Is there such a thing as at rest ?

[ChrisPeace]

Is there such a thing as "at rest"?

I've been watching some lectures on relativity from a professor at Berkley and he brings up the central idea of the "reference frame" relative to being in motion or "at rest".

He says that if I were to ask you if you were currently in motion or at rest, some of you could say "Yes, I'm at rest" whereas others could say "I'm moving at the same velocity of the Earth" and both of these would be true.

If you put me in a car, and spray paint the windows black so its pitch dark in the vehicle, then drive around, I can certainly feel that I'm not at rest, but I can't exactly prove it.

Regardless of your reference frame isn't the passing of the day to night (given what we now know about the universe around us) absolute proof that there is no "at rest" no matter my reference frame?

 

[JesseM]

If you put me in a car, and spray paint the windows black so its pitch dark in the vehicle, then drive around, I can certainly feel that I'm not at rest, but I can't exactly prove it.

You'll only feel it when the car accelerates (changes speed or direction, which results in G-forces felt inside), even small accelerations from bumps in the road. The equivalence between reference frames in special relativity is only between inertial (non-accelerating) frames. If one person is on a ship in deep space that's at rest relative to the solar system, and the other is on a ship in deep space moving at a constant velocity of 0.9c relative to the first one, both will feel totally weightless, and as long as their windows are blacked out they'll have no way to tell which ship they're on.

Regardless of your reference frame isn't the passing of the day to night (given what we now know about the universe around us) absolute proof that there is no "at rest" no matter my reference frame?

If you're on the surface of the Earth, you aren't really moving inertially since you're going in a circle. Inertial motion is either going in a straight line at constant speed through a region of spacetime that's not curved (and gravity curves spacetime in general relativity, so you'd have to be in deep space for this to be approximately true), or it's the motion of a freefalling observer in a gravitational field for a very brief period of time and in a small region of space, what's known as a "locally" inertial frame since the laws of physics here look identical to those seen by an inertial observer in the uncurved spacetime of special relativity (see the http://www.einstein-online.info/en/spotlights/equivalence_principle/index.html of general relativity).

 

[CompuChip]

You are making several mistakes.
If I put you in a blinded car and drive you around, you can feel that you are not at rest, because the car is accelerating (when it speeds up or slows down, and when it turns off). If this is the case, then you can even prove it. For example, you can hang a small ball of a rope on the ceiling of the car. When there is an acceleration, you can measure the angle of the rope with respect to the vertical axis and calculate the direction and magnitude of the acceleration.

In the passing of day and night, you are first of all accelerating, because the Earth is rotating around it's axis, and around the sun, etc. Secondly, the notion of being "at rest" here depends on an outside reference frame (namely, for example, stars very far away which are approximately fixed).

You need to carefully apply the axioms of special relativity. In particular: in the absence of fixed reference points, any two observers which are moving at constant velocity with respect to each other, are equivalent.

 

[ChrisPeace]

I guess to simplify my question:


Is there ANYTHING in the universe that isn't in motion?

 

[JesseM]

I guess to simplify my question:


Is there ANYTHING in the universe that isn't in motion?

In relativity there is no objective truth about whether something is "in motion" or "at rest", it just depends on your choice of reference frame, and all inertial reference frames are considered equally valid.

 

[CompuChip]

It is very hard (if not impossible) to find a non-accelerating frame of reference on earth. The Earth rotates around the sun, the solar system around the center of our galaxy, our galaxy ...

However, this issue already arises in classical mechanics -- just as in special relativity we usually just assume that our reference system is "sufficiently" non-accelerating. Depending on what you are calculating though, you need to be careful (for example, in meteorology, the rotation of the Earth provides the Coriolis effect and is therefore not negligible).
But as I said this is not specifically related to special relativity and the questions from your earlier post.

 

[lightarrow]

I guess to simplify my question:
Is there ANYTHING in the universe that isn't in motion?

They have already answered you, however I only add: to be "in motion" is not an intrinsic property of a system, because you can only say that you are or you are not in motion with respect to something else. So, yes, I can say that I'm not in motion with respect to my room, for example.
Instead, to say that you are or you are not in motion in an absolute sense, you should need to have a preferential frame of reference in all the universe. Maybe we could say that such a frame exist and it's the CMBR (cosmic microwave background radiation), but I would probably be speculating, so I let this subject to someone else more aknowledged than me, in case.

 

[ChrisPeace]

If I place a hand grenade on the pavement, pull the pin, and get behind something safe, when it explodes, EVERYTHING in that package that was once "at rest" is now "in motion".

If we are to ascribe to the theory that this whole universe exploded into existence from a singularity, then we HAVE to agree that there is absolutely nothing in this universe that is at rest. I don't understand how anything can be debated when it comes to this...maybe I'm being short sighted.

 

[JesseM]

If I place a hand grenade on the pavement, pull the pin, and get behind something safe, when it explodes, EVERYTHING in that package that was once "at rest" is now "in motion".

"In motion" relative to the ground, but not in any absolute sense. And suppose some pieces fly out at, say, 200 mph...that means that if the grenade was riding in a plane traveling 200 mph relative to the ground, than a piece flung off in the opposite direction as the plane's direction of motion relative to the ground might now be at rest relative to the ground. Again, the point is that there is no physical way to make sense of the terms "in motion" or "at rest" except relative to something else like the ground.

If we are to ascribe to the theory that this whole universe exploded into existence from a singularity, then we HAVE to agree that there is absolutely nothing in this universe that is at rest. I don't understand how anything can be debated when it comes to this...maybe I'm being short sighted.

The idea of the Big Bang is not an explosion at a distinct point in some preexisting space...see my post #10 here.

 

[CompuChip]

If I place a hand grenade on the pavement, pull the pin, and get behind something safe, when it explodes, EVERYTHING in that package that was once "at rest" is now "in motion".

Yes, because a force was exerted and everything has accelerated. However, suppose that you had a temporary blackout during the explosion. When you wake up looking up, and you see something flying by with a constant velocity, you cannot tell whether that object is in motion and you are at rest or vice versa. You need to verify that you are lying still with respect to the floor to recover your intuition.

Did you ever sit in a train on a station with another train right beside it? When you look out the window and you see yourself moving past the other train you would swear that it is your train moving. You can even convince yourself that you are feeling a small acceleration sometimes. Only when one of the trains is accelerating noticably and/or you can see the platform of the station, you can say whether you are moving or the other train...

If we are to ascribe to the theory that this whole universe exploded into existence from a singularity, then we HAVE to agree that there is absolutely nothing in this universe that is at rest. I don't understand how anything can be debated when it comes to this...maybe I'm being short sighted.

Nobody is debating that there is anything absolutely at rest... the whole point is that we can only talk about objects at rest relative to ... (us, the CMB, the room, etc).

 

[lightarrow]

If we are to ascribe to the theory that this whole universe exploded into existence from a singularity, then we HAVE to agree that there is absolutely nothing in this universe that is at rest.

To see the universe exploding and the pieces moving off, you MUST be in a space outside the universe. But this is not possible, since SPACE itself (and time too) were created with the Big Bang and the universe.

 

[ChrisPeace]

It really just sounds like "Sorry, this is the BEST we can do".

It doesn't make it right, it makes it the only thing we've come up with thus far.

 

[Vanadium 50]

Pity that the universe doesn't seem to work the way you would prefer. It is what it is, I'm afraid.

 

[altonhare]

I guess to simplify my question:Is there ANYTHING in the universe that isn't in motion?

It is, indeed, impossible to logically conclude that any entity is motionless. The confusion and debate comes from the lack of a clear, unambiguous definition of "motion".

Motion: Two or more locations of an object.
Location: The set of distances from an object to every other object in the universe.
Object: That which has shape.

To avoid circularity the "that" in the definition of object does not refer to an object. It is a "wildcard" for the name of whatever we are stating has shape. "Apple has shape", therefore it fulfills the definition of "object".

Now, one can see why it is impossible to conclude that s/he is motionless nor that any other entity is motionless.

 

[atyy]

Motion: Two or more locations of an object.
Location: The set of distances from an object to every other object in the universe.
Object: That which has shape.

What is "distance"?

 

[CompuChip]

It really just sounds like "Sorry, this is the BEST we can do".

It doesn't make it right, it makes it the only thing we've come up with thus far.

I think the problem is not so much in the way the principle of relativity works, just in your failure to consider things objectively and letting go of your intuition.
That's not your fault though, changing how we think about things that we see in everyday life and that are so built into us, takes time and practice.

 

[Mentz114]

Altonhare:

It is, indeed, impossible to logically conclude that any entity is motionless. The confusion and debate comes from the lack of a clear, unambiguous definition of "motion".

Relative motion - can be unambiguously defined.
Absolute motion - cannot be defined.

It is necessary to further refine the concept of motion, that's all.

 

[altonhare]

What is "distance"?

The gap or separation between two objects.

[QUOTE="Mentz114]Relative motion - can be unambiguously defined.
Absolute motion - cannot be defined.

It is necessary to further refine the concept of motion, that's all.
[/quote]

I just defined absolute motion, justify your claim that it "cannot be defined".

 

[Mentz114]

altonhare:

I just defined absolute motion, justify your claim that it "cannot be defined".

Where ? I don't see it.

 

[CompuChip]

It is necessary to further refine the concept of motion, that's all.

Errr, wait... is this a special relativity or a philosophy thread?

 

[JesseM]

The gap or separation between two objects.

That's not a physical definition unless you give a procedure for actually measuring this separation, which is problematic since in relativity rulers in motion relative to one another measure different lengths.

 

[Naty1]

Is there ANYTHING in the universe that isn't in motion?

Motion is basically an interval of position over an interval of time...so if you take a "snapshot" at an instant of time = 0, then everything is "still", like a photo.

I don't think that's really a good description, but sometimes taking the limit of an example, an extreme position, provides some understanding. Because motion is measured relative to a frame of reference and since there is no absolute frame of reference, there is no absolute motion.

You might also consider if there was motion before the universe existed...I'd suggest that without space (distance), without time (duration) , likely "motion" would likely be meaningless...

 

[altonhare]

altonhare:

Where ? I don't see it.

Motion: two or more locations of an object
Location: the set of distances from an object to every other object in the universe

That's not a physical definition unless you give a procedure for actually measuring this separation, which is problematic since in relativity rulers in motion relative to one another measure different lengths.

Are you saying a human being has to measure a couch in order for the couch to have extent? Why should Nature conform to what a human being is limited to doing? Of course Nature doesn't bend to our will, so it would behoove us to try to use our imagination. We can at least imagine what I am describing, even if a human cannot measure the distance from him/her self to every other entity without a previous measurement becoming "outdated". I can imagine freezing the universe and taking even steps from one entity to every other one. My feet will contract as I step, but then when I stop they will be a standard length. It may take me a zillion years but eventually I'd be done.

Measurement is quantitative and invokes an observer. Distance and extent are qualitative, they do not require observers. An object has extent whether a person ever looks at it or measures it. The couch is not 5 feet long until a person puts his/her measuring tape next to it, but it had size, i.e. extent, i.e. shape before anyone looked at it. Similarly an entity has location even when nobody's looking or measuring.

 

[Nabeshin]

I'm having a hard time understanding the OP's confusion on the subject. To me it seems as simple as this:

Sure, objects might appear to be at rest in certain reference frames. However, there will be other reference frames in which these objects are moving. And since all reference frames are equivalent, it is an obvious contradiction if we try to impose some kind of absolute motion. Therefore, no such thing exists.

Seems as simple as that to me.

 

[Nabeshin]

Motion: two or more locations of an object
Location: the set of distances from an object to every other object in the universe

But these two locations appear different to different observers =/ Also, the distance from an object to every other object in the universe as measured from whose reference frame?

 

[JesseM]

Are you saying a human being has to measure a couch in order for the couch to have extent?

In this case you're talking philosophy rather than physics. But philosophically I favor the 4D block universe view, where the couch is "really" a sort of 4D tube in spacetime, and spatial length can only refer to a 3D cross-section of this 4D tube--just as there is no "objective truth" about size of 2D cross-sections of a 3D tube independent of your choice of what angle to take the cross-section, similarly in the 4D view there'd be no "objective truth" about the size of 3D cross-sections of a couch's world-tube independent of an arbitrary choice of what angle to slice up 4D spacetime into a series of 3D instants (i.e. an arbitrary choice about simultaneity).

Why should Nature conform to what a human being is limited to doing? Of course Nature doesn't bend to our will, so it would behoove us to try to use our imagination.

Why should Nature conform to your preconceived ideas about time, in which you insist on imagining the universe to be a series of 3D spatial configurations at different objective moments rather than a 4D spacetime with no objective notion of simultaneity? There is nothing inherently illogical about the 4D "block time" view, and physics seems to point us in that direction.

 

[CompuChip]

But these two locations appear different to different observers =/ Also, the distance from an object to every other object in the universe as measured from whose reference frame?

Wouldn't three of them be enough to do a triangulation?
And having a reference frame anyway, what is wrong with just setting up a coordinate system in it and writing down three (or four) coordinates instead of all the distances to all other things?

 

[altonhare]

In this case you're talking philosophy rather than physics. But philosophically I favor the 4D block universe view, where the couch is "really" a sort of 4D tube in spacetime, and spatial length can only refer to a 3D cross-section of this 4D tube--just as there is no "objective truth" about size of 2D cross-sections of a 3D tube independent of your choice of what angle to take the cross-section, similarly in the 4D view there'd be no "objective truth" about the size of 3D cross-sections of a couch's world-tube independent of an arbitrary choice of what angle to slice up 4D spacetime into a series of 3D instants (i.e. an arbitrary choice about simultaneity).

Why should Nature conform to your preconceived ideas about time, in which you insist on imagining the universe to be a series of 3D spatial configurations at different objective moments rather than a 4D spacetime with no objective notion of simultaneity? There is nothing inherently illogical about the 4D "block time" view, and physics seems to point us in that direction.

There is no justification for declaring time as a "dimension". Not only does our daily experience indicate that entities are 3D in the sense that they have extent in three mutually perpendicular directions, but mathematically what we insert and label "time" behaves entirely differently. There is no reason to think that time is anything other than: relative motion i.e. motion+observer.

Indeed one cannot even make a substantial claim for time as a dimension without defining both time and dimension unambiguously. The only consistent definitions I have seen summarily rule out "time" as a dimension.

Objectively, all we have before us is the slowing of cyclical mechanisms in particular scenarios. There is no reason to think this not purely a mechanical effect that is not understood. Indeed, since radioactive decay is still considered "random" one cannot deduce any definitive conclusions about "the nature of time" from SR experiments. When the process for measuring time itself, the clock, is understood we can begin to make meaningful judgments on this matter. In the meantime we are still studying the clock itself.

Nature should not conform to my ideas, and I welcome skepticism, but not for skepticism's sake. One needs to pose an alternative definition/conception or, as a bare minimum, try to argue why the one posed is wrong.

Wouldn't three of them be enough to do a triangulation?
And having a reference frame anyway, what is wrong with just setting up a coordinate system in it and writing down three (or four) coordinates instead of all the distances to all other things?

If you measure distances to a few specific entities then all you know is the entity's distance from those. You don't know anything about its distance from everything else in the universe (its location). Since motion is defined as at least two locations of an entity, you absolutely need to know its location.

 

[altonhare]

I'm having a hard time understanding the OP's confusion on the subject. To me it seems as simple as this:

Sure, objects might appear to be at rest in certain reference frames. However, there will be other reference frames in which these objects are moving. And since all reference frames are equivalent, it is an obvious contradiction if we try to impose some kind of absolute motion. Therefore, no such thing exists.

Seems as simple as that to me.

It's not "as simple as that". The keyword, the meaning of which you have taken for granted, is move. What does it actually mean to move? Do you have to see something move for it to have moved? That doesn't make any sense.

But these two locations appear different to different observers =/ Also, the distance from an object to every other object in the universe as measured from whose reference frame?

Incorrect. Each entity has the same location no matter whose looking or not looking. If we imagine freezing the universe and I measure all the distances from A to everything else I get a list of numbers. If you do the same you get the exact same list of numbers.

 

[Nabeshin]

It's not "as simple as that". The keyword, the meaning of which you have taken for granted, is move. What does it actually mean to move? Do you have to see something move for it to have moved? That doesn't make any sense.



Incorrect. Each entity has the same location no matter whose looking or not looking. If we imagine freezing the universe and I measure all the distances from A to everything else I get a list of numbers. If you do the same you get the exact same list of numbers.

Motion is a change in position over a given time. Position is the spatial dimension measured with respect to some reference frame. (My, and I think most people's, definition of motion).

Let's examine your concept of "location" which you seem to think is invariant. Let's assume you have a universe of three objects, A, B, and C. If I am situated on A, I will measure B and C to be some distance away at some moment in time. (Say I get B=5 and C=6 units away from me). This is your concept of location. However, if I am situated on B and I attempt to measure the same quantities, namely, the distances from A to B and from A to C, I may very well arrive at B=3 and C=2. There really is no reason for you to think these numbers will be the same (and they aren't).

 

[JesseM]

There is no justification for declaring time as a "dimension".

The justification is that it is simplest to treat spacetime as a unified 4D geometrical structure in relativity.

Not only does our daily experience indicate that entities are 3D in the sense that they have extent in three mutually perpendicular directions

As long as relativity makes correct predictions about all the specific events that actually make up our daily experience, and relativity is compatible with the notion of time as a dimension, then there can be no logical way that our daily experience can contradict this idea.

but mathematically what we insert and label "time" behaves entirely differently. There is no reason to think that time is anything other than: relative motion i.e. motion+observer.

Time is obviously different from spatial dimensions (this is true in relativity for numerous reasons), but there's nothing illogical about the idea of time being a different sort of dimension in 4D spacetime. It seems to me you are failing to take your own advice here:

Why should Nature conform to what a human being is limited to doing? Of course Nature doesn't bend to our will, so it would behoove us to try to use our imagination.

Indeed one cannot even make a substantial claim for time as a dimension without defining both time and dimension unambiguously. The only consistent definitions I have seen summarily rule out "time" as a dimension.

Mathematically the idea of a 4D spacetime manifold, which is distinct from a 4D spatial manifold because of a difference in the metric signature, is quite well-defined in differential geoemetry, it's a type of psuedo-Riemannian manifold (with a Riemannian manifold being a purely spatial one).

Objectively, all we have before us is the slowing of cyclical mechanisms in particular scenarios. There is no reason to think this not purely a mechanical effect that is not understood. Indeed, since radioactive decay is still considered "random" one cannot deduce any definitive conclusions about "the nature of time" from SR experiments. When the process for measuring time itself, the clock, is understood we can begin to make meaningful judgments on this matter. In the meantime we are still studying the clock itself.

Nature should not conform to my ideas, and I welcome skepticism, but not for skepticism's sake. One needs to pose an alternative definition/conception or, as a bare minimum, try to argue why the one posed is wrong.

Why does your "series of 3D moments" view get to be treated as the default while "4D spacetime" is treated as an "alternative" that can only be considered if your view is shown wrong? Since relativity offers no preferred definition of simultaneity, any notion of absolute simultaneity must either assume new physics will eventually justify it, or it must admit to being a philosophical idea that can never be verified by experiment. If the latter, then the "4D spacetime" view and the "series of 3D moments" view are competing philosophical notions, so any argument as to why one is "wrong" would have to be a purely philosophical argument. I don't claim there are any clear-cut philosophical arguments that prove the "series of 3D moments" view is wrong, just that there are also no philosophical arguments that prove the "4D spacetime" view wrong either, so there is no basis for your seeming total confidence that the first is right and the second is wrong; it seems that your only basis for this is that you find the first more aesthetically appealing, or less counterintuitive. Given current physics, I personally find the "4D spacetime" view more elegant for the simple reason that it doesn't require us to postulate undetectable metaphysical entities beyond those described by physics. You don't have to agree, but I hope you will at least admit that there is no logical contradiction of philosophical impossibility in this view.

 

[map19]

Motion in space is measured by the frequency shift seen in the CMB. Compound motion causes the CMB frequenct to show a quadripole image. As noticed by the COBE satellite, because the Earth has motion in all axes relative to the Cosmos.
It might be possible to be "stopped in space" as mentioned on Startrek if a spaceship could manoevre to where the CMB showed no frequency shifts in any of the three spatial axes.

 

[altonhare]

Motion is a change in position over a given time. Position is the spatial dimension measured with respect to some reference frame. (My, and I think most people's, definition of motion).

Let's examine your concept of "location" which you seem to think is invariant. Let's assume you have a universe of three objects, A, B, and C. If I am situated on A, I will measure B and C to be some distance away at some moment in time. (Say I get B=5 and C=6 units away from me). This is your concept of location. However, if I am situated on B and I attempt to measure the same quantities, namely, the distances from A to B and from A to C, I may very well arrive at B=3 and C=2. There really is no reason for you to think these numbers will be the same (and they aren't).

Here we have A lying "5" away from B and "6" away from C:

000000000000000000000
0000000000A0000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
0000000000B0000000000
0000000000C0000000000
000000000000000000000

Did I make the diagram wrong? No matter how I look at it I get AB=5 and AC=6.

 

[altonhare]

The justification is that it is simplest to treat spacetime as a unified 4D geometrical structure in relativity.

As long as relativity makes correct predictions about all the specific events that actually make up our daily experience, and relativity is compatible with the notion of time as a dimension, then there can be no logical way that our daily experience can contradict this idea.

Time is obviously different from spatial dimensions (this is true in relativity for numerous reasons), but there's nothing illogical about the idea of time being a different sort of dimension in 4D spacetime. It seems to me you are failing to take your own advice here:


Mathematically the idea of a 4D spacetime manifold, which is distinct from a 4D spatial manifold because of a difference in the metric signature, is quite well-defined in differential geoemetry, it's a type of psuedo-Riemannian manifold (with a Riemannian manifold being a purely spatial one).

Objectively, all we have before us is the slowing of cyclical mechanisms in particular scenarios. There is no reason to think this not purely a mechanical effect that is not understood. Indeed, since radioactive decay is still considered "random" one cannot deduce any definitive conclusions about "the nature of time" from SR experiments. When the process for measuring time itself, the clock, is understood we can begin to make meaningful judgments on this matter. In the meantime we are still studying the clock itself.

Why does your "series of 3D moments" view get to be treated as the default while "4D spacetime" is treated as an "alternative" that can only be considered if your view is shown wrong? Since relativity offers no preferred definition of simultaneity, any notion of absolute simultaneity must either assume new physics will eventually justify it, or it must admit to being a philosophical idea that can never be verified by experiment. If the latter, then the "4D spacetime" view and the "series of 3D moments" view are competing philosophical notions, so any argument as to why one is "wrong" would have to be a purely philosophical argument. I don't claim there are any clear-cut philosophical arguments that prove the "series of 3D moments" view is wrong, just that there are also no philosophical arguments that prove the "4D spacetime" view wrong either, so there is no basis for your seeming total confidence that the first is right and the second is wrong; it seems that your only basis for this is that you find the first more aesthetically appealing, or less counterintuitive. Given current physics, I personally find the "4D spacetime" view more elegant for the simple reason that it doesn't require us to postulate undetectable metaphysical entities beyond those described by physics. You don't have to agree, but I hope you will at least admit that there is no logical contradiction of philosophical impossibility in this view.

Fundamentally, what is your justification for this symbol "time" referring to a "dimension"? An equation can have as many parameters in it as we want, they don't equal dimensions. This is not an issue that can be resolved mathematically. Mathematics deals with useful descriptive models. Time is a useful parameter to humans so we put it in our equations. When you reduce this down to essential language "time" is no more than relative motion and is, in fact, unnecessary though convenient.

 

[Nabeshin]

Here we have A lying "5" away from B and "6" away from C:

000000000000000000000
0000000000A0000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
0000000000B0000000000
0000000000C0000000000
000000000000000000000

Did I make the diagram wrong? No matter how I look at it I get AB=5 and AC=6.

Your diagram is with respect to some reference frame. In another frame, it might look like this:
000000000000000000000
0000000000A0000000000
000000000000000000000
000000000000000000000
000000000000000000000
0000000000B0000000000
000000000000000000000
000000000000000000000
0000000000C0000000000
000000000000000000000

(not the same as the numbers I gave earlier but yeah).
Just because you put a grid onto something does not make it an absolute!