Who knows

[Oldboy913]

If time is relative to speed, and all the parts of the universe are going at different speeds, how can we be sure of the distance to any other part of the universe? As we refer to light years.

[wildwood]

I believe that the speeds of heavenly objects are calculated from comparison with known points. I read about a quaser recently that was calculated to be moving at faster than lightspeed but it was just that Earth and the quasar are not moving in the same direction (sorry, can't find the link now).

What I can not comprehend is just what is out there. The nearest galaxy is 2.2 LY away and is one of countless millions. Could some of these newly discovered bodies be so far away that they've been there all the time but its light has only just reached us?

[Loosehead]

a light year is a measure of distance, not time. Time dialation is relative to relative speed. Eg it is 4 light years to the closest star, the relative speed is pretty well 0 between the 2 stars as they maintain the same position in the Galaxy relative to each other.

[Teddio]

For very distant objects such as galaxies and quasars the Red Shift of the light is calculated using a spectrometer. Spectral absorption lines obtained in the laboratory are compared with the same absorption lines in the distant object. The greater the Red Shift the greater the distance. The Hubble Constant provides a connection between the recession velocity of a galaxy and the distance from us. Its value is approximately 70 km/second per Megaparsec. This means that for a distance of 3.3 million light years the object will recede at 70km/second relative to us. This recession velocity is obtained via the Red Shift measurement hence the distance can be found.

[kujawski]

I Thought nothing apart from light could travel at the speed of light let alone faster than, that includes quasars ( i dont even know what one of them is), so some one please explain

[mibn2cweus]

Distance is relative as well. Both time and distance must specify a reference frame from which both are measured and only by virtue of the common reference of the constant velocity of light is complete agreement attainable when two points in space are in motion relative to each other, especially as the velocity of relative motion between them approaches the velocity of light.

[mibn2cweus]

kujawski, To put it in the simplest terms, the universe is expanding. Since the time the light first embarked on its journey towards us the universe has grown in size therefore the apparent paradox. This expansion is expressed in the wavelength of light shifting toward the red end of the colour spectrum. The individual photons are throughout their journey through space and time continuously traveling at the velocity of light.

[Loosehead]

Kujawski: They are not travelling faster than light but the parting speed can exceed light. eg if object A is moving at 75% C and Object B is moving at 75% C in the opposite direction then the parting speed is 1.5C but neither are exceeding C. That is what wildwood is getting at.

[jake-the-peg]

I see nobody's actually yet explained how we know how far things are away as opposed to what measurements we make.

That's a shame because it's a great story.

Astronomers tend to use parsecs rather than light years. If you hold up a finger and wink one eye then another it appears to move against the background far away. This is called paralax and we use it to tell the distance to the closest stars. Over 6 months the nearset stars seem to move a little. 1 second of arc movement is 1 parsec - it's about 3.26 light years.

In order to find the distance to the nearest galaxies Hubble needed to find a certain type of star - a Cepheid variable.

Cephid variables are special because they vary in brightness and crucially their brightness is related to the period of the variations.

That meant that when Hubble found one in the Andromeda galaxy he could measure it's period and know how bright it would be if seen from 10 parsec away. He knew how bright it seemed from Earth so he could work out it's distance a staggering 780 Kilo Parsecs or 2.5 Million light years in old money

[Robb Phoenix]

1 parsec is the distance in which the distance of earth from the sun subtends an angle of 1 arc second
1 arc second = 0.000277777778 degree
1parsec=3.08568025x10*16
1 light year = 9.4605284 � 10*15 meters
Time dilation occurs only when objects with mass travel close to light speed (relative to the object) and whenever anything travels faster than light.Time became relative (along with distance) when it was shown speed of light is an absolute constant.
The universe expands more or less uniformly so even if the velocities are different acceleration is uniform.
btw Quasars (quasi stellar radio sources) are the most distant and thus fastest moving in the universe.These objects are the very bright centers of some distant galaxies, where some sort of energetic action is occurring, most probably due to the presence of a supermassive black hole at the center of that galaxy .