Split Thread The validity of classical physics (split from: DWFTTW)

[Christian Klippel]

<Non-answers omitted>

See how your posts end up when all the non-related garbage is removed?

So, still waiting for answers/evidence to your claims. Can't give any? Well, thought that already.

 

[Christian Klippel]

I find the time. You don't.

That's just a lie. How do i know? Because you said so many times that you will come up with something, but so far you didn't show anything sensible. Do you plan to make that a running gag or what?

 

[spacediver]

The only paradox comes from someone (incorrectly) expecting the total kinetic energy of some system to be identical across different frames. I probably would have made this same mistake myself not so long ago, but as a result of spending lots of time on this thread and it's parent have learned (or possibly relearned) that way of thinking is wrong.

Thanks for the reply Clive, but you haven't really addressed my question. Yes I know that KE is relative, and not intrinsic (as I made clear in my post).

All you've done is reiterate that conservation of energy holds from one frame of reference, but not across multiple frames.

You haven't said why this is so. Why should we be allowed to violate conservation of energy across frames?

I chose my example carefully: In my universe there is a finite and unchanging amount of energy. Some of that is stored in the spheres themselves (matter = energy), and some of it is in the form of kinetic energy.

If conservation of energy is true, and all possible collisions are perfectly elastic, then the total kinetic energy component of this universe should remain unchanged, right?

If that's the case, then why do we get different values for the total kinetic energy when we analyze from different frames?

How much energy is there actually in this universe?

 

[mender]

This humberism might show where humber's definition of waterspeed is coming from:

A small battery-powered car on the belt, it would first need to achieve beltspeed to get to zero wrt the ground, and then again, to get to windspeed wrt the ground.

If his definition of waterspeed is the same as his definition of windspeed (quite likely), it isn't the same as everyone else's.

Perhaps you can elaborate, humber, and clear this up.

 

[Michael C]

The Physics Classroom Tutorial, "Relative Velocity and Riverboat Problems"

 

[Tunny]

You are right to say that objects reach a velocity dependent upon their drag, and approach waterspeed, but they do not reach it. ?

I did not say that. I said they both reach the same speed, water speed (or so close as to be irrelevant). I DID say that they would accelerate to water speed at different rates, with the object having the higher drag reaching terminal speed faster.

Pooh sticks is perhaps appropriate. In this case, it is difficult to asses the real velocity w.r.t. the water, and if one is 5% slower that the other? Surface turbulence and so forth. make that difficult.

How so? Does one object continue to move farther away from the other, or do they maintain the same relative distance over time? Not hard to see. If one were moving 5% slower, it would fall farther and farther behind. However, this isn't the case.

If you tow a dingy behind a boat, does it have a bow wave?

I don't see the relevance, but I would expect so, since it's moving relative to the water, and encountering drag as a result.

When your floating objects have reached their terminal velocity below water speed, I assume they must exhibit a stern wave, right? After all, water is flowing past them at some non-trivial speed, if they're short of water speed.

Tunny

 

[Tunny]

The Physics Classroom Tutorial, "Relative Velocity and Riverboat Problems"

Wait, that can't be true! If that were the way things worked, why, pilots would be able to compute their courses based solely on wind speed and direction, without having to factor in stuff like the KE and drag of their plane...

Oh, wait.

Tunny

 

[Clive]

I chose my example carefully: In my universe there is a finite and unchanging amount of energy. Some of that is stored in the spheres themselves (matter = energy), and some of it is in the form of kinetic energy.

If conservation of energy is true, and all possible collisions are perfectly elastic, then the total kinetic energy component of this universe should remain unchanged, right?

If that's the case, then why do we get different values for the total kinetic energy when we analyze from different frames?

How much energy is there actually in this universe?

Hello again spacediver

You've essentially created the "paradox" directly by stating as a fact that your universe has a "finite and unchanging amount of energy". That just can't always be true if kinetic energy is also measured in the usual way (based on velocity relative to some more or less arbitrary frame). In effect your description tries to set kinetic energy to some absolute value (the total energy less other forms) at the same time that you allowing it to vary by using velocity measured relative to some arbitrary inertial frame.

Contradiction in, paradox out.

To try to get around this you might choose to define the centre of your universe as the centre of mass of the three objects, and also define your version of kinetic energy to always be relative to the "centre of universe" frame. Then it all works out fine!

The usual Law of Conservation of Energy actually only holds with some conditions. Perhaps because those additional details often aren't explicitly spelled out, people tend to slip into the trap of thinking it applies in a wider sense than is actually true?

 

[spacediver]

so the total amount of energy in our universe differs according to which frame of reference we choose to analyze from?

And since any frame of reference we choose to analyze from is utterly arbitrary, that means that it is meaningless to quantify how much energy there is in the universe, right?

 

[spork]

so the total amount of energy in our universe differs according to which frame of reference we choose to analyze from?

And since any frame of reference we choose to analyze from is utterly arbitrary, that means that it is meaningless to quantify how much energy there is in the universe, right?

I'd say that's right.

 

[spacediver]

I wish they taught this more explicitly in high school. It's a bit of a conceptual leap for me to get my head around this.

We're always taught that energy cannot be created or destroyed, and that makes us think of energy as a distinct thing that has a stable and finite quantity.

I need a better way of conceptualizing energy for all this to make sense...

Let's say in this universe, you want to transform as much energy as possible into heat.

Let's say you can only use the kinetic energy available.

How do you go about analyzing how much heat you can generate, if it is meaningless to quantify the kinetic energy of a system?

 

[Christian Klippel]

How do you go about analyzing how much heat you can generate, if it is meaningless to quantify the kinetic energy of a system?

Hello spacediver,

i would say one has to find the origin if the universe, or be ouside of the universe, at an absolutely non-moving point, and then calculate all the velocities (and thus KE's) of all objects in this universe to do that.

Which again brings the problem: what is a non-moving point? How can one make sure that a given point is the absolute reference? What is outside the known universe, if anything?

In the end, it's all relative.

Greetings,

Chris

Edit: Which is probably why people invented things like a god as an absolute thing.....

 

[roger]

There are three of you saying the same thing, so I would expect that one of you could find evidence of objects that do as you say. That is, reach waterspeed, when driven only by the water.

A surfer.

 

[spacediver]

Hello spacediver,

i would say one has to find the origin if the universe, or be ouside of the universe, at an absolutely non-moving point, and then calculate all the velocities (and thus KE's) of all objects in this universe to do that.

Which again brings the problem: what is a non-moving point? How can one make sure that a given point is the absolute reference? What is outside the known universe, if anything?

In the end, it's all relative.

Greetings,

Chris

Edit: Which is probably why people invented things like a god as an absolute thing.....

that still doesn't help me... I'm sure there's a way to solve this apparent paradox, but all of you seem to just be reiterating the paradox rather than solving it.

 

[Christian Klippel]

that still doesn't help me... I'm sure there's a way to solve this apparent paradox, but all of you seem to just be reiterating the paradox rather than solving it.

Hello spacediver,

that's true. But then, there is the lack of a known absolute reference frame. So, this "paradox" can't really be solved, It is what it is. The same as the "paradox" of an "endless" universe, or the "paradox" of what is outside of this universe. Nobody knows for real, so it is what it is.

One might choose an arbitrary point in space and calculate from there to give at least an hint to the wanted outcome. But still it would never be absolute. Since we are inside it, we can not look at it, and thus are unable to make predictions as to the "whole thing". As a matter of fact, we mere humans know only very little. But we are trying hard to know more, at least.

Said that, happy new year to you all! It's been one hour into 2009 in Germany already.

Greetings,

Chris

 

[sol invictus]

that still doesn't help me... I'm sure there's a way to solve this apparent paradox, but all of you seem to just be reiterating the paradox rather than solving it.

Let me see if I can help.

First of all, the statement that "energy is conserved" means that the total energy doesn't change with time. It does not mean that energy doesn't change under a boost (a change of reference frame). This is even more obvious if you think about momentum - total momentum is conserved too, but it clearly changes when you change reference frames. Remember - changes of reference frames are not physical processes. They are not something that happens as time goes on.

As for the energy of "the universe" - no, that's not a well-defined quantity, generally. Think about Newtonian physics (forget Einstein for now) - and forget gravity. Imagine a single massive object. Then the total energy is just the kinetic energy of that object (plus its rest mass energy, if you want to keep that), which is not invariant under changes of reference frame.

With general relativity (i.e. gravity) things are a little different, but that's too far off topic.

 

[spacediver]

thanks for the replies - i'll try to meditate on this some more, and will post back here after doing so.

But I strongly suspect that this same conceptual sticking point is what is at the heart of humber's issues.

 

[sol invictus]

Capable enough not to fall for a sophism like that.

OK, that's more than enough. You're a liar and a troll, and I've stopped being entertained by this stupid exchange. You refuse to admit your manifold and basic errors, contradict yourself in consecutive sentences, ignore mathematics when it agrees with common sense and demonstrates what a fool you are, respond to questions with gibberish and lies, and maintain your pigheaded ignorance in the face of thousands of posts and tens of posters trying to help you. You're humber than I thought possible, and far too humb to bother with further.

The only cure for trolls is to starve them to death.

 

[sol invictus]

Let's say in this universe, you want to transform as much energy as possible into heat.

Let's say you can only use the kinetic energy available.

How do you go about analyzing how much heat you can generate, if it is meaningless to quantify the kinetic energy of a system?

Sorry, didn't see this - it's a good question. The amount of energy you can convert into heat does not change when you change reference frames. The reason is that it depends on relative velocity, not on absolute velocity.

As an example, consider two lumps of clay on a collision course. When they collide they stick together, conserving momentum but turning some of their KE into heat. As you can easily check, the amount of energy turned into heat is independent of reference frame - in other words if you shift both velocities by the same amount, it doesn't change.

But the total energy is not independent of frame, and so physicists often speak about the "center of mass energy" - the energy in the frame in which the center of mass of the system is at rest - to avoid this ambiguity.

In the language of special relativity, physics depends only on Lorentz invariant quantities, like the contraction of two energy-momentum 4-vectors (which is E₁E₂ - c² p₁ dot p₂). It does not and cannot depend on things like the E₁ or p₁ alone, because they are not invariant.

 

[Clive]

More for spacediver...

There's a bit more information about using the centre of mass frame to calculate kinetic energy in the Wikipedia article on Kinetic Energy that I probably should have included in my last post. Unfortunately I got called away unexpectedly so just submitted what I had written at that time, and have only now had a chance to come back to this topic.

If the universe had finite mass (and you were happy to accept various other simplifications such as sticking with Newtonian type concepts as others have previously mentioned) then, in principle you could find the centre of mass for the entire universe. The total kinetic energy that you calculate for the universe using a frame of reference where the centre of mass is stationary is also the lowest it can be amongst all the different possible inertial frames.

This total amount of kinetic energy (calculated in the "centre of mass frame - or any other "centre of momentum frame" for that matter) is also the maximum amount kinetic energy that can ever be converted to other forms of energy in that universe. I'm really just parroting Wikipedia at this point but the point is that if you use some other inertial frame (one where the centre of mass is not stationary) then you will see the whole system as having a higher total amount of kinetic energy, but the excess amount (over that seen when using the centre of mass frame for example) is in some sense "useless". So, as far as I can see, this is possibly the closest you could get to calculating a sensible total (of usable) kinetic energy in the universe - but don't forget we are also assuming there is only some finite amount of mass otherwise the calculations could clearly take too long even if you could find the centre of mass to start with!