How do you measure speed in space?

[SpeedFreek]

Please keep in mind that I'm strictly an amateur when it come to cosmology, so if the above is totally wrong, please let me know.

Chris

Everything you said is totally correct! Well done!

 

[ramparts]

Yep, that was a great post Chris!

I would add that even to the extent that the CMB defines an absolute rest frame, it still isn't the absolute rest frame any more than the ground on some patch of Earth is. It's one that's more cosmologically significant, but is still - in the end - completely arbitrary.

 

[TimeTheFinalFrontier]

Suppose (long ago) when the universe was 20 light years across, that you were in a spacecraft moving away from the center of the universe at 99% the speed of light. If the universe has now expanded to bilions of lights years across, what is your velocity relative to the center of the universe? Disregard that you would be really, really old...

 

[orionrider]

moving away from the center of the universe

The universe has no center. The popular 'Big Bang' is misleading, making you think of some kind of explosion; stuff coming out of some center point...
In fact, it is more like: 'before, there was no universe, then there was, everywhere at once, all of it'. There is no 'border', no 'outside', no 'center'.

 

[ramparts]

Exactly, space itself is expanding, rather than the objects inside it moving away from some common center.

You can think of the popular balloon analogy: put a bunch of dots on the surface of a balloon and then start to blow it up. Those dots will expand away from each other, but there's no center to the surface of the balloon, every point is expanding away from every other.

 

[TimeTheFinalFrontier]

You'll have to explain this.... Space is "expanding", like a balloon. A balloon has an outer boundary, therefore it has a center. Space is expanding, but expanding into what? If you are saying space is infinite, then I believe you that there is no center, but then how can something infinite be "expanding"?

This discussion has been worked over on SDC too many times to be remembered. My conclusion is that it is semantics: the dimension of space is inifinite, the observable material of the universe was created at a point of origin and is somehow "expanding" uniformly with respect to (at least our) POV.

For the sake of my question just assume velocity is calculated from a point in space where my spacecraft first accelerated due to thrust without gravitational influence and I was on a straight line moving away from that point at 99% c. Now it's 10 billion years later...

 

[TimeTheFinalFrontier]

Exactly, space itself is expanding, rather than the objects inside it moving away from some common center.

If space is expanding it is not infinite. If it is not infinite it has a center.

 

[SpeedFreek]

To understand what we mean when we say space "expands", I would recommend the article at the link in my signature. (It is a direct link to a pdf file)

 

[orionrider]

Unfortunately, these matters are confusing and difficult to grasp for newtonian minds. That is why it took an Einstein to sort it out in the first place. It is not something you can quickly explain on a forum, you need to read the manual on this one : http://www.amazon.com/Theory-Relativity ... 628&sr=8-1

 

[ramparts]

Exactly, space itself is expanding, rather than the objects inside it moving away from some common center.

If space is expanding it is not infinite. If it is not infinite it has a center.

The first statement is simply not true. The expansion of space simply means that the distances between points in the space are growing with time. Whether or not space is infinite has no bearing on that.

The second statement is technically true, but with some major caveats which really kill the idea. We know that the Universe is at least as big as the visible universe, and since we're probably not at the center of all creation (and for various other reasons), we can pretty safely assume that the Universe, even if finite, has quite a significant extent beyond what we can see. The first consequence of this is that even if the Universe technically has a spatial center, we'll never know where it is, since we can't know about the Universe beyond the visible universe. We're at the center of our visible universe, and anything beyond that is beyond the possibility of our knowledge. The second consequence is that, due to the equivalence of inertial reference frames, there's no unique way to define the shape and size of the Universe (as a whole) at a given time, and so there's no unique center.

I'd stress, though, that the first consequence is far more important than the second, because it tells us that even if the Universe does have a center, that center is completely physically meaningless.

 

[ramparts]

You'll have to explain this.... Space is "expanding", like a balloon. A balloon has an outer boundary, therefore it has a center. Space is expanding, but expanding into what? If you are saying space is infinite, then I believe you that there is no center, but then how can something infinite be "expanding"?

This discussion has been worked over on SDC too many times to be remembered. My conclusion is that it is semantics: the dimension of space is inifinite, the observable material of the universe was created at a point of origin and is somehow "expanding" uniformly with respect to (at least our) POV.

For the sake of my question just assume velocity is calculated from a point in space where my spacecraft first accelerated due to thrust without gravitational influence and I was on a straight line moving away from that point at 99% c. Now it's 10 billion years later...

If you haven't read the link in SpeedFreek's signature, do it! You'll find it very informative and it should answer a lot of your questions, which are really common.

You've caught onto the biggest problem with the balloon analogy, which is that the 2D surface of a balloon is embedded in a higher 3D space. Unfortunately, this is also a requirement of the analogy, since we're not used to dealing with curved 3D spaces or standalone 2D spaces in everyday life, so we can only visualize a curved space as curving into another space. But that's all it is, an analogy and helpful visualization. All analogies break down somewhere, and this analogy breaks down here.

Mathematically, a space can have what's called intrinsic curvature. It can be curved just on its own, without having to curve "into" a higher-dimensional external space. This is perfectly fine, because none of the important features of curvature (think about the angles of a triangle summing to more or less than 180 degrees, parallel lines meeting or diverging, etc.) really require that external space. It's just a very difficult thing for us to conceptualize because, again, we don't run into that sort of thing in our day-to-day lives.

I would add something to my previous post, since you seem to be pretty confused about an infinite space expanding. A space is defined by the way you measure distances between two points. For example, in a flat 3D space (such as the one we live our daily lives in), distances are measured by the Pythagorean theorem. In an expanding space, the distance between two points is multiplied by a factor which grows in time, so that as time progresses, two points naturally get further and further apart, even if they're not actually moving within the space. And as you think over this, bear in mind the caveats I mentioned in that last post saying that it doesn't actually matter whether or not space is infinite!

 

[neomaine]

Or did I miss something??? :?:

Suffice to say, whether or not you know you need more fuel to accelerate, light still travels 300,000 km/s faster than you do, whatever speed you travel at. .... absolute speed.

That would mean that if I was traveling at .1c, that the light coming out of my headlights is traveling at 1.1c? Don't think so.

Light can't travel, well..., faster than light.

 

[SpeedFreek]

Or did I miss something??? :?:

Suffice to say, whether or not you know you need more fuel to accelerate, light still travels 300,000 km/s faster than you do, whatever speed you travel at. .... absolute speed.

That would mean that if I was traveling at .1c, that the light coming out of my headlights is traveling at 1.1c? Don't think so.

Light can't travel, well..., faster than light.

Light always travels 300,000 km/s faster than you, in a vacuum, whatever speed you are travelling relative to anything else. However fast you travel, your headlights will always shine out in front of you travelling 300,000 km/s faster than you, from your point of view.

Light doesnt travel faster than light however - what happens is different observers split up space and time differently, so that light always travels at c, 300,000 km/s faster than whoever is measuring it!

If light is always measured to have the same speed relative to whoever is measuring it, whatever their motion, then what changes between different observers are their notions of space and time - which is where we get time dilation and length contraction from.

 

[neomaine]

And, that's why my status is just 'dust' on the forum folks... I've obviously got some more reading to do but:

If I'm traveling at .1c relative to a fixed point in a vacuum,

(Not relative to our motion to earth, to our sun or to the galatic center or the galaxy's relative motion with it local group or the lg's motion around... whatever's next...)

...then the light from my headlights can only travel at c. Your statement seems to indicate that an analogy of firing a bullet at 300mph from a car moving 30mph has the bullet moving at 330mph be the same.

 

[csmyth3025]

And, that's why my status is just 'dust' on the forum folks... I've obviously got some more reading to do but:

If I'm traveling at .1c relative to a fixed point in a vacuum....then the light from my headlights can only travel at c. Your statement seems to indicate that an analogy of firing a bullet at 300mph from a car moving 30mph has the bullet moving at 330mph be the same.

I'm not sure what your saying, but I can assure you - as others have already done - that the light leaving your headlights will propogate through space at c relative to you. That same light will be measured by anyone else in space - regardless of their motion - as traveling at c. The principle behind this is called Special Relativity. A Wikipedia article on it can be found here:
http://en.wikipedia.org/wiki/Special_relativity_for_beginners

Firing a bullet from your car is governed by the principles of Classical Mechanics (Newtonian Mechanics). In this system of physical laws the speed of the bullet you fire in the direction you're going and the speed of your car are additive. This set of physical laws doesn't apply to the propogation of light in a vacuum.

Chris

 

[ramparts]

Yes, if the speed of light were 300 mph and bullets were made of light, then observers both on the ground and in your car (moving at 30 mph relative to the ground) would measure the bullet as traveling at 300 mph. Normally, as you've been thinking, the observer in the car would measure its speed to be 300 mph but one on the ground would measure it at 330. That's what's so weird about the speed of light!

 

[neomaine]

... but one on the ground would measure it at 330. That's what's so weird about the speed of light!

Yup, still stumped. Thought I had a (very) basic understanding... The V+c is just killing me.

 

[ramparts]

Yeah, that's the thing. Adding velocities as v1+v2 (so in this case, v+c) works classically, but when you start dealing with any speeds near the speed of light, you've got to use a much more complicated equation for adding velocities which only reduces to v1+v2 in the limit that everything involved is moving slowly.

 

[believer_since_1956]

by orionrider » Sat Sep 18, 2010 3:44 pm
I mean, close to the Earth it's easy, it's surface or orbital speed.
But suppose you go farther, like way outside of Earth's gravity well? What does the speedometer show? Speed relative to your orbit around the Sun?

And if you were waking up from hibernation, lost somewhere in the great intergalactic void? The universe has no 'plane' or 'ecliptic', how would you measure your speed? Is there some standard point you can use as a reference?

For that matter, since the speed of light is a constant, how do you know when you are at 'zero c'? Is immobility even possible? :?:




At the risk of repeating myself here's how space navigation works. This procedure is utilized on all the space probes from the first satellites through the latest interplanetary probes and includes the Mercury, Gemini, Apollo, and probably the shuttle as well. You triangulate your position from taking a 3 star siting. Record your time. Wait a fixed period of time take another triangulation to determine your position. Now you know the distance you have traveled and the time it took to do so. Velocity = Speed = Distance/Time. Cosmic radiation, Doppler velocities with respect to light have nothing to do with it. It's simple algebra and utilizing a modification of a method that has been around for several hundreds of years. A recent example of this is the cruse stage of Phoenix it had instruments on it for star sitings to navigate with. I was involved in that mission I looked it up at the JPL and University of Arizona web site for a presentation I gave.

 

[unclefred]

» Fri Oct 01, 2010
At the risk of repeating myself here's how space navigation works. This procedure is utilized on all the space probes from the first satellites through the latest interplanetary probes and includes the Mercury, Gemini, Apollo, and probably the shuttle as well. You triangulate your position from taking a 3 star siting.

Can you explain what you mean by a "3 star siting"? What is it you are measuring?

 

[MeteorWayne]

The position in space of 3 stars from your viewpoint. This allows you to find your 3 dimensional position relative to them.

 

[adrenalynn]

Think of it as an interstellar GPS. You're sighting at least three known points, then using Doppler Shift (for one example) to figure out how you're moving in relation to those known points.

 

[unclefred]

by orionrider » Sat Sep 18, 2010 3:44 pm
At the risk of repeating myself here's how space navigation works. This procedure is utilized on all the space probes from the first satellites through the latest interplanetary probes and includes the Mercury, Gemini, Apollo, and probably the shuttle as well. You triangulate your position from taking a 3 star siting. Record your time. Wait a fixed period of time take another triangulation to determine your position. Now you know the distance you have traveled and the time it took to do so. Velocity = Speed = Distance/Time. Cosmic radiation, Doppler velocities with respect to light have nothing to do with it. It's simple algebra and utilizing a modification of a method that has been around for several hundreds of years. A recent example of this is the cruse stage of Phoenix it had instruments on it for star sitings to navigate with. I was involved in that mission I looked it up at the JPL and University of Arizona web site for a presentation I gave.

Measuring the positions of 3 stars will NOT provide your position in space. None of our satellites, interplanetary probes, or the space shuttle use this method. The reason is simple. The stars look exactly the same from the Earth, Mars, or Jupiter. You would have to travel many light years to make any measureable difference in the stars position. Doppler shift is also useless because any shift due to your velocity would be insignificant compared to the stars velocity.

Navigation is accomplished by the deep space network using the normal communications channels. The position is determined by time of flight and Doppler of the signals. Mars Reconnaissance Orbiter (MRO) had an Optical Navigation Camera as an experiment. Only when very close to the planet, it attempted to find the spacecraft position by imaging the planet against the background stars. The Phoenix Lander did not have any such experiment.

 

[believer_since_1956]

by orionrider » Sat Sep 18, 2010 3:44 pm
At the risk of repeating myself here's how space navigation works. This procedure is utilized on all the space probes from the first satellites through the latest interplanetary probes and includes the Mercury, Gemini, Apollo, and probably the shuttle as well. You triangulate your position from taking a 3 star siting. Record your time. Wait a fixed period of time take another triangulation to determine your position. Now you know the distance you have traveled and the time it took to do so. Velocity = Speed = Distance/Time. Cosmic radiation, Doppler velocities with respect to light have nothing to do with it. It's simple algebra and utilizing a modification of a method that has been around for several hundreds of years. A recent example of this is the cruse stage of Phoenix it had instruments on it for star sitings to navigate with. I was involved in that mission I looked it up at the JPL and University of Arizona web site for a presentation I gave.

Measuring the positions of 3 stars will NOT provide your position in space. None of our satellites, interplanetary probes, or the space shuttle use this method. The reason is simple. The stars look exactly the same from the Earth, Mars, or Jupiter. You would have to travel many light years to make any measureable difference in the stars position. Doppler shift is also useless because any shift due to your velocity would be insignificant compared to the stars velocity.

Navigation is accomplished by the deep space network using the normal communications channels. The position is determined by time of flight and Doppler of the signals. Mars Reconnaissance Orbiter (MRO) had an Optical Navigation Camera as an experiment. Only when very close to the planet, it attempted to find the spacecraft position by imaging the planet against the background stars. The Phoenix Lander did not have any such experiment.

On the subject of velocity
Point I was trying to make which is consistent with Deep Space Network (DSN) ranging sub-carrier method of determining velocity is if you know a position, and know a second position at a later time then you know velocity.

On the subject of triangulation:
I may stand corrected but I do know of missions that had the ability to triangulate was used for navigational purposes and at least in one Gemini mission and on Apollo 8 triangulation was utilized. A point to consider is at 10:00PM in October the constellation Orion will be at a different angle (referenced to the Eastern Horizon) when seen from a fixed point on Earth then it will be at 10:00PM in January when viewed from the same point because the Earth has moved in it orbit, and the distance traveled by the Earth is considerably less than a light year, for all intents and purposes Orion has remained fixed in space while Earth has moved. Then knowing the angular change you can compute the distance traveled by the Earth or any other object traveling in space you are residing on using Trigonometry, you will have to know the Galactic coordinates of your reference star in Orion. After you know the coordinates of your star and you know your angular differences for the different times of each observation you can now determine your relative positions and distances. I use Orion as a reference because it is a widely known constellation. I humbly submit this to your consideration as proof that I am correct. I apologize for getting long winded I have always enjoyed the subject of navigation.

Deep Space 1 utilized both methods discussed above
Reference: Autonomous Optical Navigation (Auto Nav) DS 1 Validation Report
nmp-techval-reports.jpl.nasa.gov/DS1/AutoNav_Integrated_Report_A.pdf

 

[MeteorWayne]

Yes for locations close to earth, the planet can be used as a reference. After all, we know the diameter, the size of an orbit, etc. For interstellar space, the triangulation method would work quite well.