Theoretical way to determine true velocity, as opposed to relative velocity

All right, this is mostly based on theory, but I figure GD may be the more appropriae place for this. First off, keep in mind that I’m assuming a couple things here: 1. That the universe was created as the Big Bang Theory describes, thus the center of the Universe is a single identifiable point, 2. You have in your possession a handy-dandy spaceship that 3. has sensors that are capable of detecting photons with 100% accuracy.

I don’t want no quibbles over “well, you really couldn’t do that, so the point is moot”. :smiley:

ANYway… so you’re out in the middle of space, doing what space travelers do best… trying to do stuff that’s difficult to do. You want to see if you can stop all velocity of your vessel in any direction. By conventional means, you would pick the closest big object and make sure you don’t move in relation to THAT (a planet, star, asteroid, Jennifer Lopez’s butt, etc.). However, by this method, you would still be moving (the planet orbits a star, the star moves with the galaxy, Jennifer Lopez keeps making movies, etc.). So, conventionally, you can’t stop “moving” (having a velocity).

HOWEVER… (this is where my meager understanding of the Theory of Relativity comes partially into play, feel free to correct me), the ToR says that light coming from a source moving away from you would appear to be moving slower (dimmer), while light moving towards you would appear to be moving faster (brighter)… yes? No? Well, it’s not entirely important to what I’m getting at…

Now, say that you detect the photons being emitted by two stars, both on the same line (or the closest approximation you can make) from your starship, and all three are directly in line with the Center of the Universe (CotU), assuming there is one and assuming we’ve found it by now.

Wouldn’t you be able to compute the photon speeds from these two stars to figure out how to bring your vessel to a total and complete stop, ZERO velocity in ANY direction? Even if my stuff about the ToR is way off, wouldn’t this be a plausible means to pull off something utterly useless (although I’m sure some scientist can find a way to make “Zero Velocity” sound interesting)?

Erm. Not gonna try to answer this one. But I will point out that as far as I know, there is no single identifiable “centre of the universe”. All places are the centre of the universe - that is the nature of the universe.


Er, if the universe was perfectly spherical, or hyperspherical?, sure. The problem is that what gives many bodies their initial velocity is the expansion of the universe.

They are, um, surfing on it so to speak. The universe expands and that stuff is carried with it. So this means that your ship must have a gravitational center and it too must be at the exact center of the universe.

But sure, I don’t see why you couldn’t do this in the manner suggested.

No. The speed of light is the same in all inertial frames. It doesn’t matter if you are moving relative to the source or not. The light from a source moving away from you will be shifted toward the red end of the spectrum, but the measured speed will be c.

The big bang was not a localized explosion that flung matter out into the universe. It happened everwhere. There is no center.

I wasn’t really thinking when I posted because I was thinking wavelength compression as opposed to actual change in velocity. Misread the OP.

But Dr. Matrix, are you suggesting that our universe is not closed? If it is spherical in shape, no matter how many dimensions it occupies, it has a center. If it is elliptical it has two! If it is an open universe then we’ve got problems with a center, agreed.

But the point of expansion could be measured by finding a blue-shift everywhere. At the “center” there would be no red shift…

An expanding sphere is a useful model of a two-dimensional (closed) universe, but it is important to distinguish the features of the model from the features you are modeling. The higher (third) dimension is not a part of the universe; it is a part of the model. It is the surface of the sphere that is the universe. The center of the sphere is a feature of the model. You cannot travel on the surface of the sphere and reach the center.

So many economics threads arl forgets his science!

Thanks Dr. M, reminds me that the whole idea of a closed universe is that if you kept travelling in one direction you’d get back where you started (if the universe wasn’t expanding, etc).

I am in complete agreement with you.

Isn’t the universe based on Riemannian geometry, not spherical geometry? So the surface-of-sphere model doesn’t quite work?

This is certainly on the cusp of my learning however. IANAP*



If by Reimannian geometry you mean hyperbolic, well, that depends on whether or not you feel the universe is closed or open.

Am I to assume that there is a logical/scientific basis for the closed universe* or is this a matter of faith? :slight_smile:

isn’t it theoretically possible to measure the redshift of the background radiation to get an aproximation of your absolute velocity… once again, it’s been a LONG time since I studied any astro.

Am I to assume that there is a logical/scientific basis for the closed universe or is this a matter of faith? :slight_smile:

If I’m not mistaken, Riemannian geometry is spherical geometry.

Oh, one more thing. While there is no center of the universe (or, if you prefer, every point is the center), it is possible to measure your velocity relative to the background radiation by measuring its “color”. So, I guess that you could say whether you are stopped.

As to the open/closed/flat question. It depends on the amount of mass in the universe (or is it density?) If it is above some critical value, the universe is closed and finite. If below, the universe is open and infinite. If exactly equal, the universe is flat and infinite. The universe appears to be flat, but it could be just that the curvature is too slight to measure. I don’t think the question is settled yet.

grienspace and Dr. Matrix
Last I’ve read there is no evidence one way or the other as to if the universe is closed, open, or static. The latter is believed by less and less people, and last thing I’ve read is that evidence gives more weight to the open universe (hyperbolic) than the closed one (spherical), though there is no general consensus yet. The whole “do neutrinos have mass” thing is still up in the air :wink:

As for which one Reimann had a hand in, I can’t remember. All I remember about him is how to integrate using limits of successively smaller rectangles. And I ain’t going back to that after the fundamental theorems of the calculus :smiley:

Actually, the latest evidence (from the BOOMERANG experiment) is that the universe is “flat” (i.e., open, but not diverging) (i.e., Euclidean)
FWIW - DrMatrix first response was pretty much exactly what I was going to say to the OP.

The term “Riemannian geometry” applies to both spherical and hyperbolic geometry, as well as Euclidean geometry and a whole bunch of other stuff. The term “Riemannian” in this context just refers to the existence of something called a “Riemannian metric”; both spherical and hyperbolic geometry possess such a metric.

(I think this is the first time I have ever seen topology come up in a GD thread.

BTW: excellent link, Phobos. Although I can’t help but think that the curvature of the universe alone would be enough to determine its topological shape, simply because the universe is so darn big…but I digress.)

“WOULDN’T be enough”, not “WOULD be enough”…oh, never mind.

The anisotropy in the cosmic background radiation (as discovered by George Smoot et al) indicates there is a ‘universal rest frame’ of sorts. If you look at the microwave background, it is slightly hotter in one direction than in the other.

It should be possible to accelerate your spaceship until the cosmic radiation is isotropic to first order. Then, your spaceship is at rest with respect to the universe.

For more info see Wrinkles in Time by Smoot and Davidson.

For pictures, see DMR Images from NASA. The best one is this one, but it is huge so it may take a while to download.

Anyway, there are three images, the first one is the important one for this thread. It shows something that looks like a blue-red Yin-Yang. The blue is in the direction we are moving towards, the red is in the direction we are moving away. The oval picture is a map of the entire sky, like a map of the earth. If you were to put it on a globe, it would look rather like pictures you’ve seen of the moon or earth when it’s not in full phase - half red, half blue, with a fade in the middle.

If you were to accelerate away from the blue long enough, you’d eventually get a uniform color in that first picture, and you’d be “at rest”.

The other pictures have to do with non-uniformities in the early universe and have little to do with your question.

Oh, jeez, where do I begin?

I greatly appreciate the input. While I really can’t think of any reason to try to bring about an “all-stop”, I was just wondering if it were possible, and what’s more, feasible.

Thanks, Dr. Matrix et al, for setting my young mind straight with the ToR. :smiley: