# The Naked Scientists Forum

### Author Topic: If the galaxy is expanding, how do you measure objects using the speed of light?  (Read 2231 times)

#### thedoc

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##### If the galaxy is expanding, how do you measure objects using the speed of light?
« on: 20/03/2014 20:30:01 »
Question regarding speed of light and distance to stars.

Please correct me: If a star or a galaxy is, letīs say, 2.5 million light years from the Earth, its light will take 2.5 million light years to reach us.

But under that period of time, the universe would have expanded and the origin of that light would be further away from us.

How does this affect the calculations of the astronomers regarding the distance of celestial objects?

Thank you for the programme
What do you think?
« Last Edit: 20/03/2014 20:30:01 by _system »

#### Soul Surfer

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##### Re: If the galaxy is expanding, how do you measure objects using the speed of light?
« Reply #1 on: 20/03/2014 23:54:12 »
You can only measure the distance to an object when the light from it has reached you the object is of course further away because of the time this takes.  this is what relativity is all about.

#### Pmb

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##### Re: If the galaxy is expanding, how do you measure objects using the speed of light?
« Reply #2 on: 21/03/2014 11:25:57 »
Question regarding speed of light and distance to stars.

Please correct me: If a star or a galaxy is, letīs say, 2.5 million light years from the Earth, its light will take 2.5 million light years to reach us.

But under that period of time, the universe would have expanded and the origin of that light would be further away from us.

How does this affect the calculations of the astronomers regarding the distance of celestial objects?

Thank you for the programme
What do you think?
Astronomers measure the distance to distant objects by using Hubble's law. The further away the source is then the faster it's moving away. Hubble's law relates these variables and they therefore measure the distance by using that relation.

#### Bill S

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##### Re: If the galaxy is expanding, how do you measure objects using the speed of light?
« Reply #3 on: 24/03/2014 22:12:40 »
Alfonso, you may find the chart at this link will help.

http://www.einsteins-theory-of-relat...TabCosmo7.html
« Last Edit: 24/03/2014 22:16:32 by Bill S »

#### Bill S

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##### Re: If the galaxy is expanding, how do you measure objects using the speed of light?
« Reply #4 on: 24/03/2014 22:19:48 »
I couldn't get that link to work; hopefully this will be better.

http://www.einsteins-theory-of-relativity-4engineers.com/TabCosmo7.html

#### evan_au

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##### Re: If the galaxy is expanding, how do you measure objects using the speed of light?
« Reply #5 on: 25/03/2014 18:57:56 »
Astronomers think that the galaxy is so strongly bound by its own gravity that it won't be measurably expanding due to the expansion of the universe.

Similarly, our local cluster of galaxies is strongly bound by gravity so that it isn't expanding, either. In fact, one of our larger neighbors, the Andromeda galaxy is actually moving in our direction at about 300 km/second, and is expected to collide with our galaxy in about 4 billion years.

Andromeda, at a distance of about 2.5 million light years fits your description quite well. For a galaxy at this close distance (astronomically speaking), it is possible to pick out some bright variable stars. Because we know the period and brightness of these stars in our own galaxy, we can estimate the distance to Andromeda.

This distance measurement applies at the time the measurement was made, on light arriving on Earth; the variable stars in Andromeda may now have quite different behavior. But the measurement is still a valid measurement, because the speed of approach is <.1% of the speed of light, so the distance has not changed much in the time that it took the light to arrive.

However, if you look at the most distant visible objects in the universe, whose redshift suggests that they are receding from us at a significant fraction of the speed of light: Their position has changed dramatically since the light was emitted. Again the quoted distance is based on light observed now, and does not try to estimate the distance to the actual object now (although a cosmologist could plug the reported distance into her favourite model of dark energy, and come up with an estimate of its distance and velocity now).

Note: Einstein's General Theory of Relativity suggests that the concept of "now" does not mean very much when you are discussing distant objects traveling at a relative speed amounting to a significant fraction of the speed of light.
« Last Edit: 25/03/2014 20:42:33 by evan_au »

#### yor_on

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##### Re: If the galaxy is expanding, how do you measure objects using the speed of light?
« Reply #6 on: 26/03/2014 11:03:36 »
There are two types of redshift to something moving away from you, you measuring its light. One is the 'real motion' of it, if you think of it as a wave then the wave gets stretched out, as some undulating rope getting stretched attached to the object moving away. The other type of redshift is the one created by the expansion of the universe in where this stretching (expanding?) should take place in each point of the wave/rope. Or you can think of it as energy, and then assume as two objects move from each other the energy connecting them to each other loses energy. There it doesn't matter how they move away from each other I think. I think the last is the better description, it shouldn't matter how they get 'split apart' for it. Soulsurfer got it right, you can only measure on what you get, the rest is theoretical evidence.

#### The Naked Scientists Forum

##### Re: If the galaxy is expanding, how do you measure objects using the speed of light?
« Reply #6 on: 26/03/2014 11:03:36 »