VALLEY COTTAGE LIBRARY BOOK DISCUSSION GROUP

"The Elegant Universe", by Brian Greene

Questions and Answers

Please feel free to email any questions you may have related to the book, the NOVA documentary or physics in general to:

Hal Evans & Joanne McNally.

We'll post the questions and answers (such as they are) below.

You may also find useful information in the Bibliography Section of the Physics for Poets course.

Questions from November 2

1. Question: Does string theory include a concept of what kind of energy is causing the strings to vibrate, and if so, where that energy is coming from? (If indeed energy is involved, which one could reasonably expect.)

   Answer: This is a question of cause and effect. In fact, energy does not cause strings to vibrate, rather the fact that the string vibrates means that it has energy. In other words, the vibration is the cause of the energy.
   

2. Question: What is the status of scientific investigations into the possibility that dark energy might be the source of both antigravitational and gravitational effects throughout the universe?

   Answer: Dark energy is intimately entwined with the effects of gravity. It is simply a new word for a concept that goes back to Einstein - something called the cosmological constant.

   Einstein originally introduced this idea into his equations governing the evolution of the size of the universe. It acts to counterbalance the pull of gravity, which would make the universe grow smaller (since everything is attracted to everything else). He needed this term because he was convinced (incorrectly) that the size of the universe was not changing. When it became apparent that the universe is actually expanding, but mainly because of the big bang, Einstein gave up this idea, calling it his "greatest blunder".

   Recent measurements of the speed with which supernovae at various distances from us are moving away from, however, are making people rethink Einstein's "blunder". It looks like a cosmological constant (a.k.a "dark energy") is necessary to describe what we observe - although not for the reasons Einstein proposed.

   Because we have very little idea what could be the source of this dark energy, however, we should be a bit careful of drawing conclusions from the measurements that are too strong. Right now all we can say is that it appears, from a fairly small number of measurements, that nature behaves in a way that is consistent with having a cosmological constant. But there could be other explanations...

   You can find some further information about the cosmological constant at the following links.

   • The Cosmological Constant, by Eli Michael
   • The Cosmological Constant, by Sean Carroll
   
   

Questions from October 19

Note from Hal:
These were great questions! They really made me think - which, of course, means that the answers may be wrong, or at least incomplete. If I am struck by more inspiration, I'll add to the answers.
1. Question: What is the basis of supersymmetry?

   Answer: The idea of supersymmetry was invented largely to fix a technical problem with the Higgs boson. Only later did people realize that supersymmetry was also a requirement of string theory. Remember that in the Standard Model, our best current description of the world of elementary particles, the Higgs particle is the result of something called electro-weak symmetry breaking. One of the main results of this process is to give mass to quarks, leptons and gauge bosons (W and Z). Although we haven't actually observed the Higgs in an experiment yet, we know that something like it has to exist since the electromagnetic and weak forces appear to be different to us and because most types of particles have mass.

   The problem is that if you try to calculate the mass of the Higgs boson, it turns out to be extremely large (and therefore cannot perform its job of breaking electro-weak symmetry) unless some parameters of the theory are chosen to an accuracy of 16 decimal places. This is sometimes referred to as the Naturalness Problem. It is quite annoying because past experience has shown that if you have to set a particular parameter (other than trivial ones like "1" or "2") to such a high precision or else your theory falls apart, then chances are there is something wrong with your theory.

   Supersymmetry gets around this problem by postulating that a whole new set of elementary particles exist, one for each type of particle we already know and love. So for electrons there are also super-electrons (or selectrons), for each type of quark there is a squark, etc. Each super-particle is identical to its ordinary particle partner except for its spin. Because of this, the sparticles cancel out the effects of the particles on the Higgs mass, which is why the Higgs became so heavy in the first place.

   So, at the expense of doubling the number of types of particles in the universe, we have been able to resolve a serious problem with our understanding. Of course, we have not yet observed any of these sparticles. But we're looking...

   You can learn a little more about supersymmetry and some of the other concepts discussed in the book and the video by visiting the Particle Adventure website of the Particle Data Group. Click on "Start Here" and then on the "Unsolved Mysteries" part of the side bar.
   

2. Question: How does string theory relate to the theory of dark energy?

   Answer: As far as I know, no one has a good idea of what dark energy actually is. Undoubtedly, string theorists are trying to fit this concept into their theories, but I don't know if there have been any results yet. I'll check in with my string theorist colleagues to see if they have anything more to add.

   Addendum Oct. 26: I had a talk about the string theory - dark energy connection with a colleague at Columbia, Dan Kabat, who is a string theorist. He tells me that, in the last year, people have found a way to generate something that behaves like dark energy in string theories by allowing electric and magnetic fields to act in the extra dimensions required by the theory. Unfortunately, to get the right kind of dark energy, one has to put all kinds of specific and unmotivated conditions on the way the fields work in the extra dimensions and on the geometry of space. Because of this, Dan described the work as a "proof of principle" that string theories can accomodate dark energy, but nothing approaching a real answer.
   

3. Question: In Part 3, Brian Greene used slices of bread as an analogy for the different layers, but we didn't understand what the layers were representing. Could you explain what he was talking about?

   Answer: Hmmm... I'm not sure which analogy you're referring to. I remember him using a loaf of bread to illustrate how events happening in space-time appear to different observers. But maybe you're thinking of a different analogy. Perhaps we should discuss this in person on Nov. 1?
   

4. Question: On the video it showed two branes colliding or bumping into each other and causing the big bang as a theory of how the big bang could have happened. Someone asked "if these branes are parallel universes and if so how could they collide?" As you can see it's all getting over our heads. Maybe if we took a step back and you could explain branes. I know when I'm seeing this on the documentary I'm forgetting that these strings and branes are infinitesimally tiny. Maybe that's the confusion here or we're getting it mixed up with the slices of bread.

   Answer: I think that the key point to keep in mind here is that these branes (whether they are alternate universes or not) are moving around in dimensions of space that we cannot see. They're only illustrated in the video as 2D sheets of paper floating through 3D space because we are limited to only perceive three spatial dimensions.

   A brane itself is just a fancy word for an object with less dimensions than the space it inhabits. A line in 3D space is an example. The line can be very short or very long, but it still does not fill the space. If you lived on the line, your life would be very boring: you would only know backward and forward. Up/down and East/West would be beyond your ability to see - or even comprehend. Thus, if another long line-brane was bearing down upon you from below, you would have no way of knowing about it until it actually hit your line-brane, no doubt causing havoc in your 1D world.
   

5. Question: The word "elegant". What do the scientists mean when using this word? One person asked if you would show some examples of elegance and non-elegance in science either in mathematical equations or description.

   Answer: The best general analogy I can draw to the use of "elegance" by physicists is with poetry. In both cases, something (a theory, a poem...) is considered elegant if it expresses a large number of ideas in a very self-contained and compact form. The difference between elegant physics and elegant poetry is that poetry achieves its goal by being ambiguous - open to many interpretations. Physics, on the other hand, aims to be completely unambiguous. Below are two examples of elegance (and non-elegance) in physics.

   1. The Standard Model vs String Theory:
      String Theory is generally considered to be more elegant because it (hopes to) describe all the forces of nature using only a single principle, which leads to a remarkably simple mathematical formulation.

      The Standard Model, on the other hand, cannot explain gravity and requires about 20 input parameters, whose values we must be given: we cannot predict what they are beforehand. It also suffers from the naturalness problem discussed above.

      Note though, that when the Standard Model was developed, it was the most elegant theory of its time.

   2. Sun-centered vs Earth-centered models of the solar system:
      This may appear to be a simple case of right and wrong: the planets revolve around the Sun - so the Sun is the center of the solar system. However, when this was a hot topic of debate, things were not so clear. Predictions of the positions of stars and planets in the sky were made in both sun-centered and earth-centered models. Both agreed with the fairly crude measurements that had been made prior to the 16th century. In fact an earth-centered model, based on work by Ptolemy, was used by people whose lives depended on knowing where the stars were - navigators.

      The real issue of elegance here was that the earth-centered model was so ambiguous. For each planet, predictions could only be made using a series of circles-within-circles (referred to as epicycles). Up to 80 of these circles were necessary to accurately describe the motion of any planet. If predictions of the theory were found to disagree with observation, one simply added more and more epicycles until they matched. King Alfonso X of Spain, a noted astronomer of the time, famously quipped:

      "If the Lord Almighty had consulted me before embarking upon the creation, I should have recommended something simpler."

      Sun-centered theories, such as those put forward by Copernicus and Kepler, were much less cumbersome. In fact, Newton was able to explain Kepler's Laws using a single principle - the Law of Gravity. This was a very elegant idea because it explained both motion on the Earth and that in the Heavens using a single equation.
      

   
   

   Questions from October 5

   1. Question: This may be more of an astronomy question, but I saw on the BBC recently that scientists have caught on film(?)two galaxies colliding. How is it possible for two such large bodies to collide when everything is supposed to be moving out and away from each other from the Big Bang?

      Answer: All objects in the universe are indeed moving away from each other because of the Big Bang. This is refered to as the Hubble expansion after the astronomer who first measured it. The important point is that this expansion describes the motion of galaxies on average. The motion of any single galaxy may be quite different than this.

      One of the main contributors to deviations from the average Hubble expansion is the force of gravity. If two galaxies happen to be in close proximity to each other, they will exert gravitational forces on each other, which will modify the Hubble motion and cause the two galaxies to fall toward each other.

      Galaxy collisions are important because they give us concrete evidence of how the universe achieved its current lumpy state from the very smooth distribution of matter and energy that existed just after the Big Bang. Gravity is the culprit here - it enhances small fluctuations in the density of the universe because areas where more matter is concentrated tend to fall in on themselves, just as the two close-together galaxies collide under the influence of gravity.

      You can find a NASA press release showing pictures of colliding galaxies as seen by the Hubble Space Telescope here.

   
   

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