Ask a physicist anything. (8)

[Seipai]

Yes, that's the false geometric interpretation of relativity, that gravity is not a force at all. Not worth bothering to talk about such a ridiculous idea that violates cause and affect.

What!!??


Don't tell me that you do not accept relativity. That theory has been tested many many times over and no one has found any real flaws in it to date.

If you want a Nobel Prize in physics all you have to do is to prove that relativity is wrong.

 

[Wiccan_Child]

Yes, that's the false geometric interpretation of relativity, that gravity is not a force at all. Not worth bothering to talk about such a ridiculous idea that violates cause and affect.

"I don't like causality being violated, therefore I'll throw away an exceedingly well-evidenced theory" - Justa, in true myopic style.

According to relativity, warped spacetime is the cause of gravity, which means it is technically not a force (though it can be modelled as such in classical mechanics). It is a fictitious force, not unlike the centrifugal force.

To throw away all of general relativity, despite its century of confirmation after confirmation, simply because you don't like it, is to deliberately close your mind to honest investigation and scientific inquiry.

 

[Justatruthseeker]

"I don't like causality being violated, therefore I'll throw away an exceedingly well-evidenced theory" - Justa, in true myopic style.

According to relativity, warped spacetime is the cause of gravity, which means it is technically not a force (though it can be modelled as such in classical mechanics). It is a fictitious force, not unlike the centrifugal force.

To throw away all of general relativity, despite its century of confirmation after confirmation, simply because you don't like it, is to deliberately close your mind to honest investigation and scientific inquiry.

The purely geodesic interpretation is an irrelevant theory.

Place a ball stationary on the edge of warped space. What force could possibly make it move, if gravity is NOT a force? Why would the ball roll "downhill" if you did not first conceive of a force acting beneath the sheet pulling it downwards? Two balls placed near each other in space ALWAYS come together. So if gravity is not a force, what would cause that stationary ball to begin to move in the first place? The geometric interpretation fails utterly to describe the real world, and can only be applied to previously moving bodies.

But you, you will ignore the violation of cause and effect in order to keep your Fairie Dust geometric interpretation of bent space alive. Without gravity as a force, the ball would never move from its position you placed it in. This is observed nowhere in the entire universe.

 

[Seipai]

The purely geodesic interpretation is an irrelevant theory.

Place a ball stationary on the edge of warped space. What force could possibly make it move, if gravity is NOT a force? Why would the ball roll "downhill" if you did not first conceive of a force acting beneath the sheet pulling it downwards? Two balls placed near each other in space ALWAYS come together. So if gravity is not a force, what would cause that stationary ball to begin to move in the first place? The geometric interpretation fails utterly to describe the real world, and can only be applied to previously moving bodies.

But you, you will ignore the violation of cause and effect in order to keep your Fairie Dust geometric interpretation of bent space alive. Without gravity as a force, the ball would never move from its position you placed it in. This is observed nowhere in the entire universe.

The idea of the geodesic interpretation is that the objects are following a straight line in 4D spacetime. They are not following a curve. It may seem to be a curve in 3D space, but space has at least 4 dimensions. Time is the fourth dimension and it has a literal meaning in relativity. You are making the mistake of graphing in only three D space and forgetting that for the universe time is another dimension. Just by "moving" forward in time an object is moving in 4D spacetime. Talk to essentialsaltes for more details.

 

[Justatruthseeker]

The idea of the geodesic interpretation is that the objects are following a straight line in 4D spacetime. They are not following a curve. It may seem to be a curve in 3D space, but space has at least 4 dimensions. Time is the fourth dimension and it has a literal meaning in relativity. You are making the mistake of graphing in only three D space and forgetting that for the universe time is another dimension. Just by "moving" forward in time an object is moving in 4D spacetime. Talk to essentialsaltes for more details.

That's not what I asked. I asked how a stationary object begins to move in space, if gravity is not a force? The geometric interpretation violates cause and effect.

 

[Seipai]

That's not what I asked. I asked how a stationary object begins to move in space, if gravity is not a force? The geometric interpretation violates cause and effect.

But you keep forgetting, space is 4 dimensional in relativity. If an object is not moving at the speed of light then it is moving in the time dimension in 4D space.

Any mass cannot be stationary in 4D space. It has to be moving at least in the time dimension. You cannot treat time as if it were not equivalent dimensionally to the other dimensions.

ETA: You should really ask essentialsaltes these questions but you might try reading up a bit.

Here is a starting point. You can follow the links in the references for a more thorough explanation:

http://en.wikipedia.org/wiki/Minkowski_space

 

[Wiccan_Child]

That's not what I asked. I asked how a stationary object begins to move in space, if gravity is not a force? The geometric interpretation violates cause and effect.

No, you just haven't understood what it's saying.

Don't forget that, in relativity, a) 'gravity' is a warping in space AND time, and b) all objects move at a speed c through spacetime, with a component in each dimension. An object's 4D worldline is a line through spacetime, and warping spacetime curves the worldline.

So a 'stationary' object is always moving at c through spacetime - c in the time direction and 0 in the spatial directions. Warped spacelines curve this worldline into the spatial direction, which to our eyes manifests as spatial acceleration.

The question is a good one... but don't you think it's a little presumptuous to ask a question and instantly declare there is no possible answer or resolution?

 

[Chesterton]

So a 'stationary' object is always moving at c through spacetime - c in the time direction and 0 in the spatial directions.

I'm puzzled by this sentence. It seems Justa asked about "moving in space" and you answered about "moving in spacetime".

"c" is a certain speed over distance (spatial). How/why is an object always moving at c through time?

 

[Wiccan_Child]

I'm puzzled by this sentence. It seems Justa asked about "moving in space" and you answered about "moving in spacetime".

"c" is a certain speed over distance (spatial). How/why is an object always moving at c through time?

Because c isn't a certain speed through distance, it's a speed through spacetime - don't forget that, per Einstein, space and time are actually one four-dimensional object. To use, an object is stationary, but it's actually moving - it's just moving through time at a rate of one second per second.

Imagine driving in your car along a flat plains. You're travelling at a speed V due East. If you aim your car 22.5° more north so that you drive east-northeast, you're still travelling at V, but your eastward speed is only 0.92xV, and your northward speed is now 0.38xV.

This is what happens to objects in spacetime. Objects never change their speed, they just point more in the space direction. You don't accelerate, so much as your movement is more space-y than time-y (which is why time dilates when you move fast - too much is being taken, as it were).

 

[Chesterton]

Because c isn't a certain speed through distance, it's a speed through spacetime - don't forget that, per Einstein, space and time are actually one four-dimensional object. To use, an object is stationary, but it's actually moving - it's just moving through time at a rate of one second per second.

Of course it's a speed through spacetime, but it's certainly measured as a speed over miles or meters or what have you. Some measure of space.

 

[pgp_protector]

Because c isn't a certain speed through distance, it's a speed through spacetime - don't forget that, per Einstein, space and time are actually one four-dimensional object. To use, an object is stationary, but it's actually moving - it's just moving through time at a rate of one second per second.

Imagine driving in your car along a flat plains. You're travelling at a speed V due East. If you aim your car 22.5° more north so that you drive east-northeast, you're still travelling at V, but your eastward speed is only 0.92xV, and your northward speed is now 0.38xV.

This is what happens to objects in spacetime. Objects never change their speed, they just point more in the space direction. You don't accelerate, so much as your movement is more space-y than time-y (which is why time dilates when you move fast - too much is being taken, as it were).

Too bad that won't work to get out of a speeding ticket, officer I was doing the same speed as everybody else C. I was just going slower through time than other people, are you really going to give me a ticket for going too slow through time?

 

[essentialsaltes]

Of course it's a speed through spacetime, but it's certainly measured as a speed over miles or meters or what have you. Some measure of space.

The speed limit on the freeway is measured in miles per hour (or kph). The concept of speed depends on both space and time.

 

[Chesterton]

The speed limit on the freeway is measured in miles per hour (or kph). The concept of speed depends on both space and time.

I think that's what I tried to say.

But then Wiccan also refers to something traveling "one second per second", which seems like tautological gibberish.

 

[essentialsaltes]

Well, when we say that 'time is the fourth dimension,' a consequence of that is that nothing is ever 'at rest'. To be at rest, all your coordinates have to remain fixed. But since one of the coordinates is time, and time keeps clicking along, nothing is ever at rest.

Everything is 'moving' in time. The equations of relativity mix space and time, so if we follow an object that is initially at rest in the three spatial coordinates, but still moving in time, then at a later time it may have some motion in space, because it has been 'redirected' by the influence of gravity and how it curves the entire 4 dimensional space-time.

 

[Chesterton]

Well, when we say that 'time is the fourth dimension,' a consequence of that is that nothing is ever 'at rest'. To be at rest, all your coordinates have to remain fixed. But since one of the coordinates is time, and time keeps clicking along, nothing is ever at rest.

Everything is 'moving' in time. The equations of relativity mix space and time, so if we follow an object that is initially at rest in the three spatial coordinates, but still moving in time, then at a later time it may have some motion in space, because it has been 'redirected' by the influence of gravity and how it curves the entire 4 dimensional space-time.

I don't know, I guess I don't even understand what "rest" means even in 3 spatial dimensions. I realize I and my keyboard are hurtling through space around a galactic center, which is also hurtling through space. Everything is moving. Even if you could exclude the time dimension, you're still right, nothing is ever at rest.

So when we're talking about a stationary object at rest, it seems that has to mean it is at rest in a spatial relation to another object, i.e., my keyboard is at rest on my desk; my lamp is at rest 12 inches from my computer. No?

 

[essentialsaltes]

So when we're talking about a stationary object at rest, it seems that has to mean it is at rest in a spatial relation to another object, i.e., my keyboard is at rest on my desk; my lamp is at rest 12 inches from my computer. No?

Yes, indeed. In fact, when you talk about gauging rest (or motion) in relation to some other object, that is part of the essence of relativity.

It turns out that there's no such thing [contradicting Newton] as an absolute frame of reference that is objectively 'at rest'. All motion is relative. Different observers will see different things depending on their relative motion. The trick of relativity is to uncover the set of rules that is consistent, no matter what your state of motion is.

 

[Seipai]

I don't know, I guess I don't even understand what "rest" means even in 3 spatial dimensions. I realize I and my keyboard are hurtling through space around a galactic center, which is also hurtling through space. Everything is moving. Even if you could exclude the time dimension, you're still right, nothing is ever at rest.

So when we're talking about a stationary object at rest, it seems that has to mean it is at rest in a spatial relation to another object, i.e., my keyboard is at rest on my desk; my lamp is at rest 12 inches from my computer. No?

Worse yet there is no "at rest" in 4D space. Even if you pick your frame of reference so an object is "at rest" in that frame it is moving forward in time. Time is just treated as another dimension equivalent to the other three in 4D space, if I understand how it works. And your total "speed" through 4D space can be represented as a constant when all of them are added up.

As a result a photon undergoes no time component. It is everywhere along its path at the same time. At least in its own "personal frame of reference". I will take correction gladly if I am wrong.

So when Justatruthseeker was complaining about gravity causing an "at rest" object to move along a geodesic in 4D space he was wrong since the object was never at rest. All objects move in 4D space. For an object not to move it would have to exist for an infinitesimal fraction of a second. And when an object falls in 4D space it is only following a straight line through that space when you add in the time dimension. We cannot perceive the time dimension directly we can only observe the "now" in 4D space at any moment. But we can represent it mathematically.

 

[Wiccan_Child]

Of course it's a speed through spacetime, but it's certainly measured as a speed over miles or meters or what have you. Some measure of space.

Is it? Motion through the space part of spacetime is in meters per second, sure, but relativity argues that motion is more general than that. Motion is movement through spacetime, whether it's space-y (meters per second) or time-y (seconds per second).

An object is always moving through spacetime. Whatever units we care to use, however we delineate the four axes of spacetime, an object has a line curving through 4D spacetime.

Imagining anything in four dimensions is near impossible for we humans, and it only gets more complicated when you start twisting spacetime all over the place (as gravity is wont to do).

I don't know, I guess I don't even understand what "rest" means even in 3 spatial dimensions. I realize I and my keyboard are hurtling through space around a galactic center, which is also hurtling through space. Everything is moving. Even if you could exclude the time dimension, you're still right, nothing is ever at rest.

Yes and no. One of the core principles of relativity is, well, the Principle of Relativity - there is no possible experiment that could tell you whether you are at rest or in constant motion. That is, the concept of motion (and therefore 'at rest') is entirely relative - we decide whether an object is at rest or not, and everything else settles around it. We can say how much faster one object is moving than the other, but there is no grand objective reference frame to measure everything against.

So when we're talking about a stationary object at rest, it seems that has to mean it is at rest in a spatial relation to another object, i.e., my keyboard is at rest on my desk; my lamp is at rest 12 inches from my computer. No?

Yes... in space, relative to the desk (and you). It's not at rest through time. If it was moving, it'd see you moving, and thus experiencing time dilation and such.

 

[Chesterton]

...or time-y (seconds per second).

Yes and no. One of the core principles of relativity is, well, the Principle of Relativity...

[bless and do not curse], physicists are insane.

That is, the concept of motion (and therefore 'at rest') is entirely relative - we decide whether an object is at rest or not, and everything else settles around it. We can say how much faster one object is moving than the other, but there is no grand objective reference frame to measure everything against.

Who is this "we"? Just the other day, you said the human mind plays no role in determining physical reality. Now you have me thinking I should become a geocentrist flat-earther.

 

[Wiccan_Child]

[bless and do not curse], physicists are insane.

Well, yes, but we build you GPS machines, so hush

Who is this "we"? Just the other day, you said the human mind plays no role in determining physical reality. Now you have me thinking I should become a geocentrist flat-earther.

Ah, I said velocity is relative, but acceleration is not. I can't say how fast you're moving, but I can certainly say how fast your movement is changing. So if you accelerate, it can be known. And since orbiting the Sun is a form of acceleration, it can be known. That's why geocentrism/heliocentrism isn't a matter of arbitrary choice - even if you define the Earth to be stationary, there's still the Sun's gravity pulling us towards it. Since we've defined the Earth to be stationary, we see the Sun move out from under us, hence we fall forever in a circle - we orbit.

So velocity (speed+direction) is relative, but acceleration (change in velocity) is not. So orbit (changing direction = changing velocity = acceleration) is not relative, it's absolute.