Quantum Mechanics and Relativity: Who Will Prevail?
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The Quantum-Relativity Conundrum
The ongoing rivalry between gravity and quantum physics is evolving in fascinating ways.
It represents both the largest and smallest quandaries in physics today. Currently, scientists operate under two distinct frameworks to comprehend the workings of nature. General relativity elegantly explains gravity and its far-reaching impacts, encompassing everything from orbiting celestial bodies to colliding galaxies and the overall dynamics of the universe. This is the grand scale. In contrast, quantum mechanics adeptly describes the remaining three fundamental forces: electromagnetism and the two nuclear forces. Quantum theory excels in explaining phenomena like uranium decay and the interaction of light particles with solar cells. This is the micro scale.
However, herein lies the dilemma: relativity and quantum mechanics are fundamentally divergent theories, each with its own unique structure. This discrepancy is not merely a matter of scientific jargon; it highlights a genuine conflict between incompatible interpretations of reality.
The tension between these two realms of physics has been brewing for over a century, ignited by Albert Einstein's pivotal 1905 papers—one detailing relativity and the other introducing quantum theory. Recent developments, however, signal that this debate is entering a compelling and unpredictable new chapter. Two prominent physicists have taken starkly opposing stances, conducting experiments that may ultimately determine which paradigm holds greater significance.
Just as a pixel serves as the smallest unit of an image, there may exist a fundamental minimum unit of distance: a quantum of space.
To illustrate the divide between relativity and quantum mechanics, think of it as "smooth" versus "chunky." General relativity posits that events occur continuously and deterministically, meaning each cause has a specific, local effect. In contrast, quantum mechanics depicts events resulting from interactions among subatomic particles as occurring in discrete jumps, leading to probabilistic rather than definitive outcomes. Quantum principles allow for connections that classical physics forbids, as evidenced by a recent experiment where Dutch researchers demonstrated that two electrons could influence one another instantaneously, even across a mile's distance. When attempting to interpret the smooth laws of relativity through a chunky quantum lens—or vice versa—results can be disastrous.
Relativity yields nonsensical results when applied to the quantum scale, ultimately leading to infinite values in its description of gravity. Conversely, quantum mechanics falters when scaled to cosmic dimensions. Quantum fields possess energy even in seemingly vacant space, and this energy amplifies as the fields expand. According to Einstein's famous equation, e=mc², energy and mass are interchangeable; thus, an accumulation of energy is akin to an accumulation of mass. If taken to extremes, this could result in a black hole that collapses the universe inward—quite the conundrum.
To delve deeper into this discussion, we turn to two figures: Craig Hogan, a theoretical astrophysicist at the University of Chicago and director of the Center for Particle Astrophysics at Fermilab, and Lee Smolin, a founding member of the Perimeter Institute for Theoretical Physics in Canada. Hogan is proposing a revolutionary theory suggesting that quantum units of space may be substantial enough to be studied directly. Meanwhile, Smolin aims to propel physics forward by revisiting and extending Einstein's philosophical foundations in a bold new direction.
The Stakes: A Historical Perspective
The stakes in this debate are monumental. When Einstein introduced general relativity, he didn't merely surpass Isaac Newton's theory of gravity; he also initiated a transformative perspective on physics that led to our understanding of concepts like the Big Bang, black holes, atomic bombs, and even the GPS adjustments essential for modern navigation. Similarly, quantum mechanics did more than revise James Clerk Maxwell's equations; it provided the theoretical framework for the Large Hadron Collider, solar technology, and contemporary microelectronics.
The outcomes of these inquiries could lead to nothing short of a third revolution in modern physics, with profound implications. They may shed light on the origins of the laws of nature and whether the universe operates on principles of uncertainty or determinism.
Hogan, the proponent of the quantum viewpoint, likens himself to a "lamp-post physicist" who prefers to explore areas where illumination is brightest, as that's where discoveries are most likely to occur. His current research focuses on investigating the interactions of gravity over extremely short distances. "I believe there's an experiment we can conduct that might reveal insights about the interface we still don't fully comprehend," he asserts.
A core assumption in Einstein's framework—rooted in Aristotle's philosophy—is that space is continuous and infinitely divisible, allowing any distance to be subdivided further. However, Hogan challenges this notion, proposing that, similar to how a pixel is the smallest unit of an image, there may exist an irreducible smallest unit of distance: a quantum of space.
In Hogan's vision, querying how gravity behaves at scales smaller than this fundamental unit would become nonsensical, as no such scale would exist. Consequently, general relativity would need to reconcile with quantum physics since the space in which gravitational effects are measured would be partitioned into these indivisible quantum units. The realm of gravity's influence would thus unfold on a quantum stage.
The Holometer and Experimental Insights
Hogan believes that the holometer will either clarify the correct understanding of the underlying quantum structure of space or eliminate misconceptions. Acknowledging that his concept may seem unconventional, he notes that a group of physicists has been developing string theory since the late 1960s to reconcile general relativity and quantum mechanics. Although string theory has become the predominant framework, it has yet to fulfill many of its initial promises.
Unlike string theory, which posits that every object in the universe is composed of vibrating strings of energy, Hogan's chunky space approach diverges significantly. The latter introduces a finite, smallest scale to the universe, while Hogan seeks to identify a more substantial structural element.
If Hogan's hypothesis regarding the chunkiness of space is validated, it could invalidate many current interpretations of string theory and inspire a novel approach to formulating general relativity in quantum terms. This would lead to fresh perspectives on the fundamental nature of space and time, suggesting that our three-dimensional reality may emerge from more basic two-dimensional units. Hogan takes the pixel analogy seriously, positing that just as a flat image can create an impression of depth, space itself might arise from elements that behave as if they exist solely in two dimensions.
Like various ideas at the fringes of theoretical physics, Hogan's notions may sound like fanciful musings. However, he plans to rigorously test them experimentally. Starting in 2007, Hogan began contemplating how to construct a device capable of measuring the fine granularity of space. His colleagues contributed ideas based on technology developed for gravitational wave detection. Within two years, Hogan had assembled a proposal and began collaborating with institutions like Fermilab and the University of Chicago to create a "holometer"—an apparatus designed to detect these spatial chunks.
At its core, the holometer comprises a laser beam, a half-reflective mirror to divide the beam into two perpendicular paths, and mirrors that bounce the beams along two 40-meter-long tunnels. The beams will be calibrated to detect the precise locations of the mirrors. If space is indeed granular, the positions of the mirrors would fluctuate, leading to random variations in their separation. When the two beams converge, any discrepancy would indicate the scale of the spatial chunks.
To identify the chunkiness that Hogan aims to uncover, he needs to measure distances with an accuracy of 10⁻¹⁸ meters—about 100 million times smaller than a hydrogen atom—and gather data at an extraordinary rate of 100 million readings per second. Remarkably, this experiment is not only feasible but also practical due to advancements in photonics and readily available components. "We managed to keep costs low thanks to these technological advancements," Hogan remarks. The holometer is currently operational and collecting data at the desired precision, with preliminary results expected by year-end.
Despite Hogan's optimism, he faces skepticism from many in the theoretical physics community. A successful outcome for the holometer could undermine a significant amount of ongoing string theory research. Yet, amidst this tension, there exists a shared conviction among many theorists that quantum mechanics will ultimately supersede general relativity. Given that the other three fundamental forces adhere to quantum principles, it seems reasonable to expect gravity to follow suit.
For many current theorists, the belief in the primacy of quantum mechanics is deeply entrenched. They view the classical reality of large-scale physics as an illusion—an approximation stemming from the more fundamental quantum world operating at minuscule scales. Hogan's chunky space theory aligns with this perspective.
In a historical context, Hogan likens his project to the iconic Michelson-Morley experiment of the 19th century, which searched for aether—the hypothetical medium through which light waves were thought to propagate. The experiment yielded null results, inspiring Einstein's special theory of relativity, which ultimately transformed our understanding of physics. Interestingly, the Michelson-Morley experiment utilized mirrors and split beams of light, echoing the setup of Hogan's holometer.
"We're conducting the holometer in that spirit. Whether we observe something or not, the outcome will be significant. The purpose of the experiment is to uncover insights that could guide future theories," Hogan explains. "The true measure of a theorist is how they respond to unconventional ideas. Amidst a plethora of mathematical theories, I'm hopeful for experimental findings that shift theoretical focus."
Regardless of the outcomes, Hogan believes the holometer will contribute to resolving the conundrum of big versus small in physics. It aims to establish the correct understanding—or eliminate misconceptions—regarding the quantum structure of space and its implications for the relativistic laws governing gravity.
A Radical Alternative: Smolin's Perspective
If you're seeking a radically different perspective, Lee Smolin of the Perimeter Institute may be your answer. While Hogan approaches the issue cautiously, Smolin is an unapologetic dissenter. "Richard Feynman once advised me during my graduate studies: If every attempt to validate a theory has failed, it may be because the theory is fundamentally flawed. String theory has been languishing for decades without definitive progress."
Smolin's critique extends further. He argues that the small-scale focus of contemporary physics is inherently inadequate. Current quantum field theories effectively describe the behavior of individual particles or small systems but fall short of addressing the broader cosmic picture. They fail to explain why our reality is configured as it is. In Smolin's view, quantum mechanics serves merely as "a theory of subsystems within the universe."
If you've ever desired to be part of something greater, this is the field of physics for you.
Smolin advocates for a more comprehensive approach that treats the universe as a colossal system, suggesting the development of a new theory that encompasses everything. General relativity, unlike quantum frameworks, does not accommodate an external observer or clock since there is no "outside." Instead, it defines reality through relationships among objects and regions of space. Even inertia—the tendency of an object to maintain its state of motion—can be viewed as interconnected with the gravitational field of every other particle.
This notion merits further consideration. Imagine a thought experiment reminiscent of Einstein's original inquiries from 1907: two astronauts in an empty universe, one spinning and the other stationary. The spinning astronaut feels disoriented, while each perceives the other as the one in motion. Without an external reference, Einstein argued, neither can claim superiority, and there's no reason for one to experience different effects than the other.
The distinction becomes meaningful only when the rest of the universe is considered. In this classical interpretation of general relativity, inertia exists solely due to its measurement against the overarching gravitational field. This principle applies universally: the behavior of each component is inextricably linked to that of every other component. If you've ever yearned to be part of something larger, this is the physics for you. Smolin believes this perspective may yield significant insights into the fundamental workings of nature across various scales.
"General relativity is not just a subsystem description; it encompasses the entire universe as a closed system," he asserts. In resolving the clash between relativity and quantum mechanics, physicists would do well to follow Einstein's example and adopt a macro perspective.
Smolin recognizes that his ideas may challenge the prevailing inclination toward small-scale, quantum-based thinking. "I don't intend to provoke; it just happens that way. My goal is to articulate my conclusions clearly and let the discourse unfold," he states amicably. "I hope my arguments will lead to predictions that can be tested."
At first glance, Smolin's proposals appear to provide a solid foundation for concrete experimentation. He suggests that, much like the interconnectedness of the universe across space, it may also be interconnected across time. His reasoning has led him to hypothesize that the laws of physics may evolve throughout the universe's history. He has formulated two detailed theories in this regard, including the concept of cosmological natural selection, which envisions black holes as cosmic seeds giving rise to new universes. More recently, he has developed a thought-provoking hypothesis regarding the emergence of quantum mechanics, termed the principle of precedence—one that seems ripe for experimental validation.
Smolin's principle of precedence addresses the question of why physical phenomena are reproducible. When conducting an experiment previously performed, one expects consistent outcomes (e.g., striking a match produces a flame). We typically take reproducibility for granted, attributing it to the influence of a natural "law" that behaves consistently over time. Smolin proposes that these laws may actually emerge as quantum systems replicate the behavior of similar systems from the past.
A potential method for observing this emergence is to conduct an experiment that has never been performed before, thus lacking a precedent to mimic. Such an experiment could involve creating a complex quantum system with many components in a novel entangled state. If Smolin's principle holds, the system's initial response would be largely random. However, as the experiment is repeated, precedents would accumulate, leading to more predictable responses over time. "The process through which the universe builds precedent could be indistinguishable from experimental noise," Smolin admits, "but it's not impossible."
While precedence may manifest at the atomic level, its effects would be felt on a cosmic scale. This notion ties back to Smolin's argument that reductionist approaches are insufficient for resolving larger enigmas. Addressing the interplay between the two classes of physics is crucial, but the ultimate goal is to uncover why the universe is structured as it is. Why does time flow in a single direction? How did we arrive at this specific set of laws governing our universe?
The absence of meaningful answers to these inquiries highlights the "deeply flawed" understanding of quantum field theory, according to Smolin. Like Hogan, he prioritizes the broader quest for fundamental truths over the outcome of any single experiment. For Smolin, this entails formulating a cohesive narrative about the universe, yielding predictive capabilities while also elucidating the unique properties of atoms, planets, rainbows, and human beings. He draws inspiration from Einstein yet again.
"The recurring lesson of general relativity is the triumph of relationalism," Smolin observes. The most promising path toward understanding the universe may lie in engaging with it as an interconnected whole.
The Referee: Sean Carroll
In the debate between big and small, Sean Carroll—a cosmology and gravitational physics expert at Caltech—serves as a knowledgeable arbitrator. He possesses a comprehensive understanding of both relativity and quantum mechanics, complemented by a sense of humor evident in the title of his personal blog: Preposterous Universe.
Carroll quickly aligns with the quantum perspective. "Most of us in this field believe quantum mechanics is significantly more fundamental than general relativity," he asserts. This viewpoint has prevailed since the 1920s, when Einstein sought to identify flaws in quantum theory's counterintuitive predictions but repeatedly failed. The recent Dutch experiment showcasing instantaneous quantum connections between particles—events Einstein dismissed as "spooky action at a distance"—further underscores the evidence's strength.
Nonetheless, Carroll emphasizes that the overarching issue is not simply a matter of general relativity versus quantum field theory; it's about classical versus quantum dynamics. Despite its perceived oddity, relativity operates on classical principles of cause and effect, while quantum mechanics diverges from this norm. Although Einstein hoped that deeper discoveries might reveal a deterministic reality underlying quantum mechanics, no such order has yet emerged. The demonstrated existence of "spooky action" suggests that such order may not be attainable.
"People often underestimate how thoroughly quantum mechanics dismantles our concepts of space and locality," Carroll notes. These notions may represent large-scale impressions arising from fundamentally different small-scale phenomena, akin to Hogan's argument that a three-dimensional reality stems from two-dimensional quantum units.
While Carroll generally supports Hogan's approach, he views the holometer as a long shot, albeit one outside his primary focus. Conversely, he regards Smolin's emphasis on space as a foundational element as misguided, equating it to arguing that air is more fundamental than atoms. In terms of potential pathways to advance physics, Carroll remains optimistic about string theory, which he sees as a natural extension of quantum field theory, consistent with contemporary thought.
Yet, despite endorsing the quantum perspective, Carroll acknowledges the substantial gaps in quantum theory's explanatory power. "Our failure to pinpoint the correct version of quantum mechanics is embarrassing," he admits. "Moreover, our current understanding of quantum mechanics is inadequate when applied to cosmology or the universe as a whole. We can't even define time." Both Hogan and Smolin share this sentiment, albeit with differing opinions on how to address it. Carroll prefers a bottom-up explanation, positing that time emerges from small-scale quantum interactions, while he remains "entirely agnostic" regarding Smolin's competing hypothesis that time is more fundamental and universal. Thus, the jury remains out on the nature of time.
Regardless of how these theories ultimately unfold, the macro scale is undeniably significant because it encompasses our lived experiences. Essentially, the universe as a cohesive entity presents the answer, and physicists are challenged to articulate it through their equations. Even if Hogan's hypothesis about space chunks is correct, these must ultimately average out to the smooth reality we perceive daily. Conversely, even if Smolin's views prove incorrect, the universe presents myriad unique properties that require explanation—something quantum physics alone cannot adequately address.
By challenging existing paradigms, both Hogan and Smolin contribute to bridging the gap in physics. They are guiding the field toward not only reconciling quantum mechanics with general relativity but also aligning our perceptions with the underlying realities of the universe. The next groundbreaking theory in physics promises to yield not only beautiful mathematical frameworks and astonishing technologies but also a deeper understanding that resonates with us—the observers who define the fundamental scale of existence.
The Black Hole Conundrum
In the academic realm, the clash between top-down and bottom-up physics theories is manifesting in scholarly articles and intricate experimental setups. However, theorists across the spectrum acknowledge that neither pure speculation nor feasible tests may suffice to break the deadlock. Fortunately, there are alternative avenues for finding a more definitive resolution, one of which has garnered broad acceptance among physicists, regardless of their ideological positions.
"Black hole physics provides a clear experimental target," states Craig Hogan, a theoretical astrophysicist from the University of Chicago and director of the Center for Particle Astrophysics at Fermilab. "The quantum black hole issues are critical," concurs Lee Smolin, a founding member of the Perimeter Institute for Theoretical Physics in Canada.
But why focus on black holes? These enigmatic entities are often associated more with questions than answers. They cannot be created in laboratories, manipulated with instruments, or closely examined by spacecraft. Yet, they represent the unique intersection of Hogan's and Smolin's theories—where the entirety of quantum physics collides with general relativity in an unavoidable manner.
At the event horizon—the outer boundary of a black hole—gravity is so intense that not even light can escape, presenting an extreme test case for general relativity. Atomic-scale events become dramatically stretched and slowed down at the horizon, which also delineates two distinct zones: inside and outside. Interestingly, a stellar-mass black hole's size is roughly equivalent to that of Los Angeles, whereas a black hole with Earth's mass would be comparable to the size of a marble. Black holes bring the big-small dilemma of physics into a relatable context.
The significance of black holes in resolving this issue accounts for the fervent debates surrounding them, often involving Stephen Hawking and his contemporaries. Remarkably, we don't need to approach black holes directly to study them. Quantum theory suggests that a single particle could potentially exist both within and beyond the event horizon, a paradox that defies logic. Additionally, the fate of information that enters a black hole poses another conundrum, as it appears to vanish despite theoretical assertions that information cannot be destroyed. Addressing these contradictions compels theoretical physicists to engage more rigorously with the interplay between quantum mechanics and general relativity.
Best of all, insights garnered from black hole research will not remain confined to theoretical frameworks. Astrophysicists are employing increasingly sophisticated techniques to investigate the region surrounding event horizons by observing the brilliant clouds of particles swirling around various black holes. A significant breakthrough is on the horizon: the Event Horizon Telescope, which aims to connect approximately a dozen radio dishes worldwide to create an enormous networked telescope capable of capturing clear images of Sagittarius A*, the supermassive black hole at our galaxy's center. Anticipated results from this project could emerge as early as 2020, providing critical insights to constrain black hole theories and shedding light on the big-small dilemma.
Imagine human researchers utilizing a planet-sized array of radio telescopes to study a star-sized black hole, all in pursuit of resolving the subatomic and cosmic enigmas at the heart of physics. If successful, the scale of this achievement would be unprecedented.
Corey S. Powell is the science editor at Aeon and editor-at-large at Discover magazine, situated between a proton and a galaxy in scale. He is a frequent presence on Twitter @coreyspowell.
Originally published at Nautilus on June 8, 2017.
The first video titled "General Relativity VS Quantum Mechanics | Why Are They Incompatible?" explores the tensions between these two foundational theories in physics, examining their fundamental differences and the implications for our understanding of the universe.
The second video titled "Entanglement as the Glue of Spacetime" delves into the concept of quantum entanglement and its potential role in unifying the principles of quantum mechanics and general relativity.