# 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?

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#### JesseM

Re: Is there such a thing as "at rest"?

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.
ChrisPeace said:
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 [Broken] of general relativity).

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#### CompuChip

Homework Helper
Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

I guess to simplify my question:

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

#### JesseM

Re: Is there such a thing as "at rest"?

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

Homework Helper
Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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 existance 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

Re: Is there such a thing as "at rest"?

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.
ChrisPeace said:
If we are to ascribe to the theory that this whole universe exploded into existance 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.

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#### CompuChip

Homework Helper
Re: Is there such a thing as "at rest"?

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 existance 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

Re: Is there such a thing as "at rest"?

If we are to ascribe to the theory that this whole universe exploded into existance 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

Re: Is there such a thing as "at rest"?

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.

Staff Emeritus
Re: Is there such a thing as "at rest"?

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

#### altonhare

Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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

Homework Helper
Re: Is there such a thing as "at rest"?

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

Gold Member
Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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

Gold Member
Re: Is there such a thing as "at rest"?

altonhare:
I just defined absolute motion, justify your claim that it "cannot be defined".
Where ? I don't see it.

#### CompuChip

Homework Helper
Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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

Re: Is there such a thing as "at rest"?

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?

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