Tag Archives: Physics

The Uncertainty Principle and Us

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It’s difficult to discuss physics if you aren’t a physicist or don’t understand the math involved. Nevertheless, what physicists tell us about the world is so strange that it’s hard not to discuss it sometimes, whether we understand it or not. (The brilliant physicist and all-around cool guy Richard Feynman once said that nobody understands quantum mechanics, but some understand it better than others.)

There are philosophers who specialize in the philosophy of physics and aren’t shy about discussing physics at all, among themselves and with physicists. One of these philosophers, Craig Callender, has recently written two interesting articles for the New York Times. In these articles, Callender argues that Werner Heisenberg’s uncertainty principle, probably the best-known part of quantum mechanics, shouldn’t be as famous as it is. 

Heisenberg was one of the founders of quantum mechanics. He published the uncertainty principle in 1927. If you look up “uncertainty principle” now, you’ll find statements like this: “The position and momentum of a particle cannot be simultaneously measured with arbitrarily high precision” and “The uncertainty principle is at the foundation of quantum mechanics: you can measure a particle’s position or its velocity, but not both.”

Well, here is Callender on quantum mechanics:

[Quantum mechanics is] a complex theory, but its basic structure is simple. It represents physical systems – particles, cats, planets – with abstract quantum states. These quantum states provide the chances for various things happening. Think of quantum mechanics as an oddsmaker. You consult the theory, and it provides the odds of something definite happening….

The quantum oddsmaker can answer … questions for every conceivable property of the system. Sometimes it really narrows down what might happen: for instance, “There is a 100 percent chance the particle is located here, and zero percent chance elsewhere.” Other times it spreads out its chances to varying degrees: “There is a 1 percent chance the particle is located here, a 2 percent change it is located there, a 1 percent chance over there and so on.”

According to Callender:

The uncertainty principle simply says that for some pairs of questions to the oddsmaker, the answers may be interrelated. Famously, the answer to the question of a particle’s position is constrained by the answer to the question of its velocity, and vice versa. In particular, if we have a huge ensemble of systems each prepared in the same quantum state, the more the position is narrowed down, the less the velocity is, and vice versa. In other words, the oddsmaker is stingy: it won’t give us good odds on both position and velocity at once.

Callender then points out that he hasn’t said anything about measurement or observation:

The principle is about quantum states and what odds follow from these states. To add the notion of measurement is to import extra content. And as the great physicist John S. Bell has said, formulations of quantum mechanics invoking measurement as basic are “unprofessionally vague and ambiguous.” After all, why is a concept as fuzzy as measurement part of a fundamental theory?

Callender later shares another quote from J. S. Bell (considered by some to be the greatest physicist of the second half of the 20th century):

What exactly qualifies some physical systems to play the role of “measurer”? Was the wavefunction [the quantum state] of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system … with a Ph.D.? If the theory is to apply to anything but highly idealized laboratory operations, are we not obliged to admit that more or less “measurement-like” processes are going on more or less all the time, more or less everywhere?

When physicists use their instruments to measure a subatomic particle’s position or momentum, the instruments affect the particle. It’s the interaction at the subatomic level between the instrument and the particle that’s important, not the fact that the interaction has something to do with measurement, observation, mental energy or human consciousness. We aren’t that important in the vast scheme of things.

Viewing the theory of quantum mechanics as a cosmic oddsmaker may seem unhelpful. We want to know what’s going on at the subatomic level that results in the theory calculating certain odds. Heisenberg thought physicists shouldn’t even think about an underlying reality — they should simply focus on the results of their observations. But some (many?) physicists working today believe that quantum mechanics is an incomplete theory that will eventually be replaced by a more fundamental theory, possibly one that explains away the apparent randomness that exists at the subatomic level (that’s what Einstein thought too). Their hope is that uncertainty will one day be replaced by certainty, or something closer to it.

If you do a Google search for “uncertainty principle consciousness”, you’ll probably get more than 8 million results. If you search for “uncertainty principle measurement”, you can get more than 32 million. Professor Callender thinks those numbers should be much, much smaller.

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This is Callender’s first article in the Times:

http://opinionator.blogs.nytimes.com/2013/07/21/nothing-to-see-here-demoting-the-uncertainty-principle/

Here he responds to questions and criticisms from readers:

http://opinionator.blogs.nytimes.com/2013/07/25/return-of-the-stingy-oddsmaker-a-response/

Time Reborn: From the Crisis in Physics to the Future of the Universe by Lee Smolin

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The theoretical physicist Lee Smolin has written 4 books. I’ve read 3 1/2 of them.

His first book, The Life of the Cosmos, applied the theory of evolution to cosmology. Smolin suggested that our universe might be a good home for life because universes breed new universes, which differ somewhat from their parents. Over time, a universe with lots of black holes will generate a number of new universes with lots of black holes, and universes with lots of black holes tend to be hospitable for life, since their fundamental constants (like the strength of their subatomic forces) have values that permit life to evolve.

His next book, Three Roads to Quantum Gravity, was too technical for me, but I did finish his 3rd book, The Trouble With Physics. In that one, he argued that string theory is much too popular among physicists, since it isn’t a proper scientific theory. It’s too speculative and might never generate testable predictions.

Now there is Time Reborn. This is a kind of sequel to Smolin’s earlier books. He still subscribes to the evolutionary views presented in The Life of the Cosmos, but his principal thesis now is that time is real. In fact, time is more real than space. This contradicts the common view among physicists and philosophers that space and time are the four dimensions that make up “spacetime”. The standard view among physicists is that all events, whether past, present or future, are equally real. There is nothing special about the present moment. In fact, our perception that time passes is an illusion.

Smolin argues that this consensus view of the universe as a “block universe”, in which all moments are the same, is a mistake. He agrees that the laws of physics and the equations that express them can run forwards or backwards, but only on scales smaller than the universe as a whole. The planets could revolve the other way around the sun, just like clocks can run in reverse. But the universe as a whole has a history that is real and a future that isn’t determined. Smolin thinks that treating time as real might help resolve certain issues in physics, such as the “arrow of time”, i.e., the fact that certain processes always go in one direction (entropy tends to increase in isolated systems).

Professor Smolin tries to explain how his view of time fits with Einstein’s special theory of relativity (in which temporal properties are relative to an observer) and how something can act like a particle and a wave at the same time (as shown by the famous “double-slit” experiment). I don’t know if those explanations or some of his other technical explanations make sense. But it was reassuring to read a book by a reputable physicist who believes that time is real, physicists have overemphasized the importance of mathematics in understanding the universe, and there is a reality beyond what we can observe. Smolin also believes that there are probably more fundamental, deterministic laws that underlie quantum mechanics. I believe that’s what Einstein thought too.

Time Reborn veers into philosophy at times. There is much discussion of the Principles of Sufficient Reason and the Identity of Indiscernibles. The book concludes with some comments on subjects that aren’t physics, like the nature of consciousness. Smolin’s philosophical remarks are relatively unsophisticated. I assume his physics is better.

Even if he’s wrong about the reality of time, however, I enjoyed the book. For one thing, I can now see how two particles at opposite ends of the universe could be “entangled”, such that a change to one would automatically result in an immediate change to the other. Space might have more dimensions than we recognize. In another spatial dimension, the two entangled particles might be very close neighbors, making what Einstein called “spooky action at a distance” (“spukhafte Fernwirkung“) less mysterious. That makes me feel a lot better.

Those Crazy, Mixed Up Photons

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On the website of the American Association for the Advancement of Science, a physicist recently wrote:

Suppose you have a quantum particle of light, or photon. It can be polarized so that it wriggles either vertically or horizontally. The quantum realm is … hazed over with unavoidable uncertainty, and thanks to such quantum uncertainty, a photon can … be polarized vertically and horizontally at the same time. If you then measure the photon, however, you will find it either horizontally polarized or vertically polarized, as the two-ways-at-once state randomly ‘collapses’ one way or the other.

This two-ways-at-once state is called “superposition”. The idea is that something can be in more than one state (or “position”) at one time, i.e. a super-position.

However, saying that a photon can be polarized vertically and horizontally at the same time, or that it can be in a “two-ways-at-once” state, looks extremely suspicious. It’s hard to know what such a statement means, if anything. After all, language is based on logic (it wouldn’t work otherwise) and logic is based on the law of contradiction: proposition P cannot be both true and false, assuming that P has a single, precise meaning.

The proposition that photon p is polarized vertically at time t has a single, precise meaning. So does the proposition that photon p is polarized horizontally at time t. Yet these statements certainly look contradictory. It looks as if we have to give up the law of contradiction in order to accept them both.

To avoid the contradiction, however, it might be preferable to say that a photon can be in an indeterminate state, in which its polarization is neither vertical nor horizontal. It’s potentially in either state, but it’s not in either one until its state is measured (or otherwise affected), at which point the photon randomly ends up in one state or the other.

Viewed in probabilistic terms, the fate of Schrödinger’s cat doesn’t seem to be a problem (to me anyway). It was alive when it was put in the box and presumably remained alive unless it was poisoned as the result of a random sub-atomic event. We don’t have to say that the cat is now both dead and alive (or in some twilight state). It’s just a cat that may have died and there is a certain probability that it did.

But then there is the famous double-split experiment. This experiment shows that photons don’t behave like cats (or dogs) or, in the philosopher J. L. Austin’s phrase, “medium-sized dry goods”. A single photon travels through two slits and creates a wave-pattern on the other side, even though common sense tells us that the photon can only travel through one slit or the other. The bizarre but reasonable conclusion is that the photon actually takes every possible path through the two openings, not just in theory, but in fact.

Fortunately, there isn’t any contradiction in saying that the photon goes through slit 1 and slit 2 at the same time, since saying that it goes through slit 2 doesn’t conflict with saying that it also goes through slit 1. In similar fashion, photons can be polarized horizontally and vertically at the same time, because that’s the kind of thing that can happen to the crazy little bastards (i.e. sub-atomic particles).

We are used to saying things like “a person can’t be in two places at the same time” (many episodes of Law and Order are based on that premise). Logic tells us that if the number 5 is odd, it can’t be even. Logic and experience tell us that if Miss Scarlet was in the billiard room, she wasn’t in the conservatory. That’s how numbers and people work. Photons don’t work that way. It’s extremely strange, but not incomprehensible and not contradictory.

http://news.sciencemag.org/sciencenow/2013/05/physicists-create-quantum-link-b.html

Why Does the World Exist?: An Existential Detective Story by Jim Holt

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Journalist and former philosophy grad student Jim Holt sets out to answer that long-standing philosophical/scientific question: Why is there something rather than nothing? 

His principal method is to interview a number of well-known philosophers (Adolph Grunbaum, Richard Swinburne, John Leslie and Derek Parfit) and scientists (David Deutsch, Andre Linde, Alex Vilenkin, Steven Weinberg and Roger Penrose). He also talks to John Updike, who is surprisingly knowledgeable about both science and philosophy.

Nowadays, when people ask why the world exists they are generally asking why the Big Bang occurred. Unfortunately, nobody knows. The most common answers are that there was some kind of random quantum event that made it happen or that God made it happen. Some people think that our universe is just a small part of reality and that somehow the existence of a vast, possibly infinite, collection of other universes explains why ours is here and/or why ours is the way it is. The philosopher John Leslie thinks that our universe might exist because it’s good.

As soon as a particular cause or reason for our universe to exist is suggested, it is natural to ask why that cause or reason is the explanation, rather than some other cause or reason. Why are the laws of quantum mechanics in effect? Where did God come from? This is why the answer provided by a Buddhist monk at the very end of the book is my personal favorite: “As a Buddhist, he says, he believes that the universe had no beginning….The Buddhist doctrine of a beginning-less universe makes the most metaphysical sense”.

Perhaps the reality that exists (the super-universe, whatever ultimately caused the Big Bang) has always existed and always will. It simply is. It never came into existence, so no cause, reason or explanation is necessary or possible. Perhaps it’s cyclical. Perhaps it’s not. But it’s eternal, with no beginning or end.

This book is worth reading, but not as good as it might have been. Mr. Holt writes well and seems to accurately present the ideas of the thinkers he interviews. But his own thoughts on the subject, and other subjects, such as consciousness and death, aren’t especially interesting or profound. In particular, his attempt to prove the existence of an infinite yet mediocre universe is completely unconvincing. His travel writing — where he stayed, what he ate, his strolls through Oxford and Paris — is also a bit much. He doesn’t just bump into a philosophy professor at a local grocery store; it’s a “gourmet” grocery store. He has excellent taste in food and drink as well.  (9/8/12)

Why Does E=mc2? (And Why Should We Care?) by Brian Cox and Jeff Forshaw

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Two English physicists try to explain Einstein’s famous equation and much more, including relativity and quantum mechanics. I didn’t understand quite a bit and didn’t try to do the math (which is relatively limited), but found their explanations reasonably helpful. For example, they explain that the speed of light is an upper limit because photons have no mass. It isn’t anything to do with light per se. Any particle with no mass travels at the speed of light and no faster. Gluons don’t have mass and, if they exist, neither do gravitons. So we might just as well call it “the speed of particles with no mass”. 

They also explain that mass and energy are constantly being exchanged in accordance with Einstein’s equation. Atomic weapons are just the most spectacular example of a process that is universal to nature, and occurs, for example, every time heat is generated or there is some other chemical reaction.

I’m still confused by the Twin Paradox. Why would someone in a spaceship moving close to the speed of light age more slowly than someone staying on Earth, if all motion is relative? Why not say that the person moving near the speed of light is standing still and the person who stayed at home is moving near the speed of light? The answer is that the person in the spaceship is accelerating and decelerating, and that’s why we can properly say that he or she is moving faster than the person on Earth and why he or she ages more slowly. There are formulas that explain this, but it still sounds fishy. 

I’m also bothered by the idea that the Big Bang had no location. If the universe is expanding in all directions, why can’t we say where the Big Bang occurred? And maybe put a monument there with a gift shop?  (9/8/11)