Ok... not totally sure how to do this/post on a different thread... I'm probably just being incompetent with technology again. But here goes:
Personally, I'm interested to know how we define locations in space, considering it seems to me that everything is relative. It seems that location is an important thing to astronomers, as it allows for us to make measurements and collect knowledge in databases, but it all seems arbitrary to me. How do we know where other galaxies are, except relative to us? More importantly, if the universe is expanding, how can we ever define a location if it is changing?
I remember Prof Djorgovski said something about having a spacetime grid, but I'm not exactly sure how it's implemented. Do astronomers use the simple Cartesian coordinate grid or something a lot more complex?
Well, we use spherical coordinate systems, naturally. They can be transformed from one to the other, and the choice of the center depends on your purposes: geocentric, heliocentric, galactocentric... Separating the radial distance (tricky to measure) from the apparent coordinates on the cellestial sphere helps.
Things get more complicated in cosmology, where there are two kinds of coordinates: comoving (expand with the expanding space) and proper (fixed physical sizes, relative to which the space expands).
Objects in the Solar system move relatively fast in terms of an apparent motion projected on the cellestial sphere, so their coordinates change accordingly, and we know how to compute that with an exquisite precision. Stars also move in space, but the projected motions are tiny. We still know how to measure them and account for them. Galaxies more way too slowly on the cellestial sphere for it to be detectable. Notice that we are talking about the apparent motions here, not the spatial speeds.
My intellectual two cents: Is there some point at the edge of the universe where galaxies cannot hold themselves together? If space is constantly expanding, then is there some point where the gravitational force in galaxies is overcome by a greater force of the expanding universe? And to just tack on to Chloe's question, how did Google sky even come to be remotely accurate?
First of all, you can now make new posts yourself, instead of just entering a comment here.
I sense a considerable amount of a conceptual confusion here, e.g., the universe does not have an edge, and the expansion is not due a force. And the coordinates business is not really mysterious. Maybe we should tackle these at the seminar today. These are important concepts, so all of you should get a clear understanding of them.
Anyway, please post more questions as they occur to you (as separate posts).
Ok... not totally sure how to do this/post on a different thread... I'm probably just being incompetent with technology again. But here goes:
ReplyDeletePersonally, I'm interested to know how we define locations in space, considering it seems to me that everything is relative. It seems that location is an important thing to astronomers, as it allows for us to make measurements and collect knowledge in databases, but it all seems arbitrary to me. How do we know where other galaxies are, except relative to us? More importantly, if the universe is expanding, how can we ever define a location if it is changing?
Chloe
I remember Prof Djorgovski said something about having a spacetime grid, but I'm not exactly sure how it's implemented. Do astronomers use the simple Cartesian coordinate grid or something a lot more complex?
ReplyDeleteWell, we use spherical coordinate systems, naturally. They can be transformed from one to the other, and the choice of the center depends on your purposes: geocentric, heliocentric, galactocentric... Separating the radial distance (tricky to measure) from the apparent coordinates on the cellestial sphere helps.
ReplyDeleteThings get more complicated in cosmology, where there are two kinds of coordinates: comoving (expand with the expanding space) and proper (fixed physical sizes, relative to which the space expands).
Objects in the Solar system move relatively fast in terms of an apparent motion projected on the cellestial sphere, so their coordinates change accordingly, and we know how to compute that with an exquisite precision. Stars also move in space, but the projected motions are tiny. We still know how to measure them and account for them. Galaxies more way too slowly on the cellestial sphere for it to be detectable. Notice that we are talking about the apparent motions here, not the spatial speeds.
We can talk more about this on Tuesday.
My intellectual two cents:
ReplyDeleteIs there some point at the edge of the universe where galaxies cannot hold themselves together? If space is constantly expanding, then is there some point where the gravitational force in galaxies is overcome by a greater force of the expanding universe?
And to just tack on to Chloe's question, how did Google sky even come to be remotely accurate?
First of all, you can now make new posts yourself, instead of just entering a comment here.
ReplyDeleteI sense a considerable amount of a conceptual confusion here, e.g., the universe does not have an edge, and the expansion is not due a force. And the coordinates business is not really mysterious. Maybe we should tackle these at the seminar today. These are important concepts, so all of you should get a clear understanding of them.
Anyway, please post more questions as they occur to you (as separate posts).