Gravity is a Force

Physicists are sharply divided over whether gravity is a force. Oh, they refer to it as a force all the time, such as saying it is one of the four fundamental forces, but then say it is a fictitious force.

The excellent video channel Veritasium has explanation of Why Gravity is NOT a Force.

It is like the people who say that centrifugal force is not a force, but centripetal forces is.

Gravity has been considered a force since Newton in the 1600s, so the opposite view requires explanation.

Actually, it is not so clear that Newton believed that gravity was a force. He was very much against action-at-a-distance:

Newton famously struggled to find out the cause of gravity.^[12] In a letter to Bentley, dated January 17 1692/3, he said:

You sometimes speak of Gravity as essential and inherent to Matter. Pray do not ascribe that Notion to me, for the Cause of Gravity is what I do not pretend to know, and therefore would take more Time to consider it. (Cohen 1978, p. 298)

In a subsequent letter to Bentley, dated February 25, 1692/3, he added:

It is inconceivable that inanimate Matter should, without the Mediation of something else, which is not material, operate upon, and affect other matter without mutual Contact…That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro’ a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it. Gravity must be caused by an Agent acting constantly according to certain laws; but whether this Agent be material or immaterial, I have left to the Consideration of my readers. (Cohen 1978, pp. 302-3)

Aristotle also denied that gravity was a force.

The Aristotelian explanation of gravity is that all bodies move toward their natural place.

There are two arguments that gravity is not a force. One says that you do not feel gravity in free fall. You feel it when you stand on the ground, but you are really feeling the force of the ground pushing you up.

The second is that general relativity teaches that gravity is just curvature of spacetime, not a force. This is a variation of Aristotle's argument.

A typical explanation:

“The one sentence statement of general relativity is that ‘gravity is the curvature of spacetime,’” explains Dr. Sean Carroll, assistant professor of physics at the University of Chicago. “Really, the differences come in understanding what that sentence means.”

Carroll says that origin of the theory of general relativity dates to 1905, when scientists, notably including Albert Einstein, realized that space and time are related characteristics of a four-dimensional existence. ...

However, within this new 4-D framework, says Carroll, Einstein could not understand gravity, and how it worked in spacetime. He decided that rather than being a force, like electromagnetism, gravity must be a property: a geometric curvature.

This stuff about Einstein believing that gravity is geometrical curvature is a modern invention. Yes, he used the equations for curvature, but did not subscribe to the geometric interpretation that is popular today.

General relativity differs very slightly from Newtonian gravity. It is silly to say one is a force and the other not. They are essentially the same.

My biggest quibble is with those who say electromagnetism is a force, but gravity is not. In modern physics, all of the four fundamental forces have geometrical interpretations, where the field strength is given by curvature. Test particles follow curvature, in all cases. Here is a recent paper explaining it. So if gravitational forces are fictitious because particles are just following curvature, then nothing else is a force either.

Those who deny that gravity is a force sometimes go one step farther, and deny causality. Eg, from the Stanford Encyclopedia:

Causation in Physics

What role, if any, do causal notions play in physics? On the one hand, it might appear intuitively obvious that physics aims to provide us with causal knowledge of the world and that causal claims are an integral part of physics. On the other hand, there is an influential philosophical tradition, dating back to Ernst Mach and to Bertrand Russell’s extremely influential article “On the Notion of Cause” (1912), denying the applicability or at least the usefulness of causal notions in physics. While this tradition is perhaps not as dominant today than it once was, there continues to be a lively and active philosophical debate on whether causal notions can play a legitimate role in physics and, if yes, what role that might be.

One part of this is that if you believe in determinism and the block universe, then the Big Bang caused everything, and nothing else had any influence.

I take the view that we have forces and causes. I am all in favor of the geometrical interpretation, but not to deny forces and causes.

While I think most physicists take a geometrical view, here is a new oddball paper:

A Puzzle About General Covariance and Gauge

Eleanor March, James Owen Weatherall

We consider two simple criteria for when a physical theory should be said to be "generally covariant", and we argue that these criteria are not met by Yang-Mills theory, even on geometric formulations of that theory. The reason, we show, is that the bundles encountered in Yang-Mills theory are not natural bundles; instead, they are gauge-natural.

Of course Yang-Mill (gauge) theories are generally convariant, as the theory is independent of any particular coordinates. If you change coordinates, then the equations of motion transform as you expect from vectors and tensors.

The paper makes the trivial point that if you change the coordinates, that does not necessarily tell you how to change the gauge. Yang-Mill theories are covariant over a change in coordinates and gauge.

These confusing arguments only obscure the fact that gravity, electromagnetism, strong, and weak forces all use the same geometrical constructions. Just the bundles are different. Gravity uses the tangent bundle on spacetime, while the other forces use U(1), SU(3), and SU(2) bundles. They are all covariant.

The Wikipedia article on general covariance says that Einstein popularized the term, but did not use it precisely. So I guess that is why some might think that it applies to general relativity, but not to other bundles.

Wilczek says Falling Cats have Free Will

Physicist Frank Wilczek is famous for helping explain the short range nature of the strong force, and now he wrote a strange paper on cats. No, not Schroedinger cats.

Free Will and Falling Cats

If we consider a cat to be an isolated mechanical system governed by T-invariant mechanics, then its ability to land on its feet after being released from rest is incomprehensible. It is more appropriate to treat the cat as a creature that can change its shape in order to accomplish a purpose. Within that framework we can construct a useful and informative of the observed motion. One can learn from this example.

He seems to be saying that it is impossible for a falling cat to land on its feet, unless it has free will.

He proves it is impossible, but then says cats have a loophole because "They can readily and selectively consume stored energy, notably by converting ATP into ADP, empowering mechanical motion accompanied by radiation of heat."

This is bizarre. The physics of falling cats is well-understood, and does not require free will or ATP or heat or anything like that. See A simple model for the falling cat problem. There is even a Wikipedia article on it.

The paradox is that the cat falls with zero angular momentum, a conserved quantity, and it is hard to see how it can get its feet down without some angular momentum. But as the above paper explains, the cat can twist its body without any net angular momentum.

Wilczek has some philosophical comments that went over my head, so maybe I am not fully appreciating his paper. After all, he has a Nobel Prize and I don't. What am I missing? He does cite a book on "Falling Felines and Fundamental Physics", so he must know that this is understood.

Carroll Eulogy for Dan Dennett

Physicist Sean M. Carroll released his latest AMA podcast. He starts with a eulogy.

Dan Dennett you know was not only a hugely important 1:02 philosopher for the 20 and 21st century uh and a mindscape guest but also a personal friend so that hit very hard I 1:09 was sitting at a table with Dan and a bunch of other philosophers just a few 1:15 weeks ago at Santa Fe

He particularly praised Dennett for giving philosophical defenses of Atheism and Darwinism, and saying the mind is just a machine, so consciousness and free will are just illusions. But he was very much against telling the hoi polloi that they have no free will, or they may decide that they have no moral responsibility for their actions.

In past generations, philosophers did not want to inform us that God is dead, or that we are descended from apes.

Next, Carroll praised counterfactual reasoning.

ml pickle says your recent guests have 21:43 pointed to counterfactual reasoning as a key element of human advancement either as mental time travel or considering the 21:49 consequences of different initial conditions or empathy for the plights of others Etc I've begun to notice how 21:55 often I do it and to suspect it's true that Ed ability for example and perhaps 22:00 our overall success as a species depend upon it however I can't tell if it is required or most efficient or if it just 22:07 happens to be what I personally do how important do you think counterfactual reasoning is to effective prediction as 22:13 individuals or to our past and future advancement as a species it's hard to answer I mean that 22:20 the answer is super important basically but when you ask you know how important is it I'm not quite sure what how to 22:26 quantify it right um but I do do think that this idea of counterfactual reasoning maybe the first time we talked 22:31 about it was with Malcolm mcgyver when he was talking about the the initial evolutionary development of the capacity 22:38 for counterfactual reasoning when fish climbed onto land and could suddenly see a lot further than they could before and 22:46 yeah you're right we've talked about um people who have been thinking more specifically about how humans use those 22:52 capacities um I you know it's hard to imagine something oh Carl friston we 22:58 actually also talked with about that issue cuz I was I remember joking with him that of my two cats Ariel and 23:04 calaban one seems to be capable of counterfactual reasoning and one does not calaban just seeks the local minimum 23:10 he just wants to be as happy as possible in the moment I don't think the idea of other moments ever occurs to him whereas 23:15 Ariel is the stereotypical cat who you know when you open a door that she demands to have open she's like I don't 23:21 know do I want to open that do I want to walk to that threshold I'm not quite sure she's always thinking about the 23:26 possible bad consequences Etc and our uh feral cat our outdoor cat Puck who has 23:32 uh hung around outside our yard for a while um he's super cautious right 23:38 because this is what is keeping him alive so he doesn't do anything until he like thinks about it 20 different ways 23:43 so somewhere that's that's close to when this capacity in some minor uh key was 23:49 first developed evolutionarily but anyway yeah absolutely possible AB absolutely crucial because I think that 23:57 I suspect and I'm just making stuff up I'm not an expert on this but I suspect that the parts of the brain that are 24:03 good at in general abstract symbolic reasoning are either the same as or 24:09 closely related to the parts of the brain that are that are good at counterfactual reasoning

I would agree with this, except that there is no way to reconcile this with his views on many-worlds theory. In many-worlds, all counterfactual scenarios happen, so there is no such thing as a counterfactual.

If he really believed in many-worlds, there would be no free will either. Every decision is just a splitting of the worlds, with both branches equally real.

He also expresses support for the anti-Israel protests currently going on at college campuses.

The podcast goes on for three more hours. Let me know if I missed anything good.

Unscientific American

James B. Meigs writes:

Science journalism surrenders to progressive ideology.

Michael Shermer got his first clue that things were changing at Scientific American in late 2018. The author had been writing his “Skeptic” column for the magazine since 2001. ...

Shermer dug his grave deeper by quoting Manhattan Institute fellow Heather Mac Donald and The Coddling of the American Mind authors Greg Lukianoff and Jonathan Haidt, who argue that the rise of identity-group politics undermines the goal of equal rights for all. Shermer wrote that intersectional theory, which lumps individuals into aggregate identity groups based on race, sex, and other immutable characteristics, “is a perverse inversion” of Martin Luther King’s dream of a color-blind society. For Shermer’s editors, apparently, this was the last straw. The column was killed and Shermer’s contract terminated. Apparently, SciAm no longer had the ideological bandwidth to publish such a heterodox thinker.

The article gives many examples.

I have criticized these SciAm trends for years.

Maybe the title should be Unscientific Unamerican, since the magazine is now owned by a British company.

Many-worlds does not Solve the Measurement Problem

Brian Greene's third interview is with Carlos Rovelli:

I'll now give you a brief 0:21 summary of where we have gotten so far all right in our first conversation with 0:26 philosopher Elise Crow we discussed the basics of quantum mechanics namely that 0:33 the theory has at its core the idea that the best you can ever do in our reality 0:39 is make probabilistic predictions ...

we also explored how through quantum 1:36 entanglement reality has what we call a nonlocal quality that is what you do 1:42 here can have an instantaneous Quantum impact on something way over there and 1:48 such non-locality we found can thread not just through space but also through time

all right in 1:56 our second conversation with physicist and author Sean Carol we explored a 2:01 number of proposed resolutions to the quantum measurement problem most notably 2:07 spontaneous collapse theories as well as the many worlds interpretation of 2:13 quantum mechanics in which every possible Quantum outcome actually 2:18 happens but each takes place in its own Quantum Universe

No. Nothing can have an instantaneous nonlocal impact, and many-worlds does not solve the measurement problem.

I am surprised Greene would say this, because the advocates of many-worlds theory do not even claim to solve the measurement problem.

They argue that there is a wave function of the universe, and that the function will explain everything we see. Any mystery might be answered by understanding this function better. But it does not tell us why we see one outcome.

Or so I thought. I asked ChatGPT:

The many-worlds interpretation (MWI) is a fascinating and controversial view in quantum mechanics. Let’s delve into it and explore how it relates to the quantum measurement problem.

The Quantum Measurement Problem

The measurement problem arises from the mysterious behavior of quantum systems during measurement or observation. Here’s a brief overview:

1. Wavefunction Collapse: Before a measurement, a quantum system exists in a superposition of all possible states described by its wave function. However, upon measurement, the wave function appears to “collapse” to a specific state, leading to the observed outcome. This collapse is puzzling because it seems to violate the unitary evolution of quantum mechanics.

2. Where Do Other Possibilities Go?: If all possible outcomes of a quantum measurement have real existence, why do we observe only one outcome? What happens to the other possibilities?

Many-Worlds Interpretation (MWI)

The MWI, proposed by physicist Hugh Everett, offers an intriguing solution to the measurement problem. Here are the key ideas:

1. Parallel Universes: According to the MWI, the universe doesn’t collapse into a single state during measurement. Instead, it branches into multiple parallel universes, each corresponding to a different measurement outcome. In other words, every possible outcome occurs in a separate universe.

2. Wave Function Never Collapses: In the MWI, there is no collapse of the wave function. Instead, the entire universe (including the observer) evolves as a superposition of all possible states. Each branch represents a different version of reality, with no need for collapse.

3. Replicas of You and Me: In these parallel universes, there exist replicas of you and me, leading other lives. Each replica experiences a different outcome of the measurement. These universes coexist but remain mutually isolated.

Controversy and Philosophical Implications

The MWI challenges our understanding of knowledge and reality:

Controversy: Debates about quantum interpretations can be passionate, and the MWI is no exception. Some physicists find it elegant, while others reject it due to its radical departure from classical intuition.

Epistemological Question: The MWI raises fundamental questions about what constitutes knowledge. It challenges us to consider what kind of theory we accept as a claim to understand the world.

In summary, the MWI provides an alternative perspective that avoids wavefunction collapse, but it remains a topic of ongoing debate. Whether it fully solves the measurement problem is still a matter of interpretation and personal preference12. Feel free to explore further or ask more questions! 😊

I cites this Quanta article:

Why the Many-Worlds Interpretation Has Many Problems ...

it seemed that the central problem for quantum mechanics was the mysterious rupture created by observation or measurement, which was packaged up into the rubric of “collapse of the wave function.” ...

But Bohr and colleagues didn’t bring wave function collapse into the picture just to make things difficult. They did it because that’s what seems to happen. When we make a measurement, we really do get just one result out of the many that quantum mechanics offers. Wave function collapse seemed to be demanded in order to connect quantum theory to reality.

So what Everett was saying was that it’s our concept of reality that’s at fault. We only think that there’s a single outcome of a measurement. But in fact all of them occur. We only see one of those realities, but the others have a separate physical existence too.

Okay, I guess the Everett do claim that they have solved somethiung here. As you can see, they have not.

New Greene Videos spread Quandum Misinformation

Physicist Brian Greene, known mainly for overhyping string theory, has new interviews of Elise Crull and Sean Carroll on quantum entanglement and related topics.

You should know to be suspicious when they say the Nobel Prize committee got something wrong. Carroll complains that they gave a prize for disproving hidden variables, even though Einstein, Bohm, and Bell believed in hidden variables.

Yes, the prize was for doing experiments to prove them wrong.

Greene lets Carroll plug his favorite, many-worlds theory. They claim it is somehow simpler to assume zillions of unobservale worlds.

They do put their finger on the heart of the problem -- the theory requires rejecting probabilities.

Usually, when a theory assigns a probability to an event, it is an estimate of the likelihood. of the event occurring versus not occurring. Carroll has to reject that whole way of thinking. He has to say all events occur, and deny that probabilities mean anything.

He admits that many-worlds does not match our real-world experience, but says we need more research on the foundations of Physics to figure it out.

He complains that universities do not want people like Bohm and Bell who are concerned with this stuff. Actually he admits that they did not want Bohm because he was a Commie, and got his degree under the Commie J. R. Oppenheimer.

They all give misleading descriptions of nonlocality. There is no quantum nonlocality in the sense that doing something in one place has an immediate effect on a distant place.

There has been no Nobel Prize for quantum nonlocality. They also complain that the textbooks do not explain quantum mechanics adequately. This should make you suspicious. How is it that these guys have some profound quantum insights that are not recognized by the Nobel committee or the textbooks?

I am afraid that people watch these videos and think that they are learning something. No, they are getting some fringe ideas that mainstream physicists say is wrong.

Update: Carroll says:

just say we have a wave function and not only the electron has a wave function but you and I are part of the wave 11:14 function of the universe and we just ask what would be predicted by the [Schroedinger] equation if if we went back to the has a wave function but you and I are part of the wave 11:21 classical Paradigm where we just have stuff and an equation yeah what would happen and the answer is that that part 11:28 of the wave function let's say that has the electrons spin up and spin down it doesn't disappear when you do a 11:34` measurement a measurement is clearly defined in this picture as a physical interaction between the Observer and the 11:40 system and the decoherence part is also important and what happens is you end up in a 11:46 superposition of the electron was spin up and I saw it spin up plus the 11:51 electron was spin down and I saw it spin down and ever's entire contribution was to say and that's okay and ... yeah it's the leanest and meanest version of quantum mechanics it's just there's a wave function and an equation everything else pops

This is a little hard to follow, but he says classical mechanics uses equations to make predictions, and many-worlds is just doing the same thing to use the Schroedinger equation to make predictions.

But the equations of classical mechanics can also be used to predict probabilities. When you do, and make an observation that rules out some possibilites, then the appropriate state functions are updated accordingly. No one says that the equations create parallel worlds. So classical and quantum mechanics are essentially the same in this respect.

Carroll would say that hte difference is that the probabilities are real and fundamental in quantum mechanics, but not classical mechanics. But that is just his personal philosophical opinion.