Quantum gravity is a part of theoretical physics that describes gravity according to the quantum mechanics principle, and also describes circumstances where quantum effects cannot be disregarded, like in the black holes vicinity or similar compact astrophysical objects like the neutron star with strong gravity effects. Physics and experiment prove that there are four fundamental forces describing the universe where three of the four which are electromagnetic force, and two nuclear forces being the strong force and the weak force respectively are quantum in nature while the fourth fundamental force which is gravitational force appears to be incompatible with quantum mechanics.
Searching for a theory to explain quantum gravity has been an ongoing experiment for decades but the most fundamental problem in all of theoretical physics is that there are yet to be no proven experimental results at all to guide this persistent endeavor. The utmost priority in quantum gravity is testing the existence of gravity between quantum objects to prove if there is gravitation or not. Scientist has been experimenting and trying to prove a theory of quantum gravity but it seems almost impossible over the years. It occurs that quantum gravity effects which is the observable interaction of gravity between quantum objects could be seen using existing technology if they occur in nature.
Physicists have been experimenting and exploring for decades how to combine the understanding of gravity with quantum mechanics via a proven theory and experimentation of quantum gravity. The question of whether gravity can be placed within the framework of quantum mechanics might soon be answered. UCLQ and the University of Groningen team in their recent work outlined improvement to a potential experiment that could be used to witness quantum gravity.
Testing quantum gravity using Quantum Gravity induced Entangled of Masses
One of the proposed experiments that could provide evidence of the gravitation between quantum objects is called QGEM (Quantum Gravity induced Entangled of Masses). QGEM is a protocol to experiment whether gravity is quantum or classical on a table-top, and discusses what aspects of quantum gravity can be tested in a lab.
QGEM uses the concept of entanglement to test for quantum gravity and Entanglement is explain as the property that gives strong correlations between two or more quantum systems, even when separated by large distances apart from each other.
The QGEM experiment would use two separated quantum masses with neither magnetic field nor electric charge. In the absence of electromagnetic force and the two nuclear force leaving only gravitational force the two masses should not entangle as described in classical physics given that the masses are neutral and aren’t close enough for nuclear forces to interact but if the masses entangle, there must be an interaction via quantum gravity.
The UCL physicist and agronomist, UCLQ, Ryan Marshman pointed out that there were two main difficulties with the original QGEM proposal where one is the experimental feasibility, as all of the parameters required are extremely ambitious in scope and the other is making sure it is only gravitational interaction that occurs between the two masses because the presence of electromagnetic interaction between the masses could be the cause of the entanglement which would Interfere with the proof gravitational interaction.
Testing quantum gravity with a Single Quantum system
Testing quantum gravity on table-top has been said to be impossible until recently were a new approach using the Quantum Information Theory and Quantum Technology which makes testing seem more possible. In Quantum Information Theory, quantum information can be encoded in distinct variables like continuous variables or qubits. Continuous variable Quantum Information Theory is more powerful than the latter which makes it very effective in using Quantum Information Theory to test quantum field theory. In this experiment, continuous-variable quantum information theory is applied to quantum gravity and shows the creation of a non-Gaussianity by another quantum gravity signature. Non-Gaussianity is a continuous variable resource that is important in universal quantum computing. Compared to the Quantum Gravity induced Entanglement of Masses, the non-Gaussianity does not rely on the interaction between two or more quantum systems so it can be applied to a single quantum system rather than a multi-partite quantum system.
This advantage makes it possible to test quantum gravity in a single quantum system that is not in a superposition location. The quantum gravity signature, a fundamental quantum rather than a classical, the non-Gaussianity is been found by probing an ultra-cold gas of billions of cesium atom existing in a state called Bose-Einstein condensate (BEC). This research was done by physicists in Uk, Hong Kong, and France, Vlatko Vedral, Richard Howl, and colleagues showing theoretically that a system that shows non-Gaussianity has a gravitation interaction that is quantum mechanical. Since the challenge has been creating a real system that can remain in a coherent quantum state on this macroscopic scale of Planck mass which is about 22 µg, Vedral and colleagues’ research shows that the BEC is that possible solution to the challenge that could be set up using existing technology.
BEC is a state of matter in which all the atoms are brought to a low temperature making them share the same quantum state. 0.2 mm diameter condensation of 4 billion atoms of caesium-133 that is held in a spherical optical trap for 2seconds was the specific suggestion by the researchers. This system could be studied in different ways where an option involves releasing the condensate from its trap and then sending it through a matter-wave beam-splitter. The measurement of the atom number of the two sent-out beams over and over again will show differences between the number which should follow a non-Gaussianity if gravity is quantum mechanical.
The researchers have maintained that the BEC system has several advantages and also disadvantages over quantum entanglement between two microspheres provided that it involves a single quantum system in a single location, unlike the latter that involves a multi-partite system. The team also argued that BEC naturally displaces electromagnetic interaction would also exhibit non-Gaussianity, therefore, generating a false positive signal. The disadvantage of non-Gaussianity is the quantum noise that arises from non-Gaussianity in the condensate itself.