It's always fun to take a basic principle everybody knows from grade school and realize it's one of the deepest and most profound mysteries of the universe. This particular principle is old, encapsulated in Newton's very first law, the law of inertia:

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

In other words, its tough to change an object's state of motion, whether you be trying to slow down a speeding el train Spiderman(2)-style or start a motionless boulder rolling. The point of this thread is to ask why? Where does inertia come from? We're going to cover a lot of ground in this one so let's get started.

Equivalence Principle

More massive things have more inertia (it's more difficult to change their state of motion); in fact, that's actually a way to define mass. Everybody's seen Newton's Second Law at some point: F=ma. The force acting on some mass produces an acceleration that depends on the mass. The same force acting on a golf ball and a bowling ball will produce two very different accelerations—obviously a much smaller one for the bowling ball According to that equation that means the bowling must have a bigger “m.” Bowling balls have more inertial mass than golf balls.

But Newton was pretty prolific when it came to physical laws. He also came up with the well known law of universal gravitation which describes the forces two masses exert on each other across space. Not only did he come up with inertial mass, now's he given us gravitational mass, too. Things with more gravitational mass (like, say, the sun) have a more powerful pull than something with less (like the moon). So we can describe an object's inertial mass (how hard it is to change its state of motion) or its gravitational mass (how it reacts gravitationally with other masses).

What's the relationship between these two types of mass? Well, it turns out they're exactly equal. In fact, there's a (likely apocryphal) story of Galileo demonstrating this before Newton was even born! This is the famous experiment where he dropped different masses off the leaning tower of Pisa and found that they all fell at the same rate and hit the ground at the same time (contrary to Aristotle's notion that heavier masses should fall faster). This was shown again in an even grander experiment 4 centuries later when Commander David Scott dropped a hammer and a feather at the same time on the moon during Apollo 15. In the absence of air resistance, both hit the ground at the same time.

How does that demonstrate those two types of mass are equivalent? Well, according to the law of universal gravitation, if two masses are falling due to the Earth's gravity, the more massive one is feeling a bigger gravitational force. Yet it doesn't fall faster. The reason, it turns out, is that even though it's feeling a bigger gravitational force pulling it down, its inertial mass is also bigger so it's more resistant to changes in its state of motion. Triple the gravitational mass and you triple the gravitational force pulling it down. But you've also tripled the inertial mass, so it's three times as hard to get it going (that is, you'll need three times the force to acquire a given acceleration). Since gravitational and inertial mass are equal these effects cancel out and everything falls at the same rate. This is even incorporated into Einstein's theory of gravity, general relativity, inside a principle called the equivalence principle (this equivalence was verified experimentally by a guy named Roland von Eotvos to one part in a billion over a hundred years ago and Robert Dicke verified it to an even greater accuracy in 1962). Clearly there's an inertia-gravity connection.

Mach's Principle

Einstein was a big fan of the philosopher Ernst Mach and he sought to incorporate what became known as Mach's principle into his general theory of relativity. Mach wasn't a fan of the absolute space Newton had dealt with. Mach didn't think it made much sense to speak of accelerations relative to some absolute space but rather with respect to the distant stars (ultimately the whole rest of the universe). Without them acceleration would be meaningless and there would be no inertia—that is, if there was only one mass in the universe it wouldn't experience inertia. So Mach's principle is essentially that inertia is the result of gravitational forces from every other mass in the universe.

Philosophy's all well and good, but you can see this can get a little sticky when it comes to physics. We're suggesting that this instantaneous opposition to acceleration that every mass experiences could be due to all the rest of the masses in the universe—masses thousands, millions, even billions of light years away. But it's instantaneous: as soon as we decide to accelate something we're hit with inertial reaction forces (that pushing back on us we experience when we push on something).Supposedly influences are limited to the speed of light and below so how can this be? Did the stars somehow know a thousand, million, or billion years ago that that we were going to try to accelerate our dresser to push it to the other side of the room today and accordingly they acted in just the right way eons ago to make everything work out correctly today? Or did their influence cross the gap between us instantaneously (somehow)?

If we pull a Newton we can just say inertia is an inherent property of matter that arises internally and can't be explained any more than that; we don't have to look for an outside source. But there's something unsatisfying about that one.

The Quantum Vacuum

Perhaps we don't like this idea that inertia comes from the distant matter in the cosmos. Is it possible that inertia arises due to some kind of local field? Something that doesn't involve connections across the universe, yet provides an real explanation of how inertia arises externally from the bodies experiencing it? Maybe.

The famous Heisenberg uncertainty principle deals with a kind of knowledge seesaw that exists between certain conjugate variables—the better you can pin down the value of one, the more fuzzy and undefined the other one becomes. The most famous pair of variables is position and momentum but a similar relationship exists between energy and time. As you focus in more and more clearly on an increasingly tiny sliver of time, the amount of energy present gets a little fuzzy. This is often taken to imply that so-called virtual particles can and do pop in and out of existence all around us on exceedingly brief timescales, even out in the vacuum of space. These virtual particles can have measurable effects (maybe the most famous is called the Casimir effect in which they actually push two metal plates together but virtual particles must also be taken into account when calculating the magnetic moment of the electron—a spinning charge like an electron produces a magnetic field and the magnetic moment is just the strength of it). Sometimes they are called quantum fluctuations, sometimes zero point energy/fields. All pretty much the same.

A physicist named Bernard Haisch (along with a couple other guys) proposed that inertial reaction forces are actually resistance offered by the vacuum itself when something tries to accelerate. He thinks it's a kind of electromagnetic drag effect, with the particles of the zero point fields acting on the individual particles of the accelerating mass opposite the direction the object is accelerating. It's an interesting idea but there seem to be some problems with it. Since it's an electromagnetic effect it depends a lot on charge and two particles (say, protons and neutrons) can have different total charges yet have inertial masses that are very close. That and one of their early papers from a few years ago has the line “The neutrino, being apparently a truly neutral particle, should not have any inertial mass, consistent with current expectations.” Neutrinos are indeed now known to have mass so that kind of strikes right at the heart of their idea.

But perhaps a similar effect could arise that isn't so wholly electromagnetic. Maybe there are other local fields that could be responsible for inertia in pretty much the same vein as this idea. Let's switch gears, though, and go back to the idea that the distant masses in the universe play a role in inertia right here.

Wheeler-Feynman

Push on a charged particle and it pushes back; it resists changes in its state of motion more than an uncharged particle with the same inertial mass. These radiation reaction forces are often thought to be the result of a particle interacting with its own field—as it accelerates it experiences a force pushing back against it from the field that it generates. But there's a problem with the notion that a charge like an electron can act on itself, as mathpages explains:

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Remember that two electrons repel each other with a force (statically) proportional to the reciprocal of the square of the distance between them. This is traditionally understood in terms of each particle interacting with the field of the other particle. The intensity of each electron's field increases to infinity as the distance goes to zero (assuming point-like particles), so the force with which an electron is repelled increases to infinity as it approach the location of an electron - but therein lies a conceptual difficulty. According to this description, each electron is located in a place where there is an infinite force of repulsion against electrons!

Clearly if electrons can be acted upon by their own field then there could be come problems. It was in trying to deal with this conundrum in the '40s that Richard Feynman and John Wheeler decided to consider a scenario in which electrons don't act on themselves. That avoids the infinities but now you need to explain where the radiation reaction forces come from if not from the electon itself (they are, after all, instantaneous, occuring right when you start accelerating). Hopefully you're picking up on the parallel between these radiation reaction forces that resist acceleration and the inertial reaction force.

Wheeler and Feynman used the fact that the equations of electromagnetism are symmetric in time, working backwards as well as forwards. They allow not only the so-called retarded waves of radiation that we're used to (that arrive somewhere after they left) but also “advanced” waves (that arive somewhere before they were emitted—that is, travel backwards through time). In their model, the electron accelerates and emits radiation that goes forward in time (ignoring advanced waves) and eventually causes another particle (called the absorber) to accelerate and, in turn, emit retarded and advanced waves. The advanced waves are going backwards through time so they arrive back at the original electron at the exact instant it accelerated in the first place, providing the radiation reaction force that makes the electron resist acceleration.

Perhaps an analogous idea could be adopted to account for inertia. As this article suggests:

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Precisely the same thing evidently happens with inertial reaction forces. The act of pushing on something causes a disturbance in the gravitational field to go propagating off into the future. It makes stuff (the "absorber") out there wiggle. When the stuff wiggles it sends disturbances backward (and forward) in time. All the backward traveling disturbances converge on what we're pushing and generate the inertial reaction force we feel. No physical law is violated in any of this. And nothing moves faster than the speed of light. It only seems so because of the advanced waves traveling at the speed of light in the backward time direction.

Mach's notion of inertia being due to masses across the universe would then be realized, in a way. And a mass alone in the universe would have no absorbers to count on so it wouldn't feel a inertial reaction force, just as he suggested.

Anyway, interesting area. Just wanted to share a few of the ideas about how something as basic as inertia arises. Thoughts?


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"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman

Great Article Penthar,

Very well written (By You?)
A lot of interesting information there , keep up the good work


Domino

Quote:

Very well written (By You?)

Yeah, thanks.


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"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman

For what it's worth which won't be very much, inertia or the idea of it could be completely wrong.

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Supposedly influences are limited to the speed of light and below so how can this be? Did the stars somehow know a thousand, million, or billion years ago that that we were going to try to accelerate our dresser to push it to the other side of the room today and accordingly they acted in just the right way eons ago to make everything work out correctly today? Or did their influence cross the gap between us instantaneously (somehow)?

I don't see a problem with that, its sticky stuff for physics but I can accept that it can happen instantaneously at multiples of the speed of light so close as to be indistinguishable. But then again, I think thought itself can traverse any distance instantaneously and be received.

It's my view that whatever happens (moving objects or simply thinking) is both the result of an action and the cause of others however they be defined. Inertia as a principle in that case could be one of the noticeables of that?

Just in the part that THoTH quoted, I thought of quantum entanglement, and their supposed instantaneous "communication." Honestly, I'm too tired today to read the whole thing, but I'll give it a look over soon. Thanks Penthar.


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"There is no adequate defense, except stupidity, against the impact of a new idea."
Percy Williams Bridgman (1882-1961) U. S. physicist, Nobel Prize, 1946.

I have a blog!

The possibility does exist that there's some kind of instant nonlocal connection between masses that would indeed satisfy Mach. The author of that last article I linked to briefly touches on efforts to coerce some kind of instantaneous link between distant masses (to account for inertia) out of general relativity:

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Roughly, the modern instantaneous action argument goes as follows. In general relativity theory matter "there" tells space "here" how to curve, and space "here" tells matter "here" how to move. (Matter "here" also tells space "there" how to curve.) Thus, in order to talk about any situation in dynamics we must specify the distribution and motion of matter throughout space. (Strictly speaking, we must provide "initial data" on some suitably chosen "three dimensional spacelike hypersurface".) The usual field equations for gravity (Einstein's equations) are not enough, by themselves, to do this it turns out. Because of the finite propagation velocity built into them, we might specify some distribution of matter that subsequently leads to idiotic results. To make sure this doesn't happen, our distribution of matter has to satisfy some additional equations called "constraint" equations. The neat thing about these constraint equations is that, unlike the field equations, they're instantaneous. (Technically, they're "elliptic" rather than "hyperbolic" differential equations.) It's then claimed that inertia is conveyed by the constraint equations -- instantaneously. The use of constraint equations to communicate real physical influences instantaneously is justified by appeal to the instantaneous propagation of stationary electric fields in the Coulomb gauge.

Getting mildly technical there. Anyway, the author then dismisses this idea as unsatisfying, artificial, and ultimately untenable (though, of course, that's far from the final word on it).

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For what it's worth which won't be very much, inertia or the idea of it could be completely wrong.

It's my view that whatever happens (moving objects or simply thinking) is both the result of an action and the cause of others however they be defined. Inertia as a principle in that case could be one of the noticeables of that?

I'm not sure what you're getting at with this. Go on...


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"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman

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Is jiggling vacuum the origin of mass?

Where mass comes from is one of the deepest mysteries of nature. Now a controversial theory suggests that mass comes from the interaction of matter with the quantum vacuum that pervades the universe.

The theory was previously used to explain inertial mass – the property of matter that resists acceleration – but it has been extended to gravitational mass, which is the property of matter that feels the tug of gravity.

For decades, mainstream opinion has held that something called the Higgs field gives matter its mass, mediated by a particle called the Higgs boson. But no one has yet seen the Higgs boson, despite considerable time and money spent looking for it in particle accelerators.

In the 1990s, Alfonso Rueda of California State University in Long Beach and Bernard Haisch, who was then at the California Institute for Physics and Astrophysics in Scotts Valley and is now with ManyOne Networks, suggested that a very different kind of field known as the quantum vacuum might be responsible for mass. This field, which is predicted by quantum theory, is the lowest energy state of space-time and is made of residual electromagnetic vibrations at every point in the universe. It is also called a zeropoint field and is thought to manifest itself as a sea of virtual photons that continually pop into and out of existence.

Rueda and Haisch argued that charged matter particles such as electrons and quarks are unceasingly jiggled around by the zero-point field. If they are at rest, or travelling at a constant speed with respect to the field, then the net effect of all this jiggling is zero: there is no force acting on the particle. But if a particle is accelerating, their calculations in 1994 showed that it would encounter more photons from the quantum vacuum in front than behind it (see Diagram). This would result in a net force pushing against the particle, giving rise to its inertial mass (Physical Review A, vol 49, p 678).

But this work only explained one type of mass. Now the researchers say that the same process can explain gravitational mass. Imagine a massive body that warps the fabric of space-time around it. The object would also warp the zero-point field such that a particle in its vicinity would encounter more photons on the side away from the object than on the nearer side. This would result in a net force towards the massive object, so the particle would feel the tug of gravity. This would be its gravitational mass, or weight (Annalen der Physik, vol 14, p 479).

Rueda and Haisch say this demonstrates the equivalence of inertial and gravitational mass – something that Einstein argued for in his theory of general relativity. "In place of having the particle accelerate through the zero-point field, you have the zero-point field accelerating past the particle," says Haisch. "So the generation of weight is the same as the generation of inertial mass."

The idea is far from winning wide acceptance. To begin with, there's a conundrum about the zero-point field that needs to be solved. The total energy contained in the field is staggeringly large – enough to warp space-time and make the universe collapse in a heartbeat. Obviously this is not happening. Also, the pair's work can only account for the mass of charged particles.

Nobel laureate Sheldon Glashow of Boston University is dismissive. "This stuff, as Wolfgang Pauli would say, is not even wrong," he says. But physicist Paul Wesson of Stanford University in California says Rueda and Haisch's unorthodox approach shows promise, though he adds that the theory needs to be backed up by experimental evidence. "If Haisch [and Rueda] could come up with a concrete prediction, then that would make people sit up and take notice," he says. "We're all looking for something we can measure."

>

Just throwing that in there. Interesting but when the reigning king of quantum field theory quotes Pauli to call it not even wrong you don't want to put any money on it...


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"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman

The original post focused on inertia and not precisely the origin of mass but the two don't exist in a vacuum from each other, so to speak. That news article certainly blurred the lines a little. It mentioned the Higgs field, which in standard lore is the mass-giver, though it hasn't be observed yet. The July issue of Scientific American had a pretty good article on this which is available in its entirety online. The Mysteries of Mass. A snippet:

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Most people think they know what mass is, but they understand only part of the story. For instance, an elephant is clearly bulkier and weighs more than an ant. Even in the absence of gravity, the elephant would have greater mass--it would be harder to push and set in motion. Obviously the elephant is more massive because it is made of many more atoms than the ant is, but what determines the masses of the individual atoms? What about the elementary particles that make up the atoms--what determines their masses? Indeed, why do they even have mass?

We see that the problem of mass has two independent aspects. First, we need to learn how mass arises at all. It turns out mass results from at least three different mechanisms, which I will describe below. A key player in physicists' tentative theories about mass is a new kind of field that permeates all of reality, called the Higgs field. Elementary particle masses are thought to come about from the interaction with the Higgs field. If the Higgs field exists, theory demands that it have an associated particle, the Higgs boson. Using particle accelerators, scientists are now hunting for the Higgs.

The second aspect is that scientists want to know why different species of elementary particles have their specific quantities of mass. Their intrinsic masses span at least 11 orders of magnitude, but we do not yet know why that should be so. For comparison, an elephant and the smallest of ants differ by about 11 orders of magnitude of mass.

What Is Mass?
Isaac newton presented the earliest scientific definition of mass in 1687 in his landmark Principia: "The quantity of matter is the measure of the same, arising from its density and bulk conjointly." That very basic definition was good enough for Newton and other scientists for more than 200 years. They understood that science should proceed first by describing how things work and later by understanding why. In recent years, however, the why of mass has become a research topic in physics. Understanding the meaning and origins of mass will complete and extend the Standard Model of particle physics, the well-established theory that describes the known elementary particles and their interactions. It will also resolve mysteries such as dark matter, which makes up about 25 percent of the universe.

The foundation of our modern understanding of mass is far more intricate than Newton's definition and is based on the Standard Model. At the heart of the Standard Model is a mathematical function called a Lagrangian, which represents how the various particles interact. From that function, by following rules known as relativistic quantum theory, physicists can calculate the behavior of the elementary particles, including how they come together to form compound particles, such as protons. For both the elementary particles and the compound ones, we can then calculate how they will respond to forces, and for a force F, we can write Newton's equation F = ma, which relates the force, the mass and the resulting acceleration. The Lagrangian tells us what to use for m here, and that is what is meant by the mass of the particle.

But mass, as we ordinarily understand it, shows up in more than just F = ma. For example, Einstein's special relativity theory predicts that massless particles in a vacuum travel at the speed of light and that particles with mass travel more slowly, in a way that can be calculated if we know their mass. The laws of gravity predict that gravity acts on mass and energy as well, in a precise manner. The quantity m deduced from the Lagrangian for each particle behaves correctly in all those ways, just as we expect for a given mass.

Fundamental particles have an intrinsic mass known as their rest mass (those with zero rest mass are called massless). For a compound particle, the constituents' rest mass and also their kinetic energy of motion and potential energy of interactions contribute to the particle's total mass. Energy and mass are related, as described by Einstein's famous equation, E = mc2 (energy equals mass times the speed of light squared).

...

Long snippet. But there's a lot more.


_________________
"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman

Is it possible that tachyons appear to move backwards in time, because they travel faster than light? Or am I whizzing up a rope?


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Evil: "When I have the map, I will be free, and the world will be different, because I have understanding."

Robert: "Understanding of what, master?"

Evil: "Digital watches."

There was a thread on that a ways back: here. The short answer is yes. Though they aren't known to physically exist.


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"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman

OK, if light can be stopped, and then restarted at full speed, couldn't you then encode tachyons with information, slow them down or stop them the way you do with light, and then gather that information? In this manner, you could communicate with the past. It's supposed that tachyons can't go slower than light, but if light can be slowed down or stopped, why not tachyons?

But at that point, does the timeline that would have carried on in the future come to a dead end, or rather, a loopback to the moment the message is received, and a new, different timeline emerge? Or does the original timeline carry on unhindered, and a new alternate timeline branch off and proceed from the point of contact?


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Evil: "When I have the map, I will be free, and the world will be different, because I have understanding."

Robert: "Understanding of what, master?"

Evil: "Digital watches."

I'm not a big fan of alternate/multiple timelines and all that. It's a lot easier to maintain consistency when nothing changes. I laid my thoughts out in these two threads (especially the first one):

Back...to the future

time mining


_________________
"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman

Where do you stand on membrane theory? Does that pop up in one of the threads anywhere?


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Evil: "When I have the map, I will be free, and the world will be different, because I have understanding."

Robert: "Understanding of what, master?"

Evil: "Digital watches."

It's interesting, cutting edge stuff but not yet on firm enough ground. Some things can be fun but so far in the realm of the theoretical that in some respects they almost don't qualify as science. And no, it hasn't come up in any threads that I know of.


_________________
"There is no harm in doubt and skepticism, for it is through these that new discoveries are made." -- Richard Feynman