What causes Inertia?
May 11, 2011 9:19 AM   Subscribe

I've always wondered about this and never found a satisfactory answer anywhere - what causes inertia?
posted by enamon to Science & Nature (27 answers total) 2 users marked this as a favorite
Nothing causes inertia. It's simply the observation that things tend to remain in motion unless something acts on them.
posted by blue_beetle at 9:24 AM on May 11, 2011

Just to clarify, think of it this way: Everything is moving right now. Everything. See that stapler on your desk? It's moving, even if it doesn't look like it, you're just moving with it. If you want to move it you have to slow it down or redirect the movement, that takes energy because it has mass. The energy you apply to the stapler to redirect its movement is inertia.
posted by blue_beetle at 9:27 AM on May 11, 2011 [1 favorite]

An absence of forces acting on an object "causes" intertia. Can you explain your thinking a little more? It would help pinpoint the source of confusion.
posted by qxntpqbbbqxl at 9:27 AM on May 11, 2011

Perhaps I should clarify what I mean by "inertia":

From Wikipedia:

"Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass."
posted by enamon at 9:31 AM on May 11, 2011

This article on the recent Gravity Probe B project may be relevant:
Furthermore, general relativity has brought us nearer to an understanding of the observational fact that the local inertial compass is fixed relative to the most distant cosmic objects, but there is surely desire for still deeper understanding." Thus does direct detection of frame-dragging by Gravity Probe B gain new importance: it will shine experimental light on what has heretofore been a theoretical mystery, namely the origin of inertia. For some, this is perhaps the most beautiful and profound manifestation of spin in Einstein's spacetime: it binds us here to the universe out there, in such a way that you, standing at night under the stars on a planet known as earth, cannot turn so much as around without feeling a tug from the rest of the universe.
posted by odinsdream at 9:36 AM on May 11, 2011

I'm not an expert, but I think asking "what causes inertia" is as unanswerable as asking "what causes gravity?" or asking "why does matter exist?" These things are so fundamental that there's no explanation (at least, no satisfying explanation from modern science).

Inertia is just a fundamental property of matter in the universe. There's no cause, it just is. It's turtles all the way down.
posted by Tehhund at 9:39 AM on May 11, 2011

Well Einstein's theory was that gravity is caused by spacetime curvature and that it's not so much a force as objects trying to continue moving in a straight line that is no longer straight but has been curved by a massive object.
posted by enamon at 9:42 AM on May 11, 2011

Newton's First Law.
posted by plep at 9:48 AM on May 11, 2011

odinsdream: I don't buy it. That "tug from the rest of the universe" seems to imply an absolute reference frame which isn't the case.
posted by enamon at 9:48 AM on May 11, 2011


Newton's First Law describes it. It doesn't explain the cause.
posted by enamon at 9:48 AM on May 11, 2011

Inertia is a fundamental property of matter. It isn't caused by anything.
posted by Chocolate Pickle at 10:00 AM on May 11, 2011 [1 favorite]

I'm no physics guru but I'm not sure one would say there is so much of a cause of inertia, but rather it is the result of how the laws of physics operate.
posted by starman at 10:00 AM on May 11, 2011

Inertia isn't caused by a specific external force acting on objects. Inertia is part of the basic description of space-time, mass, and gravity.
posted by alms at 10:02 AM on May 11, 2011

This video is about trying to explain how magnetism works. It's analogous to the problem of explaining how inertia works:

posted by zeek321 at 10:07 AM on May 11, 2011 [1 favorite]

The origin of inertia is not a settled matter and does receive some attention from the physics community. My understanding here is a bit antiquated, but here is my best shot:

The Standard Model (the current accepted set of equations and explanations we use to "fit" to our measurements in particle physics that we know is not yet perfect but is the best we have got) does not explain mass. Particles have energy but the Standard Model does not explain why we have such problems shoving them about and why they don't all zip through the universe like photons, which have no rest mass, do.

If Peter Higgs is right (and we find Higgs bosons), inertia is caused by particles (fermions, such as electrons or the gluon and quark constituents of, say, protons) interacting with the Higgs field under the conditions of acceleration. (Think of this as vaguely similar to charged particles emitting photons when accelerated.) We call that measurement of inertia mass ("rest mass" or "inertial mass").

There is another mass, called gravitational mass, and so far, experimentally, mi is very, very close to mg in our measurements, and that expected identity is the foundation of Einstein's General Relativity.

Suppose we never find the Higgs boson in our nice accelerators. Back to the drawing board! I believe there are some competing theories out there as to "Why inertia?" One has to do with the non-zero lowest energy state of the vacuum. Rotational inertia was tackled by Ernst Mach and some bits of it made their way into GR.
posted by adipocere at 10:14 AM on May 11, 2011 [4 favorites]

Any description of the universe has to be based on some parameters upon which laws are based. For example, our universe appears to be well described by a 4 dimensional spacetime (3 spacelike dimensions and 1 timelike). Some people try to come up with theories that rely on fewer parameters so that the spacetime dimensionality could be derived. However, any description of OUR universe (not a fictitious alternative one) has to be based upon some fundamental observations. There have been attempts at explaining how inertia was the result of the interaction of the universe as a whole upon individual objects (search for Mach Principle). However, no theory has come up with a satisfactory demonstration of how inertia was just a derived quantity.

You can think of Newton's three laws of motion as akin to the basic postulates of geometry. Change one postulate and you can go from euclidean to non-euclidean geometry.

The standard model of particle physics might, subject to the potential discovery of the Higgs boson, provide us with the beginning of an explanation. I say "might" as it would (in theory) give us a way to calculate the numerical value of the parameter m (mass) of individual particles, but it would not provide us with an explanation as to why this parameter gives rise to Newton's laws of motion.
posted by aroberge at 10:22 AM on May 11, 2011

You could say that it's necessary if momentum is to be conserved (which we observe to be the case), but that's no more an explanation as anything else that's been said. I don't think you can give a 'why' to this question any more than you can explain why two charged particles experience a Coulomb force.
posted by Rhomboid at 10:26 AM on May 11, 2011

zeek321 has the best answer here, I came in to post the same thing myself. You either need to accept that it's there and Newton's laws are Newton's laws or you need to spend a whole lot of time narrowly defining (and understand that definition) of a large number of terms and concepts.

Also, digging through my DevonThink this stands out: On the origin of inertia by D. Sciama.
posted by Brian Puccio at 10:47 AM on May 11, 2011

(Absurd layperson's musing.)

Isn't inertia kind of the absence of cause? Inertia describes what a thing is doing (i.e. nothing) when it's not currently being affected by a force (cause.)
posted by callmejay at 11:33 AM on May 11, 2011 [1 favorite]

odinsdream: I don't buy it. That "tug from the rest of the universe" seems to imply an absolute reference frame which isn't the case.

I have no way to argue this - I just linked to the article.
posted by odinsdream at 11:52 AM on May 11, 2011

Regarding the statement that the "tug from the rest of the universe" seems to imply an absolute reference frame which isn't the case.

The absence of an absolute reference frame is something that we assume. (Yes, I know about the Michelson-Morley experiment, but I am not thinking of the aether...) However, an important observation should be noted: we are moving with respect to the cosmic microwave background. Once you correct for this motion, this background appears to be rather isotropic. If one were to assert that an absolute reference frame exists in which the cosmic microwave background is isotropic, I have no idea how one would go about to disprove such an assertion. What we (believe we) know, is that the laws of physics appear to be the same in all inertial frame - including the one in which the cosmic microwave background is isotropic. Since all inertial frames are deemed to be equivalent, we usually conclude that no absolute reference frame exists ... but I'm not convinced that one could not conclude that inertia arises from the tug of the rest of the universe as defined in the frame of reference in which the cosmic microwave background is isotropic.
posted by aroberge at 12:22 PM on May 11, 2011 [4 favorites]

This question seems oddly related to this recent xkcd.
posted by switchsonic at 1:47 PM on May 11, 2011

Conservation of momentum can also be understood as a consequence of translational symmetry in space (ie the laws of physics are the same here as over there). This is Noether's theorem, as mentioned in another xkcd. You can derive it starting from the principle of least action, which is the idea that nature is lazy, and hence minimises some integral called the action, the properties of which can be derived from simple assumptions. One of these assumptions is time symmetry, which leads to conservation of energy, and translational symmetry, which leads to conservation of momentum in classical mechanics. Now, I've yet to read a good justification of the principle of least action, so again we're just pushing the ball someplace else, but it's a an intensely beautiful way of thinking about physics.

disclaimer: most of this is probably wrong in some way or another
posted by spaghettification at 3:53 PM on May 11, 2011

Now, I've yet to read a good justification of the principle of least action...

A possible sketch of a hint of an explanation: in Feynman's formulation of QM using a sum over histories, with the phase of the exponential being the action divided by h-bar (all for dimensional reasons), one finds the most probable path(s) using essentially steepest descent methods which amounts to finding the local minima of the action. When there is only one minimum, this amounts to finding the minimum of the action i.e. the principle of least action.

When quantum effects are negligible, the most probable path is the classical path...
posted by aroberge at 4:11 PM on May 11, 2011

The short answer is that your question is one of the unsolved problem in physics. We know, or at least we theorize, that inertial mass is equivalent to gravitational mass, but, having said that, we don't really understand what mass is, nor how a given piece of matter acquires it.

If you would like to break your brain thinking about this issue; and before you just dismissively wave your hand and assert there are no privileged reference frames; start with the Wikipedia article on Mach's principle.
posted by Protocols of the Elders of Sockpuppetry at 9:39 PM on May 11, 2011

I came in here to post the article on Mach's principle. But it's already posted, thanks, Protocols. Yes, that's where you'd like to start.
posted by nat at 5:23 PM on May 12, 2011

Here's another great article on Mach's principle, this one written by Paul Davies. It starts out with a wonderful grabber:
Fill a bucket with water, grab it by the handle and whirl it in an arc above your head. If you do it right, you will stay dry. A mysterious force seems to glue the water into the upside down bucket. Scientists are still unsure about where this force comes from.
posted by Protocols of the Elders of Sockpuppetry at 5:12 PM on May 16, 2011

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