This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. Could black holes like these (which represent those detected by LIGO on Dec. 26, 2015) collide in the dusty disk around a quasar's supermassive black hole explain gravitational waves, too? Credit: LIGO/T. Pyle
Perhaps the most surprising prediction of general relativity is that of gravitational waves. Ripples in space and time that spread through the universe at the speed of light. Gravitational waves are so faint that for decades their detection was thought impossible. Even today, it takes an array of laser interferometers several kilometers long to see their effect. But what if we could detect them with a table-top experiment in a university lab?
In a recent paper published in the New Journal of Physics, a team of physicists proposes just such a device. Rather than using beams of light, they suggest using the quantum superposition of a single electron.
A gamma ray burst offers a rare opportunity to assess the nature of the apparent 'empty space' vacuum that exists between you an it. In GRB 041219A's case, that's 300 million light years of vacuum. Credit: ESA.
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The very small wavelength of gamma ray light offers the potential to gain high resolution data about very fine detail – perhaps even detail about the quantum substructure of a vacuum – or in other words, the granularity of empty space.
Quantum physics suggests that a vacuum is anything but empty, with virtual particles regularly popping in and out of existence within Planck instants of time. The proposed particle nature of gravity also requires graviton particles to mediate gravitational interactions. So, to support a theory of quantum gravity we should expect to find evidence of a degree of granularity in the substructure of space-time.
There is a lot of current interest in finding evidence of Lorentz invariance violations – where Lorentz invariance is a fundamental principle of relativity theory – and (amongst other things) requires that the speed of light in a vacuum should always be constant.
Light is slowed when it passes through materials that have a refractive index – like glass or water. However, we don’t expect such properties to be exhibited by a vacuum – except, according to quantum theory, at exceedingly tiny Planck units of scale.
So theoretically, we might expect a light source that broadcasts across all wavelengths – that is, all energy levels – to have the very high energy, very short wavelength portion of its spectrum affected by the vacuum substructure – while the rest of its spectrum isn’t so affected.
There are at least philosophical problems with assigning a structural composition to the vacuum of space, since it then becomes a background reference frame – similar to the hypothetical luminiferous ether which Einstein dismissed the need for by establishing general relativity.
Nonetheless, theorists hope to unify the current schism between large scale general relativity and small scale quantum physics by establishing an evidence-based theory of quantum gravity. It may be that small scale Lorentz invariance violations will be found to exist, but that such violations will become irrelevant at large scales – perhaps as a result of quantum decoherence.
Quantum decoherence might permit the large scale universe to remain consistent with general relativity, but still be explainable by a unifying quantum gravity theory.
The ESA INTEGRAL gamma ray observatory - devoting a proportion of its observing time to searching for the underlying quantum nature of the cosmos. Credit: ESA
On 19 December 2004, the space-based INTEGRAL gamma ray observatory detected Gamma Ray Burst GRB 041219A, one of the brightest such bursts on record. The radiative output of the gamma ray burst showed indications of polarisation – and we can be confident that any quantum level effects were emphasised by the fact that the burst occurred in a different galaxy and the light from it has travelled through more than 300 million light years of vacuum to reach us.
Whatever extent of polarisation that can be attributed to the substructure of the vacuum, would only be visible in the gamma ray portion of the light spectrum – and it was found that the difference between polarisation of the gamma ray wavelengths and the rest of the spectrum was… well, undetectable.
The authors of a recent paper on the INTEGRAL data claim it achieved resolution down to Planck scales, being 10-35 metres. Indeed, INTEGRAL’s observations constrain the possibility of any quantum granularity down to a level of 10-48 metres or smaller.
Elvis might not have left the building, but the authors claim that this finding should have a major impact on current theoretical options for a quantum gravity theory – sending quite a few theorists back to the drawing board.
