# Why don't we define absolute coordinates?

Why don't we chose a random point in the void present between galaxy clusters and define it as the absolute origin?

I know that rest is not absolute and that space expands, but we can easily keep that point at origin and explain the movement of all other bodies in both space and time, if it is acceptable to deal with negative time.

Is it not that useful to define an origin in spacetime coordinates on such a big scale? Or am I missing something?

Addendum : Every answer seems to think I am talking about defining absolute coordinates and then use it for "everything", that is not what I meant. I meant establishing them just as we have made SI units, it does not stop people from using foot, hectare, lightyear, gallons, and other units but it makes it easy to compare with various systems etc.

Similarly I do not intend to say that if we make an absolute standard we would start measuring earthly speeds according to it, I just meant that it may be useful while talking about galaxy clusters or other universal phenomenon. Since we do not have such a system that it is only why I asked why we haven't defined it, I am not at that level in sciences where I would be arguing that motion of ants on earth should be calculated with respect to absolute standards.

This question is just to ask that is such a thing as absolute standards "absolutely" of no use? And if it does find some use why havent we defined it? And should we define it?

• Why a point somewhere in space, why not our galactic center? Or more anthropic, why not earth? May 24, 2014 at 14:40
• As you have already explained, this idea is not convenient and quite arbitrary. May 24, 2014 at 14:42
• @KyleKanos : I suppose chosing earth and similarly galactic center would create mathematical difficulties in explaining the motion of the entire galaxy cluster etc, thats why I preferred to say a void rather than the earth! May 24, 2014 at 14:48
• If we used your point as absolute origin reference, then you'd get one heck of a speeding ticket next time you drive by a cop! May 24, 2014 at 15:56
• @rijulgupta: Placing some random point as "the origin" might be useful to the astrophysicists who model the local cluster, but to everyone else it would create the mathematical difficulties! Since all non-accelerating frames are equal it is much more appropriate for the modelers to choose for themselves the convenient origin. May 24, 2014 at 19:58

Why don't we chose a random point in the void present between galaxy clusters and define it as the absolute origin?

Why don't we choose a random bachelor and define him as married?

The point is this: if you understand the concept bachelor, you know that there is no married bachelor.

And, if you understand the term absolute origin, you know that such a thing cannot be chosen, randomly or otherwise.

If there were an absolute origin in spacetime, that event would have the intrinsic, objective property of being the absolute origin; the nature of that event would different from every other event.

• If the spacetime coordinates and the entire coordinate system we use is man-made and not natural why is it incomprehensible to define an absolute origin along with so much we have defined? I am sorry if I misunderstood, but your answer seems to say that if there were an absolute origin we would find it and not define it. May 24, 2014 at 15:03
• @rijulgupta, I'm saying that it the notion of choosing an absolute origin is incoherent - a contradiction in terms. May 24, 2014 at 15:18
• @rijulgupta, what jargon are you referring to? Do you not see that a randomly chosen event is an arbitrary origin, not an absolute origin? Arbitrary: "based on or determined by individual preference or convenience rather than by necessity or the intrinsic nature of something" May 24, 2014 at 16:33
• No, I get how a bachelor can't be married, and if I didn't I could look in a dictionary. But I look up "absolute origin" and I don't see any history of this term being mutually exclusive with "arbitrarily chosen origin". Did I miss something? Does "absolute" imply more than I understood it to? (as I understand it the OPs question reads exactly the same if "absolute" is removed)
– Shep
May 26, 2014 at 23:40
• I think both of these terms are a confused mouthful, but the general point is that this answer fixates too much on the "absolute" in the original post. My guess would be that @rijulgupta meant "absolute" to mean something like "standard" or "official", whereas your interpenetration seems to hinge on "absolute" meaning "non-arbitrary".
– Shep
May 27, 2014 at 0:48

We do, but of course there are a lot of convenient choices, so there's no good reason for everyone to choose one. Furthermore, since (as far as we can tell) space is isotropic and homogeneous on a large scale, it's nearly always more convenient to use relative coordinates.

Say we choose a "universal" origin roughly 1,000,000 light years from earth. It causes a few practical issues if (for example) we want to talk about our solar system, which is less than 1/1000th of a light year across:

• We have to carry around more than 9 significant digits to give any meaningful coordinates
• We have to deal with floating point round off error when we make any calculations
• Any numbers we use constantly change as the point drifts relative to the sun, or as the planets move around the sun.
• Since this point is in a void, there's going to be some ambiguity in measuring it anyway. I suppose you'd have to define it relative to the rest of the observed universe, then extrapolate backward / forward in time assuming it remains in an inertial reference frame. This introduces significant uncertainty in your origin at any time other than the present, which in turn means you have to add a large uncertainty to any coordinates you quote.

Instead, we could just choose the sun (in this example) as the origin, and these issues cease to be such a problem. Of course for other problems there will be more appropriate coordinate frames, but the point is that there's no perfect one for every occasion.

• You seem to only consider earthly problems, we define frames according to need. A frame that large may not be of use to define motion of a measly ant on earth but certainly useful to describe motion of entire clusters. May 24, 2014 at 15:57
• I don't know if I'd consider the solar system to be "earthly", but sure, most of my arguments apply only to things smaller than the entire cosmos. For the larger system, as I said we've already defined several of these. They tend to put the earth / sun in the center, but that makes perfect sense given that the universe extends equally in all directions.
– Shep
May 25, 2014 at 18:32

In general, we define coordinate systems appropriate for the task at hand. Imagine, for example, that someone asks you what is the speed of your car when you're driving. If we had defined that the origin of the coordinate system would be outside Earth, we would need to take into consideration the velocity of the car relative to Earth, Earth's motion about the centre of mass of the solar system and the motion of the solar system with respect to the point you describe, which is too much work for little benefit. Or you could just check the speed dial, which measures the speed relative to the road.

That said, there exists a preferred coordinate system in the universe at cosmological scale, the one known as the cosmic microwave background radiation frame, which is special, because CMBR permeates the entire universe and can be used to define motion relative to it everywhere.

• I never argued it would be beneficial for all purposes, but asserting that it will overall have no use seems to be a bit much. May 24, 2014 at 16:06