Can Spacetime be a Reference Frame?

In summary: As most of the posters indicated, pop sci sources are misleading if you want anything more than a superficial understanding.
  • #1
Arman777
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I was watching one of the documentaries of Brian Greene (Beyond Cosmos).

here at (12:00-12:10) he says.The skater spins respect to space.

My question is How can space-time can be a referance frame itself ?
Is it means we can just pick a coordinate and make it a referance frame ? (Sounds wrong to me )

Thanks
 
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  • #2
Arman777 said:
I was watching one of the documentaries of Brian Greene
NEVER take these pop-sci things seriously.
My question is How can space-time can be a referance frame itself ?
It cannot
Is it means we can just pick a coordinate and make it a referance frame ? (Sounds wrong to me )
Any coordinate system you choose is by definition a reference frame.
 
  • #3
So any point on space-time can be a referance frame, but space-time itself cannot be a referance frame ?

Then what he is talking about ?
 
  • #4
Arman777 said:
So any point on space-time can be a referance frame, but space-time itself cannot be a referance frame ?

Then what he is talking about ?
I have no idea what he is talking about. I learned long ago NOT to watch Brian Green. One of our moderators here commented once that Green's pop-sci shows cause more confusion than those of any other pop-sci person. Personally I think Kaku is worse, but it's close.
 
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  • #6
Arman777 said:
So any point on space-time can be a referance frame, but space-time itself cannot be a referance frame ?

Then what he is talking about ?

As phinds says, stay away from popsci things if you want to learn. They many times state things that although are correct in essence - not always, they're stated in an overly abstract, unspecified and unjustified manner, cutting corners that cannot be cut.

For the video you mention (12:00 - 12:10), don't take what he is saying at its face value. He is talking about space in the abstract.
 
  • #7
Ok then,If skater spins its spins to what ?
 
  • #8
Seems like the video at that mark is trying to convey the idea that you can always tell whether you are or aren't in an inertial reference frame, i.e. whether you are spinning or not - you can tell this by looking for fictitious forces, like centrifugal or Coriolis force. Setting up a Foucault pendulum is one good way of doing that.
So rotational motion is not relative in the above sense. There is a preferred type of frame - an inertial frame.
This is in contrast to linear motion, which is always relative. You can set up an infinite number of inertial reference frames, all of which are equivalent.

The answer to
Arman777 said:
Ok then,If skater spins its spins to what ?
is then: 'she spins w/r to any inertial reference frame'.
 
  • #9
Arman777 said:
Is it means we can just pick a coordinate and make it a referance frame ? (Sounds wrong to me )
As most of the posters indicated, pop sci sources are misleading if you want anything more than a superficial understanding.

There is a somewhat subtle distinction between a reference frame and a coordinate system, but for most discussions you can consider them the same. Modern physics likes to write the laws of physics in a form that is independent of the coordinate system. This makes it so that if you are analyzing a system with spherical symmetry then you can use spherical coordinates and if you are analyzing a system with some other symmetry you can use some other coordinates. Either way you use the same laws of physics.

The key mathematical object that allows this is the metric. This is the thing that relates the coordinates to physical distances and durations. If you use coordinates that are "rotating" then the metric has terms that don't show up in inertial frames, and those are measurable, E.g. with accelerometers.
 
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  • #10
Bandersnatch said:
is then: 'she spins w/r to any inertial reference frame'.

Theres nothing in space (Its empty).She can't spin to any inertial referance frame.Theres no referance frame at all.And we said there's no object except us but we need an object to define an inertial referance frame cause as Newton pointed out the referance frame that we took must have a constant velocity for an observer.If there's no object,theres no velocity.so there's no inertial referance frame.

phinds said:
Any coordinate system you choose is by definition a reference frame.

So we pick an arbitrary point on space-time.That point belongs to space-time itself.Isnt that mean space time can be a referance frame ?

Dale said:
If you use coordinates that are "rotating" then the metric has terms that don't show up in inertial frames, and those are measurable, E.g. with accelerometers.

I didnt understand this part
 
  • #11
Arman777 said:
Theres nothing in space (Its empty).She can't spin to any inertial referance frame.Theres no referance frame at all.And we said there's no object except us but we need an object to define an inertial referance frame cause as Newton pointed out the referance frame that we took must have a constant velocity for an observer.If there's no object,theres no velocity.so there's no inertial referance frame.
Reference frames are mental constructs we conjure up to analyze a situation. They have no physical existence and need none.
 
  • #12
jbriggs444 said:
Reference frames are mental constructs we conjure up to analyze a situation. They have no physical existence and need none.

I know that.I am just trying to say we need an object to and a point on that object to make an inertial referance frame
 
  • #13
Arman777 said:
I know that.I am just trying to say we need an object to and a point on that object to make an inertial referance frame
You have one. The person. That is enough to define a state of motion. If the person is subject to stresses and if those stresses vary depending on the way she waves her hands, that is enough to detect a state of rotation.

If you have no person then there is no question to ask or answer and no worry about whether there is or is not a reference frame within which to analyze... nothing.
 
  • #14
Arman777 said:
Theres nothing in space (Its empty).She can't spin to any inertial referance frame.Theres no referance frame at all.
Seems you are asking about is the Machian principle. Use the search function to find many threads on this.
 
  • #15
A.T. said:
Seems you are asking about is the Machian principle. Use the search function to find many threads on this.

Not really I guess.I just wonder the know basic facts about inertial referance frames.If its includes mach principle I don't know
1-I wrote some questions above and I am still waiting the answers

jbriggs444 said:
You have one. The person. That is enough to define a state of motion. If the person is subject to stresses and if those stresses vary depending on the way she waves her hands, that is enough to detect a state of rotation.

If you have no person then there is no question to ask or answer and no worry about whether there is or is not a reference frame within which to analyze... nothing.

2-We can be a referance frame for sure.But I don't think we can be an inertial referance frame ? Is this true ?
 
  • #16
Arman777 said:
I just wonder the know basic facts about inertial referance frames.If its includes mach principle I don't know
It's basically about whether the inertial frames (e.g. non-rotating) are defined by the matter in the universe. We cannot really tell, because we cannot remove all that matter to test it.
 
  • #17
A.T. said:
It's basically about whether the inertial frames (e.g. non-rotating) are defined by the matter in the universe. We cannot really tell, because we cannot remove all that matter to test it.
I think that saying "for example, non rotating", while correct, could be confusing and it would be better to say "non-accelerating".
 
  • #18
A.T. said:
It's basically about whether the inertial frames (e.g. non-rotating) are defined by the matter in the universe. We cannot really tell, because we cannot remove all that matter to test it.

I see your point ok but I am open further discussions
 
  • #19
Arman777 said:
I see your point ok but I am open further discussions
I don't really see why. It seems to me that jbriggs nailed it:

jbriggs444 said:
If you have no person then there is no question to ask or answer and no worry about whether there is or is not a reference frame within which to analyze... nothing.
He could just have well said "matter" instead of "person".
 
  • #20
phinds said:
I don't really see why. It seems to me that jbriggs nailed it:

He could just have well said "matter" instead of "person".

Yeah ok just I don't know maybe a question rises again.

I understand the main idea
 
  • #21
Here's what I suggest you do.

Write to him (http://physics.columbia.edu/people/profile/406) with a link to the video and the exact time, and then ask him directly! Tell him

"Professor Greene,

I found this video and am trying to understand what you mean when you said "...""

Zz.
 
  • #22
Haha I will. If he replies (probably he will not ), I ll share it in here.Lets see the answer
 
  • #23
Arman777 said:
I didnt understand this part
The metric for an inertial frame is ##ds^2=-dt^2+dx^2+dy^2+dz^2##. The metric for a rotating reference frame is ##ds^2 = -(1-\omega^2(x'^2 + y'^2)) dt'^2 + 2\omega(-y' dx' dt' + x' dy' dt') + dx'^2 + dy'^2 + dz'^2##. Those extra terms are measurable.
 
  • #24
Dale said:
The metric for an inertial frame is ##ds^2=-dt^2+dx^2+dy^2+dz^2##. The metric for a rotating reference frame is ##ds^2 = -(1-\omega^2(x'^2 + y'^2)) dt'^2 + 2\omega(-y' dx' dt' + x' dy' dt') + dx'^2 + dy'^2 + dz'^2##. Those extra terms are measurable.
Ohhh I see now.Thanks ( I don't quite now the metric but I understand the main idea )
 

1. Can spacetime be considered a reference frame?

Yes, according to Einstein's Theory of Relativity, spacetime can be considered a reference frame. In this theory, spacetime is a four-dimensional concept that combines the three dimensions of space with the dimension of time.

2. How does spacetime act as a reference frame?

Spacetime acts as a reference frame by providing a way to measure the position, velocity, and acceleration of objects in the universe. It also allows for the calculation of distances and durations between events.

3. Is spacetime the only reference frame in the universe?

No, there are other reference frames in the universe, such as inertial frames and non-inertial frames. However, spacetime is a universal reference frame that applies to all objects and events in the universe.

4. Can spacetime be distorted or affected by objects in the universe?

Yes, according to the theory of General Relativity, massive objects can distort spacetime, causing the curvature of space. This distortion affects the motion of objects and the passage of time in that region of spacetime.

5. How does the concept of spacetime help us understand the universe?

The concept of spacetime helps us understand the universe by providing a framework for understanding how objects and events interact and move in the universe. It also helps us explain phenomena such as gravity and the behavior of light.

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