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Is cosmic evolution a single track with no choice about the destination?Credit: Getty

Was there ever any choice in the Universe being as it is? Albert Einstein could have been wondering about this when he remarked to mathematician Ernst Strauss: What Im really interested in is whether God could have made the world in a different way; that is, whether the necessity of logical simplicity leaves any freedom at all.

US physicist James Hartle, who died earlier this year aged 83, made seminal contributions to this continuing debate. Early in the twentieth century, the advent of quantum theory seemed to have blown out of the water ideas from classical physics that the evolution of the Universe is deterministic. Hartle contributed to a remarkable proposal that, if correct, completely reverses a conventional story about determinisms rise with classical physics, and its subsequent fall with quantum theory. A quantum Universe might, in fact, be more deterministic than a classical one and for all its apparent uncertainties, quantum theory might better explain why the Universe is the one it is, and not some other version.

The brazen science that paved the way for the Higgs boson (and a lot more)

In physics, determinism means that the state of the Universe at any given time and the basic laws of physics fully determine the Universes backward history and forward evolution. This idea reached its peak with the strict, precise laws about how the Universe behaves introduced by classical physics. Take Isaac Newtons laws of motion. If someone knew the present positions and momenta of all particles, they could in theory use Newtons laws to deduce all facts about the Universe, past and future. Its only a lack of knowledge (or computational power) that prevents scientists from doing so.

Along with this distinctive predictive power, determinism underwrites scientific explanations that come close to the principle of sufficient reason most famously articulated by German polymath Gottfried Leibniz: that everything has an explanation. Every state of the Universe (with one obvious exception, which well come to) can be completely explained by an earlier one. If the Universe is a train, determinism says that its running on a track, with no option to switch to any other path because different tracks never cross.

Physicists have conventionally liked determinisms predictive and explanatory power. Others, including some philosophers, have generally been more divided, not least because of how determinism might seem to preclude human free will: if the laws of physics are deterministic, and our actions are just the summation of particle interactions, there seems to be no room for us to freely choose A instead of B, because the earlier states of the Universe will already have determined the outcome of our choice. And if we are not free, how can we be praised or blamed for our actions? Neuroendocrinologist Robert Sapolskys 2023 book Determined touches on this fascinating and controversial issue.

The strange behaviours of quantum particles that began to emerge in the twentieth century fundamentally shifted the debate surrounding determinism in physics. The laws of quantum mechanics give only the probabilities of outcomes, which can be illustrated with the thought experiment devised by Austrian physicist Erwin Schrdinger in 1935 (although when he devised it, he was concerned mainly with how the wavefunction represents reality). A cat is trapped in a box with a vial of poison that might or might not have been broken by a random event because of radioactive decay, for example. If quantum mechanics applied to the cat, it would be described by a wavefunction in a superposition of alive and dead. The wavefunction, when measured, randomly jumps to one of the two states, and quantum mechanics specifies only the probability of either possibility occurring. One consequence of the arrival of quantum mechanics was that it seemed to throw determinism out of the window.

How Stephen Hawking flip-flopped on whether the Universe has a beginning

But this accepted idea might not be the whole story, as developments in the second half of the twentieth century suggested. The quantum Universe could actually be more deterministic than a classical one, for two reasons. The first is technical. Newtons laws allow situations in which the past does not determine how things will move in the future. For example, the laws do not provide an upper bound on how much an object can be accelerated, so in theory a classical object can reach spatial infinity in finite time. Reverse this process, and you get what have been called space invaders objects that come from spatial infinity with no causal connection to anything else in the Universe, and which cant be predicted from any of the Universes past states.

In practice, this problem is solved by the universal speed limit, the speed of light, introduced by Einsteins special theory of relativity. But unruly infinities also plague Einsteinian relativity, which is a classical theory. The equations of general relativity lead to singularities of infinite curvature, most notoriously in black holes and at the Big Bang at the beginning of the Universe. Singularities are like gaps in space-time where the theory no longer applies; in some cases, anything can come out of them (or disappear into them), threatening determinism.

Many physicists think that quantum theory can come to the rescue by removing such singularities for example, by converting the Big Bang into a Big Bounce, with a Universe that continues to evolve smoothly on the other side of the singularity. If they are right, a theory of quantum gravity that fully unifies quantum theory, which predicts the behaviour of matter on the smallest scales, and Einsteins relativity, which encapsulates the large-scale evolution of the Universe, will smooth out the gaps in space-time and restore determinism.

Space-time singularities inside black holes could threaten a deterministic cosmic order.Credit: ESO/SPL

But there is a deeper reason why the quantum Universe might be more deterministic, to which Hartles scientific legacies are relevant. With US physicist Murray Gell-Mann, Hartle developed an influential approach to quantum theory, called decoherent histories1. This attempted to explain the usefulness of probabilistic statements in quantum physics, and the emergence of a familiar, classical realm of everyday experience from quantum superpositions. In their picture, the wavefunction never randomly jumps. Instead, it always obeys a deterministic law given by Schrdingers equation, which characterizes the smooth and continuous evolution of quantum states. In this respect, it is similar to US physicist Hugh Everett IIIs popular many worlds interpretation of quantum mechanics, which proposes that the quantum Universe splits into different branches according to the possibilities encoded in the wavefunction whenever anything is measured2. In what follows I assume, as Everett did, that the Universe can be completely described by a quantum wavefunction with no hidden variables that operate on a more fundamental level.

With Stephen Hawking, Hartle went on to become one of the founders of quantum cosmology, which applies quantum theory to the entire Universe. In a classical Universe, there is freedom in choosing how it all started. Even setting aside the extreme situations mentioned earlier, classical mechanics is deterministic merely in that it lays down many possible evolutionary histories for the Universe, and offers conditional statements about them: if this happens, then that must happen next. To return to the train analogy, a deterministic theory does not, by itself, say why the train is on any one given track out of many: why it is going from A to B via C, rather than from X to Y via Z. We can go back to earlier states to explain the current state, and do that all the way back to the initial state but this initial state is not explained by anything that precedes it. Ultimately, standard determinism fails to fully satisfy Leibnizs principle of sufficient reason: when it comes to the initial state, something remains without an explanation.

See me here, see me there

This failure is not just philosophical. A complete theory of the Universe should predict the phenomena we observe in it, including its large-scale structure and the existence of galaxies and stars. The dynamic equations we have, whether from Newtonian physics or Einsteinian relativity, cannot do this by themselves. Which phenomena show up in our observations depend sensitively on the initial conditions. We must look at what we see in the Universe around us, and use this information to determine the initial condition that might have given rise to such observations.

A theory that specifies deterministic laws of both the Universes temporal evolution and its exact initial condition satisfies what English physicist Roger Penrose called strong determinism in his 1989 book The Emperors New Mind. This is, according to Penrose, not just a matter of the future being determined by the past; the entire history of the universe is fixed, according to some precise mathematical scheme, for all time. Let us say that a Universe is strongly deterministic if its basic laws of physics fix a unique cosmic history. If determinism provides a set of non-crossing train tracks, without specifying which one is being used, then strong determinism lays down a single track that has no choice even about where it starts.

Strong determinism is hard to implement in classical physics. You might consider doing it by specifying the initial condition of the Universe as a law. But although the dynamical laws of classical physics are simple, the Universe itself is complex and so its initial condition must have been, too. Describing the precise positions and momenta of all the particles involved requires so much information that any statement of the initial condition is too complex to be a law.

Hartle suggested3 that quantum mechanics can solve this complexity problem. Because a quantum objects wavefunction is spread out across many classical states (cat alive or cat dead, for instance), you could propose a simple initial condition that includes all the complexities as emergent structures in the quantum superposition of these states. All the observed complexities can be regarded as partial descriptions of a simple fundamental reality: the Universes wavefunction. As an analogy, a perfect sphere can be cut into many chunks with complicated shapes, yet they can be put back together to form a simple sphere.

In 1983, Hartle and Hawking introduced4 one of the first (and highly influential) proposals about the quantum Universes initial state. Their no boundary wavefunction idea suggests that the shape of the Universe is like that of a shuttlecock: towards the past, it rounds off smoothly and shrinks to a single point. As Hawking said in a 1981 talk on the origin of the Universe in the Vatican: There ought to be something very special about the boundary conditions of the Universe, and what can be more special than the condition that there is no boundary?

Unique, or not unique?

In this perspective, the quantum Universe has two basic laws: a deterministic one of temporal evolution and a simple one that picks an initial wavefunction for the Universe. Hence, the quantum Universe satisfies strong determinism. The physical laws permit exactly one cosmic history of the Universe, albeit one described by a wavefunction that superposes many classical trajectories. There is no contingency in what the Universe as a whole could have been, and no alternative possibility for how it could have started. Every event, including the first one, is explained; the entire wavefunction of the Universe for all times is pinned down by the laws. The probabilities of quantum mechanics do not exist at the level of the basic physical laws, but can nonetheless be assigned to coarse-grained and partial descriptions of bits of the Universe.

This leads to a more predictive and explanatory theory. For example, the no-boundary proposal makes predictions for a relatively simple early Universe and for the occurrence of inflation a period of rapid expansion that the Universe seems to have undergone in its first instants.

There are still many wrinkles to this proposal, not least because some studies have shown that, contrary to initial expectations, the theory might not single out a unique wavefunction for the Universe5,6. But studies in quantum foundations research that is mostly independent from that of quantum cosmology could offer yet another method for implementing strong determinism. Several researchers have considered the controversial idea that quantum states of closed systems, including the Universe, need not be restricted to wavefunctions, but instead can come from a broader category: the space of density matrices710.

Density matrices can be thought of as superpositions of superpositions, and they provide extra options for the initial condition of the Universe. For example, if we have reasons to adopt the past hypothesis the idea, which seems likely, that the Universe began in a low-entropy state (and its entropy has been increasing steadily since) and that this theory corresponds to a set of wavefunctions, then we can choose a simple density matrix that corresponds to the uniform mixture of that set. As I have argued10, if we regard the density matrix as the initial state of the Universe and accept that it is specified by a law, then this choice, together with the deterministic von Neumann equation (a generalization of Schrdingers equation), can satisfy strong determinism. However, in this case, the laws fix a cosmic history of a quantum Universe that has many evolving branches a multiverse.

So how deterministic is the Universe? The answer will depend on the final theory that bridges the divide between quantum physics and relativity and that remains a far-off prospect. But if Hartle is right, the story of the rise and fall of determinism until now might be the reverse of the conventional tale. From a certain perspective, the quantum Universe is more deterministic than a classical one, providing stronger explanations and better predictions. That has consequences for humans, too, because that makes it harder to appeal to quantum theory to defend free will11. If the quantum Universe is strongly deterministic, then there is no other path to make the Universe than the way it is. The ultimate laws of the quantum cosmos might tell us why it is this one.

Excerpt from:

Does quantum theory imply the entire Universe is preordained? - Nature.com

On the smallest level, the universe operates in such a bizarre way that even Albert Einstein had a difficult time making sense of it. An example of the strangeness in the quantum realmone that has no equivalent in the world as we experience itis the phenomenon of quantum entanglement.

In our classical world, if Iman flips a coin in Indonesia and Olaf flips a coin in Norway, both Iman and Olaf have a 50% chance of landing on heads and a 50% chance of landing on tails. The result of one coin flip will have no effect on the result of the other.

In the quantum world, that wouldnt necessarily be true. Two particles can become entangled, which in this case, would mean the coins would be connected by the outcome of their flips. If one landed on heads, the other would land on tails, and vice versa. So if Iman saw her coin land on heads, she would immediately know that Olaf had gotten tails.

This seemed impossible to Einstein. The problem was: For one entangled particle to affect another, even if separated by a great distance, it appeared that informationperhaps the result of a coin flip, or the resolution of a particles quantum statewould need to travel between them faster than the speed of light.

But quantum entanglement, which Einstein famously described as spooky action at a distance, is now an accepted fact of nature.

Quantum entanglement is the most distinctive signature of quantum mechanics, says Juan R. Muoz de Nova, a condensed-matter physicist at the Complutense University of Madrid. It contradicts the intuitions we have on a daily basis, he says. That is why entanglement is so intrinsic to quantum mechanics.

This phenomenon has been observed by researchers around the world, and the 2022 Nobel Prize in physics was awarded to three scientists for experimentally advancing our understanding of it. Scientists have detected quantum entanglement through experiments involving macroscopic diamonds and ultracold gases.

In September 2023, the ATLAS collaboration made another advancement when they unveiled the highest-energy measurement of quantum entanglement ever, using top quarks produced in the Large Hadron Collider at CERN. Interestingly, the measurement turned out a bit differently than expected.

At the LHC, scientists accelerate protons to relativistic speeds at high energies, bringing them into collision with one another about 40 million times a second. The energy of the collisions can generate new particles, allowing experiments like ATLAS to study their properties.

When particles are created in collisions at the LHC, some of them naturally become entangled. For instance: A characteristic of a particle that can be measured is its spin; two particles produced in the same collision can become entangled so that they have opposite spin.

One of the types of particles that scientists study at the LHC are top quarks, which are heavier than any other known fundamental particle. Due to their large mass, top quarks decay extremely quickly after they are produced. This quick decay prevents a process that quarks typically go through called hadronization.

Hadronization causes quarks to grab other quarks and combine into more stable quark bundles like protons and neutrons. The fact that top quarks cant hadronize is important, because once a quark hadronizes, it is a lot more challenging to measure its spin.

Instead of hadronizing, top quarks decay, transferring their energy into less massive, more stable particles. They pass their spins on to their decay products, which ATLAS researchers can detect and measure. By reconstructing the spin correlations between top-quark pairs, the scientists can confirm whether they were entangled.

The ATLAS collaboration faced many challenges as they brought this science from theory to experimentation.

The biggest issue was accounting for neutrinos. Top quarks decay into leptonssome leptons, like electrons, are easy for the ATLAS detector to spot. Others, however, are practically invisible. Neutrinos, which can pass through the entire planet without ever interacting, easily escape the ATLAS detector without leaving a trace.

Their presence can be inferred by the amount of input and output energy from the collision. If energy has gone missing, scientists can attribute it to neutrinos. But its not a perfect solution, as a decaying top quark pair can produce more than one neutrino, and theres only one measurement of the collisions total energy. We use some clever, quite complicated bits of math, and some experimental hacks to correct for this, says James Howarth, a physics lecturer at the University of Glasgow and a coordinator of the analysis.

Scientists placed strict requirements on the collisions to carefully reconstruct the motions of top quark pairs. This problem took the most time to solve since reconstruction is crucial for detecting entanglement.

A second issue lay in the detector itself. When measuring entanglement, scientists paid close attention to the way decay products were distributed after a collision. But they needed to do so quickly because imperfections in the detector can affect the experiment. The detector actually kind of distorts the shape of the distribution that were interested in, says Yoav Afik, a physicist at the University of Chicago and a coordinator of the analysis.

To counteract the distortion, ATLAS collaborators used simulations to model what the collision looked like without the detectors effects.

A third issue: To measure entanglement experimentally, scientists needed a way to test alternative hypotheses using modeled data. The way the simulations were constructed, testing different scenarios was tricky.

With help from de Nova, who collaborated as a theorist during the analysis, researchers developed a weighted method in the model that allowed them to change the distribution of the decay products. In doing so, they could test varying hypotheses that would ultimately indicate whether entanglement existed in the system.

After a grueling three years of testing and analyzing, the ATLAS collaboration came to a rewarding yet puzzling result. What we found is that the data shows us stronger entanglement than expected by the simulations, Afik says.

It could be that the physicists have found an intrinsic difference between prediction and reality that points to a misunderstanding in our theory of quantum entanglement. Or, it could be a sign that something needs to be tweaked in the analysis. Howarth reasons that the simulations may not fully describe the region in which the collisions are taking place, so the modeling could be incomplete.

Whether or not the measurement eventually lines up with predictions, it still stands as the highest-energy detection of quantum entanglement and the first measurement of entanglement between a pair of quarks.

We never observed entanglement in such a high-energy relativistic system as the LHC, says de Nova, so this is like opening a whole new world to quantum information.

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Scientists measure entanglement at the LHC | symmetry magazine - Symmetry magazine

Quantum batteries, with their innovative charging methods, represent a leap in battery technology, promising higher efficiency and wider applications in sustainable energy solutions. Credit: SciTechDaily.com

A new way to charge batteries harnesses the power of indefinite causal order.

Batteries that exploit quantum phenomena to gain, distribute, and store power promise to surpass the abilities and usefulness of conventional chemical batteries in certain low-power applications. For the first time, researchers including those from the University of Tokyo take advantage of an unintuitive quantum process that disregards the conventional notion of causality to improve the performance of so-called quantum batteries, bringing this future technology a little closer to reality.

When you hear the word quantum, the physics governing the subatomic world, developments in quantum computers tend to steal the headlines, but there are other upcoming quantum technologies worth paying attention to. One such item is the quantum battery which, though initially puzzling in name, holds unexplored potential for sustainable energy solutions and possible integration into future electric vehicles. Nevertheless, these new devices are poised to find use in various portable and low-power applications, especially when opportunities to recharge are scarce.

In the classical world, if you tried to charge a battery using two chargers, you would have to do so in sequence, limiting the available options to just two possible orders. However, leveraging the novel quantum effect called ICO opens the possibility to charge quantum batteries in a distinctively unconventional way. Here, multiple chargers arranged in different orders can exist simultaneously, forming a quantum superposition. Credit: 2023 Chen et al.

At present, quantum batteries only exist as laboratory experiments, and researchers around the world are working on the different aspects that are hoped to one day combine into a fully functioning and practical application. Graduate student Yuanbo Chen and Associate Professor Yoshihiko Hasegawa from the Department of Information and Communication Engineering at the University of Tokyo are investigating the best way to charge a quantum battery, and this is where time comes into play. One of the advantages of quantum batteries is that they should be incredibly efficient, but that hinges on the way they are charged.

Current batteries for low-power devices, such as smartphones or sensors, typically use chemicals such as lithium to store charge, whereas a quantum battery uses microscopic particles like arrays of atoms, said Chen. While chemical batteries are governed by classical laws of physics, microscopic particles are quantum in nature, so we have a chance to explore ways of using them that bend or even break our intuitive notions of what takes place at small scales. Im particularly interested in the way quantum particles can work to violate one of our most fundamental experiences, that of time.

While its still quite a bit bigger than the AA battery you might find around the home, the experimental apparatus acting as a quantum battery demonstrated charging characteristics that could one day improve upon the battery in your smartphone. Credit: 2023 Zhu et al.

In collaboration with researcher Gaoyan Zhu and Professor Peng Xue from Beijing Computational Science Research Center, the team experimented with ways to charge a quantum battery using optical apparatuses such as lasers, lenses, and mirrors, but the way they achieved it necessitated a quantum effect where events are not causally connected the way everyday things are. Earlier methods to charge a quantum battery involved a series of charging stages one after the other. However, here, the team instead used a novel quantum effect they call indefinite causal order, or ICO. In the classical realm, causality follows a clear path, meaning that if event A leads to event B, then the possibility of B causing A is excluded. However, at the quantum scale, ICO allows both directions of causality to exist in whats known as a quantum superposition, where both can be simultaneously true.

Common intuition suggests that a more powerful charger results in a battery with a stronger charge. However, the discovery stemming from ICO introduces a remarkable reversal in this relationship; now, it becomes possible to charge a more energetic battery with significantly less power. Credit: 2023 Chen et al.

With ICO, we demonstrated that the way you charge a battery made up of quantum particles could drastically impact its performance, said Chen. We saw huge gains in both the energy stored in the system and the thermal efficiency. And somewhat counterintuitively, we discovered the surprising effect of an interaction thats the inverse of what you might expect: A lower-power charger could provide higher energies with greater efficiency than a comparably higher-power charger using the same apparatus.

The phenomenon of ICO the team explored could find uses beyond charging a new generation of low-power devices. The underlying principles, including the inverse interaction effect uncovered here, could improve the performance of other tasks involving thermodynamics or processes that involve the transfer of heat. One promising example is solar panels, where heat effects can reduce their efficiency, but ICO could be used to mitigate those and lead to gains in efficiency instead.

Reference: Charging Quantum Batteries via Indefinite Causal Order: Theory and Experiment by Gaoyan Zhu, Yuanbo Chen, Yoshihiko Hasegawa and Peng Xue, 13 December 2023,Physical Review Letters.DOI: 10.1103/PhysRevLett.131.240401

This work has been supported by the National Natural Science Foundation of China (Grant Nos. 92265209 and 12025401). Y. H. acknowledges support by JSPS KAKENHI Grant Number JP22H03659. Y.C. acknowledges support by JST SPRING, Grant Number JPMJSP2108.

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Breaking Causality: The Revolutionary Power of Quantum Batteries - SciTechDaily

Photo by Jordan McDonald on Unsplash

All that we think is obvious in the physical world is, in reality, merely the physical traits that we can see, and which we then measure against the standards we are familiar with. For instance, our common sense would appear to tell us that a big system (such as a forest ecosystem) could only be made up of subsystems (the trees, herbage, animals, water etc) that create a set of rules that linearly correlate with the overall workings of the big system (the forest). But what if we break down the subsystems further. For instance, lets take a leaf from a tree and photosynthesis. Photosynthesis, as we should have learnt by middle school (if the school is doing its job), is the process by which a plant uses carbon dioxide, water, and the sun in combination to produce energy in the form of glucose, with the aid of a machinery known as chlorophyll.

All of these are explained using biological and chemical logic that do not, for instance, explain why the energy production of the leaf must work in such combination to produce the chemical kinetics that would then lead to the intricate production of the different energy compounds needed to power the life of the tree. Rather, what we get is the expected: every base component (water + carbon dioxide + sunlight (with the aid of chlorophyll) = ATP and NADPH (the energy molecules).

Admittedly, I am taking shortcuts around the steps needed to get to the final product, but even if you were to lay out all the steps, there is still a missing answer to the question of how the photons and neutrinos from the sun cooperate with an even more complex range of particles found in carbon dioxide and water through the absorption of sunlight through the chloroplasts where the chlorophyll are located? This is even after we take into account the breaking down of those complex entities into subatomic particles found in hydrogen, oxygen, and carbon. For some scientists, the answers lie in the quantum systems within each of these component subsystems, one represented by quantum biology.

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If Quantum Physics is Queer, What Does it Mean for Quantum Technologies? - Medium

The X-ray beam from the worlds largest X-ray laser, the European XFEL, only becomes as clearly visible as in the photo in complete darkness and with an exposure time of 90 seconds. In 2024, the first experiments to detect quantum fluctuations in vacuum will take place here. Credit: European XFEL / Jan Hosan

Absolutely empty that is how most of us envision the vacuum. Yet, in reality, it is filled with an energetic flickering: the quantum fluctuations. Scientists are currently scientists are gearing up for a laser experiment intended to verify these vacuum fluctuations in a novel way, which could potentially provide clues to new laws in physics.

A research team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has developed a series of proposals designed to help conduct the experiment more effectively thus increasing the chances of success. The team presents its findings in the scientific journalPhysical Review D.

The physics world has long been aware that the vacuum is not entirely void but is filled with vacuum fluctuations an ominous quantum flickering in time and space. Although it cannot be captured directly, its influence can be indirectly observed, for example, through changes in the electromagnetic fields of tiny particles.

However, it has not yet been possible to verify vacuum fluctuations without the presence of any particles. If this could be accomplished, one of the fundamental theories of physics, namely quantum electrodynamics (QED), would be proven in a hitherto untested area. Should such an experiment reveal deviations from the theory, however, it would suggest the existence of new, previously undiscovered particles.

Dr. Ulf Zastrau heads the HED (High Energy Density Science) experimental station at the European XFEL. In the HED beam chamber the flashes from the worlds largest X-ray laser must meet the light pulses from the ReLaX high-power laser operated by the HZDR in order to detect vacuum fluctuations. Credit: European XFEL / Jan Hosan

The experiment intended to accomplish this is planned as part of the Helmholtz International Beamline for Extreme Fields (HIBEF), a research consortium led by the HZDR at the HED experimental station of the European XFEL in Hamburg, the largest X-ray laser in the world. The underlying principle is that an ultra-powerful laser fires short, intense flashes of light into an evacuated stainless steel chamber. The aim is to manipulate the vacuum fluctuations so that they, seemingly magically, change the polarization of an X-ray flash from the European XFEL, i.e., rotate its direction of oscillation.

It would be like sliding a transparent plastic ruler between two polarizing filters and bending it back and forth, explains HZDR theorist Prof. Ralf Schtzhold. The filters are originally set up so that no light passes through them. Bending the ruler would now change the direction of the lights oscillation in such a way that something could be seen as a result. In this analogy, the ruler corresponds to the vacuum fluctuations while the ultra-powerful laser flash bends them.

The original concept involved shooting just one optical laser flash into the chamber and using specialized measurement techniques to register whether it changes the X-ray flashs polarization. But there is a problem: The signal is likely to be extremely weak, explains Schtzhold. It is possible that only one in a trillion X-ray photons will change its polarization.

But this might be below the current measurement limit the event could simply fall through the cracks undetected. Therefore, Schtzhold and his team are relying on a variant: instead of just one, they intend to shoot two optical laser pulses simultaneously into the evacuated chamber.

Both flashes will strike there and literally collide. The X-ray pulse of the European XFEL is set to fire precisely into their collision point. The decisive factor: The colliding laser flashes affect the X-ray pulse like a type of crystal. Just as X-rays are diffracted, i.e., deflected, when passing through a natural crystal, the XFEL X-ray pulse should also be deflected by the briefly existing light crystal of the two colliding laser flashes.

That would not only change the polarization of the X-ray pulse but also slightly deflect it at the same time, explains Ralf Schtzhold. This combination could increase the chances of actually being able to measure the effect so the researchers hope. The team has calculated various options for the striking angle of the two laser flashes colliding in the chamber. Experiments will show which variant proves to be most suitable.

The prospects could even be improved further if the two laser flashes shot into the chamber were not of the same color but of two different wavelengths. This would also allow the energy of the X-ray flash to change slightly, which would, likewise, help to measure the effect. But this is technically quite challenging and may only be implemented at a later date, says Schtzhold.

The project is currently in the planning stages in Hamburg together with the European XFEL team at the HED experimental station, and the first trials are scheduled to launch in 2024. If successful, they could confirm QED once more.

But perhaps the experiments will reveal deviations from the established theory. This could be due to previously undiscovered particles for example, ultra-light ghost particles known as axions. And that, says Schtzhold, would be a clear indication of additional, previously unknown laws of nature.

Reference: Detection schemes for quantum vacuum diffraction and birefringence by N. Ahmadiniaz, T. E. Cowan, J. Grenzer, S. Franchino-Vias, A. Laso Garcia, M. md, T. Toncian, M. A. Trejo and R. Schtzhold, 10 October 2023,Physical Review D.DOI: 10.1103/PhysRevD.108.076005

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Beyond the Void: New Experiment Challenges Quantum Electrodynamics - SciTechDaily