Les électrons en chute libre rêvent-ils de gravité quantique? (3)
L'homme ne pourra (vraisemblablement) jamais détecter un graviton unique
Voici une conférence de Freeman Dyson qui porte précisément sur la possibilité de détecter un graviton, autrement dit un quantum de champ gravitationnel (et non sur la question de savoir s'il existe une théorique quantique renormalisable de la gravitation). Il y expose précisément les points suivant selon ses propres mots:
I can prove that detectors with the LIGO design, detecting gravitational waves by measuring their effects on the distance between two mirrors, cannot detect single gravitons. To reduce quantum ﬂuctuations in the measurement of distance, the mirrors must be heavy. To make the quantum noise small enough to observe the signal from a single graviton, the mirrors must be so heavy that they collapse together into a black hole. The laws of general relativity and quantum mechanics conspire to make the measurement impossible.
La conjecture pessimiste de Dyson sur l'impossibilité de détecter un graviton est plus ancienne puisqu'elle remonte à 2004 au moins. Pour saisir la subtilité des problèmes physiques impliqués on peut lire cet article de Tony Rothman et Stephen Boughn qui confirme les idées de Dyson :
... we can say that to detect a single graviton was a priori going to be a difficult proposition, but it was not obvious that it was fundamentally impossible. Although, as we stated at the outset, we have found no basic principle ruling out graviton detection, reasonable physics appears to do so. Perhaps the most interesting aspect of the investigation is that it leads to some fairly subtle physics, which ... has caused signiﬁcant confusion in the literature. Certainly, if a “no graviton” law appears elusive, we do feel entitled to predict that no one will ever detect one in our universe.
Tony Rothman et Stephen Boughn, Can Gravtion Be Detected ? 2006
On peut aussi découvrir l'argumentation technique de Dyson dans le texte, par exemple dans un compte-rendu de 2012 qui souligne bien les enjeux de sa problématique :
One hypothesis is that gravity is a quantum field and gravitons exist. A second hypothesis is that the gravitational field is a statistical concept like entropy or temperature, only de fined for gravitational effects of matter in bulk and not for effects of individual elementary particles. If the second hypothesis is true, then the gravitational field is not a local field like the electromagnetic field. The second hypothesis implies that the gravitational field at a point in space-time does not exist, either as a classical or as a quantum field.
Now I assert that both of the two hypotheses may or may not be experimentally testable. Analysis of the properties of graviton-detectors, following the methods of this paper, might be able to throw light on both hypotheses. Three outcomes are logically possible. If a graviton detector is possible and succeeds in detecting gravitons, then the rst hypothesis is true and the second is false. If a graviton detector is possible and fails to detect gravitons, then the rst hypothesis is false and the second is open. If a graviton detector is in principle impossible, then both hypotheses remain open. Even if their existence is not experimentally testable, gravitons may still exist.
Freeman Dyson, Is a Graviton Detectable? 06/08/2012
Heureusement, dans un lointain passé, la nature a (peut-être) fabriqué un amplificateur cosmique
Si le graviton quantique échappe aux outils du physicien peut-être peut-il le saisir par des idées ? C'est en l'occurrence la proposition audacieuse de Lawrence M. Krauss et Franck Wilczek à travers deux courts articles dont le dernier est d'autant plus intéressant qu'il propose d'étayer son argumentation sur un résultat expérimental récent, celui de BICEP2 :
Freeman Dyson has emphasized that no conventional experiment is capable of detecting individual gravitons. However, as we describe, if inﬂation occurred, the Universe, by acting as an ideal graviton ampliﬁer, aﬀords such access. It produces a classical signal, in the form of macroscopic gravitational waves, in response to spontaneous (not induced) emission of gravitons. Thus recent BICEP2 observations of polarization in the cosmic microwave background will, if conﬁrmed, provide ﬁrm empirical evidence for the quantization of gravity
Lawrence M. Krauss, Frank Wilczek, From B Modes to Quantum Gravity and Unification of Forces, 04/2014
... cosmology provides a realistic observable that is directly tied to the quantization of gravity. Speciﬁcally, observation of a cosmological gravitational wave background associated with an inﬂationary phase would provide, as a bonus, compelling evidence for the quantization of the gravitational ﬁeld. It does so in a way which is at least heuristically equivalent to all laboratory experiments that probe quantum phenomena–it
couples quantum mechanical phenomena to a classical detector, effectively amplifying quantum mechanical ffects so that they are classically measurable. The classical detector, in this case, is the expanding Universe.
The fact that quantization associated with gravity appears to be an essential feature of a gravitational wave background generated by inﬂation can be understood as follows: A period of inﬂation in the early universe results from a period of quasi-de Sitter expansion associated with a scalar ﬁeld in an almost ﬂat potential. If one considers a quantized approximately massless scalar ﬁeld in de-Sitter space, expanded into Fourier components with quantized mode functions ... then it is straightforward to calculate the zero-point quantum ﬂuctuations of these mode functions...
... if one treats these Fourier modes as quantum modes then there will be zero-point ﬂuctuations in each of the two modes that can be directly derived from equation ... Once these modes leave the horizon during the Inﬂationary expansion, they freeze in, effectively amplifying the mode number while outside the horizon, and they return inside the horizon as a coherent superposition of many quanta, e.g. as a classical wave. These waves, originating as quantum ﬂuctuations, then have a dimensionless power spectrum at the horizon ... The fact that this calculation relies on gravitational modes originating as quantum ﬂuctuations strongly suggests that the effect is essentially quantum-mechanical, but that conclusion is not logically forced. After all, many – in principle, all! – classical effects can also be calculated quantum mechanically, and sometimes that approach is simpler or more direct....
Through inﬂation, the Universe can act effectively as a graviton detector built on an “impractical scale”. It ampliﬁes a quantum mechanical effect to where it can be detected as a classical, observable signal, and may provide compelling empirical support for the quantization of gravity. Thus we both illustrate and transcend, rather than contradict, the arguments of T. Rothman and S. Boughn.
Lawrence M. Krauss et Franck Wilczek, Using Cosmology to Establish the Quantization of Gravity, 2013
Remarque : c'est moi qui ai mis en caractère gras certains passages. Par contre le texte souligné est une mise en exergue par les auteurs de l'article d'une hypothèse fondamentale sur laquelle repose leur argumentation.