Google researchers are figuring out how to study some of the weirdest theorised physics phenomena, like wormholes that link pairs of black holes, using experiments in a lab.
One central question driving theoretical physics today is how to use the same theory to explain both gravity and the rules that atoms follow, called quantum mechanics. The two haven’t played nicely yet, since gravity is an incredibly weak force, so probing it at the smallest scales is effectively impossible with today’s technology. But theoretical work has demonstrated that hints of this “quantum gravity” might emerge in certain quantum systems, ones that would one day be possible to create in the lab. One such experiment proposed by Google physicists posits that a quantum state reproducible in the physics lab can be explained as information travelling through a wormhole between two black holes.
“The experimental study of such situations therefore offers a path toward a deeper understanding of quantum gravity,” the authors write in the paper published on the arXiv.
It seems that gravity simply refuses to play nicely with quantum mechanics, and theorists have worked hard to string the two together—yet there are places where both concepts must exist simultaneously, such as on the surface of or inside black holes and at the moment of the Big Bang. One of the most popular theories linking the two is string theory, which replaces subatomic particles with tiny strings vibrating in a higher-dimensional space. String theory exists on scales far smaller than can be probed with particle accelerators, making it hard to test. However, a two-decade-old conjecture called the AdS/CFT correspondence essentially says that you can understand the higher-dimensional gravity in this higher-dimensional world as if it were a hologram produced by quantum mechanical particles. So a team of physicists at Google, as well as CalTech, the University of Maryland, and the University of Amsterdam, think that studying extreme quantum behaviours might provide stronger evidence of string theory’s existence. Maybe quantum computers could produce string theory-probing behaviours.
Among this decades’ most important physical advances has been the development of machines that control and manipulate quantum states, what we call quantum computers and quantum simulators. The smallest objects, like electrons orbiting atoms, can only take on certain values of properties, but when you’re not looking at them, they can have different values simultaneously (until you measure them, at least, when they go back to only having one value). Two or more particles can also entangle, meaning they and their properties must be described as a single mathematical object, even if you separate the atoms across space.
Google’s proposal suggests creating a circuit with two sets of connected qubits, the artificial “atoms” of the quantum computer, and dividing it into a left and right group. Pulses of inputted energy do the mathematical equivalent of evolving the qubits’ state backward in time, while another pulse is used to encode a “message” by altering the lefthand atoms’ quantum states in a specific way. Another pulse then plays the role of speeding up the qubits’ behaviour. Crucial to the black hole analogy, this scrambles the message among the qubits in a mathematically similar way to how information about a particle’s properties is scrambled and potentially lost upon entering a black hole. Once the information is scrambled, each qubit on the left is entangled with its mirror-image qubit on the right. Finally, after some amount of time, the message mysteriously should reappear in the righthand qubits, without requiring any decoding.
“It is not at all obvious how the message made it [to the other side of the system], and the most surprising fact of all is that the simplest explanation lies in the physics of black holes,” the authors write in the paper. Essentially, the researchers think that the information travelling between groups of qubits in the system is analogous to a message entering a black hole, travelling through a wormhole, and emerging outside of a second black hole. The researchers then go on to introduce a mathematical framework for understanding what’s going on and how it serves as an analogy to a traversable wormhole that doesn’t collapse.
According to the paper, there are potential setups where this system can be realised. One setup consists of arrays of atoms’ electrons are either in the lowest-energy or a very-high “Rydberg” state, controlled by laser pulses. Another is made from arrays of trapped charged ions. Either might one day be able to realise the experiment proposed by Google.
Basically, scientists think they can make a quantum computer act mathematically similar to information passing between two black holes via a wormhole. No wormholes will actually be created here on Earth. This is just a model, and like other analogue systems, just because the mathematical description of a lab experiment looks like someone’s theory describing space doesn’t mean that the theory is automatically correct. These models are just a way to produce stronger mathematical evidence that a theory might be correct. None of the researchers nor Google have responded to Gizmodo’s request for comment; I’ll update the post if I hear back.
This work builds on research into quantum information scrambling over time, as well as connections between this scrambling and black holes. But it has physicists buzzing with excitement nonetheless. Last week, dozens of physicists met at a Google X conference to discuss how quantum technology could be useful for quantum gravity researchers. “That was quite a moment, hearing about this experiment,” Guillaume Verdon, quantum resident at the Google-founded X who was not involved in this work, told Gizmodo. Studying quantum gravity “was the dream that brought me into quantum computing.”
Quantum computers that can create these wormhole-mimicking “thermofield-double” qubit states described in the paper are on the horizon, Christopher Monroe, a University of Maryland physics professor who consulted on this research, told Gizmodo. He hopes that the trapped-ion quantum computer that his group is working on could soon serve as a platform upon which to create the quantum states required to test these ideas. “Papers like this are motivating us, and giving us a push in university, company, and government labs to build these things.”