Understanding Relativity Concepts in Interstellar Space

[rg414]

Hello everyone, I'm new to the forum and I'm starting to learn about Relativity. I'm confused about a couple of concept that I've recently been reading up on. If two non-rotating spaceships are floating freely in interstellar space well outside the influence of any other objects in the universe, then both spaceships must be in the same inertial frame or does the distance between the two spaceships must change at a constant rate. That part of relativity is a little confusing to me right now.

One more concept that is a a little fuzzy to me in general relativity is Black Holes and Gravitational waves from supernovae, based on experimental observations what has not been used to confirm one or another aspect of general relativity? This seems relatively simple, I don't think gravitational waves have been detected from a supernovae?

Hopefully someone can clear up this basic concept confusions, I'm having. I'm trying to build a strong foundation.

I look forward to staying a part of the forum, and contributing in the future.

 

[bcrowell]

Hi, rg414,

Welcome to Physics Forums!

Hello everyone, I'm new to the forum and I'm starting to learn about Relativity. I'm confused about a couple of concept that I've recently been reading up on. If two non-rotating spaceships are floating freely in interstellar space well outside the influence of any other objects in the universe, then both spaceships must be in the same inertial frame or does the distance between the two spaceships must change at a constant rate. That part of relativity is a little confusing to me right now.

There is no real concept that an object is "in" an inertial frame. A measurement or a statement *about* an object is what is "in" an inertial frame. So let's say the question is this: "If two non-rotating spaceships are floating freely in interstellar space well outside the influence of any other objects in the universe, then must both spaceships be at rest with respect to one another?" The answer is no. Either spaceship could be moving at constant velocity relative to the other, as allowed by Newton's first law.

One more concept that is a a little fuzzy to me in general relativity is Black Holes and Gravitational waves from supernovae, based on experimental observations what has not been used to confirm one or another aspect of general relativity? This seems relatively simple, I don't think gravitational waves have been detected from a supernovae?

There is strong empirical evidence for black holes, especially the supermassive one at the center of our galaxy. However, nobody has resolved the event horizon of a black hole, which would really be the smoking gun.

There is also strong empirical evidence for gravitational waves, from the Hulse-Taylor binary pulsar. However, work is still under way to detect gravitational waves directly.

-Ben

 

[bobc2]

Some physicists like to think of the Special Relativity effects simply as a consequence of the 4-dimensional structure of the universe. The upper left sketch represents two spaceships floating away from each other out in empty space. The motions of the red and blue ships are seen to be a manifestation of the orientation of the 4-dimensional ships in 4 dimensions. Each observer (blue and red) is living in a different 3-D cross-section view of the 4-D universe. At position 9 the blue guy's 3-D universe includes a 3-D cross-section view of the red rocket at position 8 (it looks to the blue guy that the red guy's clock is slow). However, when the red guy is at his position 9, his 3-D cross-section of the 4-D universe includes a 3-D cross-section of the blue rocket at the blue position 8 (it looks to the red guy that the blue guy's clock is slow). That's time dilation.

The length contraction is illustrated in the lower left sketch. At some instant of time the 3-D cross-section of the 4-D universe gives him a world in which the red rocket appears to be shorter than his own blue rocket.

The upper right sketch just presents a short graphical derivation of the time dilation equation. You need only the high school level Pythagorean theorem to develop this equation. Since the speed of light is a constant, c, it is implied that all observers move along their own X4 dimension at the speed of light (X4 = ct).

 

[ghwellsjr]

Hello everyone, I'm new to the forum and I'm starting to learn about Relativity. I'm confused about a couple of concept that I've recently been reading up on. If two non-rotating spaceships are floating freely in interstellar space well outside the influence of any other objects in the universe, then both spaceships must be in the same inertial frame or does the distance between the two spaceships must change at a constant rate. That part of relativity is a little confusing to me right now.
...
Hopefully someone can clear up this basic concept confusions, I'm having. I'm trying to build a strong foundation.

I look forward to staying a part of the forum, and contributing in the future.

In Special Relativity, every object that you want to consider is equally "in" every inertial frame of reference that you want to consider. If the distance between the two spaceships is not changing, then you can define an inertial frame of reference in which they are both stationary. You could also define an inertial frame of reference in which they are both traveling at any speed in any direction. Frames of reference are for our convenience in defining and analyzing problems and scenarios. They are merely spatial co-ordinate systems with time added.

If the distance between the two spaceships is changing at a constant rate, then you cannot come up with an inertial frame in which both spaceships are at rest, but you could do this for either one of them and then the other one will be in motion at a constant rate. Or you could pick the frame of reference that is "half way" between them and then they will each be traveling in the opposite direction at a constant speed (but not at one-half the relative speed, you have to use the relativistic addition of velocity formula to figure out what the exact speed will be). Or you could pick any other frame of reference with both spaceships moving at different speeds and different directions.

The important thing is that you should "view" both spaceships "in" the same inertial frame of reference, whichever one you want.