# Is "inertial frame of reference" the same as spacetime?

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1. Jan 8, 2016

### Pjpic

Is the inertial frame of reference the same thing as space time?

2. Jan 8, 2016

### Staff: Mentor

No. Reference frames are to spacetime as latitude and longitude line are to the surface of the earth: The surface of the earth is there and we can move across it as we please; the latitude and longitude lines are just conventions for attaching numeric labels to points on that surface.

3. Jan 8, 2016

### Pjpic

If the term "reference frame" is dropped, is inertia the same as spacetime?

4. Jan 8, 2016

### Staff: Mentor

No. Inertia is something completely different - it refers to an object's tendency to keep moving in a straight line if you apply no external forces to it.

5. Jan 8, 2016

### Ibix

Take a simple Euclidean plane. You can draw many coordinate systems on it - for example a Cartesian system with the origin at some point, a Cartesian system with the origin at some other point and rotated with respect to the first, or circular polars with the origin at yet a third point, and many more.

A reference frame is simply a coordinate system. Any one of the ones I listed above is a valid reference frame for a Euclidean plane (and there are many more that I did not list). An inertial reference frame is a particular class of coordinate systems for space time, as all the possible Cartesian systems on the plane are a class of coordinate systems. Inertial reference frames happen to be a natural way to label events in spacetime for observers who are not accelerating, and they happen to make maths with the coordinates relatively easy as well. That's all they are.

To put it another way, imagine a piece of paper. You can turn it into a piece of graph paper by printing a grid of some kind on it. Choosing a reference frame is like choosing which grid to print. Choosing an inertial reference frame is like choosing which Cartesian grid to print. Actually building a network of rods with clocks at each junction (as Einstein spoke about) is like printing the grid on the paper (you don't have to do this to use the coordinates). The paper is spacetime.

What spacetime "actually is" is a matter of philosophy at this point (it's probably not actually a piece of paper). I understand that a proper theory of quantum gravity might shed some light, but that's beyond my level of understanding.

6. Jan 8, 2016

### Pjpic

When star light bends around galaxy would it not be considered a change in inertia because the light is following the natural curve of space?

7. Jan 8, 2016

### phinds

No it would not be considered a change in inertia. A change in inertia would be if it were NOT following the natural "curve" of space time.