How can any physical body truly be at rest?

  • #1
Can we truly have a rest frame or should it be a close to rest frame?

Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity.

The earth is moving at 67,000 mph and the galaxy is moving at 250,000 mph. I'm never in a single frame. Whether it's the next second, picosecond or yoctosecond.

When is anything in the universe truly in a rest frame?

When am I or anything in the universe every frozen in a single frame? Wouldn't it just be a blur of frames?

When am I in a frame of a picosecond or a millisecond or a minute? How is it possible that I'm at rest in any of those frames?

I understand how an observer can be at rest relative to an observer that's moving as a way to make the equations work but how can anything truly be at 0 velocity? We're always in motion from frame to frame.
 

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  • #2
PeterDonis
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Can we truly have a rest frame
You can always choose a frame in which you, or any chosen object, is at rest.

Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity.
You aren't relative to the observer in the car, but you are relative to the frame in which you are at rest. In that frame, you are at rest and the observer in the car is moving past you.

If you don't understand this, you have a very, very, very basic misunderstanding of how relativity works.

The earth is moving at 67,000 mph and the galaxy is moving at 250,000 mph
Relative to what? There is no such thing as absolute motion.

When is anything in the universe truly in a rest frame?
A frame is something you choose. It's not built into the universe. It's a tool we use to describe the universe. You can always choose a frame in which you, or any chosen object, is at rest. That's all there is to it.

Your use of the word "truly" indicates that you are trying to find some sort of absolute definition of rest. There isn't one. Rest, like motion, is relative; it depends on your choice of frame.

When am I or anything in the universe every frozen in a single frame? Wouldn't it just be a blur of frames?

When am I in a frame of a picosecond or a millisecond or a minute? How is it possible that I'm at rest in any of those frames?
None of this makes any sense. You are just confusing yourself by trying to find an absolute definition of rest. There isn't one. See above.
 
  • #3
Orodruin
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There is no such thing as "absolute rest". This is an absolutely fundamental part of relativity and classical mechanics alike. You can only be at rest in a frame of reference, which is just to say that you are not moving relative to its origin. However, there is nothing that selects one inertial frame over another so as to define a preferred rest frame where anything would be "truly at rest", that concept is simply not useful.

I'm never in a single frame.
This points towards a misconception of what a frame is. It is just a convention for labelling positions and times. There is no meaning to the phrase "be in a frame", anything that exists has coordinates in all frames. An object cannot exist in one frame and not exist in another.

but how can anything truly be at 0 velocity?
Again, this (absolute velocity) is simply not a useful concept.
 
  • #4
Grinkle
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Relativity has no preferred frame from which to assess whether anything is at rest or not. Except for c I think, there is no way to discuss the 'true' speed of anything using relativity.
 
  • #5
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Can we truly have a rest frame or should it be a close to rest frame?

Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity.

The earth is moving at 67,000 mph and the galaxy is moving at 250,000 mph. I'm never in a single frame. Whether it's the next second, picosecond or yoctosecond.

When is anything in the universe truly in a rest frame?

When am I or anything in the universe every frozen in a single frame? Wouldn't it just be a blur of frames?

When am I in a frame of a picosecond or a millisecond or a minute? How is it possible that I'm at rest in any of those frames?

I understand how an observer can be at rest relative to an observer that's moving as a way to make the equations work but how can anything truly be at 0 velocity? We're always in motion from frame to frame.
If you're interested in a fun bit of scientific history about how these ideas came to be understood, here is Galileo Galilei's "Dialogue Concerning the Two Chief World Systems":

https://svetlogike.files.wordpress....d-by-drake-1953-abridged-by-s-e-sciortino.pdf
 
  • #6
Relativity has no preferred frame from which to assess whether anything is at rest or not. Except for c I think, there is no way to discuss the 'true' speed of anything using relativity.
This is my point!

Words matter and there's no such thing as at rest. You can say at relative rest to a moving observer. It's like the word particles. Like Heisenberg said subatomic particles are not particles in the sense of the word. When you hear particles it implies particles of sand or particles of salt. They're really subatomic states of probability that can be in one state or the other when measured.

“[T]he atoms or elementary particles themselves are not real; they form a world of potentialities or possibilities rather than one of things or facts.”
Werner Heisenberg
Scientist complained about calling imaginary time imaginary because it implies something mysterious.

So saying something is at rest implies that it's in a singular reference frame. The truth is nothing is at rest and everything is in constant motion. It's a blurred worldline of events that are in the past, present and future simultaneously. We're just wired to see these blurred out events in a continuous rate that allows us to perceive a distinction between past, present and future.

The distinction between the past, present and future is only a stubbornly persistent illusion.
- Albert Einstein
 
  • #7
Grinkle
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This is my point!
Ok. Why the "!"?

The truth is nothing is at rest
The truth according to relativity is that there is no valid definition of globally 'at rest'.

Welcome to the real world, I guess? ;-)
 
  • #8
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Ok. Why the "!"?



The truth according to relativity is that there is no valid definition of globally 'at rest'.

Welcome to the real world, I guess? ;-)
You forgot to post a picture of Morpheus looking down in concern.
 
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  • #9
phinds
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Words matter and there's no such thing as at rest.
Yes, words matter. For example the words "frame of reference" matter and more to the point have meaning. You seem to be unwilling to recognize that and to accept that the term "rest frame" is a frame of reference in which IN THAT FRAME something is at rest. That is EVERYTHING is at rest relative to itself. That's all that we've been telling you.
 
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  • #10
Dale
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Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity.
You are truly at rest in your own reference frame, by definition
 
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  • #11
russ_watters
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This is my point!

Words matter and there's no such thing as at rest. You can say at relative rest to a moving observer.
Right. This is not news; it is the first postulate of Relativity!
https://en.m.wikipedia.org/wiki/Principle_of_relativity
So saying something is at rest implies that it's in a singular reference frame. The truth is nothing is at rest and everything is in constant motion.
No, the simple way out of this conundrum is to always define your reference frame so people don't have to guess about what is being implied. Again: this isn't news.
 
  • #12
Nugatory
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So saying something is at rest implies that it's in a singular reference frame.
Everything is always in all reference frames.

Saying that something is at rest is just a convenient way of saying that we have chosen to use a reference frame in which that something's speed is zero to describe the physical situation. But there's nothing magical or "singular" about that frame, it's just a choice that we made when we set up the problem. For example, if I throw a ball into the air and I want to calculate how long it takes to fall back to earth..... it's easy to set up and solve this problem if I use a frame in which the patch of earth under my feet is at rest.... but I could choose to work the problem using a frame in which Mars is at rest and the earth is rotating and moving around the sun at several kilometers per second.
 
  • #13
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The words “at rest” are meaningless without qualification. “An object at rest with respect to reference frame S” has meaning, and it means the object being examined in the reference frame S has a speed of zero with respect to reference frame S.

That’s pretty much it.
 
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  • #14
CWatters
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It the same with position. You always have to pick an origin to measure position from.
 
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  • #15
ZapperZ
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Can we truly have a rest frame or should it be a close to rest frame?

Even if I'm stationary and sitting on my porch and the observer in the car passing is moving, I'm still not at 0 velocity.

The earth is moving at 67,000 mph and the galaxy is moving at 250,000 mph. I'm never in a single frame. Whether it's the next second, picosecond or yoctosecond.

When is anything in the universe truly in a rest frame?

When am I or anything in the universe every frozen in a single frame? Wouldn't it just be a blur of frames?

When am I in a frame of a picosecond or a millisecond or a minute? How is it possible that I'm at rest in any of those frames?

I understand how an observer can be at rest relative to an observer that's moving as a way to make the equations work but how can anything truly be at 0 velocity? We're always in motion from frame to frame.
It appears that you're asking if there is an absolute rest frame. Both Newtonian physics and Relativity do not require those, and essentially indicated that ALL inertial reference frames are equally valid and will have the identical physics.

It means that when you were tying your post, you and your device are in the same frame, and thus, the device is in your rest frame, while you are in the device's rest frame equally. You two are at rest with respect to one another.

So, have I put this issue to rest?

Zz.
 
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  • #16
Ibix
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It means that when you were tying your post, you and your device are in the same frame, and thus, the device is in your rest frame, while you are in the device's rest frame equally.
To be pedantic, as Nugatory says, everything is always in every frame. The special thing about you and your device is that you are both at rest in each others' inertial rest frames. I think the precise wording is important here, given that the OP seems confused about what frames are, as you pointed out.
So, have I put this issue to rest?
Depends - were you at rest with respect to the drumkit on which the rimshot was played?
 
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  • #17
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Words matter and there's no such thing as at rest. You can say at relative rest to a moving observer.



Scientist complained about calling imaginary time imaginary because it implies something mysterious.

So saying something is at rest implies that it's in a singular reference frame. The truth is nothing is at rest and everything is in constant motion. It's a blurred worldline of events that are in the past, present and future simultaneously. We're just wired to see these blurred out events in a continuous rate that allows us to perceive a distinction between past, present and future.
Doesn't at rest simply mean the accelerometer reads zero? is it the two peeps at rest but in relative motion that is an issue? imo that's geometry.

There is another popular quote, space is what keeps everything from happening at the same place and time from it happening at the same time.
 
  • #18
PeterDonis
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Doesn't at rest simply mean the accelerometer reads zero?
No. That is geodesic (free fall) motion. Whether it is "at rest" depends on your choice of coordinates.
 
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  • #19
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Can we truly have a rest frame or should it be a close to rest frame?
We can find a coordinate where any appointed particle is at rest, i.e. coordinate of its position is (x,y,z,t)=(a,b,c,t) where a,b,c are constants and t is arbitrary. Such a coordinate is not an IFR in general. When it is an IFR, the law of inertia motion is applied to the particle i.e. no force e.g. electromagnetic or gravitational forces are working on it.

In relativistic quantum mechanics, an interesting phenomena called Zitterbewegung tells us that electrons or other particles are always moving with light speed c but changing direction frequently. Even in this case long time average velocity is defined with which its rest frame is identified.
 
  • #20
phinds
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a)
How can any physical body truly be at rest?
instead to say ''rest'' is better to say ''potential state''
No, it is not. It is better to "at rest in a particular rest frame". I can't see that saying "potential state" is even meaningful.
b)
Can we truly have a ''rest'' reference frame ?
Every inertial reference frame is not a ''rest'' system.
I'm not sure what you mean by "rest system" but every inertial frame of reference IS in fact a rest frame.
 
  • #21
Orodruin
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I'm not sure what you mean by "rest system" but every inertial frame of reference IS in fact a rest frame.
I believe he means to say that there does not need to be an object that is at rest in every inertial frame. That much is true, but it does not answer the question of the OP, which has already been answered above.
 
  • #22
phinds
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I believe he means to say that there does not need to be an object that is at rest in every inertial frame. That much is true ...
Yes, I agree w/ that.
 
  • #23
Mister T
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This is my point!

Words matter and there's no such thing as at rest.
There's no way to distinguish between a state of rest and a state of uniform motion, so the two are equivalent. If there's no such thing as at rest then there's no such thing as uniform motion. You can always find a new name for the thing, if that's your preference.
 
  • #24
PeterDonis
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Words matter and there's no such thing as at rest.
No, there's no such thing as absolute rest. "Rest" in a particular frame of reference is perfectly well defined and is used all the time in physics.

saying something is at rest implies that it's in a singular reference frame
No, it doesn't. That has already been addressed in this thread.
 
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  • #25
PeterDonis
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When you hear particles it implies particles of sand or particles of salt.
Maybe it does to you, but it doesn't to the physicists who use the term.

Basically you want scientists to always use words the way your ordinary naive untutored intuition says they should be used. That's not going to happen. You need to retrain your intuition if you want to understand how scientists use words; there are good reasons why words don't always have the same meanings in scientific discourse as they do in ordinary language.

The truth is nothing is at rest and everything is in constant motion. It's a blurred worldline of events that are in the past, present and future simultaneously. We're just wired to see these blurred out events in a continuous rate that allows us to perceive a distinction between past, present and future.
At this point you've degenerated into personal theory. Your actual physics question has been addressed. Thread closed.
 

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