And a inertial mass is a measurement of the 'unwillingness' of matter to accelerate, (that's not exactly right, because a uniform motion contain 'inertia' too, but any deviation of that 'uniform motion', except a head on head collision, can be seen as a acceleration, and I'm not sure how to see that 'head on head collision' as actually? It's a deceleration and ..)? Gravity is a description of accelerations, gravitational like Earths or 'motion wise' as from 'A' to 'B', as measured in some positional system, and time.

If you turn it around you now will find that accelerations is gravity. Uniform motion on the other hand measured by the absence of gravity locally ('black room scenarios' all of it)

So if all uniform motions are equal inside that room, how can you state that you 'move'? There is no way I know of without using a outside frame of reference. And that is what motion is, a comparison between frames. 'A' and 'B' is two different frames of reference here, you 'moving' is the third, using them as a definition of a 'distance'. And the 'distance' you define uses a constant 'c', defining durations, aka a 'clock'.

So, do motion exist? Well, it must, as everything we do (and are) involve 'motion' in some way. But what is it, really?

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Remember what we call water becoming ice, good, because I don't

That's 'inertia' maybe, a transitionally state defining a demarcation between two other states. Uniform motion to -> acceleration -> and back.

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And as I already destroyed the threads original question, by questioning what the Higgs particle/field describes Super (Sorry about that). This is very good too, and adds a little more to JP:s description

By Marcus;

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"When something is moving it has a "longitudinal" inertia and a sideways or "transverse" inertia. But it no longer has a mass, because mass is a directionless quantity. So the custom is to assign to each object the "invariant" mass which is the inertia it WOULD have if it were sitting still. Lorentz discovered this ambiguity of inertia of a moving object back in 1904 even before Einstein.. .

The equations (GR) that model gravity do not have mass in them they have *energy density* and related pressures. Energy is what causes gravity in GR. Energy tells space how to curve and curved space tells energy how to move...

When something is moving it has a different "longitudinal" inertia from its sideways or "transverse" inertia. It takes more force (measured in the lab frame) to produce a given acceleration vector in the direction of motion than the same acceleration sideways. It is harder to speed a moving body up than it is to deflect it---even if the observer at rest can see that the size of the acceleration vectors are the same. People used sometimes to talk about the "transverse mass" (gamma m) as opposed to the "longitudinal mass" (gamma3 m). But nowadays most physicists when they say mass just mean "rest mass"----there is no other kind. But if you google with keywords "longitudinal mass" and "transverse mass" you can still find these gamma formulas and some discussion of these things.

The factor gamma = (1 - beta2 )-1/2 can be quite large for beta near one. So there can be a big difference between gamma and (gamma3 ! The difference between forwards inertia and sideways inertia can be very large. Like, if gamma is 2, then the thing is 4 times more resistant to speeding up than it is to deflection (where the same size acceleration is to be produced) Or if gamma is 10, the thing is 100 times more resistant to speeding up than to deflection. Nowadays the use of the term "relativistic mass" is more of an endearing eccentricity than anything else. Like wearing a sword, or having suits of armor in one's livingroom. For a moving body, the "relativistic mass" is essentially the same as transverse"------inertia measured as resistance to deflection-----and the formula for it is gamma m."

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