Questions and answers for Origin of the Universe

Fall 97

Q: I was wondering if you could explain what duality is and what it means with respect to the origin of the universe.

A: Duality means that a theory (and hopefully the world it describes) has two aspects, or two sets of properties, that seem quite different but are in fact related in a deep way. If we understand one set of properties rather well, we can use this understanding to discuss, or calculate, the "dual" set of properties, which may be very hard to calculate directly. Duality has found important applications in string theory, and is relevant to the origin of the universe because the very early universe must be described by quantum gravity, and "strings" seem to occur naturally in quantum gravity. Actuallly, the latest trend is to go beyond string theory, to M-theory.
For details on duality in string theory, and web links, click here.

------

Q: Are there an infinite number of dimensions?

A: The short answer is no. In quantum gravity the number of dimensions is fluctuating, and could be arbitrarily large in some parts of the universe. Prevailing theories indicate that in our corner (in the visible universe) the number of dimensions is 10+1 (ten spacelike dimensions plus time), because a stable and richly structured region of the universe must have this dimensionality. We know for sure that there are 3 extended space dimensions, so the other 7 must be compacted or hidden in some way. However, somewhere else there could be any number of spacial dimensions (for instance, 5 extended and 9 compacted), and perhaps more than one time-like dimension (but this is debatable).
For details on fluctuating dimensions and spaces with an infinite number of dimensions, click here.

------

Q: Why are physicists so sure that there is a Theory of Everything (TOE)? Do they seriously believe that, as Einstein put it, we can understand the mind of God?

A: It is very reasonable to search for a theory that describes everything we know about the universe today, in the same sense that today’s theories are capable of describing everything known to science a century ago. General relativity describes very precisely all the motions of the planets, and the quantum theory of electrons and nuclei is all we need, in principle, to describe all chemistry and the behavior of matter, as it was known in 1895. We also have a pretty good theory of the inner structure of nuclei, of radioactive decays, and many other things that nobody had dreamed of in 1895 and were a total mystery at the turn of the century (radioactivity was discovered by Becquerel in 1896). Today’s physics cannot describe what happens when the energy concentration is so high that space-time is violently distorted. The mass-energy of an elementary particle is concentrated in such a small volume that its description eludes us, just as the internal structure of atoms eluded our ancestors in 1895. Remember that the existence of electrons was established in 1897 (by J.J. Thomson). When the behavior of electrons was accurately described by quantum theory, we did not just get a good theory of chemistry, we also had to change our ideas about determinism in science and about reality itself, in ways that no philosopher or theologian had ever dreamed of. It is not farfetched to say that we got a better understanding of the mind of God, if we assume with Einstein that the laws of nature reflect the mind of God. With quantum electromagnetic theory, we can describe more than isolated atoms and molecules; we can also understand fire and lightning and complex structures such as a cell, although we cannot, yet, construct a self-reproducing cell from scratch in the lab. Most things are so complicated that they outstrip our ability to apply the theory, but we are pushing farther every day with clever math and better computers. Similarly, with quantum gravity we can hope to understand not only the constitution of electrons and quarks, but also what goes on inside a black hole, and how the Big Bang came about. At the center of a black hole, electrons and quarks are pushed close together and dissolve, somewhat like atoms fuse into a solid or become ionized in a fire. It is almost certain that radically new notions will emerge from quantum gravity, like the notions of quantization and spacetime emerged a century ago. We will master a deeper level of reality, but of course the question is: will it be the ultimate level of reality? Or will we just have a theory of everything we know now, a 1997-TOE?
My personal opinion is that superstring theories are already breaking into a deeper level of reality. I have no idea how far we are from the ultimate level, if it exists.

Fall 98

Mass of radiation from the sun.

Q: The video Einstein revealed says that light from the sun adds 4 pounds per second to the mass of the earth. How do we compute that?

Answer: We use E = mc², or rather E/t = (m/t)c², where E is the energy of the radiation that arrives during time t and m is the corresponding mass. We can take t=1 second. The quantity E/t is known as power, and is measured in watts in the metric system [1]. A standard light bulb delivers 100 watts (however, most of it is infrared radiation, only about 5% is in the form of visible light). Solar radiation arriving on earth delivers 1360 watts to an area of 1 square meter. This makes sense: if you hang 10 light bulbs above your desktop, you get about as much power on it as in full sun at noon [2] . The area of the earth that receives sunlight is effectively , where R = 6400 km = 6.4×10⁶ meters is the (mean) radius of the earth [3]. So, you can get E/t by multiplying 1.36 kw/m², which is known as the "solar radiation constant", times R². The result should be in watts. Divide by c² (where, of course, c = 300000 km/sec = 3×10⁸ m/sec and you will get m/t in kg/sec. Convert to lb/sec if you wish.
Thus, 1360×3.14 ×6400²/300000² = 1.944 is the answer in kg/sec. In pounds/sec, it is 1.944×2.205 = 4.286.
For footnotes [1]-[3], and related questions, click here.

Questions answered by students

Click and see 21 questions with sometime conflicting answers by the class. Inaccuracies have been edited out (hopefully), but there are conflicting answers to debatable questions.

Short Answers:

Q: Are there any theories about the actual machine that would enable time travel to be possible? Such as, would it be like a rocket or more of a pod, or is this just totally science fiction with no scientific merit?

Answer: Papers on time travel are now accepted in official journals like the Physical Review, but usually they just discuss whether time travel is possible, in principle, for a fairly complex object. Some discuss whether an advanced civilization could ever build a time-travel machine. I do not know of any proposal for such a machine that could be built now. One would have to create a wormhole or two, and be able to carry their openings around.

Q: Does the sun revolve around anything?

Answer: The sun revolves around the center of our Galaxy (the Milky Way), along with all the other stars in it. It is moving on its orbit at half a million miles per hour (220 km/sec), and will complete a revolution in 240 million years (7.5*10¹⁵ seconds). The distance from the sun to the center of the Milky Way is 28 thousand light years (2.6*10¹⁷ km). It is also true that the sun and the earth, for instance, revolve around each other in such a way that their center of mass does not wobble. Because the sun is 300 thousand times more massive than the earth, the net result of this mutual revolution is that the sun barely wobbles, while the earth swings around on a large orbit. Since each planet interacts with the sun in this way, the overall wobble of the sun is rather complicated, but very small.

Q: What is the current estimate as to how many stars/galaxies in the universe?

Answer: In round numbers, there are 10 to 100 billion stars in a typical galaxy, and between 10 and 100 billion galaxies in the visible universe. So the total number of stars is somewhere between 10²⁰ and 10²⁴, not counting "dead" stars.

Q: Is the search for the unified theory the biggest quest of physicists right now?

Answer: It is the biggest quest for those involved in it. Other physicists have a variety of opinions. My guess is that, if a poll of all physicists were taken, unification and the origin of the universe would take the top spots (and perhaps they are related, as you know).

Q: Do temperatures come close to absolute zero in areas of the universe distant from stars, where there is little heat or light?

Answer: The microwave background radiation is everywhere and has a temperature of 2.7 degrees Kelvin (5 degrees Fahrenheit above absolute zero). Unless some advanced civilization has built very-low-temperature refrigerators, as we have, it is hard to imagine that there could be lower temperatures anywhere.