Tag Archives: Physics

Three Weeks of Time and an Unknown Amount of Space

Posted on by

It’s been more than three weeks since I graced the internet with a post on this blog. That could be the longest time I’ve ever stayed away. The relatively encouraging results of last month’s election may have left me with the feeling that I should leave well enough alone. Plus, the urge to share thoughts — mine or somebody else’s — can come and go.

But to get going: There’s something that bothers me about time travel. When a fictional character travels through time, they always land standing up or sitting down in a relatively comfortable location. Arnold, for example, landed in an alley, naked, the first time we saw him. Time travelers never end up a mile deep in the earth’s crust or a million miles out in space.

One problem here is that the surface of the earth in the distant past or future is nowhere near wherever the time machine is. The earth, not a perfect sphere, is revolving on its axis and revolving around the sun; the sun and the rest of the solar system is going around the galaxy; the galaxy is moving quickly away from other galaxies as the universe expands. That means calculating the location of the traveler’s destination in space, not just in time, must be quite a challenge. A tiny mistake and Arnold lands six feet under or in the wrong solar system. Naked.

This detail concerning time travel came to mind because I’ve been skimming a TV series that makes use of time travel (in a surprising way) and because I read something that actually seems worth sharing.

Sean Carroll, famous physicist and now the Homewood Professor of Natural Philosophy at Johns Hopkins, has a new book out called The Biggest Ideas in the Universe: Space, Time and Motion. It’s the first volume in a planned trilogy that is supposed to make the fundamental equations of physics understandable to those of us who got through high school math (which may be a problem for me, since trigonometry convinced me to avoid calculus).

Quanta magazine an article adapted from Prof. Carroll’s book that helped me think about space and time differently. Maybe it will have the same effect on you. The article isn’t very long, so you might want to visit Quanta (it’s free). If not, here are selections that mainly leave out some historical background and may (or may not) clarify a few of Carroll’s remarks:

… In relativity, it’s no longer true that space and time have separate, objective meanings. What really exists is space-time, and slicing it up into space and time is merely a useful human convention.

One of the major reasons why relativity has a reputation for being difficult to understand is that our intuitions train us to think of space and time as separate things. We experience objects as having extent in “space,” and that seems like a pretty objective fact. Ultimately it suffices for us because we generally travel through space at velocities far lower than the speed of light, so pre-relativistic physics works.

But this mismatch between intuition and theory makes the leap to a space-time perspective somewhat intimidating. What’s worse, presentations of relativity often take a bottom-up approach — they start with our everyday conceptions of space and time and alter them in the new context of relativity.

We’re going to be a little different. Our route into special relativity might be thought of as top-down, taking the idea of a unified space-time seriously from the get-go and seeing what that implies. We’ll have to stretch our brains a bit, but the result will be a much deeper understanding of the relativistic perspective on our universe….

Einstein’s contribution in 1905 was to point out that [to] better understand the laws of physics … all we had to do was accept a completely new conception of space and time. (OK, that’s a lot, but it turned out to be totally worth it.)

Einstein’s theory came to be known as the special theory of relativity, or simply special relativity. [Einstein] argued for new ways of thinking about length and duration. He explained the special role of the speed of light by positing that there is an absolute speed limit in the universe — a speed at which light just happens to travel when moving through empty space — and that everyone would measure that speed to be the same, no matter how they were moving. To make that work out, he had to alter our conventional notions of time and space.

But he didn’t go quite so far as to advocate joining space and time into a single unified space-time. That step was left to his former university professor, Hermann Minkowski…. Once you have the idea of thinking of space-time as a unified four-dimensional continuum, you can start asking questions about its shape. Is space-time flat or curved, static or dynamic, finite or infinite? Minkowski space-time is flat, static and infinite.

Einstein worked for a decade to understand how the force of gravity could be incorporated into his theory. His eventual breakthrough was to realize that space-time could be dynamic and curved, and that the effects of that curvature are what you and I experience as “gravity”. The fruits of this inspiration are what we now call general relativity.

So special relativity is the theory of a fixed, flat space-time, without gravity; general relativity is the theory of dynamic, curved space-time, in which curvature gives rise to gravity….

We should be willing to let go of our pre-relativity fondness for the separateness of space and time, and allow them to dissolve into the unified arena of space-time. The best way to get there is to think even more carefully about what we mean by “time”. And the best way to do that is to hark back, once again, to how we think about space.

Consider two locations in space, such as your home and your favorite restaurant. What is the distance between them?

Well, that depends… There is the distance “as the crow flies”, if we could imagine taking a perfectly straight-line path between the two points. But there is also the distance you would travel on a real-world journey … avoiding buildings and other obstacles along the way. The route you take is always going to be longer than the distance as the crow flies, since a straight line is the shortest distance between two points.

Now consider two events in space-time. In the technical jargon of relativity theory, an “event” is just a single point in the universe, specified by locations in both space and time. One event, call it A, might be “at home at 6 p.m.” and event B might be “at the restaurant at 7 p.m.” 

… We can ask ourselves, just as we did for the spatial distance between home and restaurant, how much time elapses between these two events…. If one event is at 6 p.m. and the other is at 7 p.m., there is one hour between them, right?

Not so fast, says Einstein. In an antiquated, Newtonian conception of the world, sure. Time is absolute and universal, and if the time between two events is one hour, that’s all there is to be said.

Relativity tells a different story. Now there are two distinct notions of what is meant by “time”. One notion of time is as a coordinate on space-time. Space-time is a four-dimensional continuum, and if we want to specify locations within it, it’s convenient to attach a number called “the time” to every point within it. That’s generally what we have in mind when we think of “6 p.m.” and “7 p.m.” Those are … labels that help us locate events….

But, says relativity, just as the distance as the crow flies is generally different from the distance you actually travel between two points in space, the duration of time you experience [on the journey between A and B] generally won’t be the same as the [one-hour difference between the universal coordinate times, A and B]. You experience an amount of time that can be measured by a clock that you carry with you on the journey. This is the proper time along the path. And the duration measured by a clock, just like the distance traveled as measured by the odometer on your car, will depend on the path you take.

That’s one aspect of what it means to say that “time is relative”. We can think both about a common time in terms of a [space-time coordinate] and about a personal time that we individually experience [or measure] along our path. And time is like space — those two notions need not coincide.

By a “straight path” in space-time, we mean both a straight line in space and a constant velocity of travel … with no acceleration. Fix two events in space-time — two locations in space and corresponding moments in time. A traveler could make the journey between them in a straight line at constant velocity … or they could zip back and forth. The back-and-forth route will always involve more spatial distance, but less proper time elapsed, than the straight version [i.e. a clock along for the ride will run more slowly on the back-and-forth route — really?].

Why is it like that? Because physics says so. Or, if you prefer, because that’s the way the universe is. Maybe we will eventually uncover some deeper reason why it had to be this way, but in our current state of knowledge it’s one of the bedrock assumptions upon which we build physics, not a conclusion we derive from deeper principles. Straight lines in space are the shortest possible distance; straight paths in space-time are the longest possible time. It might seem counterintuitive that paths of greater distance take less proper time. That’s OK. If it were intuitive, you wouldn’t have needed to be Einstein to come up with the idea.

Philosophizing Naturally

Posted on by

Science used to be called “philosophy”. More specifically, it was called “natural philosophy”:

From the ancient world (at least since Aristotle) until the 19th century, natural philosophy was the common term for the study of physics (nature), a broad term that included botany, zoology, anthropology, and chemistry as well as what we now call physics. It was in the 19th century that the concept of science received its modern shape, with different scientific subjects emerging, such as astronomy, biology, and physics…. Isaac Newton’s book PhilosophiĂŠ Naturalis Principia Mathematica (1687) (Mathematical Principles of Natural Philosophy) reflects the use of the term natural philosophy in the 17th century [Wikipedia].

It makes some sense, therefore, that well-known physicist Sean Carroll decided to promote “natural philosophy”. This is from the transcript of one of Prof. Carroll’s podcasts:

… One of the bonuses of my new job here at Johns Hopkins is that I got to choose my own title. My title is Homewood professor, but then Homewood professor of what? … Knowing that I would both be involved in the physics department and the philosophy department, I thought it would be fun to call myself a professor of natural philosophy….

Back in the day, before we had separated out something called science and something called physics from philosophy, people like Isaac Newton or Galileo would have been considered to be philosophers. [He then mentions the full title of Newton’s Principia] …There’s a certain kind of philosophy and a certain kind of physics that really, really overlap, that are almost indistinguishable from each other, asking the biggest questions about, what is the world? What is it made of? Where did it come from? Why does it exist? Those kinds of things that really intersect with more down-to-earth physics questions like, “How does quantum mechanics work? What is fine-tuning in cosmology?” Things like that.

After reading that, I came upon an article from Quanta Magazine: “Inside the Proton, the ‘Most Complicated Thing You Could Possibly Imagine’”. Here’s how it starts:

The positively charged particle at the heart of the atom is an object of unspeakable complexity, one that changes its appearance depending on how it is probed
.

High school physics teachers describe them as featureless balls with one unit each of positive electric charge — the perfect foils for the negatively charged electrons that buzz around them. College students learn that the ball is actually a bundle of three elementary particles called quarks. But decades of research have revealed a deeper truth, one that’s too bizarre to fully capture with words or images.

“This is the most complicated thing that you could possibly imagine,” said Mike Williams, a physicist at the Massachusetts Institute of Technology. “In fact, you can’t even imagine how complicated it is.”

Reading further made me want to do some philosophy:

The proton is a quantum mechanical object that exists as a haze of probabilities until an experiment forces it to take a concrete form. And its forms differ drastically depending on how researchers set up their experiment. Connecting the particle’s many faces has been the work of generations. “We’re kind of just starting to understand this system in a complete way,” said Richard Milner, a nuclear physicist at MIT.

As the pursuit continues, the proton’s secrets keep tumbling out. Most recently, a monumental data analysis published in August found that the proton contains traces of particles called charm quarks that are heavier than the proton itself.

The proton “has been humbling to humans,” Williams said. “Every time you think you kind of have a handle on it, it throws you some curveballs.”

There are two things here that don’t sound right. First, what is a “haze of probabilities”? Physicists (and philosophers) disagree about what exists when we refer to a quantum entity. Is there something relatively substantial underlying it that we can’t (yet) identify? Or is there nothing there except “probabilities” that become real or substantial when we do a measurement (or when some other quantum entity interferes)? Speaking philosophically, it makes no sense that probabilities exist in some sort of “haze”. A probability is a possibility. How could a possibility exist without anything to separate it from other possibilities? Why would a possibility be in one place (say, Switzerland) as opposed to another (perhaps Johns Hopkins)? Most physicists would reply that I just don’t understand the quantum world. Unfortunately, according to physicist Richard Feynman’s well-known remark, neither do they:

I think I can safely say that nobody understands quantum mechanics. So do not take [this] lecture too seriously, feeling that you really have to understand in terms of some model what I am going to describe, but just relax and enjoy it. I am going to tell you what nature behaves like. If you will simply admit that maybe she does behave like this, you will find her a delightful, entrancing thing. Do not keep saying to yourself, if you can possible avoid it, “But how can it be like that?” because you will get ‘down the drain’, into a blind alley from which nobody has escaped. Nobody knows how it can be like that.

But, Prof. Feynman, going down blind alleys from which nobody has escaped is something philosophers do! That’s what they do most of the time! In this case, however, instead of going down the alley, we might suggest that “exists as” be replaced by “appears to be” or perhaps “manifests itself as”: the proton manifests itself as a haze of probabilities.

This brings me to the second thing that doesn’t sound right. The Quanta article says “the proton contains traces of particles … heavier than the proton itself”. The author meant “more massive than” rather than “heavier than”, but putting that aside, how can something’s contents be more massive than the thing itself?

The original study published in Nature says it this way:

Both light and heavy quarks, whose mass is respectively smaller or bigger than the mass of the proton, are revealed inside the proton in high-energy collisions.

It would be clearer to say that when measured, the proton has a certain mass, but when heavy quarks are measured outside the proton, their mass is greater than the proton’s. That’s certainly puzzling, and obviously justifies further investigation, but it’s not as contradictory as saying the proton’s contents are more massive than the proton.

Commenting About What Exists

Posted on by

I’ve recommended the 3 Quarks Daily site before:

3 Quarks Daily is a good place to visit for online intellectual stimulation. They publish original content on Mondays; the rest of the week they link to articles on “science, arts, philosophy, politics, literature”. Even the (moderated) comments are often worth reading. 

I mention this because there were two somewhat-related posts in the past two days that especially interested me. Both concerned what exists or is real (philosophers call the study of existence or being “ontology”).

The first article was from the BBC: “Why Does Time Go Forwards, Not Backwards?” It’s a perennial question in physics and philosophy: the nature of time. Here’s the part that bothered me:

The difference between hot things and cold things is how agitated their molecules are – in a hot steam engine, water molecules are very excited, careening around and colliding into each other rapidly. The very same water molecules are less agitated when they coalesce as condensation on a windowpane.

Here’s the problem: when you zoom in to the level of, say, one water molecule colliding and bouncing off another, the arrow of time disappears. If you watched a microscopic video of that collision and then you rewound it, it wouldn’t be obvious which way was forwards and which backwards. At the very smallest scale, the phenomenon that produces heat – collisions of molecules – is time-symmetric.

This means that the arrow of time from past to future only emerges when you take a step back from the microscopic world to the macroscopic….

“So the direction of time comes from the fact that we look at big things, we don’t look at the details,” says [physicist Carlo Rovelli]. “From this step, from the fundamental microscopic vision of the world to the coarse-grained, the approximate description of the macroscopic world – this is where the direction of time comes in.

“It’s not that the world is fundamentally oriented in space and time,” Rovelli says. It’s that when we look around, we see a direction in which medium-sized, everyday things have more entropy – the ripened apple fallen from the tree, the shuffled pack of cards.

While entropy does seem to be inextricably bound up with the arrow of time, it feels a bit surprising – perhaps even disconcerting – that the one law of physics that has a strong directionality of time built into it loses this directionality when you look at very small things.

My response:

Compared to people like Carlo Rovelli …, I’m a scientific ignoramus. Nevertheless:

The fundamental physical laws humanity has discovered, except for the one concerning entropy (the second law of thermodynamics), don’t refer to time. From this, most physicists conclude that time isn’t fundamental, or that it’s illusory or somehow less than real, even though it’s an obvious feature of the universe. Consider the Big Bang, then consider the city of Philadelphia. Is that contrast simply a matter of our perception?

Why assume that if something is a fundamental feature of the universe, it must be a variable in more than one law? Why assume that it has to be a variable in other laws we’ve discovered?

In the other fundamental laws we know about, there’s no distinction between past and future. So what? There seems to be an assumption here that time should appear in these other laws if it’s real. It looks like many physicists have turned their belief in (or desire for) simplicity, or universality, or uniformity, into a conclusion about how the universe is.

The author [of the BBC article] writes:

“Here’s the problem. when you zoom in to the level of, say, one water molecule colliding and bouncing off another, the arrow of time disappears. If you watched a microscopic video of that collision and then you rewound it, it wouldn’t be obvious which way was forwards and which backwards.”

So pay attention to what happened and don’t rewind the video.

It may be hard to believe, but neither the BBC person nor Prof. Rovelli have responded so far.

The second article is oddly titled: “Do You Want To Die With Me?” It’s by a history professor at Towson University, Akim Reinhardt, although it’s not about history. His topic is what exists (matter? energy? something else?). I’ll give you my comment first and then his response, which is clearer and shorter than the original article:

Another way to categorize our existence is to distinguish between our experiences (what we sense, including the testimony of other people) and our thoughts. It’s the empiricism/rationalism distinction. We need both categories to construct a semblance of reality. That relates to your matter/energy/ideas threesome.

So you say: “Matter, energy, and ideas: everything you observe, everything you think you know, falls into one of these three categories. Matter has mass and weight. Energy moves something against a force. Everything else is an idea.”

But a few sentences later, ideas seem to disappear: “There is no meaning. Only matter and energy, … all the conservable energy transferring elsewhere, all the matter unhinging and recombining into other things, and all the ideas as they ever were, never really here except as we imagined them.”

It’s interesting, however, that further down the page, in today’s first post, “How Civilization Inevitably Gives Rise To A Battle Between Good And Evil”, Andy Schmookler writes:


 Many in our contemporary secular culture hold the belief that Values are not really “real”…. That idea goes something like this: We cannot find Value “out there” in the cosmos, therefore Value isn’t part of reality…. But, when it comes to Value, there is a big logical flaw in that way of thinking….That’s because Value must mean that something matters, and there’s no way that anything could matter unless it matters to someone. (In a lifeless universe, there could be no Value…If there were no one who cared, there would be no way any such events would register on the dimension of “Value”. Which points to the logical non sequitur involved in dismissing Value as “not real” for not being “out there” in the “objective” world…. Value can only exist in terms of the subjective experience of creatures to whom things matter.

So [I asked], do ideas exist? Do values? Do numbers? Does Sherlock Holmes? Does meaning? Deciding how to answer such questions isn’t easy. Deciding not to try is easier.

Although I think it’s sensible to say meaning exists as long as somebody finds something meaningful.

Professor Reinhardt’s response:

I think all the stuff that’s not matter or energy (values, ideas, etc.) only exists if we believe they exist. I also believe we’re wired to believe they exist. And finally, I think we can intellectually overcome that and acknowledge that all the non-energy/non-matter stuff is make believe.

However, I don’t think we can fully overcome our wiring; in the vast majority of moments in which we exist, we are doomed to have ideas, values, etc. b/c that’s how our brains work. Once one accepts this, once one peers backstage at the proverbial puppet show, I think there are only two real options. One, think it through (ironic, I know) and come up with ideas, values, etc. that work for you (while forever knowing in the back of your mind that it’s a sham on some level), or two, choose ceasing to exist, which of course will inevitably happen at some point whether one chooses it or not, thereby freeing yourself of the conundrum as your matter and energy to disperse into non-sentient forms.

I got involved with another article today, this one by a physicist: “What Entanglement Doesn’t Imply”. I don’t know if I understood his position, but there’s no denying that the 3 Quarks Daily site exists.

A Nice Explanation of Quantum Mechanics, with Thoughts on What Makes Science Special

Posted on by

Michael Strevens teaches philosophy at New York University. In his book, The Knowledge Machine: How Irrationality Created Modern Science, he argues that what makes modern science so productive is the peculiar behavior of scientists. From the publisher’s site:

Like such classic works as Karl Popper’s The Logic of Scientific Discovery and Thomas Kuhn’s The Structure of Scientific Revolutions, The Knowledge Machine grapples with the meaning and origins of science, using a plethora of . . .  examples to demonstrate that scientists willfully ignore religion, theoretical beauty, and . . . philosophy to embrace a constricted code of argument whose very narrowness channels unprecedented energy into empirical observation and experimentation. Strevens calls this scientific code the iron rule of explanation, and reveals the way in which the rule, precisely because it is unreasonably close-minded, overcomes individual prejudices to lead humanity inexorably toward the secrets of nature.

Here Strevens presents a very helpful explanation of quantum mechanics, while explaining that physicists (most of them anyway) are following Newton’s example when they use the theory to make exceptionally accurate predictions, even though the theory’s fundamental meaning is mysterious (in the well-known phrase, they “shut up and calculate”):

To be scientific simply was to be Newtonian. The investigation of nature [had] changed forever. No longer were deep philosophical insights of the sort that founded Descartes’s system considered to be the keys to the kingdom of knowledge. Put foundational matters aside, Newton’s example seemed to urge, and devote your days instead to the construction of causal principles that, in their forecasts, follow precisely the contours of the observable world. . . .

[This is] Newton’s own interpretation of his method, laid out in a postscript to the Principia’s second edition of 1713. There Newton summarizes the fundamental properties of gravitational attraction—that it increases “in proportion to the quantity of solid matter” and decreases in proportion to distance squared—and then continues:

I have not as yet been able to deduce from phenomena the reason for these properties of gravity, and I do not feign hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. . . . It is enough that gravity really exists and acts according to the laws that we have set forth and is sufficient to explain all the motions of the heavenly bodies and of our sea.

The thinkers around and after Newton got the message, one by one.

[Jumping ahead three centuries:]

According to Roger Penrose, one of the late twentieth century’s foremost mathematical physicists, quantum mechanics “makes absolutely no sense.” “I think I can safely say that nobody understands quantum mechanics,” remarked Richard Feynman. How can a theory be widely regarded both as incomprehensible and also as the best explanation we have of the physical world we live in?

. . . Quantum theory derives accurate predictions from a notion, superposition, that is quite beyond our human understanding. Matter, says quantum mechanics, occupies the state called superposition when it is not being observed [or measured]. An electron in superposition occupies no particular point in space. It is typically, rather, in a kind of “mix” of being in many places at once. The mix is not perfectly balanced: some places are far more heavily represented than others. So a particular electron’s superposition might be almost all made up from positions near a certain atomic nucleus and just a little bit from positions elsewhere. That is the closest that quantum mechanics comes to saying that the electron is orbiting the nucleus.

As to the nature of this “mix”—it is a mystery. We give it a name: superposition. But we can’t give it a philosophical explanation. What we can do is to represent any superposition with a mathematical formula, called a “wave function.” An electron’s wave function represents its physical state with the same exactitude that, in Newton’s physics, its state would be represented by numbers specifying its precise position and velocity. You may have heard of quantum mechanics’ “uncertainty principle,” but forget about uncertainty here: the wave function is a complete description that captures every matter of fact about an electron’s physical state without remainder.

So far, we have a mathematical representation of the state of any particular piece of matter, but we haven’t said how that state changes in time. This is the job of Schrödinger’s equation, which is the quantum equivalent of Newton’s famous second law of motion F = ma, in that it spells out how forces of any sort—gravitational, electrical, and so on—will affect a quantum particle. According to Schrödinger’s equation, the wave function will behave in what physicists immediately recognize as a “wavelike” way. That is why, according to quantum mechanics, even particles such as electrons conduct themselves as though they are waves.

In the early days of quantum mechanics, Erwin Schrödinger, the Austrian physicist who formulated the equation in 1926, and Louis de Broglie, a French physicist—both eventual Nobel Prize winners—wondered whether the waves described by quantum mechanics might be literal waves traveling through a sea of “quantum ether” that pervades our universe. They attempted to understand quantum mechanics, then, using the old model of the fluid.

This turned out to be impossible for a startling reason: it is often necessary to assign a wave function not to a single particle, like an electron, but to a whole system of particles. Such a wave function is defined in a space that has three dimensions for every particle in the system: for a 2-particle system, then, it has 6 dimensions; for a 10-particle system, 30 dimensions. Were the wave to be a real entity made of vibrations in the ether, it would therefore have to be flowing around a space of 6, or 30, or even more dimensions. But our universe rather stingily supplies only three dimensions for things to happen in. In quantum mechanics, as Schrödinger and de Broglie soon realized, the notion of substance as fluid fails completely.

There is a further component to quantum mechanics. It is called Born’s rule, and it says what happens when a particle’s position or other state is measured. Suppose that an electron is in a superposition, a mix of being “everywhere and nowhere.” You use the appropriate instruments to take a look at it; what do you see? Eerily, you see it occupying a definite position. Born’s rule says that the position is a matter of chance: the probability that a particle appears in a certain place is proportional to the degree to which that place is represented in the mix.

It is as though the superposition is an extremely complex cocktail, a combination of various amounts of infinitely many ingredients, each representing the electron’s being in a particular place. Taste the cocktail, and instead of an infinitely complex flavor you will—according to Born’s rule—taste only a single ingredient. The chance of tasting that ingredient is proportional to the amount of the ingredient contained in the mixture that makes up the superposition. If an electron’s state is mostly a blend of positions near a certain atomic nucleus, for example, then when you observe it, it will most likely pop up near the nucleus.

One more thing: an observed particle’s apparently definite position is not merely a fleeting glimpse of something more complex. Once you see the particle in a certain position, it goes on to act as though it really is in that position (until something happens to change its state). In mixological terms, once you have sampled your cocktail, every subsequent sip will taste the same, as though the entire cocktail has transformed into a simple simple solution of this single ingredient. It is this strange disposition for matter, when observed, to snap into a determinate place that accounts for its “particle-like” behavior.

To sum up, quantum mechanical matter—the matter from which we’re all made—spends almost all its time in a superposition. As long as it’s not observed, the superposition, and so the matter, behaves like an old-fashioned wave, an exemplar of liquidity (albeit in indefinitely many dimensions). If it is observed, the matter jumps randomly out of its superposition and into a definite position like an old-fashioned particle, the epitome of solidity.

Nobody can explain what kind of substance this quantum mechanical matter is, such that it behaves in so uncanny a way. It seems that it can be neither solid nor fluid—yet these exhaust the possibilities that our human minds can grasp. Quantum mechanics does not, then, provide the kind of deep understanding of the way the world works that was sought by philosophers from Aristotle to Descartes. What it does supply is a precise mathematical apparatus for deriving effects from their causes. Take the initial state of a physical system, represented by a wave function; apply Schrödinger’s equation and if appropriate Born’s rule, and the theory tells you how the system will behave (with, if Born’s rule is invoked, a probabilistic twist). In this way, quantum theory explains why electrons sometimes behave as waves, why photons (the stuff of light) sometimes behave as particles, and why atoms have the structure that they do and interact in the way they do.

Thus, quantum mechanics may not offer deep understanding, but it can still account for observable phenomena by way of . . . the kind of explanation favored by Newton . . . Had Newton [engaged with scientists like Bohr and Einstein at conferences on quantum mechanics] he would perhaps have proclaimed:

I have not as yet been able to deduce from phenomena the nature of quantum superposition, and I do not feign hypotheses. It is enough that superposition really exists and acts according to the laws that we have set forth and is sufficient to explain all the motions of the microscopic bodies of which matter is made.

Newton . . .  was the chief architect of modern science’s first great innovation. Rather than deep philosophical understanding, Newton pursued shallow explanatory power, that is, the ability to derive correct descriptions of phenomena from a theory’s causal principles, regardless of their ultimate nature and indeed regardless of their very intelligibility. In so doing, he was able to build a gravitational theory of immense capability, setting an example that his successors were eager to follow.

Predictive power thereby came to override metaphysical insight. Or as the historian of science John Heilbron, writing of the study of electricity after Newton, put it:

When confronted with a choice between a qualitative model deemed intelligible and an exact description lacking clear physical foundations, the leading physicists of the Enlightenment preferred exactness.

So it continued to be, as the development and acceptance of quantum mechanics, as unerring as it is incomprehensible, goes to show. The criterion for explanatory success inherent in Newton’s practice became fixed for all time, founding the procedural consensus that lies at the heart of modern science.

Could This Be a More Sensible Timeline? (Quantum Mechanics Edition)

Posted on by

This post has nothing to do with politics.

One of the big mysteries or surprises in quantum physics is “non-locality”. This is the apparent fact that two particles can be “entangled”, such that even though they’re a billion miles apart, when something happens to one, something automatically happens to the other. The particles are far away in spacetime, but it’s as if they’re somehow right next to each other, possibly in some “deeper” reality. If that’s true, it’s pretty damn cool. 

From Quanta Magazine:

In a series of breakthrough papers, theoretical physicists have come tantalizingly close to resolving the black hole information paradox that has entranced and bedeviled them for nearly 50 years. Information, they now say with confidence, does escape a black hole. If you jump into one, you will not be gone for good. Particle by particle, the information needed to reconstitute your body will reemerge. Most physicists have long assumed it would; that was the upshot of string theory, their leading candidate for a unified theory of nature. But the new calculations, though inspired by string theory, stand on their own, with nary a string in sight. Information gets out through the workings of gravity itself — just ordinary gravity with a single layer of quantum effects. . . .

What it all means is being intensely debated in Zoom calls and webinars. The work is highly mathematical and has a Rube Goldberg quality to it, stringing together one calculational trick after another in a way that is hard to interpret. Wormholes, the holographic principle, emergent space-time, quantum entanglement, quantum computers: Nearly every concept in fundamental physics these days makes an appearance, making the subject both captivating and confounding.

And not everyone is convinced. . . .

But almost everyone appears to agree on one thing. In some way or other, space-time itself seems to fall apart at a black hole, implying that space-time is not the root level of reality, but an emergent structure from something deeper. Although Einstein conceived of gravity as the geometry of space-time, his theory also entails the dissolution of space-time, which is ultimately why information can escape its gravitational prison. . . .

The researchers drew on a concept that Richard Feynman had developed in the 1940s. Known as the path integral, it is the mathematical expression of a core quantum mechanical principle: Anything that can happen does happen. In quantum physics, a particle going from point A to point B takes all possible paths, which are combined in a weighted sum. The highest-weighted path is generally the one you’d expect from ordinary classical physics, but not always. If the weights change, the particle can abruptly lurch from one path to another, undergoing a transition that would be impossible in old-fashioned physics.

The path integral works so well for particle motion that theorists in the ’50s proposed it as a quantum theory of gravity. That meant replacing a single space-time geometry with a mĂ©lange of possible shapes. To us, space-time appears to have a single well-defined shape — near Earth, it is curved just enough that objects tend to orbit the center of our planet, for example. But in quantum gravity, other shapes, including much curvier ones, are latent, and they can make an appearance under the right circumstances. . . .

For [Stephen Hawking], space-time might knot itself into doughnut- or pretzel-like shapes. The extra connectivity creates tunnels, or “wormholes,” between otherwise far-flung places and moments. These come in different types.

Spatial wormholes are like the portals beloved of science-fiction writers, linking one star system to another. So-called space-time wormholes are little universes that bud off our own and reunite with it sometime later. Astronomers have never seen either type, but general relativity permits these structures, and the theory has a good track record of making seemingly bizarre predictions, such as black holes and gravitational waves, that are later vindicated. Not everyone agreed with Hawking that these exotic shapes belong in the mix, but the researchers doing the new analyses of black holes adopted the idea provisionally. . . .

Theorists in the West Coast group imagined sending [radiation escaping from a black hole] into a quantum computer. After all, a computer simulation is itself a physical system; a quantum simulation, in particular, is not altogether different from what it is simulating. So the physicists imagined collecting all the radiation, feeding it into a massive quantum computer, and running a full simulation of the black hole.

And that led to a remarkable twist in the story. Because the radiation is highly entangled with the black hole it came from, the quantum computer, too, becomes highly entangled with the hole. Within the simulation, the entanglement translates into a geometric link between the simulated black hole and the original. Put simply, the two are connected by a wormhole. “There’s the physical black hole and then there’s the simulated one in the quantum computer, and there can be a replica wormhole connecting those,” said Douglas Stanford, . . . a member of the West Coast team. This idea is an example of a proposal by [Juan Martin] Maldacena and Leonard Susskind  . . . in 2013 that quantum entanglement can be thought of as a wormhole. The wormhole, in turn, provides a secret tunnel through which information can escape the interior [of the black hole] . . .

Theorists have been intensely debating how literally to take all these wormholes. The wormholes are so deeply buried in the equations that their connection to reality seems tenuous, yet they do have tangible consequences. . . .

But rather than think of the wormholes as actual portals sitting out there in the universe, [some physicists] speculate that they are a sign of new, nonlocal physics. By connecting two distant locations, wormholes allow occurrences at one place to affect a distant place directly, without a particle, force or other influence having to cross the intervening distance — making this an instance of what physicists call nonlocality. “They seem to suggest that you have nonlocal effects that come in” [one physicist] said. In the black hole calculations, the island and radiation are one system seen in two places, which amounts to a failure of the concept of “place.” “We’ve always known that some kind of nonlocal effects have to be involved in gravity, and this is one of them . . . Things you thought were independent are not really independent.”

At first glance, this is very surprising. Einstein constructed general relativity with the express purpose of eliminating nonlocality from physics. Gravity does not reach out across space instantly. It has to propagate from one place to another at finite speed, like any other interaction in nature. But over the decades it has dawned on physicists that the symmetries on which relativity is based create a new breed of nonlocal effects. . . 

All this reinforces many physicists’ hunch that space-time is not the root level of nature, but instead emerges from some underlying mechanism that is not spatial or temporal. . . 

Skepticism is warranted if for no other reason than because the recent work is complicated and raw. It will take time for physicists to digest it and either find a fatal flaw in the arguments or become convinced that they work. . . .

Unquote.

So it looks like the universe is making more sense. Maybe we’ve entered a more sensible timeline, in which physics is less paradoxical and the president behaves like an adult politician/human being!