# I Can Spacetime be a Reference Frame?

1. Jan 20, 2017

### Arman777

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

2. Jan 20, 2017

### phinds

NEVER take these pop-sci things seriously.
It cannot
Any coordinate system you choose is by definition a reference frame.

3. Jan 20, 2017

### Arman777

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. Jan 20, 2017

### phinds

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.

5. Jan 20, 2017

### Arman777

I see ok

6. Jan 20, 2017

### QuantumQuest

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. Jan 20, 2017

### Arman777

Ok then,If skater spins its spins to what ?

8. Jan 20, 2017

### Bandersnatch

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.

is then: 'she spins w/r to any inertial reference frame'.

9. Jan 20, 2017

### Staff: Mentor

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.

10. Jan 20, 2017

### Arman777

Theres nothing in space (Its empty).She cant spin to any inertial referance frame.Theres no referance frame at all.And we said theres 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 theres no object,theres no velocity.so theres no inertial referance 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 ?

I didnt understand this part

11. Jan 20, 2017

### jbriggs444

Reference frames are mental constructs we conjure up to analyze a situation. They have no physical existence and need none.

12. Jan 20, 2017

### Arman777

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. Jan 20, 2017

### jbriggs444

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. Jan 20, 2017

### A.T.

Seems you are asking about is the Machian principle. Use the search function to find many threads on this.

15. Jan 20, 2017

### Arman777

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

2-We can be a referance frame for sure.But I dont think we can be an inertial referance frame ? Is this true ?

16. Jan 20, 2017

### A.T.

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. Jan 20, 2017

### phinds

I think that saying "for example, non rotating", while correct, could be confusing and it would be better to say "non-accelerating".

18. Jan 20, 2017

### Arman777

I see your point ok but I am open further discussions

19. Jan 20, 2017

### phinds

I don't really see why. It seems to me that jbriggs nailed it:

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

20. Jan 20, 2017

### Arman777

Yeah ok just I dont know maybe a question rises again.

I understand the main idea

21. Jan 20, 2017

### ZapperZ

Staff Emeritus
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. Jan 20, 2017

### Arman777

Haha I will. If he replies (probably he will not ), I ll share it in here.Lets see the answer

23. Jan 20, 2017

### Staff: Mentor

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. Jan 20, 2017

### Arman777

Ohhh I see now.Thanks ( I dont quite now the metric but I understand the main idea )