11 Jun 08
Relative to what? All motion is relative, however, in our normal day to day life we measure all speeds relative to a common frame of reference (the Earth's surface / in the rotating frame of the Earth , or however you care to describe it).
Anyway, my point is you need to specify the relative motion between two frames, there is no such thing as an absolute speed.
12 Jun 08
Originally posted by MattPSounds to me like your over-analyzing this. Since we are approximately 93 million miles away from the sun (a radius), can't we use that to figure out our orbit's length (I know it's elliptical, not circular; I'm just saying we CAN approximate fairly easily) using 2 pi r squared? Then divide that by 365.25 x 24 (the number of hours in a year) and voila! We have the (very approximated) speed of the earth through space!
Relative to what? All motion is relative, however, in our normal day to day life we measure all speeds relative to a common frame of reference (the Earth's surface / in the rotating frame of the Earth , or however you care to describe it).
Anyway, my point is you need to specify the relative motion between two frames, there is no such thing as an absolute speed.
12 Jun 08
Originally posted by eamon oHow fast we're going through the universe, does that mean speed? We should use velocity.
how fast are we travelling given our speed of rotation around the sun + the movement through space of the solar system + the big bang effect etc etc ?
Velocity is a vector, both speed and direction should be accounted for. We can't add speed together because certain speeds cancel out each other if the go in opposite direction.
Think of what will happen if you go through a slow train. If you go 10 km/h back in train, and the train go 10 km/h forward, then with the respect of the ground, you would actually stand still. Then you use ground as a referent frame. But if you use the train floor as a referent frame, then you actually go with a 10 km/h speed, disregard the train speed.
Your question was "how fast are we travelling" and I would like to analyze our velocity and its components.
(1) The Earth is spinning around its own axis. Every point has its velocity. If you live at the equator you make a compete revolution in 24 hours. You know the circumference of the Earth, divide this with 24 hours and you get a speed. If you don't live at the equator, then you have to multiply this speed with cosine your latitude to get a real value.
(2) The earth goes around the sun with one orbit in one year. We know the distance from the sun, from that you get the circumference of the orbit. Divide this with the number of hours in one year.
(3) The solar system goes one revolution around the center of our galaxy, the Milky Way, in 250 000 years. We are approx. 30,000 light years from the center of the galaxy. Find the length of the orbit, knowing that one light-year is the distance the light will travel in one year and the speed of light is 300,000 km per second.
(4) The entire galaxy is moving in one direction, don't know where, but I think we are influenced by the gravity of our local galaxy cluster, and in a larger respect our local super cluster of galaxies.
(5) Then the whole Universe is expanding, but not in respect to anything. There is no centre of universe so this speed is meaningless.
Now, I haven't done any calculation of (1) to (5) because it is easy to do them for yourself. You will however notice that the speeds can't be sum up because you don't know the direction of them. The vary with the day, the year, the position in the galaxy etc so if you just sum the speeds, not taken the direction into account, the result will be meaningless, some cancel out, some adds up.
But you will also notice that the speed will increase from (1) to (5) so what happens here on Earth is miniscule compared to what happen in galactic space. So the velocity of the velocity of our galactic super cluster through the universe is prominent. If you don't know this value, you actually don't know anything.
So to calculate "how fast we're travelling" is more or less pointless.
12 Jun 08
1 edit
Originally posted by FabianFnasI agree with your analysis, though it might not infact be true that the velocity of our super cluster relative to other super clusters is greater than that of the local cluster relative to the super cluster.
But you will also notice that the speed will increase from (1) to (5) so what happens here on Earth is miniscule compared to what happen in galactic space. So the velocity of the velocity of our galactic super cluster through the universe is prominent. If you don't know this value, you actually don't know anything.
However I dont see why we couldnt get an approximate figure for the final answer by measuring the redshifts of the light we recieve from distant galaxies (outside our supercluster) and averaging it out by direction. I don't know if our instruments are accurate enough though to get much of an answer.
I beleve the velocity of other superclusters in relation to us is greater due to the expansion of the universe than it is due to the movement of the superclusters through space (hence all distant galaxies appear to be moving away from us).
12 Jun 08
Originally posted by FabianFnasThanks, I take your point about velocity, but I dont agree that to calculate how fast we are travelling is pointless. It is about getting at the truth. I find the concept that we are travelling fast through space very interesting and it would be good to have a number for it. I heard a british female scientist on the radio some months ago giving an approximation of it but I didnt write it down. I guess that many of these vectors need to be factored-in when we plan the trajectory for a space craft on an interplanetary trip. The concept of multiple motion and speed is interesting partly because much of what is really going on is undetectible to our senses, that is why for me the truth of it becomes more intriguing.
How fast we're going through the universe, does that mean speed? We should use velocity.
Velocity is a vector, both speed and direction should be accounted for. We can't add speed together because certain speeds cancel out each other if the go in opposite direction.
Think of what will happen if you go through a slow train. If you go 10 km/h back in tr ...[text shortened]... nything.
So to calculate "how fast we're travelling" is more or less pointless.
12 Jun 08
Originally posted by eamon oBecause it is all relative, for an interplanetary trip we only need to know our velocity relative to the solar system (sun) and the relative veolcities of whichever planets we are aiming for. The velocity of the solar system through the galaxy is irrelevant.
Thanks, I take your point about velocity, but I dont agree that to calculate how fast we are travelling is pointless. It is about getting at the truth. I find the concept that we are travelling fast through space very interesting and it would be good to have a number for it. I heard a british female scientist on the radio some months ago giving an approxi ...[text shortened]... ng on is undetectible to our senses, that is why for me the truth of it becomes more intriguing.
We could even ignore our velocity relative to the solar system and just deal with our velocity relative to a planet we are aiming for thought the calculations are more complicated that way.
To get to near by stars we would again need there velocity relative to us and the velocity of the galaxy relative to other galaxies would be irrelevant.
14 Jun 08
Originally posted by twhiteheadi think thats ok provided the velocities we are factoring are in 3d and take account of rotation, orbit and expansion etc.
Because it is all relative, for an interplanetary trip we only need to know our velocity relative to the solar system (sun) and the relative veolcities of whichever planets we are aiming for. The velocity of the solar system through the galaxy is irrelevant.
We could even ignore our velocity relative to the solar system and just deal with our velocity re ...[text shortened]... ty relative to us and the velocity of the galaxy relative to other galaxies would be irrelevant.
if you had to pre programme a journey with 100% accuracy you would need to take into account tiny influences like the interplay of minute gravitational effects, the big bang expansion, solar rain etc
14 Jun 08
Originally posted by eamon oI don't think the expansion of the entire universe has to do with local velocities. There is no center in the universe that you can see as some kind of a zero point. Or you can see any point of the universe as being the center, even our own position.
i think thats ok provided the velocities we are factoring are in 3d and take account of rotation, orbit and expansion etc.
if you had to pre programme a journey with 100% accuracy you would need to take into account tiny influences like the interplay of minute gravitational effects, the big bang expansion, solar rain etc
14 Jun 08
Originally posted by FabianFnasdepends on the degree of accuracy you are aiming at. i reckon you need to take a hypothetical fixed point to calculate everything accurately
I don't think the expansion of the entire universe has to do with local velocities. There is no center in the universe that you can see as some kind of a zero point. Or you can see any point of the universe as being the center, even our own position.
15 Jun 08
Originally posted by eamon oDepending on what universe center point you chose, you get different velocity due to universe expansion. So the center of universe is not a good idea.
depends on the degree of accuracy you are aiming at. i reckon you need to take a hypothetical fixed point to calculate everything accurately
15 Jun 08
1 edit
Originally posted by dizzyfingersThat may compute the velocity of the Earth around the sun, but that does not take into account of 1) the velocity of the solar system around the Milky Way, which you can compute easily enough, we are about 30,000 LY from the center of the galaxy, and one rev takes about 100,000,000 years. Then there is the velocity of Andromeda and the Milky Way collision and then there is the general velocity of both Andromeda and the Milky Way towards the Virgo Cluster (I think thats the cluster we are crashing into). Around the Milky Way, roughly 100,000 Ly journey in 100,000,000 years= about 1000 years per ly which would be about 186 miles per second or so.
Sounds to me like your over-analyzing this. Since we are approximately 93 million miles away from the sun (a radius), can't we use that to figure out our orbit's length (I know it's elliptical, not circular; I'm just saying we CAN approximate fairly easily) using 2 pi r squared? Then divide that by 365.25 x 24 (the number of hours in a year) and voila! We have the (very approximated) speed of the earth through space!
Not sure of the other velocity vectors but these movement describe a curving corkscrew-like movement through the cosmos. A corkscrew within a corkscrew.
17 Jun 08
I like the velocity relative to the CMB as a reference point. It doesn't define a "center" to the universe that doesn't really exist, it measures our movement relative to our observable patch of the universe.
http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation#Velocity_relative_to_CMB_anisotropy
It gives ~600 km/s, by the way.