Akerai

Lasers intense enough to actually force electron-positron pairs out of the vacuum.

There is a whole new world of interesting physics to be measured with the advent of ultraintense lasers.

It would be way less exciting than you think though :D

In an indirect way anyway.

I think it wouldn't shock me if someone managed to measure superluminal virtual particles at some point.

without transmitting information

but here you have a whole list of things that propagate faster than light

Not sure, that is way beyond the area of my expertise.

that nothing is faster than light

and only if one averages them over sufficiently (macroscopically) long times then one gets the GR rule

while quantum field theory implies that one can get particles that move faster than light (for very short periods of time).

The clashes arise from GR setting a fixed upper limit.

It doesn't matter what the upper limit is, as long as there is an upper limit the physics is going to be the same.

as far as I know anyway.

Information cannot be sent faster than light is the rule, there is no particular reason why.

because things can move faster than light

I just wrote out a more sensible definition when it comes to some speed of light constraints.

Yeah, physics is imperfect.

'If you average over a sufficiently long time then it is not possible to send information at a speed faster than light.'

also there are many technical nitpicks I could make when it comes to that statement as well, it would probably be more true if one formulated it as

It's simply a descriptive model, and it clashes with many things in other branches of physics.

Nonetheless, it does not explain why.

That's fair enough, it seems to be a reasonable assumption that the space as-is has a finite limit to the speed of a particle.

In what sense

but that does not then have a feedback on the particle itself.

i.e. a massive particle makes some gravitational field (warps the spacetime around itself)

the nice thing about gravity is that it does not interact with itself.

There are things worse than gravity.

The real question is, do you care about feeling effectively no force or exactly no force in the particular framework you work in.

so if you are sufficiently far away in the empty space out there you will feel effectively no force.

but then again, the gravitational force falls off as 1/r^2

You are right then, it is unlikely

It's all quite complicated, but in short it's something you shouldn't care about. It's enough that things don't feel 'forces' to the extent we care about. For all intents and purposes it is a valid statement to say that the paperweight on my table feels no net forces.

It gets rather involved, but essentially it is nearly impossible to get a real particle (such as the electron) that would not interact with any other force field.

the entities that are physical and change in time are fields, i.e. some 'probability densities' that a particle is somewhere (don't take me too literally here) that are a function of (x,y,z,t)

Essentially the formalism does not know "particles" as dynamical entities, particles are simply something that is measured.

It is however the correct approximation to use if you are speaking of an extremely small particle.

Now in the Quantum case things get a little fuzzy, as the 'motion' cannot be particularly well defined and nor can the 'force' be.

In that formalism you can have exactly force-free motion for the body. If you speak of our universe, then there is too many bodies and it is highly unlikely they conspire to be in exactly the kind of configuration that would give exactly zero force somewhere.

then the body in the middle will feel exactly no force as the forces cancel by symmetry.

o ---- o ---- o

for example if you have three massive bodies like this:

Now in the Newtonian sense to have a particle with exactly zero force on it you essentially need all of the force components acting on the particle to cancel.

Force is simply something that causes acceleration - and there are many ad-hoc equations for different forces. i.e. the gravitational force, the coulomb force. (Weak and Strong force don't appear in the Newtonian description as they can be approximated to be zero).

In Newtonian physics a particle is something that has a position uniquely defined by three coordinates (x,y,z) at a time 0. Then it's easy to define the 'motion' as the pace at which these coordinates change with time.

It turns out that we have just such descriptive models - and we use different models for different approximations. For example for human length and time scales the best approximation might be the classical Newtonian physics.

Now physics should never be understood as the 'truth' or as the way 'the world works'. It is simply a descriptive model that given some input data about a physical system should lead to some output data about the same physical system at a later time / different position.

Essentially there are two tricky things here to define one of them is "force" and the other is "motion".

Alright, I think the question, and my uncertainty about the answer comes down to definitions really.

@acros