What fundamental aspect of nature causes objects to resist a change in velocity? -as observed in Newtons First Law of motion.
The force required to get something to move/change is proportional to its mass (second law), but what is it about mass that does this resisting?
Is it something to do with subatomic particles?
There are several answers given here: Calphysics Institute: Inertia Research
Nobody knows.
Isn’t that part of what they’re trying to find out at CERN? The higgs-bosun, etc.?
Boson, not bosun. A bosun is someone on a ship.
And the Higgs boson is associated with one of the mechanisms by which a particle can have mass, but it’s not even the most significant one, and anyway that doesn’t really address why mass is the ratio of force to acceleration. Ultimately, it comes down to the symmetries of the Universe.
One answer (really, more of a non-answer) that I have seen somewhere (sorry, no cite) is more like a cart-before-the-horse definition of “mass”:
So what is mass? Mass is simply THAT which resists a change in its velocity.
Helpful, no?
So mass is something that (a) resists change in velocity and (b) is attracted to other mass. Are these two qualities part of a more fundamental quality of mass?
Chronos. Isn’t is boatswain?
Same thing. Bos’n=Bosun=boatswain
Sailors whittle, whether it’s wood or words. Cf. Fo’c’s’le.
Now you’re asking the good question. In physicist language, “Why is inertial mass equal to gravitational mass?”. It’s a big question in physics.
And, so far, the answer is pretty much “Nobody has shown why it has to be this way”. Some new theory may well show why they’re really the same thing, but we don’t have one yet. And we might never; it could turn out more or less that the answer is “because that’s the way the universe is”.
It’s pretty intuitive to me: No one (and no thing) likes change. You wanna change something, you’ll have to push it, pull it, or whatever it needs, but it’s gonna need something to force that change.
(Note that the above covers both cases: Whether the object is at rest and you want it to move, or it is already in motion and you want to change that motion (to either another direction, another velocity, or to stop it. Any kind of change will be resisted without some sort of impetus.)
I don’t mean the above humorously, but rather very seriously:
Let’s works backwards. You want to presume that it is logical for a change in velocity to occur without any outside cause. Isn’t that identical to saying that the change would occur randomly, for no reason whatsoever? Well, if so, that what will determine what the new velocity will be, and when the change will occur? Will they be random too?
You have just claimed that it is logical for everything in the universe to change its velocity constantly and randomly. That’s ridiculous. Therefore, it is NOT logical for the change in velocity to occur without any outside cause, but that it IS logical for the change in velocity to occur only if there is an outside cause.
And how come an object in motion can move through space effortlessly? An object with mass will warp the fabric of space-time. Why doesn’t it take any energy to warp space-time as the object is moving with a fixed speed? It takes energy to accelerate the object, but not to keep it moving. Strange.
As with all “why” questions, it depends on what you take as fundamental. It follows straightforwardly from the Equivalence Principle, which is at the heart of General Relativity. Why is the Equivalence Principle true? Well, why wouldn’t it be?
It only seems strange to us, because we are so used to things that slow us down. Push a wagon or a tricycle, and it will stop after a few feet or inches. Float in a pool, and kick against the side, and you’ll stop in a couple of yards. But in space, there is absolutely nothing in the way. Just the slightest tap, and you’ll just keep on going forever, or until something stops you, whichever comes first.
Like Chronos just said - “it depends on what you take as fundamental.”
Keep in mind that when the energy stops, so does the acceleration. You don’t keep on accelerating. You do keep moving, but in the exact same direction and speed as when the energy stopped. Maybe that will help you to understand it.
I do see the point **filmore **made though. Here’s my take FWIW …
Certainly the vacuum of free space lacks macroscopic friction, unlike our everyday experience. No problems there. So the idea that merely changing position, but not *velocity *, consumes no energy is easy to accept when you just look at the macroscopic stuff.
But it’s widely asserted that objects which have mass & hence gravity distort spacetime. And if they are moving through space, they are exerting a changing effect on spacetime over time. The gravitational dimple I create *here * & *now *will be gone when in 5 minutes I’m there then. And I’ll have created a dimple there then instead.
All this flexing of spacetime ought to consume some sort of energy somehow. If not, then we can summon the very reasonable argument of post 11 that if it can flex without consuming energy, then why isn’t it flexing randomly all the time? And if it offers no resistance to flexing, then the magnitude of those random flexs ought to be unboundedly large.
Yet we don’t observe ginormous random fluctuations of spacetime = gravitational anomalies.
I suspect it’s rather like the assertion that a magnetic field can do no work. The gravitational “effort” spent to make my spacetime dimple *here *& *now *is 100% recovered by me when that spacetime dimple moves to a new spot when I do. So as an object travels in unaccelerated motion it’s continuously recovering the “effort” spent creating the dimple which is collapsing behind it and spending the same “effort” creating the new dimple ahead of it.
Unlike a boat in water, we leave no wake in the gravitational field /spacetime behind us when we move. Nor do we deposit waste heat in there. If somehow we did either of those things, then that *would *consume “effort” in some form. But the Universe (apparently) doesn’t work that way.
Spacetime flexure is sorta analagous to superconductivity. You get back out exactly 100% of what you put in. But you do have to put some in at the beginning. Electricity does not spontaneously appear in a conductor as it goes critical; neither does spacetime flexure just appear here & there willy nilly.
As I understand it, within GR curved spacetime is mass, and mass is curved spacetime. There is no important distinction, until you start to ask how that curved spacetime interacts with the electromagnetic force – at which point you must step outside GR, and indeed classical theory, entirely.
So the question of a moving mass dimpling and undimpling spacetime is rather more the question of why the traveling dimple doesn’t disperse. It can’t simply vanish because, of course, that would violate conservation of energy. But traveling waves in perfectly elastic media do tend to disperse, if they are not eigenmodes of the media, and a dimple in spacetime is a priori not a eigenmode of the entire Universe ha ha. But of course if the traveling dimple dispersed, that would be equivalent to the mass breaking up and dispersing, and we know very well that doesn’t happen.
About all I can suggest is that there is something about spacetime which is not like an elastic sheet, and that therefore we cannot apply our intuition about what happens to traveling waves in ordinary elastic media to what happens to traveling dimples in spacetime, a.k.a. moving masses.
Also, it may be that there is a very weak retarding force, because as I understand it, two orbiting bodies emit gravitational waves and lose orbital energy, albeit very very slowly, and will eventually coalesce. Since gravity has infinite range, in principle every body in the universe is orbting every other body, and will presumably lose very tiny amounts of energy to gravitational waves over time. Alas, while this tends to suggest an eventual coalescence of all the mass in the universe, this comes into direct conflict with the expansion of the universe, or indeed its accelerating expansion…and then there’s dark matter and energy…it’s a God-damned mess, I say. The successor to Einstein is long overdue.
But ultimately, of course, science gets to a dead end when asking “Why?” As **beowulf **said upthread, the ultimate answer is “nobody knows.” Most scientific answers aren’t explanations of why, but rather descriptions of what. Newton didn’t really say Why, he clarified What. The farthest one can go with “why” ends with “that’s the way the universe is.”
This great episode of SpaceTime on Youtube explains how the Higgs field gives elementary particles mass and how inertial mass and gravitational mass are related (the same).
Many people find this intuitive - but that may simply be because they exist in a universe where this is true.
In other universes, intuition may be different.