Tag Archives: dark matter

The Mystery of Gravity

In grade school we learned that gravity is an always-attractive force that acts between particles of matter. Later on, we learn that it has an infinite range through space, weakens as the inverse-square of the distance between bodies, and travels exactly at the speed of light.

But wait….there’s more!


It doesn’t take a rocket scientist to remind you that humans have always known about gravity! Its first mathematical description as a ‘universal’ force was by Sir Isaac Newton in 1666. Newton’s description remained unchanged until Albert Einstein published his General Theory of Relativity in 1915. Ninety years later, physicists, such as Edward Witten, Steven Hawkings, Brian Greene and Lee Smolin among others, are finding ways to improve our description of ‘GR’ to accommodate the strange rules of quantum mechanics. Ironically, although gravity is produced by matter, General Relativity does not really describe matter in any detail – certainly not with the detail of the modern quantum theory of atomic structure. In the mathematics, all of the details of a planet or a star are hidden in a single variable, m, representing its total mass.


The most amazing thing about gravity is that is a force like no other known in Nature. It is a property of the curvature of space-time and how particles react to this distorted space. Even more bizarrely, space and time are described by the mathematics of  GR as qualities of the gravitational field of the cosmos that have no independent existence. Gravity does not exist like the frosting on a cake, embedded in some larger arena of space and time. Instead, the ‘frosting’ is everything, and matter is embedded and intimately and indivisibly connected to it. If you could turn off gravity, it is mathematically predicted that space and time would also vanish! You can turn off electromagnetic forces by neutralizing the charges on material particles, but you cannot neutralize gravity without eliminating spacetime itself.  Its geometric relationship to space and time is the single most challenging aspect of gravity that has prevented generations of physicists from mathematically describing it in the same way we do the other three forces in the Standard Model.

Einstein’s General Relativity, published in 1915, is our most detailed mathematical theory for how gravity works. With it, astronomers and physicists have explored the origin and evolution of the universe, its future destiny, and the mysterious landscape of black holes and neutron stars. General Relativity has survived many different tests, and it has made many predictions that have been confirmed. So far, after 90 years of detailed study, no error has yet been discovered in Einstein’s original, simple theory.

Currently, physicists have explored two of its most fundamental and exotic predictions: The first is that gravity waves exist and behave as the theory predicts. The second is that a phenomenon called ‘frame-dragging’ exists around rotating massive objects.

Theoretically, gravity waves must exist in order for Einstein’s theory to be correct. They are distortions in the curvature of spacetime caused by accelerating matter, just as electromagnetic waves are distortions in the electromagnetic field of a charged particle produced by its acceleration. Gravity waves carry energy and travel at light-speed. At first they were detected indirectly. By 2004, astronomical bodies such as the  Hulse-Taylor orbiting pulsars were found to be losing energy by gravity waves emission at exactly the predicted rates. Then  in 2016, the  twin  LIGO gravity wave detectors detected the unmistakable and nearly simultaneous pulses of geometry distortion created by colliding black holes billions of light years away.

Astronomers also detected by 1997 the ‘frame-dragging’ phenomenon in  X-ray studies of distant black holes. As a black hole (or any other body) rotates, it actually ‘drags’ space around with it. This means that you cannot have stable orbits around a rotating body, which is something totally unexpected in Newton’s theory of gravity. The  Gravity Probe-B satellite orbiting Earth also confirmed in 2011 this exotic spacetime effect at precisely the magnitude expected by the theory for the rotating Earth.

Gravity also doesn’t care if you have matter or anti-matter; both will behave identically as they fall and move under gravity’s influence. This quantum-scale phenomenon was searched for at the Large Hadron Collider ALPHA experiment, and in 2013 researchers placed the first limits on how matter and antimatter ‘fall’ in Earth’s gravity. Future experiments will place even more stringent limits on just how gravitationally similar matter and antimatter are. Well, at least we know that antimatter doesn’t ‘fall up’!

There is only one possible problem with our understanding of gravity known at this time.

Applying general relativity, and even Newton’s Universal Gravitation, to large systems like galaxies and the universe leads to the discovery of a new ingredient called Dark Matter. There do not seem to be any verifiable elementary particles that account for this gravitating substance. Lacking a particle, some physicists have proposed modifying Newtonian gravity and general relativity themselves to account for this phenomenon without introducing a new form of matter. But none of the proposed theories leave the other verified predictions of general relativity experimentally intact. So is Dark Matter a figment of an incomplete theory of gravity, or is it a here-to-fore undiscovered fundamental particle of nature? It took 50 years for physicists to discover the lynchpin particle called the Higgs boson. This is definitely a story we will hear more about in the decades to come!

There is much that we now know about gravity, yet as we strive to unify it with the other elementary forces and particles in nature, it still remains an enigma. But then, even the briefest glance across the landscape of the quantum world fills you with a sense of awe and wonderment at the improbability of it all. At its root, our physical world is filled with improbable and logic-twisting phenomena and it simply amazing that they have lent themselves to human logic to the extent that they have!


Return here on Monday, March 13 for my next blog!

Cancer and Cosmology

For the treatment of my particular cancer, small B-cell follicular non-Hodgkins Lymphoma, I will soon be starting a 6-month course of infusions of Rituximab and Bendamustine. The biology of these miracle drugs seems to be very solid and logically sound. This one-two chemical punch to my lymphatic system will use targeted antibodies to bind with the CD20 receptor on the cancerous B-cells. This will set in motion several cellular mechanisms that will kill the cells. First, the antibody bound to the CD20 receptor attracts T-cells in the immune system to treat the cancerous B-cell as an invader. Thus begins my immune system’s process of killing the invader. The antibody also triggers a reaction in the cell to commit suicide called apoptosis. Even better, Rituximab does not set in motion the process to kill normal B-cells!

The promise is that my many enlarged lymph nodes chock-a-block with the cancerous B-cells will be dramatically reduced in size to near-normal levels as they are depopulated of the cancerous cells. So why do some patients not all show the same dramatic reductions? About 70% respond to this therapy to various degrees while 10% do not. Why, given the impeccable logic of the process, aren’t the response rates closer to 100%?

Meanwhile, in high-energy physics, supersymmetry is a deeply beautiful and lynch-pin mathematical principle upon which the next generations of theories about matter and gravity are based. By adding a teaspoon of it to the Standard Model, which currently accounts in great mathematical detail for all known particles and forces, supersymmetry provides an elegant way to explore an even larger universe that includes dark matter, unifying all natural forces, and explaining many of the existing mysteries not answered by the Standard Model.
Called the Minimal Supersymmetric Standard Model (MSSM), Nature consistently rewards the simplest explanations for physical phenomena, so why has there been absolutely no sign of supersymmetry at the energies predicted by MSSM, and being explored by the CERN Large Hadron Collider?

In both cases, I have a huge personal interest in these logically compelling strategies and ideas: One to literally save my life, and the other to save the intellectual integrity of the physical world I have so deeply explored as an astronomer during my entire 40 year career. In each case, the logic seems to be flawless, and it is hard to see how Nature would not avail itself of these simple and elegant solutions with high fidelity. But for some reason it chooses not to do so. Rituximab works only imperfectly, while supersymmetry seems an un-tapped logical property of the world.

So what’s going on here?

In physics, we deal with dumb matter locked into simple systems controlled by forces that can be specified with high mathematical accuracy. The fly in the ointment is that, although huge collections of matter on the astronomical scale follow one set of well-known laws first discovered by Sir Isaac Newton and others, at the atomic scale we have another set of laws that operate on individual elementary particles like electrons and photons. This is still not actually a problem, and thanks to some intense mathematical reasoning and remarkable experiments carried out between 1920 and 1980, our Standard Model is a huge success. One of the last hold-outs in this model was the discovery of the Higgs Boson in 2012, some 50 years after its existence was predicted! But as good as the Standard Model is, there seem to be many loose ends that are like red flags to the inquiring human mind.

One major loose end is that astronomers have discovered what is popularly called ‘dark matter’, and there is no known particle or force in the Standard Model to account for it. Supersymmetry answers the question, why does nature have two families of particles when one would be even simpler? Amazingly, and elegantly, supersymmetry answers this question by showing how electrons, and quarks, which are elementary matter particles, are related to photons and gluons, which are elementary force-carrying particles. But in beautifully unifying the particles and forces, it also offers up a new family of particles, the lightest of which would fit the bill as missing dark matter particles!

This is why physicists are desperately trying to verify supersymmetry, not only to simplify physics, but to explain dark matter on the cosmological scale. As an astronomer, I am rooting for supersymmetry because I do not like the idea that 80% of the gravitating stuff in the universe is not stars and dust, but inscrutable dark matter. Nature seems not to want to offer us this simple option that dark matter is produced by ‘supersymmetric neutralinos’. But apparently Nature may have another solution in mind that we have yet to stumble upon. Time will tell, but it will not be for my generation to discover.

On the cancer-side of the equation, biological systems are gears-within-gears in a plethora of processes and influences. A logically simple idea like the Rituximab treatment looks compelling if you do not look too closely at what the rest of the cancerous B-cells are doing, or how well they like being glommed onto by a monoclonal antibody like Rituximab. No two individuals apparently have the same B-cell surfaces, or the same lymphatic ecology in a nearly-infinite set of genetic permutations, so a direct chemical hit by a Rituximab antibody to one cancerous B-cell may be only a glancing blow to another. This is why I am also rooting for my upcoming Rituximab treatments to be a whopping success. Like supersymmetry, it sure would simplify my life!

The bottom line seems to be that, although our mathematical and logical ideas seem elegant, they are never complete. It is this incompleteness that defeats us, sometimes by literally killing us and sometimes by making our entire careers run through dark forests for decades before stumbling into the light.


Check back here on Wednesday, December 28 for the next installment!

Rainbow image credit: Daily Mail: UK