Chapter 1: The Physicist Who Questions Dark Matter

Mordehai Milgrom, a prominent figure at the Weizmann Institute of Science, refers to a colleague as one of the "dark matter people." This colleague is on a quest for evidence of dark matter just down the hall. "There are no 'dark matter people' and 'MOND people,'" the colleague counters. Milgrom proudly identifies as "MOND people," advocating for Modified Newtonian Dynamics (MOND), a theory that seeks to amend Newtonian physics rather than assume the existence of dark matter and dark energy—elements that constitute a staggering 95.1% of the universe's total mass-energy according to conventional cosmology.

This exchange reflects Milgrom's gentle yet determined character. At 70, dressed in shorts amidst the Israeli summer heat, he appears unassuming. Yet, he asserts that he is the third scientist to challenge Newtonian physics, following Max Planck and Einstein. This year marks Milgrom's 50th anniversary at the Weizmann Institute, and I sat down with him to understand the experience of being a scientific outlier, his thoughts on Thomas Kuhn's The Structure of Scientific Revolutions, and his stance against the existence of dark matter and dark energy.

What sparked your interest in the motion of stars?

I distinctly recall how physics captivated me at 16; it seemed a profound way to comprehend the workings of the universe, far beyond what my peers understood. It wasn’t a planned endeavor, just a daily fascination. I cherished physics much like others do art or sports. I never envisioned making a significant discovery, such as correcting Newton.

Perhaps we should reconsider Newton's laws instead of fabricating an entirely new category of matter to match our observations.

In school, I had an excellent physics teacher, yet textbook material represents final products. You miss the struggle involved in groundbreaking science, where clarity is absent, and progress is often intuitive and fraught with errors. Education tends to present science as a linear trajectory: a body of knowledge expands with each new discovery. However, real scientific advancement is anything but straightforward.

How did you become involved with dark matter?

As I neared the end of my Ph.D., the physics department at Weizmann sought to broaden its focus, prompting three leading Ph.D. students in particle physics to select a new area. We chose astrophysics, leading me to Cornell for further training. After several years in high-energy astrophysics, I shifted to studying galaxy dynamics, shortly after the first detailed measurements of star velocities in spiral galaxies emerged. This data revealed a significant discrepancy.

To grasp this issue, one must consider celestial rotations. The Earth orbits the sun, which in turn orbits the Milky Way's center. Gravitational forces between the sun and planets maintain balance. Based on Newtonian principles, Mercury orbits the sun at over 100,000 miles per hour, while Neptune moves at a mere 10,000 miles per hour.

One might assume that similar logic applies to galaxies: stars farther from the center should revolve more slowly. While the measurements for inner stars aligned with Newtonian physics, outer stars exhibited unexpectedly rapid speeds, defying gravitational expectations based on visible mass. The discrepancy intensified in the late 1970s, when radio telescopes detected cold gas clouds at galaxy outskirts, orbiting the center much farther than stars, complicating the scientific puzzle.

One approach to resolving this anomaly is to propose additional unseen matter—dark matter.

What led you to doubt the existence of dark matter?

I observed a pattern in the anomaly: rotational velocities were not only higher than anticipated but also consistent across radii. Why was this the case? If dark matter were present, star speeds would indeed rise, but the rotation curves—speed plotted against radius—could still fluctuate depending on distribution. The lack of variability struck me as peculiar. In 1980, during my sabbatical at the Institute for Advanced Studies in Princeton, I hypothesized that if rotational speeds are constant, we may be witnessing a new natural law. Instead of postulating dark matter, we might need to revise Newton's laws.

To modify established laws, one must identify factors distinguishing solar systems from galaxies. I created a chart comparing various attributes—size, mass, rotation speed, etc.—for Earth, our solar system, and various galaxies. For instance, galaxies are larger than solar systems, suggesting a potential breakdown of Newton's laws over vast distances. However, if that were true, the rotation anomalies should increase in larger galaxies, which did not occur.

Ultimately, I discovered a key factor in acceleration—the rate at which an object's velocity changes.

In contrast to typical vehicles that accelerate in a straight line, think of a merry-go-round; one can maintain circular motion while accelerating. The same principle applies to celestial bodies. The acceleration for stars orbiting a galaxy is approximately a hundred million times less than that of Earth around the sun.

For these minor accelerations, MOND introduces a new constant of nature, denoted a₀. Recall Newton's second law: force equals mass times acceleration (F=ma). This law is effective for higher accelerations, but I proposed that for much lower accelerations—below that of our sun around the galactic center—the force could be expressed as F=ma²/a₀.

In simpler terms, Newton's laws suggest that a star's rotational speed around a galaxy's center should diminish as the star moves farther from the center. If MOND holds true, rotational speeds would stabilize, eliminating the necessity for dark matter.

What were your colleagues' reactions at Princeton?

I hesitated to share my thoughts with Princeton colleagues, fearing they might label me as irrational. By 1981, when I had a clearer concept of MOND, I wanted to avoid others co-opting my ideas, which seems absurd in hindsight. Not surprisingly, no one gravitated toward my ideas, even when I desperately hoped they would.

At 35, you proposed a revision to Newton's laws. Why not?

If something is flawed, it deserves correction. I didn't see the boldness in my proposition; I was rather naïve. I underestimated how deeply entrenched conventions and interests influence scientists, just as they do in other fields.

Like the ideas presented in Kuhn's The Structure of Scientific Revolutions?

I cherish that book, having read it multiple times. It illustrates how many scientists, including myself, experience similar narratives throughout history. It's easy to dismiss those who opposed what we now consider sound science, but are we truly different? Kuhn emphasizes that dissenters often possess valid concerns, albeit through unique lenses not widely recognized. I find humor in it now, especially as MOND has gained traction, but there were challenging moments of isolation.

What is it like to be a scientific maverick?

Overall, the past 35 years have been exhilarating and rewarding due to my advocacy for an unconventional paradigm. I'm naturally a loner, and despite the doubting periods, I prefer this path to simply following the mainstream. My confidence in MOND's fundamental validity from the outset helped me navigate these challenges. The ongoing resistance to MOND has two advantages: it afforded me time to make more contributions than I might have had the community embraced it early, and once MOND gains acceptance, its initial opposition will highlight its complexity.

By the end of my Princeton sabbatical, I had secretly written three papers introducing MOND. However, publishing them proved difficult. I initially submitted my foundational paper to journals like Nature and Astrophysical Journal Letters, but they were swiftly rejected. Eventually, all three papers were published together in Astrophysical Journal.

The first person I confided in about MOND was my wife, Yvonne. I feel emotional discussing this—she's not a scientist, yet she has been my greatest supporter.

Integrating MOND with Einsteinian physics has become more feasible.

The first advocate for MOND was another scientific maverick: the late Professor Jacob Bekenstein, who proposed that black holes possess a distinct entropy, later known as Bekenstein-Hawking entropy. After submitting my initial MOND trilogy, I circulated preprints among various astrophysicists, and Jacob was the first to engage with me on MOND, offering enthusiasm and encouragement from the start.

Gradually, the initial opposition to dark matter evolved from merely two physicists to several hundred supporters, or at least those who take MOND seriously. While dark matter remains the prevailing scientific consensus, MOND stands as a formidable challenger, asserting that dark matter is akin to the ether of our time.

So what has transpired regarding dark matter? Essentially, nothing significant. Numerous experiments aimed at detecting dark matter—including those at the Large Hadron Collider, various underground studies, and space missions—have failed to yield direct evidence of its existence. In contrast, MOND has consistently predicted the rotational behaviors of an increasing number of spiral galaxies—over 150 to date.

What about the claim that some galaxies don't align with MOND's predictions?

That assertion holds truth, but it's acceptable. MOND's predictions depend on observable measurements. Using the visible matter distribution, MOND can forecast galaxy dynamics. However, our methods of estimating galaxy mass often rely on light measurements without precise distance data, leading to uncertainty in mass estimations. Other variables, such as molecular gas, are also unobservable. While some galaxies might not perfectly align with MOND's predictions, the abundance of data supporting MOND's validity is remarkable.

Critics argue that MOND's primary flaw lies in its incompatibility with relativistic physics.

In 2004, Bekenstein proposed TeVeS, or Relativistic Gravitational Theory for MOND. Since then, various relativistic MOND formulations have emerged, including my own, known as Bimetric MOND or BIMOND.

Thus, integrating MOND into Einsteinian physics is no longer an issue. I still hear claims to the contrary, but they often stem from individuals who are not updated on recent developments.

Another common argument from cosmologists is that dark matter is necessary for understanding motion on even larger scales. How does MOND address this?

According to the Big Bang theory, the universe originated from a singularity 13.8 billion years ago. Observations of cosmic background radiation suggest that the gravitational influence of all matter is insufficient to form the structures we observe today within that timeframe. Thus, dark matter was positioned as a solution, as it does not emit radiation but interacts gravitationally with visible matter. By the 1980s, the prevailing cosmological perspective was that dark matter constituted an astonishing 95% of the universe's matter—until the unexpected revelation in 1998.

We discovered that the universe's expansion is accelerating, contrary to previous assumptions of deceleration. This led to the introduction of dark energy—an entirely new entity. The current cosmological framework posits that the universe comprises 70% dark energy, 25% dark matter, and 5% ordinary matter.

MOND's constant aligns with the speed of light squared divided by the universe's radius.

However, dark energy is merely a stopgap, similar to dark matter. Just as with dark matter, one can either create an entirely new type of energy and expend years deciphering its properties or attempt to refine existing theories.

Among other insights, MOND reveals a profound connection between the structure and dynamics of galaxies and cosmology. This connection is unexpected in accepted physics; galaxies, as small structures, should behave differently from the larger universe without contradicting cosmological consensus. Yet, MOND forges this link, integrating the two realms.

Interestingly, MOND's constant, a₀, closely approximates the acceleration characterizing the universe itself. In fact, MOND's constant equates to the speed of light squared divided by the universe's radius.

To your earlier question, the current conundrum is indeed valid. MOND lacks a comprehensive cosmological framework, but we are actively working on it. Once MOND is fully understood, I believe we will also grasp the universe's expansion, and vice versa: a novel cosmological theory could elucidate MOND. Wouldn't that be remarkable?

What are your thoughts on proposed unified theories of physics that integrate MOND with quantum mechanics?

These ideas echo my 1999 paper on "MOND as a vacuum effect," suggesting that the quantum vacuum in our universe might produce MOND behavior within galaxies, where the cosmological constant manifests as the MOND acceleration constant, a₀. I'm thrilled to witness these proposals emerging, especially from researchers outside the traditional MOND community. It's crucial that scholars from diverse backgrounds engage with MOND and contribute fresh perspectives to deepen our understanding of its origins.

And what if you achieved a unified theory of physics that explained everything? Then what?

While I'm not religious, I often reflect on our small planet and the meticulous work we physicists undertake. Who knows? Perhaps somewhere out there, in one of those galaxies I’ve studied, a unified theory of physics exists with a variation of MOND embedded within it. But then I think: So what? We still enjoyed the process of mathematical exploration. The thrill of grappling with the universe's complexities is what matters, even if the universe remains indifferent to our efforts.

Oded Carmeli is a science journalist and poet based in Tel Aviv, Israel.

Chapter 2: Understanding the Universe Through Dark Matter

The first video, "Dark Energy and Dark Matter: The Truth No Scientists Can Deny," delves into the ongoing debate surrounding dark energy and dark matter, exploring the implications of these concepts on our understanding of the universe.

Chapter 3: New Perspectives on Cosmology

The second video, "Sir Roger Penrose: New Cosmological View of Dark Matter, which Strangely and Slowly Decays," presents insights from Sir Roger Penrose, offering a fresh perspective on dark matter and its role in the cosmos.