Cosmologists

"Obvious results provoke opposition. The more obvious the result, the stronger the opposition."

"What struck me was some regularity in the anomaly. The rotational velocities were not just larger than expected, they became constant with radius. Why? Sure, if there was dark matter, the speed of stars would be greater, but the rotation curves, meaning the rotational speed drawn as a function of the radius, could still go up and down depending on its distribution. But they didn't. That really struck me as odd. So, in 1980, I went on my Sabbatical in the Institute for Advance Studies in Princeton with the following hunch: If the rotational speeds are constant, then perhaps we’re looking at a new law of nature. If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements."

"It is often stated that the evidence for is overwhelming. This is not quite correct: the evidence for mass discrepancies is overwhelming. These might be attributed to either dark matter or a modification of gravity."

"I did encounter much opposition and sheer disregard, and this was against my expectation. I was surprised that people thought that this was not a legitimate avenue to explore. Many times I heard people say that it is too early to start considering such heretical ideas. Why? I wondered. I thought it was legitimate to consider anything."

"I have experienced time and again people dismissing the data because they think MOND is wrong, so I am very consciously drawing a red line between the theory and the data."

"The nice thing is that MOND has not had to change or adjust itself to the many observations that came after it was proposed. … In the end, the process that might sway people's minds could be a slow one, in which they become disillusioned and dissatisfied with DM, while MOND explains more and more of the observations. And, if MOND does turn out to have some truth to it, the fact that it encountered so much opposition will just show how nontrivial a thought it was."

"To explain the appearance in MOND of a cosmological acceleration constant, a0, I suggest that MOND inertia — as embodied in the actions of free particles and fields — is due to effects of the vacuum. The same vacuum effects enter both MOND (through a0) and cosmology e.g. through a cosmological constant Λ)."

"It seems to me that in understanding MOND and its fundamentals we have only scratched the surface. If the developments of quantum mechanics and relativity are any lesson here, departures of such magnitude from long- and well-tested physics may bring with them completely new concepts, not summarized by mere modifications of the governing actions or the equations of motion. MOND may also turn out to bring in concepts that are presently beyond our ken (as hinted perhaps by the cosmological connotations of a0)."

"Dark matter cannot be any sort of matter that is known to science."

"Of all of the many mysteries of modern astronomy, none is more vexing than the nature of dark matter... This dark matter has eluded every effort by astronomers and physicists to bring it out of the shadows. A handful of us suspect that it might not really exist, and others are beginning to consider this possibility seriously."

"What makes MOND particularly intriguing is that it predicted many effects that could not even be tested when I formulated it."

"Why does MOND get any predictions right? It has had many a priori predictions come true. Why does this happen?"

"... I was deeply shocked when it was not my predictions or any of my immediate colleagues' predictions that came true in my data — but Milgrom's. This was completely outside my conceptual framework."

"Having begun my life in science searching for the equation beyond time, I now believe that the deepest secret of the universe is that its essence rests in how it unfolds moment by moment in time."

"We seem to have an ingrained idea that if something is valuable, it exists outside of time."

"Special pleading that the standards of science should be lessened to admit explanations with no falsifiable consequences, in order to keep alive a bold speculative idea, should be strongly resisted. ...ultimately science is not interested in what might be true, it is interested only in what can be convincingly demonstrated by deductions from observational evidence."

"I... propose that time and its passage are fundamental and real and the hopes and beliefs about timeless truths and timeless realms are mythology."

"The landscape problem and the problem of background independence are closely linked. The latter is the only route the former has to experimental confirmation."

"The hypothesis underlying all approaches to the landscape is that there is a cosmological setting in which different regions or epochs of the universe can have different effective laws. This implies the existence of spacetime regions not directly observable... These regions must either be in the past of our big bang, or far enough away from us to be causally unrelated."

"If we believe that the task of physics is the discovery of a timeless mathematical equation that captures every aspect of the universe, then we believe that the truth about the universe lies outside the universe."

"The landscape problem represents a serious problem in the development of science. Its solution requires... the construction of speculative cosmological scenarios, which posit regions or epochs of our universe for which we presently have no observable evidence. Nonetheless we must insist on taking seriously only scenarios and hypotheses that make falsifiable or strongly verifiable predictions, otherwise people can just make stuff up and the distinction between science and mythology becomes porous. ...there are already candidate solutions that make real, falsifiable predictions."

"Quantum theory can be described as a new kind of language to be used in a dialogue between us and the systems we study with our instruments. ...It tells us nothing about what the world would be like in our absence."

"Many of the founders of quantum mechanics, including Einstein, Erwin Schrödinger, and Louis de Broglie... were realists. For them quantum theory... was not a complete theory, because it did not provide a picture of reality absent our interaction with it. On the other side were Niels Bohr, Werner Heisenberg, and many others. Rather than being appalled, they embraced this new way of doing science."

"In both quantum theory and general relativity, we encounter predictions of physically sensible quantities becoming infinite. This is likely the way that nature punishes impudent theorists who dare to break her unity. ...If infinities are signs of missing unification, a unified theory will have none. It will be what we call a finite theory."

"Many of us believed in the possibility of a principled explanation for the laws of nature. We hoped to discover a short list of principles, which could be realized in a unique theory, which would retrodict the standard model and uniquely predict the physics to be discovered beyond it. The shocking implication of the results of Strominger reported in 1986 was that it was not to be, at least within the confines of string theory. ...String theory offered more, however... It offered the promise of a setting in which the different perturbative string theories are realized as expansions around solutions of a still more fundamental theory. ...That more fundamental theory would have to be background independent..."

"Relativity and... quantum... remain incomplete. ...the main reason each is incomplete is the existence of the other. The mind calls for a third theory to unify all of physics, and for a simple reason. Nature is... "unified." …interconnected, in that everything interacts with everything else."

"Unfortunately, so far... a truly background independent formulation of string theory has not been achieved... [It is] often called the search for M theory..."

"Thinking in time is not relativism but a form of relationalism... the truest description of something consists of specifying its relationships to other parts of the system it is part of."

"Combine general relativity and quantum theory into a single theory that can claim to be the complete theory of nature. This is called the problem of quantum gravity."

"Jacob Bekenstein found... in 1971 that every black hole must have an entropy proportional to the area of its horizon... Stephen Hawking then refined this by showing that the constant of proportionality must be... exactly one quarter. ...entropy is supposed to correspond to a measure of information ...Loop quantum gravity... [gives] a detailed description of the microscopic structure of a black hole. ...a horizon can have, for each quantized unit of area, a finite number of states. Counting them, we get exactly Bekenstein's result..."

"Spacetime... turns out to be discrete, described by a structure called spin foam."

"Some of the effects predicted by the theory [of loop quantum gravity] appear to be in conflict with one of the principles of Einstein's special theory of relativity... that the speed of light is a universal constant. ...Photons of higher energy travel slightly slower than low-energy photons. ...the principle of [general] relativity is preserved but Einstein's special theory of relativity requires modification. ...A photon can have an energy-dependent speed without violating the principle of [general] relativity!"

"In string theory one studies strings moving in a fixed classical spacetime. ...what we call a background-dependent approach. ...One of the fundamental discoveries of Einstein is that there is no fixed background. The very geometry of space and time is a dynamical system that evolves in time. The experimental observations that energy leaks from binary pulsars in the form of gravitational waves—at the rate predicted by general relativity to the... accuracy of eleven decimal places—tell us that there is no more a fixed background of spacetime geometry than there are fixed crystal spheres holding the planets up."

"I was joined by Carlo Rovelli, and we were able to make a full-fledged quantum theory of gravity... This became loop quantum gravity."

"While most people... were seeking to modify the principles of either relativity or quantum theory, we surprised ourselves (and many other people) by succeeding in putting them together without modifying their principles."

"String theory seems to be incompatible with a world in which a cosmological constant has a positive sign, which is what the observations indicate."

"There is a smallest unit of space. Its minimum value is given by the cube of the Planck length... If you take a volume of space and measure it to a very fine precision... It has to fall into some discrete series of numbers, just like the energy of an electron in an atom. ...we can calculate the discrete areas and volumes from the theory."

"Neither space nor time has any existence outside the system of evolving relationships that comprises the universe. Physicists refer to this feature of general relativity as background independence."

"In quantum theory, distance is inverse to energy, because you need particles of very high energy to probe very short distances. The inverse of the Planck energy is the Planck length."

"It is absurd in general relativity to speak of a universe in which nothing happens."

"We detect light and particles that have traveled billions of light years on their way across the universe to us. During the billions of years of travel, very small effects due to quantum gravity can be amplified to the point that we can detect them."

"Since the 1950s, the key equation of quantum gravity has been called the Wheeler-DeWitt equation. Bryce DeWitt and John Wheeler wrote it down, but in all the time since then, no one had been able to solve it. We found we could solve it exactly, and in fact we found an infinite number of exact solutions."

"From the beginning of physics, there have been those who imagined they would be the last generation to face the unknown. Physics has always seemed to its practitioners to be almost complete. This complacency is shattered only during revolutions, when honest people are forced to admit that they don't know the basics."

"To be human is to be suspended between danger and opportunity. ...The challenge of life is to choose wisely, from the enormous number of possible dangers, what's worth worrying about. It is also about choosing, from all the opportunities... always in the face of incomplete knowledge of the consequences."

"One of the evident facts about the world is the stability of empty space-time. In classical general relativity we can explain this as a consequence of the positive energy theorem... the positive energy theorem must extend in some suitable form to any viable quantum theory of gravity."

"Calculation establishes that a quantum positive energy theorem may be possible using a representation based on the Ashtekar connection. Left open is a key question of whether this use of the Ashtekar connection is necessary or whether a positive quantum energy result can be achieved for representations based on other connections, i.e., for [other] values of the Immirzi parameter."

"We show that some known classical results have particularly simple derivations within the Ashtekar formalism. These include Witten's positive energy theorem..."

"The positive energy theorem was for half a century or more an open challenge to relativists. Many attempts were made to prove flat spacetime was stable, but none completely succeeded completely until a majestic tour de force of geometric reasoning of Shoen and Yau. This was followed two years later by a proof of Witten, which was as elegant as it was short. It is this proof of Witten’s that we take as a template here for the quantum theory."

"The suggestion was that a positive energy proof for general relativity could be gotten by restricting supergravity to its bosonic sector, which is general relativity."

"We have known since the middle of the nineteenth century that the world is not composed only of particles. ...the world is also composed of fields. ...General relativity is a theory of... the gravitational field. ...Because there are three sets of field lines, the gravitational field defines a network of relationships having to do with how the... lines link with one another. ...This is why we call relativity a relational theory."