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S'està carregant… ## The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (1999)## de Brian Greene
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First, despite being 20 years old, this book isn't particularly dated. Lots of questions about string theory still remain. Greene writes very clearly as he describes first the theories of Einstein and of Quantum Mechanics, then why they don't reconcile with each other and how string theory brings them together--at least in theory. As a non-scientist, I am still not 100 percent certain how physicists can derive so many facts from an unproven theory, but Greene explains things about as well as it is possible to do so. I found it a compelling and not particularly difficult read. Before I read the book, I had watched Greene's PBS series based on it (and Greene's second PBS series based on his later book, The Fabric of the Cosmos.) This predisposed me to like the book and its author, since Greene's star power and ability to communicate complex ideas quite clearly is pretty amazing. If you're buying this book used, try to get a copy with the updated preface and epilogue.
In the great tradition of physicists writing for the masses, ''The Elegant Universe'' sets a standard that will be hard to beat. ## Contingut a## Conté## Té l'adaptació## Abreujat a## Premis## Distincions## Llistes notables
Relates the scientific story and the human struggle behind the search for the string theory--the ultimate theory which scientists believe is capable of describing all physical phenomena, large and small; and discusses how the theory is impacting human understanding of space and time. No s'han trobat descripcions de biblioteca. |
## Debats actualsCap## Cobertes populars
Google Books — S'està carregant… ## Gèneres## Classificació Decimal de Dewey (DDC)539.7258Natural sciences and mathematics Physics Matter; Molecular Physics; Atomic and Nuclear physics; Radiation; Quantum Physics Atomic and nuclear physics Particle Physics## LCC (Clas. Bibl. Congrés EUA)## ValoracióMitjana:
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"..During the past half-century, physicists of each new generation, through fits and starts, and diversions down blind alleys-have been building steadily on the discoveries of their predecessors to piece together an ever fuller understanding of how the universe works.........And now, long after Einstein articulated his quest for a unified theory but came up empty-handed, physicists believe they have finally found a framework for stitching these insights together into a seamless whole a single theory". [Presumably, Greene thinks this is string theory].

"We can speak about the motion of an object, but only relative to or by comparison with another. There is thus no meaning to the statement "George is traveling at 10 miles per hour," as we have not specified any other object for comparison. There is meaning to the statement "George is traveling at 10 miles per hour past Gracie," as we have now specified Gracie as the benchmark........Although at first it sounds completely ridiculous, unlike what happens if one runs from an oncoming baseball, grenade. or avalanche, the speed of approaching photons is always 670 million miles per hour. The same is true if you run toward oncoming photons or chase after them -their speed of approach or recession is completely unchanged; they still appear to travel at 670 million miles per hour".

For instance, Newton's theory of gravity claims that if the sun were suddenly to explode, the earth- some 93 million miles away would instantaneously suffer a departure from its usual elliptical orbit.....Einstein called the indistinguishability between accelerated motion and gravity the equivalence principle? It plays a central role in general relativity.

With the classic example of a ball on a rubber membrane....An important point to note is that the ball bearing itself warps the rubber membrane, although only slightly. Similarly, the earth, being a massive body in its own right, also warps the fabric of space, although far less than the sun.....When the sun causes the fabric of space around it to warp this is not due to its "being pulled downward" by gravity as in the case of the bowling ball, which warps the rubber membrane because it is pulled earthward by gravity. In the case of the sun, there is no other object to "do the pulling." Instead, Einstein has taught us that the warping of space is gravity. The mere presence of an object with mass causes space to respond by warping.....Einstein showed that objects move through space (spacetime, more precisely) along the shortest possible paths- the "easiest possible paths" or the "paths of least resistance." If the space is warped, such paths will be curved. (I assume that this is with respect to some hypothetical outside observer because a "straight line" in this space time is the "line of least resistance". There is no other way to measure a straight line .....). ..........A second shortcoming of the analogy stems from the rubber membrane's being two-dimensional. In reality, although harder to visualize, the sun (and all other massive objects) actually warps the three-dimensional space surrounding it. Figure 3.6 is a rough attempt to depict this; all of the space surrounding the sun- "below," "on the sides," on "top"-suffers the same kind of distortion. I give credit to Greene here. I've never seen any other author make this point and it's always worried me that the examples given of the bowling ball on a trampoline etc were woefully inadequate to explain the warping of space time in multiple dimensions........A third, related shortcoming of the analogy is that we have suppressed the time dimension....... But....acceleration- and hence gravity warps both space and time.

Space: When no mass is present, space is flat, and a small object [seems to be a mass to me] will blissfully be at rest or will travel at a constant velocity. If a large mass comes on the scene, space will warp.....but, as in the case of the membrane, the distortion will not be instantaneous. Rather, it will spread outward from the massive body, ultimately settling down into a warped shape that communicates the gravitational pull of the new body. [Presumably this distortion can only travel at the speed of light?]

By offering the explanation for the expansion of the universe, Einstein achieved one of the greatest intellectual feats of all time. Extrapolating all the way back to "the beginning," the universe would appear to have begun as a point--an image we will critically re-examine in later chapters- in which all matter and energy is squeezed together to unimaginable density and temperature. It is believed that a cosmic fireball, the big bang, erupted from this volatile mixture spewing forth the seeds from which the universe as we know it evolved. In the big bang. there is no surrounding space...... the big bang is the eruption of compressed space whose unfurling, like a tidal wave, carries along matter and energy even to this day. [I've often wondered why we have to extrapolate backwards all the way to a point. Yes we can do that but maybe the universe actually expanded from some a sphere of some finite magnitude....maybe the size postulated for the universe at the end of the hypothesised rapid inflation. Why do we have to assume we have to extrapolate back to a point?]

It's pretty impressive that "No deviations from the predictions of general relativity have been found in experiments performed with our present level of technology".

The frequency of the light (its color) determines the speed of the ejected electrons; the total intensity of the light determines the number of ejected electrons. And so Einstein showed that Planck's guess of lumpy energy actually reflects a fundamental feature of electromagnetic waves: They are composed of particles: photons- that are little bundles, or quanta, of light. The photoelectric effect shows that light has particle properties. The double-slit experiment shows that light manifests the interference properties of waves. Together they show that light has both wave-like and particle-like properties.......Inspired by a chain of reasoning rooted in Einstein's special relativity, de Broglie suggested that the wave particle duality applied not only to light but to matter as well.

Davisson and Germer examined electrons making it through the two slits....... Their experiment therefore showed that electrons exhibit interference phenomena, the telltale sign of waves...... similar experiments lead to the conclusion that all matter has a wave-like character...... de Broglie set down a formula for the wavelength of matter waves, and it shows that the wavelength is proportional to Planck's constant h. (More precisely, the wavelength is given by h divided by the material body's momentum.) Since this so small, the resulting wavelengths are similarly minuscule compared with everyday scales.

But waves of what?...... Born's suggestion is one of the strangest features of quantum theory, but is supported nonetheless by an enormous amount of experimental data. He asserted that an electron wave must be interpreted from the standpoint of probability.

Feynman argued that in traveling from the source to a given point on the phosphorescent screen each individual electron actually traverses every possible trajectory simultaneously;......Feynman showed that he could assign a number to each of these paths in such a way that their combined average yields exactly the same result for the probability calculated using the wave-function approach. (I recall that he did this using the calculations of multiple graduate students...must have been hell working for him).

Just as Heisenberg showed that there is a trade-off between the precision of measurements of position and velocity, he also showed that there is a similar trade-off in the precision of energy measurements and how long one takes to do the measurement. Quantum mechanics asserts that you can't say that a particle has precisely such-and-such energy at precisely such-and-such moment in time. Ever increasing precision of energy measurements require ever longer durations to carry them out.

Since the borrowing and repaying on average cancel each other out, an empty region of space looks calm and placid when examined with all but microscopic precision. The uncertainty principle, however, reveals that macroscopic averaging obscures a wealth of microscopic activity. 'As we will see shortly, this frenzy is the obstacle to merging general relativity and quantum mechanics.

They [physicists in 1930’s-1940’s] found that Schrödinger's quantum wave equation was actually only an approximate description of microscopic physics- an approximation that works extremely well when one does not probe too deeply into the microscopic frenzy (either experimentally or theoretically), but that certainly fails if one does.......... Through a series of inspirational developments, they created quantum electrodynamics. This is an example of what has come to be called a relativistic quantum field theory, or a quantum field theory, The success of quantum electrodynamics inspired other physicists in the 1960s and 1970s to try an analogous approach for developing a quantum-mechanical understanding of the weak, the strong, and the gravitational forces. For the weak and the strong forces, this proved to be an immensely fruitful line of attack. In analogy with quantum electrodynamics, physicists were able to construct quantum field theories for the strong and the weak forces, called quantum chromodynamics and quantum electroweak theory.

Glashow, Salam, and Weinberg showed, in essence, that at high enough energy and temperature such as occurred a mere fraction of a second after the big bang electromagnetic and weak force fields dissolve into one another, take on indistinguishable characteristics, and are more accurately called electroweak fields. When the temperature drops, as it has done steadily since the big bang, the electromagnetic and weak forces crystallize out.

According to the standard model, just as the photon is the smallest constituent of an electromagnetic field, the strong and the weak force fields have smallest constituents as well. As we discussed briefly in Chapter 1, the smallest bundles of the strong force are known as gluons, and those of the weak force are known as weak gauge bosons (or more precisely, the W and Z bosons)....For like-charged particles, the photon carries the message "move apart," while for oppositely charged particles it carries the message "come together."....... . Similarly, the gluons and weak gauge bosons are the messenger particles for the strong and weak nuclear forces....... the odd man out in our discussion of the quantum theory of the forces of nature is gravity.

If the three colors- the three different strong charges -that a quark can carry were all shifted in a particular manner (roughly speaking, in our fanciful chromatic language, if red, green, and blue were shifted, for instance, to yellow, indigo, and violet). and even if the details of this shift were to change from moment to moment or from place to place, the interactions between the quarks would be, again, completely unchanged. For this reason, just as we say that a sphere exemplifies rotational symmetry because it looks the same regardless of how we rotate it around in our hands or how we shift the angle from which we view it, we say that the universe exemplifies strong force symmetry: Physics is unchanged by...... it is completely insensitive to...... these force-charge shifts. For historical reasons, physicists also say that the strong force symmetry is an example of a gauge symmetry........ For the case of the gauge symmetry associated with shifting quark-color charges, the required force is none other than the strong force itself.

I found the following explanations of "quantum foam" to be especially helpful....."from a purely classical standpoint, we would expect this placid and flat image of space to persist all the way to arbitrarily small length scales. But quantum mechanics changes this conclusion radically. Everything is subject to the quantum fluctuations inherent in the uncertainty principle even the gravitational field. Although classical reasoning implies that empty space has zero gravitational field, quantum mechanics shows that on average it is zero, but that its actual value undulates up and down due to quantum fluctuations. Moreover, the uncertainty principle tells us that the size of the undulations of the gravitational field gets larger as we focus our attention on smaller regions of space"........It is on such short distance scales that we encounter the fundamental incompatibility between general relativity and quantum mechanics. The notion of a smooth spatial geometry, the central principle of general relativity, is destroyed by the violent fluctuations of the quantum world on short distance scales. On ultramicroscopic scales, the central feature of quantum mechanics-the uncertainty principle is in direct conflict with the central feature of general relativity the smooth geometrical model of space (and of spacetime)......But note that you become aware of the discrete nature of the picture only when you examine it on the smallest of scales; from far away it looks smooth. Similarly, the fabric of space-time appears to be smooth except when examined with ultramicroscopie precision. This is why general relativity works on large enough distance (and time) scales the scales relevant for many typical astronomical applications but is rendered inconsistent on short distance (and time).

All objects undergo quantum jitter,....... This holds true for the loops in string theory as well; no matter how placid a string appears it will always experience some amount of quantum vibration. The remarkable thing, as originally worked out in the 1970s, is that there can be energy cancellations between these quantum jitters and the more intuitive kind of string vibrations....... As an important example, Scherk and Schwarz found that for the vibrational pattern whose properties make it a candidate for the graviton messenger particle, the energy cancellations are perfect, resulting in a zero-mass gravitational-force particle. This is precisely what is expected for the graviton; the gravitational force is transmitted at light speed and only massless particles travel at this maximal velocity.

If string theory is right, each of the infinitely many resonant patterns of string vibration should correspond to an elementary particle. An essential point, however, is that the high string tension ensures that all but a few of these vibrational patterns will correspond to extremely heavy particles (the few being the lowest-energy vibrations that have near-perfect cancellations with quantum string jitters). And again, the term "heavy" here means many times heavier than the Planck mass......when quantum mechanics is taken into account, continually increasing the energy of a string does not continually increase its ability to probe finer structures, in direct contrast with what happens for a point particle........ The upshot is that no matter how hard you try, the extended nature of a string prevents you from using it to probe phenomena on sub-Planck-length distances. [This seems to me to be a significant problem for string theory].

In a universe governed by the laws of string theory, the conventional notion that we can always dissect nature on ever smaller distances, without limit, is not true. There is a limit, and it comes into play before we encounter the devastating quantum foam of Figure 5.1. Therefore, in a sense ....., one can even say that the supposed tempestuous sub-Planckian quantum undulations do not exist. A positivist would say that something exists only if it can at least in principle-be probed and measured..... String theory tells us that we encountered these problems only because we did not understand the true rules of the game; the new rules tell us that there is a limit to how finely we can probe the universe and, in a real sense, a limit to how finely our conventional notion of distance can even be applied to the ultramicroscopic structure of the cosmos. The supposed pernicious spatial fluctuations are now seen to have arisen in our theories because we were unaware of these limits and were thus led by a point particle approach to grossly overstep the bounds of physical reality.

Physicists describe these two properties of physical laws —that they do not depend on when or where you use them —as symmetries of nature. By this usage physicists mean that nature treats every moment in time and every location in space identically symmetrically—by ensuring that the same fundamental laws are in operation........ Through the equivalence principle of general relativity, Einstein sig nificantly extended this symmetry by showing that the laws of physics are actually identical for all observers, even if they are undergoing complicated accelerated motion...... if you perform some experiment and then decide to rotate all of your equipment and do the experiment again, the same laws should apply....This is known as rotational symmetry, and it means that the laws of physics treat all possible orientations on equal footing........ there is [also] a quantum-mechanical notion of spin that is somewhat akin to the usual image but inherently quantum mechanical in nature.

If the particular Calabi-Yau shape singled out by the equations of the theory were to have three holes, we would have found an impressive postdiction from string theory explaining a known feature of the world that is otherwise completely mysterious. But finding the principle for choosing among Calabi-Yau shapes is a problem that as yet remains unsolved. Nevertheless—and this is the important point—we see that string theory provides the potential for answering this basic puzzle of particle physics, and this in itself is substantial progress....... there are examples of Calabi-Yau spaces that, when chosen for the curled-up dimensions required by string theory, give rise to string vibrations that are closely akin to the particles of the standard model. [This is starting to sound like a bit of an apology for string theory]

"....the density of the universe at the Planck time was simply colossal. At such energies and densities gravity and quantum mechanics can no longer be treated as two separate entities as they are in point-particle quantum field theory. Instead, the central message of this book is that at and beyond these enormous energies we must invoke string theory. In temporal terms, we encounter these energies and densities when we probe earlier than the Planck time of 10+3 seconds ATB, and hence this earliest epoch is the cosmological arena of string theory.

For our present purpose, the important thing to note is that the phase transition results in a decrease in the amount of symmetry displayed by the H2O molecules. Whereas liquid water looks the same regardless of the angle from which it is viewed it appears to be rotationally symmetric-solid ice is different. It has a crystalline block structure, which means that if you examine it with adequate precision, it will, like any crystal, look different from different angles. The phase transition has resulted in a decrease in the amount of rotational symmetry that is manifest.

At time zero, as the size of the universe vanishes, the temperature and density soar to infinity, giving us the most extreme signal that this theoretical model of the universe, firmly rooted in the classical gravitational framework of general relativity, has completely broken down.

Nature is telling us emphatically that under such conditions we must merge general relativity and quantum mechanics-in other words, we must make use of string theory. [I'm surprised that Greene harbours no other ideas...just string theory]

Valiant attempts by physicists such as Hawking and James Hartle of the University of California at Santa Barbara have tried to bring the question of cosmological initial conditions within the umbrella of physical theory, but all such attempts remain inconclusive. In the context of string/M-theory, our cosmological understanding is, at present, just too primitive to determine whether our candidate "theory of everything" truly lives up to its name and determines its own cosmological initial conditions,....Rather, it appears that the complete formulation of string/M-theory must break the traditional mold and spring into existence as a full-fledged quantum-mechanical theory........Currently, no one knows how to do this........ With guarded optimism, we can envision that a reframing of the principles of quantum mechanics within string theory may yield a more powerful formalism that is capable of giving us an answer to the question of how the universe began and why there are such things as space and time.

Among the many features of string theory that we have discussed in the preceding chapters, the following three are perhaps the most important ones to keep firmly in mind. First, gravity and quantum mechanics are part and parcel of how the universe works and therefore any purported unified theory must incorporate both. String theory accomplishes this. Second, studies by physicists over the past century have revealed that there are other key ideas many of which have been experimentally confirmed that appear central to our understanding of the universe. These include the concepts of spin, the family structure of matter particles, messenger particles, gauge symmetry, the equivalence principle, symmetry breaking, and supersymmetry, to name a few. All of these concepts emerge naturally from string theory. Third, unlike more conventional theories such as the standard model, which has 19 free parameters that can be adjusted to ensure agreement with experimental measurements, string theory has no adjustable parameters. In principle, its implications should be thoroughly definitive they should provide an unambiguous test of whether the theory is right or wrong........ No doubt, achieving a full understanding of string/M-theory will require a great deal of hard work and an equal amount of ingenuity.

And that sort of sums it up. Greene reckons string theory has the potential to give us the answers but we still don't have the answers. My concern is that many other eminent researchers think that string theory leads nowhere and that there are so many parameters that you can "prove" anything. So I'm left with very mixed feelings about the book. The universe might be elegant but Greene's explanations are not. I still give it five stars. ( )