# Lee Smolin

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### "Loop Quantum Gravity,"

### "A perspective on the landscape problem"

### "Positive energy in quantum gravity"

**Lee Smolin** (born June 6, 1955) is an American theoretical physicist, academic and author known for his contributions to quantum gravity theory, in particular the approach known as loop quantum gravity. His research interests include cosmology, elementary particle theory, the foundations of quantum mechanics, and theoretical biology.

## Contents

- 1 Quotes
- 1.1
*The Life of the Cosmos*(1997) - 1.2
*Three Roads to Quantum Gravity*(2000) - 1.3 "Loop Quantum Gravity,"
*The New Humanists: Science at the Edge*(2003) - 1.4
*The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next*(2007) - 1.5 "A perspective on the landscape problem"
*arXiv*(Feb 15, 2012) - 1.6
*Time Reborn: From the Crisis in Physics to the Future of the Universe*(2013) - 1.7 "Positive energy in quantum gravity"
*arXiv*(Jun 10, 2014)

- 1.1
- 2 External links

## Quotes[edit]

- Is the flow of time something real, or might our sense of time passing be just an illusion that hides the fact that what is real is only a vast collection of memories?
- as quoted by Ray Kurzweil,
*The Age of Spiritual Machines: When Computers Exceed Human Intelligence*(2000)

- as quoted by Ray Kurzweil,

*The Life of the Cosmos* (1997)[edit]

- Since the 1960s, particle theory had been split into two groups: those following the atomism of the quark theory and those who had followed the anti-atomism of that had led from the bootstrap program to the string theory. What happened in 1984 was that it was realised that string theory could combine and satisfy the aspirations of both approaches to fundamental physics. Thus, the community of gauge theorists, driven by the failure of the proton decay experiments to search for new ideas that could unify physics, all of a sudden encountered their old friends, the string theorists, in the middle of what might be called a desert of disappointed expectations.

*Three Roads to Quantum Gravity* (2000)[edit]

- The first principle of cosmology must be 'There is nothing outside the universe'. This is not to exclude religion or mysticism... But if it is knowledge that we desire... we need to seek answers to questions about the things we can see... only things that exist in the universe.

- There is no meaning to space that is independent of the relationships among real things of the world. ...Space is nothing apart from the things that exist. ...If we take out all the words we are not left with an empty sentence, we are left with nothing.

- The geometry of space changes when things in the universe change their relationships to one another.

- There are unfortunately not a few good professional physicists who still think about the world as if space and time had an absolute meaning.

- I believe that the main lesson of relativity and quantum theory is that the world is nothing but an evolving network of relationships.

- The relational picture of space and time has implications that are as radical as those of natural selection, not only for science but for our perspective on who we are and how we came to exist in this evolving universe of relations.

- We are the result of processes much more complicated than the small aspects of our lives and societies over which we have some control.

- There is no fixed, eternal frame to the universe to define what may or may not exist.

- There is nothing beyond the world except what we see, no background to it except its particular history.

- 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.

- In the theory of electric fields it is assumed that points have meaning. ...Physicists using general relativity... cannot speak of a point, except by naming some features of the field lines that will uniquely distinguish that point. ...the network of relationships evolve with time... constantly changing.

- There is no time apart from change. There is no such thing as a clock outside the network of changing relationships. ...one can only compare how fast one thing is happening with the rate of some other process.

- Time is described only in terms of change in the network of relationships that describes space.

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

- 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*.

### "Loop Quantum Gravity," *The New Humanists: Science at the Edge* (2003)[edit]

- John Brockman, Editor

- 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.

- 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.
- Describing work with Ted Jacobson

- 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.

- 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.

- 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!

- 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...

- 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.

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

*The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next* (2007)[edit]

- 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.

- 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.

*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*.

- 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*.

- 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.

### "A perspective on the landscape problem" *arXiv* (Feb 15, 2012)[edit]

- 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...

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

- 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.

- 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.

- 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.

*Time Reborn: From the Crisis in Physics to the Future of the Universe* (2013)[edit]

- 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.

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

- 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.

- 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.

- 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.

### "Positive energy in quantum gravity" *arXiv* (Jun 10, 2014)[edit]

- arXiv:1406.2611v1 based partly on results derived with Artem Starodubtsev

- 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.

- 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.

- 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.