**Ask a monkey a physics question thread**

[OngBonga]

Except that being at a different location in time does not imply being at a different location in space, necessarily. Here on the surface of a planet, it certainly does, but not necessarily in the abstract discussion of the nature of motion.

Well, surely it does imply different locations if time and space are one and the same, or to put that in better language, coordinates of the same field.

We can still have our 3 spacial components as 0-vectors, though, showing that motion in time, but not space is possible.

But that 4th dimension of time isn't a 0-vector. So there is always motion through spacetime.

...inertial mass and gravitational mass... they don't have to be the same.

Sure they do. Gravity is acceleration, it's basically resistance to a change in state of motion caused by this acceleration. How is that not exactly the same as inertial mass? Gravity just does the acceleration for us.

Surely you at least expect it to be the same? I mean isn't this what GR is? An attempt to unify graviational and inertial mass? That's my interpretation of GR.

Whether or not the future exists before we arrive in it seems unmeasurable.

Surely we only need to prove space and time are essentially one and the same? Because once we do that, then how can we arrive in a pre existing region of spacetime? Something else existed in that part of space at that moment in time.

If space and time are seperate, then ok we can be in a region of pre existing space. But if we can't say space without actually meaning spacetime, then how can the region of spacetime that I am approaching already exist? If it does, why can't I observe it? One nanosecond in the future isn't that far away, but every direction I look is in the past. Where is one nanosecond in the future? Why can't I see it?

And of course, we can measure a great deal that we once would have called "immeasureable". What does atom mean? Indivisible. When we named the atom, it was unthinkable that there was anything smaller, let alone that we could measure such things.

I'm not sure how one could measure something "in the future" in that regard.

What does it tell us if we prove that we cannot measure the future?

The many worlds interpretation of QM would imply that all possible futures exist, but that's not science

Yeah I don't get on with this. I'm happy enough to say all possible futures have an equal probability, and this one is happening.

You're kind of on to something big, though. In a very real and practical sense, the reason for gravitational attraction toward massive bodies is due to time dilation

This only strengthens the idea that space and time are the same thing, in the same sense that electricity and magnetism are the same. If space is curved, then so too is time, to a proportional degree. If this is an unavoidable truth, then space and time are one, and motion through spacetime is, for everything with mass, into the unseen.

[MadMojoMonkey]

Well, surely it does imply different locations if time and space are one and the same, or to put that in better language, coordinates of the same field.

They are coordinates in the same field, but moving in any one direction doesn't necessitate movement in any other direction.
That's like saying, "You moved forward, so therefore you've moved up and to the side." but in a way that says there is no rotation of the coordinate system in which a straight line lies parallel to one of the coordinate axes.
That's bonkers, even to a quantum physicist.

Give me at least one line (the direction of motion) and I can call it the z-axis in cartesian, cylindrical, or polar coordinates. Then my first guess is to see if I can solve it in 1 dimension (or 2 if we're counting time) by that choice. By choosing an orientation of the coordinate system, I can eliminate motion in 2 axes.
Therefore, motion in one axis does not necessitate motion in another axis.

Space and time are coordinates of the same field, but they are not one and the same. Stuff can move freely in any direction in space, but not time.

But that 4th dimension of time isn't a 0-vector. So there is always motion through spacetime.

Yes, but motion through spacetime does not necessarily imply motion through space.

Sure they do. Gravity is acceleration, it's basically resistance to a change in state of motion caused by this acceleration. How is that not exactly the same as inertial mass? Gravity just does the acceleration for us.

Surely you at least expect it to be the same?

There is nothing in the theory which says the [something] which causes an effect will be, itself, affected by the effect.

Particles are not affected by their own electric charge. If you tell me the direction of the electric field in some region, I can tell you how a charged particle will accelerate in that region due to that field. I do not need to work out how that particle's charge alters or adds to the incident field, because all of those effects cancel out for that particle.

I mean isn't this what GR is? An attempt to unify graviational and inertial mass? That's my interpretation of GR.

No. GR is an attempt to answer the question, "What do all observers agree on?"
Specifically, and differentiating it from Special Relativity, GR considers accelerations.

Importantly, SR describes gravitation, and does not consider acceleration. Object follow orbits because those are logically straight lines in curved spacetime.

Surely we only need to prove space and time are essentially one and the same? Because once we do that, then how can we arrive in a pre existing region of spacetime? Something else existed in that part of space at that moment in time.

We cannot move freely in time the way we can in space, though. Proving that they are the same means we need to be able to do that.

If space and time are seperate, then ok we can be in a region of pre existing space. But if we can't say space without actually meaning spacetime, then how can the region of spacetime that I am approaching already exist? If it does, why can't I observe it? One nanosecond in the future isn't that far away, but every direction I look is in the past. Where is one nanosecond in the future? Why can't I see it?

These are juicy questions.
Why can I send information to the future, but not the past?

Arguments for the arrow of time never really satisfy me. I don't have any good answers to these questions.

And of course, we can measure a great deal that we once would have called "immeasureable". What does atom mean? Indivisible. When we named the atom, it was unthinkable that there was anything smaller, let alone that we could measure such things.

Of course this is an excellent point, and maybe someday we will figure out how to move freely through time.

I kinda like Stephen Hawking's cheeky one-liner on the subject.
"If time travel is possible, where are all the tourists from the future?"

What does it tell us if we prove that we cannot measure the future?

That all the science fiction about time travel was fiction after all?
That the grandfather paradox is truly untestable?

Yeah I don't get on with this. I'm happy enough to say all possible futures have an equal probability, and this one is happening.

:/
Surely futures in which at least one major world government will be probing Uranus are more probable.

This only strengthens the idea that space and time are the same thing, in the same sense that electricity and magnetism are the same. If space is curved, then so too is time, to a proportional degree. If this is an unavoidable truth, then space and time are one, and motion through spacetime is, for everything with mass, into the unseen.

ummm... what?

[OngBonga]

ummm... what?

idk

I mean if we change an electric current, the magnetic field responds instantly. We can say it's the same thing... we do, we call it electromagnetism. Spacetime is no different. If you suddenly plonk an object of mass in empty space, well anything that suddenly experiences this gravity will also immediately experience time dilation. It's unavoidable, like the magnetic field changing when we change the current. Space and time, they're the same just like just left and right are, sort of, the same thing... left and right are different directions that rely entirely on each other... you can't define one without immediately defining the other.

So if space and time are essentially the same, then it can be seen as a 4d graph with x, y, z and t axes. And when we consider our motion relative to this 4d graph, well our motion is in the direction where nothing exists, because all points we can travel to are in the future.