Is Absolute Space and Time Really Absolute?

[Nugatory]

Like the counter > floor in height no matter which of the views you presented you adopt.

That's not necessarily true - no law requires that I choose the direction of increasing height values in such a way that they increase in the direction of the Earth's sky. A physicist on Mars is unlikely to care about the height of either my kitchen floor or my kitchen counter, but if he did then (depending on where Earth and Mars happen to be at the moment), he would say that the counter is 54,600,000,000 meters high and the floor is 54,600,000,001 meters high.

The frame-independent truth that you can take to the bank is that the the gravitational potential at the counter is less than that at the floor by a particular amount. From this, you can calculate the result of any experiment involving the floor and the counter.

 

[name123]

That's not necessarily true - no law requires that I choose the direction of increasing height values in such a way that they increase in the direction of the Earth's sky. A physicist on Mars is unlikely to care about the height of either my kitchen floor or my kitchen counter, but if he did then (depending on where Earth and Mars happen to be at the moment), he would say that the counter is 54,600,000,000 meters high and the floor is 54,600,000,001 meters high.

The frame-independent truth that you can take to the bank is that the the gravitational potential at the counter is less than that at the floor by a particular amount. From this, you can calculate the result of any experiment involving the floor and the counter.

I specifically stated that what I was saying regarding the height related to the two options you had previously given. And it was true for them. I thought on Earth height roughly did mean in the direction of Earth's sky. I couldn't think of a counter example where it wouldn't, in English anyway. Though I can see that there would be problems in comparing heights on Mars and Earth, but I guess if you were to have some definition, then maybe it would fix the comparison answer.

 

[PeterDonis]

You are saying that there is no such thing as energy, or that there is no such thing as absolute energy?

There is no such thing as absolute energy.

 

[DEvens]

Wow. As with so many threads connected to relativity, this is experiencing rather severe topic drift. The original question has been left behind and seemingly is forgotten, while people are talking past each other about quite tangential ideas.

Let's get back to the original question.

Even at low speeds such as 1 m/s to 3 m/s, would you expect the energy to get to 3 m/s to be three times the energy to get to 1 m/s? If you had a rocket that could give you three 1 m/s kicks, do you expect your kinetic energy to be three times as much after three 1m/s kicks as it was after one 1 m/s kick?

Of course, the answer is that at 3 m/s you have 9 times as much kinetic energy as at 1 m/s. Not even close to 3 times as much. So what happens to conservation of energy? Surely you will burn just about three times as much fuel to get to 3 m/s as you would to get to 1 m/s. (With some minor differences because the expulsion of stuff out the back of the rocket changes its mass.) How can the rocket be producing more kinetic energy from later changes in velocity?

0 to 1 increases by v^2 = 1
1 to 2 increases by 2^2 - 1^2 = 3
2 to 3 increases by 3^2 - 2^2 = 5

Well, of course, the answer is that the rocket is not the only thing with kinetic energy. If the rocket masses 100 kg, and flings 1 kg out the back at 100 m/s to gain 1 m/s in velocity, the reaction mass winds up with kinetic energy as well. If the rocket starts at 1 m/s then the reaction mass also starts at 1 m/s, and so winds up going 99 m/s backwards instead of 100 m/s. If you work out the total change in kinetic energy you find that it is invariant under changes of velocity. (Don't forget to include the starting kinetic energy of the ship and the reaction mass.)

So the fact that the velocity composition means three increases of 0.1 c does not quite get you to 0.3 c really isn't helping understand the kinetic energy of the situation. It's a complication on the end, but there are more fundamental ideas at work here.

 

[name123]

Well thanks for the help everyone gave there, that's cleared a few things up for me. I hadn't realized that energy was being considered truly relative, but I guess it follows from the metaphysical assumption that presentism isn't true.

 

[PeterDonis]

I hadn't realized that energy was being considered truly relative, but I guess it follows from the metaphysical assumption that presentism isn't true.

I'm not sure what you mean by "the metaphysical assumption that presentism isn't true", but if you mean that energy being relative is something that goes with the theory of relativity, as multiple people have pointed out in this thread, that's not the case: energy is relative (i.e., coordinate-dependent) in Newtonian mechanics.

 

[name123]

I'm not sure what you mean by "the metaphysical assumption that presentism isn't true", but if you mean that energy being relative is something that goes with the theory of relativity, as multiple people have pointed out in this thread, that's not the case: energy is relative (i.e., coordinate-dependent) in Newtonian mechanics.

You can use frames of reference for motion in Newtonian mechanics, and only consider relative motion. Though I assume that metaphysically there was supposed to be absolute motion. I assume it was the similar for energy (you could use equations for relative motion or energy but metaphysically absolute motion and energy was assumed).

 

[PeterDonis]

You can use frames of reference for motion in Newtonian mechanics, and only consider relative motion.

It's stronger than that: all motion is relative in Newtonian mechanics, just as in relativity. The relativity of motion was established by Galileo and has been a part of physics ever since; it was not introduced by relativity.

Though I assume that metaphysically there was supposed to be absolute motion.

No. Newtonian mechanics assumed absolute space and time, but not absolute motion. All Galilean coordinate systems (the Newtonian analogue of Einstein's inertial frames with their rods and clocks--the word "Galilean" is used because the transformations between them are called Galilean transformations) are equivalent in Newtonian mechanics, just as all Lorentzian inertial frames ("Lorentzian" because they are related by Lorentz transformations) are equivalent in SR.

 

[A.T.]

You can use frames of reference for motion in Newtonian mechanics, and only consider relative motion.

What do you mean by "you can"? Since there is nothing in Newtonian mechanics that allows you to determine absolute motion, you have to use relative motion.

 

[name123]

[QUOTE="PeterDonis, post: 5007707, member: 197831"
No. Newtonian mechanics assumed absolute space and time, but not absolute motion.[/QUOTE]

Doesn't absolute space and time imply absolute motion? If at any point in absolute time an entity has an absolute location, wouldn't the changes in its absolute spatial location in absolute time count as absolute motion?

 

[jbriggs444]

Doesn't absolute space and time imply absolute motion? If at any point in absolute time an entity has an absolute location, wouldn't the changes in its absolute spatial location in absolute time count as absolute motion?

Under Newtonian mechanics, the distance between two co-moving points is fixed regardless of whether you adopt a frame of reference where those points are moving or one in which they are at rest. In that sense, Newtonian space is "absolute".

 

[A.T.]

Doesn't absolute space and time imply absolute motion?

In Newtonian mechanics distances, clock rates and simultaneity are frame independent, while in Relativity they are frame dependent. Velocities are frame dependent in both models. The first postulate of SR is just restating Galilean relativity, not introducing something new.

 

[name123]

Under Newtonian mechanics, the distance between two co-moving points is fixed regardless of whether you adopt a frame of reference where those points are moving or one in which they are at rest. In that sense, Newtonian space is "absolute".

In the sense that you can imagine whatever coordinate system you like and no matter what frame of reference the human you experience being is in, the experience could be imagined to be an experience of a physical reality in which that distance was manifest as a property (regardless of what method of measurement you use to measure it), and that it's proportion to other physical distances wouldn't be dependent upon the frame of reference of your human.

 

[Nugatory]

Doesn't absolute space and time imply absolute motion? If at any point in absolute time an entity has an absolute location, wouldn't the changes in its absolute spatial location in absolute time count as absolute motion?

No. Google for "Galilean relativity".

 

[PeterDonis]

Doesn't absolute space and time imply absolute motion?

No.

If at any point in absolute time an entity has an absolute location

That's not what "absolute space" means, at least not as that term is used in Newtonian mechanics. "Absolute space" means that spatial distances at a given instant of time are the same for all observers, regardless of their state of motion. (Note that the qualifier "at a given instant of time" is only possible because that phrase has the same meaning for all observers, i.e., because Newtonian mechanics has absolute time.) It does not mean that spatial positions are the same; those are relative. Mathematically, Galilean transformations preserve spatial distances but not spatial positions (whereas Lorentz transformations do not preserve either spatial distances or spatial locations).