Space Duel! The key to survival is Quantum Gravity

Two spacecraft face each other for a desperate space duel. The survival of each crew depends on their respective captain's ability to unify quantum physics and general relativity and formulate a quantum theory of gravity.

Physicists have spent the last five decades attempting to unify two of the most revolutionary theories in science — quantum physics and general relativity — in the process, providing a quantum theory of gravity. The lack of such a theory being the major hurdle in said unification.

One of the other areas in which the two theories also diverge is how the concept of time should be approached. Time has a fundamentally different character in quantum mechanics than it does in general relativity.

However, a new study suggests, that by examining one of these conundrums, it may be possible to solve the other. So, finding an explanation to unite the different interpretations of time, may lead to a quantum theory of gravity.

An international team of researchers, led by physicists from the University of Vienna, the Austrian Academy of Sciences as well as the University of Queensland (AUS) and the Stevens Institute of Technology (USA), believe they have done just this.

The team have combined the key elements of the two theories which describe the flow of time — finding, in the process, that the order in time in which events occur can display quantum features.

Conflicting views on time

One of Einstein’s most revolutionary and controversial ideas was the concept that massive objects have an effect on, not just space, but time as well.

In his theory of general relativity, Einstein states that the presence of a massive object slows time’s flow. The greater the mass, the more drastic the effect. To picture this, imagine taking two identical, synchronised clocks and placing them at varying distances from a supermassive black hole.

When we bring the clocks back together — neglecting effects from things like acceleration in our simple thought experiment — we should find that the clock which placed closer to the black hole displays an earlier time than the one which was placed further away.

The conflict arises when considering the idea of superposition which exists in quantum mechanics.

Using wave mechanics, quantum physics suggests that it should be possible to place any object into a superposition of states — theoretically at least. To better understand what a superposition is, imagine a beam of light travelling through a beam splitter. The splitter sends a photon either to the left or to the right.

But, immediately after the photon passes through the beam splitter and until it is measured — ie. it interacts with something — the photon can only be considered to be in a superposition of states. We describe it with an equation that suggests it went in both directions.

So, rerunning our clock thought experiment with this principle in mind — it suggests that we can’t ever state for sure which clock was at which position. Wave mechanics suggests it existed at both.

This is an idea that is not without controversy.

Firstly, it raises the question “at what limits do quantum effects cease to apply?” We know that quantum physics is a theory of the very small, but to sat that the delineation of quantum and classical mechanics is ‘fuzzy’ is an understatement.

Researchers are finding particle-wave duality present in larger and larger molecules all the time. A famous example of this is Carbon-60 molecules, which when used in the double-slit experiment display the same duality in nature as electrons and photons.

Many researchers believe that there is some mechanism that must block quantum effects — in particular, superposition — from applying in the macroscopic world. Whilst others are forming theories that hinge on the idea.

What if…?

Magdalena Zych from the University of Queensland, explains that tackling this curious disparity head-on inspired the team in their research: “We started by tackling a question: what would a clock measure if it was influenced by a massive object in a quantum superposition state?”

The team was surprised to discover that they could use pretty standard, textbook physics to describe what would happen in such a situation. They discovered that when a massive object is placed in a superposition in the neighbourhood of a set of syncronised clocks — their order in time can become genuinely ‘quantum.’

Thus defying any classical description.

Caslav Brukner, a Professor from the University of Vienna who specialises in the foundations of quantum physics, adds that the regime where quantum time order could arise is quite remote from our everyday experience.

Professor Bruker — a co-author of the paper, continues: “The most important insight from our work is that quantum time order is at all possible and that it results in new physical effects.”

Quantum Quarrel: Duelling starships and quantum gravity

Let’s return to our opening dilemma. Two spacecraft from different races — one that really likes yellow, the other that prefers greyscale — locked in an intergalactic duel.

The rules of the duel state, that both must fire at the other at a specified time — 00:000 — and immediately start their engines in an attempt to dodge the other’s attack. Should either ship’s gunner fire earlier than 00:00 they will destroy the other ship before they have the chance to engage their engines.

This establishes a strict time order between events.

In empty space, both ship’s fire at precisely at 00:00 and engage their engines for an evasive manoeuvre at the same time. As a result, neither ship is destroyed.

Rerunning the duel, the captain of the yellow ship has a plan. He chooses the field of the duel so that the grey ship will be sat closer to the edge of a black hole.

This would result in time slowing for the ship closest to the black hole and the furthest ship firing before the closest ship can engage their engines, even if they fire at precisely 00:00.

But, the researchers say, by manipulating the quantum superposition of the black hole — allowing it to be placed simultaneously at varied locations — the laws of quantum physics and general relativity state that both ships can be placed in an entangled state.

Thus meaning they are in a superposition in which either ship could have been destroyed depending on which was closer to the black hole.

War… huh…what is it good for? Formulating a viable theory of quantum gravity.

What the new study suggests is that the temporal order among events can exhibit superposition and entanglement. Both of which are genuinely quantum features.

These features are particularly useful for testing quantum theory.

This means that the result could potentially be used as a theoretical testing ground for frameworks for quantum gravity, and thus help to move forward in formulating the correct theory of quantum gravity.

Aside from this, and advanced space warfare, the study will also be relevant for future quantum technologies. For example, quantum computers that exploit the quantum order of performing operations might beat devices that operate using only fixed sequences.

Practical implementations of quantum temporal order do not require extreme conditions — such as black holes in superposition — and can be simulated without the use of gravity. Thus, the discovery of quantum properties of time can lead to better quantum devices in the upcoming era of quantum computers.