Inertial frames in special relativity

In summary, the definition of an inertial frame in special relativity is that it is the frame of reference of a freely moving observer.
  • #1
JustinLevy
895
1
What is used to define an inertial frame in special relativity?
Do I need to take one of the postulates as defining an inertial frame?

What is bothering me is that I used to use Newton\'s first law to define inertial frames (a freely moving object will have a constant velocity). This can not be the definition in SR because I could use the galilean transformations and Newton\'s first law would still hold (yet these are not inertial frames according to SR).

So how do you define an inertial frame in SR?
 
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  • #2
JustinLevy said:
What is used to define an inertial frame in special relativity?
Do I need to take one of the postulates as defining an inertial frame?

What is bothering me is that I used to use Newton\'s first law to define inertial frames (a freely moving object will have a constant velocity). This can not be the definition in SR because I could use the galilean transformations and Newton\'s first law would still hold (yet these are not inertial frames according to SR).

So how do you define an inertial frame in SR?
An inertial frame in SR is the same as in Newtonian mechanics: the frame of reference of an observer who is experiencing no external force (dp/dt=0).

AM
 
  • #3
Andrew Mason said:
An inertial frame in SR is the same as in Newtonian mechanics: the frame of reference of an observer who is experiencing no external force (dp/dt=0).
Specifying a freely moving observer only specifies one point (an origin if you will), but not an entire coordinate system. So unfortunately, that does not answer the question.

As I explained above, there are an infinite number of coordinate systems in which Newton\'s first law is true, yet the coordinate system is not an inertial one. You merely picked one of these freely moving points. If Newton\'s first law is not sufficient to define an inertial frame, definitely picking just one point will not be.


The more I think about it, I believe the answer is: an inertial frame in SR is defined by the Minkowski metric. Which in essence means the second postulate of SR is used to define an inertial frame.

Is this correct? Or are there other ways of looking at it?
 

1. What are inertial frames in special relativity?

Inertial frames in special relativity are frames of reference in which an object is at rest or moving at a constant velocity. They are used to describe the motion of objects in the absence of any external forces.

2. How do inertial frames relate to the theory of relativity?

Inertial frames are an important concept in the theory of relativity because they provide a way to measure and compare the motion of objects in different frames of reference. This allows for the principles of relativity, such as the constancy of the speed of light, to be applied.

3. Can there be more than one inertial frame in a given situation?

Yes, there can be multiple inertial frames in a given situation. In fact, any frame of reference that is moving at a constant velocity relative to another frame can be considered an inertial frame.

4. How does special relativity change our understanding of inertial frames?

Special relativity introduces the concept of spacetime, where space and time are intertwined. This means that the measurement of an object's motion in an inertial frame can vary depending on the observer's relative motion. This is in contrast to classical mechanics, where time and space are treated as separate entities.

5. What is the principle of relativity in relation to inertial frames?

The principle of relativity states that the laws of physics are the same in all inertial frames. This means that the fundamental laws and principles governing the behavior of objects, such as the laws of motion, are the same regardless of the observer's frame of reference.

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