also
identified targets that will allow us to improve this limit of
quantum foam by more than an order of magnitude with future
ESPRESSO observations.

* ESPRESSO stands for Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations.