# B Relative maps?

1. Sep 22, 2016

### Brian E

As I understand relativity, length gets contracted when speed raises. Does that imply that the maps we make of the universe actually describes the universe seen from our perspective, and that these maps would have looked different in size for an observer moving at different speed??

2. Sep 22, 2016

### Ibix

Yes and no. Any observer could note that there is a particular choice of velocity at which the universe is isotropic, and would probably choose to make maps from that perspective - same as we do. It's not required, though, just as there's no requirement for north to be up on a geographic map, and you can draw different maps and convert from one to another easily enough.

I should point out that this isn't quite due to length contraction, which is a special relativistic phenomenon. You need general relativity to describe the universe accurately. But it is a related concept.

3. Sep 22, 2016

### Brian E

Thank you for the answer :-)

As I understand your answer, one can draw observer dependent maps if one insists, but that it really wouldn't change anything if one konws relativity...

What confused me, and kind of still does is the following nonsense ;-)

If I and a particle were to compete in the same 100 m race on the same track with me running really slow, and the particle travelling near the speed of light. Then as I understand it, the length of the track the particle travelled ,would get contracted from my perspective. That would, in my silly mind, imply two very different "realities" .

I do see a problem in the fact that a lot of particles pass me everyday and that I havent noticed anything really wierd yet,

4. Sep 22, 2016

### Ibix

You are getting confused. Basically, anything moving with respect to you will be length contracted (according to your measurements) by an amount dependent on its speed. This means that the track is contracted from the particle's perspective, since the track is moving near lightspeed according to the particle. But it is not significantly contracted from your perspective (at a 10m/s sprint I make a 100m track to be on the order of one ten thousandth of an atomic diameter shorter). I'm not sure particles have anything you could call a length to contract, but you would certainly measure the distance between two particles moving at the same speed as length contracted.

This may seem a little contradictory, but it's not. You also need to take into account time dilation and the relativity of simultaneity in order to construct a completely coherent picture of what is going on here.

Also, please be aware that we're talking special relativity here. Which is fine and appropriate, but a bit different from your original post which was about cosmological scales where we needed to invoke general relativity.

5. Sep 22, 2016

### pervect

Staff Emeritus
If the universe worked according to special relativity, we could draw different maps for "moving" observers, as you describe, though I must say we probably wouldn't (assuming there were still some frame in which the universe were isotropic).

But the universe doesn't work according to special relativity, so things are more complicated. The default standard we use when describing distance isn't based on any single observer, but a chain of them. And while nearby observers in the chain have negligible relative motion, there is appreciable relative motion in the chain when you get far enough away.

6. Sep 28, 2016

### GeorgeDishman

The speed of the Solar System is fairly low compared to the speed of light so length contraction isn't significant. Of much more importance is the expansion of the universe. For nearby galaxies it is again small but at larger scales it would badly distort distances. In fact earlier than 9.7 billion years ago, we see objects closer than those we see at more recent times.

The usual convention is that we map them in "proper distance" which means how far away we think they are now. That is, we plot their observed distance multiplied by the amount by which the universe has expanded since the time when the light we see left them.

7. Sep 28, 2016

### Staff: Mentor

This statement needs clarification. What particular observations are you describing by "see objects closer"?

8. Sep 28, 2016

### GeorgeDishman

I was referring to angular size distance which reduces for redshift above about z=1.6 (based on the Planck 2013 parameters).