How do you measure speed in space?

[orionrider]

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? :?:

 

[kg]

You can tell your speed towards or away from other objects by means of the doppler effect by observing the spectrum and looking for changes in the position of absorption lines.
http://en.wikipedia.org/wiki/Red_shift#Doppler_effect

The other more practicle method would be to keep track of fuel consumption and what ever gravitaional influences you might come accross. It's best when traveling in space to have a destination in mind before you leave and have a plan on how to get there. A planet for example that you are trying to visit is not going to be where you see it when you set off to visit it. It takes alot of effort to figure out where that planet is going to be when you get there.

 

[bdewoody]

For most of any human space flights you won't be constantly consuming fuel. That is one of my pet peeves with a lot of Sci-Fi movies or TV shows. I remember on the original BSG that they stopped in space once when they ran out of fuel. Babylon 5 got it right, if you are travelling at any velocity and run out of fuel you will continue in whatever direction you were going at the speed you were going until you hit something or are influenced by some objects gravity well.

Speed calculation in our solar system should be easy but I'm not sure about interstellar space as everything is moving in some direction. I guess you would start out with the dopplar readings from as many stars as possible and take readings on those stars and hopefully measure the differences.

I think most people including a lot of space buffs don't understand the mechanics of space travel. The amount of fuel you would have to carry to burn your engines for the entire flight is tremendous and it will take as much fuel to slow down as it does to accelerate. To change direction in space also requires fuel burns as there is no atmosphere to make turns by kiicking over the rudder.

 

[MeteorWayne]

One of the things that bothered me most about Star Trek. You're in "orbit", but the second you run out of fuel, or some alien shuts off the engines, you have hours before you all die. If that's the case, you were never in orbit in the first place!!!

If you are in orbit, no additional thrust is required, If you run out of propellant (since as s_g has pointed out, fuel is useless without an oxidizer) who cares? If you are really in orbit, no additional thrust is required.

 

[ZenGalacticore]

M.Wayne-

I know this might be a revelation to you, but "Star Trek" was science-fiction. And there's such a thing as creative license.


God forbid only amateur astronomers and "scientists" wrote science-fiction. Otherwise, we'd have nothing but "Star Trek: The Next Generation", "Voyager", "Deep Space Nine", and "Stargate".

All sci-fi greats! :roll:

(Yeah, I know. I'm an opinionated kind of guy!)

 

[orionrider]

Aha, the Enterprise was in a very low orbit, and remember it is a huge ship. Of course they had to regularly start the proton powerbeams to counter orbital decay.

Now back to the speed in space:

If you want to measure the 'absolute' speed of an object in space, doppler shift is useless because it could be that you are dead in the water (sic) and the targets are moving.

Scientists say some distant galaxies are receding at relativistic speeds. But then, maybe they don't move at all and we are hurtling through space at 0.5c!

From: http://www.time.com/time/magazine/artic ... z0zyCPtPcN

Since red shift is caused by motion, 3C-147, Schmidt decided, must be speeding away from the earth at 76,000 miles per second, almost half the speed of light. He had taken a picture of the fastest object known to man.

I think absolute speed is impossible to measure, unless you accelerate. Then you can measure the power required to change velocity relative to some targets. Relativity says the required power increases with your speed, gravity and time dilation. Maybe you could compute your absolute velocity from the power needed to accelerate? :? :?:

 

[SpeedFreek]

There is no such thing as absolute speed. Speed is relative.

 

[emperor_of_localgroup]

Would it be so difficult?
How do we measure speed of a car? Correct me if I'm wrong. Isn't it all rpm of the tires, gear ratios, translated into speed? Nothing relative is considered.

When time comes for such space flights, we can determine the amount of thrust (for acceleration) on a space ship for an amount of fuel consumed. Then from duration of thrust and a motion equation of physics can give us the speed. Am I way off?

 

[drwayne]

Would it be so difficult?
How do we measure speed of a car? Correct me if I'm wrong. Isn't it all rpm of the tires, gear ratios, translated into speed? Nothing relative is considered.

Actually, there IS something relative here. The speed of the car is measured in the reference frame of a "static" Earth,
effectively, though not entirely in a coordinate frame called Earth Centered Rotating (ECR). Now, if you simply looked in a
frame that is Earth Centered Inertial (ECI), you would have a different speed that your equipment would have a tough time
measuring. Similarly for any sun centered or galaxy centered reference frame.

So, when you measure speed, you are always measuring it in some reference frame. Most of the time we choose one
that is most convenient to measure or interpret with our sensors, but that doesn't mean that is the only speed that
can accurately characterize the motion.

 

[orionrider]

There is no such thing as absolute speed. Speed is relative.

But there is an absolute speed and that is the speed of light... But relative to what? To nothing it seems. Whatever your velocity relative to whatever reference system, light in the vacuum always seems to go at 300.000km/sec, be it in front of the ship, or behind. And your front nav lights emit photons traveling at c. Not V+c.

Say you set the engines to 100% to produce what is initially 1G acceleration. During your journey, relativistic effects become more and more significant and you slowly loose acceleration. After 6 month and having reached a speed of nearly 0.5c, you accelerate at just 0.87G, as predicted by Einstein (http://www.1728.com/reltivty.htm).

But what if you were already moving at 0.4c before starting the engine?
If you followed the same vector as your departure point, you would have more trouble accelerating than predicted. And if you went against the vector, your acceleration would slowly improve, being the highest when in fact the ship's absolute velocity is zero.

That would be the only way to measure the absolute velocity of the departure point.

Or did I miss something??? :?:

 

[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. Speed is not absolute in any global sense, but it is relative - in this case, to your departure point, but in general, to the contents of the universe!

What I mean is, however fast you manage to travel at, relative to any departure point in the universe, a photon will still overtake you travelling at 300,000 km/s relative to you, so light does not propagate relative to any absolute frame of reference, therefore there is no such thing as absolute speed, in that sense.

As to your comment about having more trouble accelerating than expected, have a look at the link below, it is interesting to think of starship travel in terms of the amount of fuel required to accelerate and decelerate.
http://math.ucr.edu/home/baez/physics/R ... ocket.html

Of course, if you know how long you have been accelerating for, and what your acceleration is, you know your speed relative to your departure point. But if you don't know that, you would have to make observations of the spectra of your departure point or your destination, or the impact energy of the interstellar medium for instance, and then try to calculate your speed. It is easiest to make these observations and calculations with the engines off!

But all you would be measuring was your speed relative to the universe around you, you would not be measuring your absolute speed.

 

[believer_since_1956]

A space craft measures speed in space by triangulation off of 3 reference stars which for the are "relatively fixed in space" for our time frame. You take a position wait a fixed length of time take a new position now you have distance.
Velocity = Distance/Time.

Another method is using a ranging signal provided by Deep Space Network to calculate distance.

 

[ramparts]

Orionrider - as has been pointed out, you can't really ask what your absolute speed is in space. The speed your speedometer reads in your car isn't an absolute speed, either - it's a speed relative to the ground. But if you factor in the Earth's movement around the Sun, suddenly your car is moving a whole lot faster, no? And it goes on. Without defining a reference point, which is pretty much arbitrary, you can't talk about speed.

Light's speed isn't relative to nothing - it's relative to everything. This is definitely not the way a car works, for example. If your speedometer reads 40 mph, it means you're moving at 40 miles per hour relative to the ground, but relative to a car moving in the same direction at 10 mph, you're only moving at 30 mph. That's how it works with most things - a moving object's speed depends on the motion of other observers, and yet with light, any observer moving in any direction at any speed will measure the same speed.

So if you're not traveling at the speed of light, there's not much use to talking about absolute speed - it's all relative.

That said, there are (as have been discussed in this thread) lots of interesting ways to measure your speed relative to some "fixed" reference point of your choosing, and it's an interesting question altogether as to which reference points it's reasonable to call fixed. There's no absolute preferred frame, but there are some which are more interesting to look at than others, just as it's more useful for a car to know its speed relative to the ground than to the Sun, or to a passing bird.

 

[bdewoody]

Think of the two Voyagers. They have basically left our solar system and are heading out into deep space somewhere. We measure their speed relative to the sun, but they also have built into their momentum the speed that the sun is orbiting the galaxy and also the speed that the galaxy is traveling away from other galaxies (except Andromeda). So As Obe wan said it all depends on your point of view.

 

[StarRider1701]

If you want to measure the 'absolute' speed of an object in space, doppler shift is useless because it could be that you are dead in the water (sic) and the targets are moving.
I think absolute speed is impossible to measure, unless you accelerate.

First of all, how did you get somewhere out in the interstellar void of THIS GALAXY without having been in motion away from Earth and this Solar System? Where were you going? Or did you just point the ship in any old direction and blast away?!? And how did you STOP? Sorry, but there is really no way to become "dead in space" without choosing to do so and decelerating. Not a wise idea since no ship, not even on Star Trek had unlimited resources with regards to food and water.

How can you tell the difference between your motion and the motion of other stars? Simple, you measure several of them. There are such things as Star Maps. We have knowledge of many stars in our Galaxy and thier relationship to each other. Heck, even primiitive sailors were capable of navigation using known stars as reference points.

I'm pretty sure that no spaceships have an actual "speedometer." Speed is calculated by the amount of thrust produced by your engines and the amount of fuel used or the amount of time that the engines were on and thrusting.
Much depends upon your starting point. In space, your speed will nearly always be measured in relationship to where you came from and where you are going. Stopping, or becomming dead in the water, out in interstellar space before reaching your destination would either be impossible or be a matter of utmost dire catastrophe!

 

[orionrider]

If speed depends ont you choosing a reference frame (the ground, the planet, the Sun, the galaxy, etc.) it means it is virtual, not real, because you don't know how your reference point moves. One cannot really stop because someone else will aways see you moving.

:arrow: Speed is an illusion.

 

[csmyth3025]

Is it possible to measure one's speed relative to the Cosmic Microwave Background? In this sense, I mean that if you look in one direction and you detect an overall blue shift in the frequency of the background and then you look in the opposite direction and detect an overall red shift in the frequency of the background, will that tell you in what direction and at what speed you're moving relative to the CMB? I believe this is called a dipole anisotropy.

If it's possible for you to make this measurement in your spaceship and for some distant observer to also make this measurement, will these two measurements provide any meaningful information about your motion relative to that of the distant opserver?

Chris

 

[Technetium]

Speed is always relative.

You can't have a speed of something from itself. Otherwise it would calculate as 0, Not moving.

It has to be related to another object otherwise you can't say it's moving.

 

[drwayne]

One cannot really stop because someone else will aways see you moving.

We have had some interesting discussions with respect to Star Trek and helm orders on this very topic.

 

[bdewoody]

Is it possible to measure one's speed relative to the Cosmic Microwave Background? In this sense, I mean that if you look in one direction and you detect an overall blue shift in the frequency of the background and then you look in the opposite direction and detect an overall red shift in the frequency of the background, will that tell you in what direction and at what speed you're moving relative to the CMB? I believe this is called a dipole anisotropy.

If it's possible for you to make this measurement in your spaceship and for some distant observer to also make this measurement, will these two measurements provide any meaningful information about your motion relative to that of the distant opserver?

Chris

I don't think so. The cosmic background radiation comes from every direction and I doubt that the speeds we can attain would register a difference. Now if we are talking about the distant future when hopefully light speed is common maybe that will be a different story.

 

[neilsox]

Hopefully most of the decision makers know how fast the craft is going with respect to something. Radar and inertial guidance can give very accurate numbers. Doppler shift of the light from a nearby sun is probably not very accurate, but does confirm that the other data is approximately correct. The computer can calculate the speed with respect to other things, and in fact makes most of the decisions. As others have typed it involves changing vectors and is often far from Earth. Typically the media just gives a number without specifying, with respect to. The motion of Earth's Equator becomes quite meaningless, if you are more than 1000 kilometers away and moving out. Speed is relative, so there is really no absolute speed. It typically takes a lot of delta v to stop with respect to anything, and we can only define stop with respect to one thing, such as the moon's surface, if we are about to land on the moon.
You can go zero c with respect to the planet you are landed on, but esentually everything is at at least a tiny fraction of c when you are in space. That is one reason the crew is not aware of increased mass, length shortening, nor time dilatation. The crew's speed is zero with respect to the ship, even if the speed is about 0.9999 c with respect to many other things. Neil

 

[SpeedFreek]

Is it possible to measure one's speed relative to the Cosmic Microwave Background? In this sense, I mean that if you look in one direction and you detect an overall blue shift in the frequency of the background and then you look in the opposite direction and detect an overall red shift in the frequency of the background, will that tell you in what direction and at what speed you're moving relative to the CMB? I believe this is called a dipole anisotropy.

If it's possible for you to make this measurement in your spaceship and for some distant observer to also make this measurement, will these two measurements provide any meaningful information about your motion relative to that of the distant opserver?

Chris

Yes, it is possible to measure one's speed relative to the Cosmic Microwave background, as you say. Our local group of galaxies are moving at something around 600 km/s relative to what is known as the CMBR reference frame or CMBR rest frame, which is why the CMBR is hotter in one direction than it is in another, to us.

In order to for two distant observers to use their own measurements of the CMBR to derive information about their motion relative to each other, they would need to know both their speed and their direction, relative to the CMBR. This is far more tricky, as you need to establish a global coordinate system with which both are moving in respect to - you need a "landmark" to fix your direction in relation to. And once you are a certain distance away from another observer, the expansion of the universe would mean that both observers could be at rest in relation to the CMBR rest frame, but still be receding from each other!

 

[ramparts]

Yeah, in fact, measuring our speed relative to the CMB rest frame is really, really easy. You can determine it from the CMB dipoles, the largest-scale CMB anisotropy.

 

[orionrider]

So there is a reference frame for the whole universe? And that would define an 'absolute speed', being 600km/sec for us? :?:

 

[csmyth3025]

So there is a reference frame for the whole universe? And that would define an 'absolute speed', being 600km/sec for us?

There are two aspects of measuring one's speed and direction relative to the CMB that render such measurements meaningless in terms of a universal frame of reference.

First, one can measure one's own local motion relative to the CMB and describe that motion in terms of the coordinates of the ship in which you're traveling (x km/sec towards the front, back, left, right, "up", or "down" - x,y,z coordinates). If you send a radio message to a fellow space traveler and give him your velocity based on these coordinates, he won't know anything about your motion relative to his own ship because he doesnt know how your coordinates line up with his ship. For instance. you could be going 600 km/sec "up" against the CMB and he could be going 600 km/sec "forward" against the CMB when, in fact, you could both be heading straight for each other - or away from each other - depending on the orientation of your respective ships. In this case you both need to agree on a common point of reference in space (a star, perhaps) that you can both measure your relative speed and direction against. Once you've done this, you can determine whether you're approaching each other or going in opposite directions.

The other aspect of the CMB that makes it useless in determining "absolute" velocity the is issue of "co-moving coordinates". Let's say we measure our speed relative to the CMB to be 600 km/sec. Keep in mind that this is a local measurement in our little patch of space. Another observer in a very distant galaxy measures his speed to also be about 600 km/sec relative to the CMB in his little patch of space. Here, again, neither he nor you have any idea what direction your going unless you both have a common point in space that you can use for reference.

More importantly, the distant observer's speed relative to the CMB where he is doesn't make much difference to his velocity away from you. At large cosmic distances he may well be going 400,000 km/sec away from you even though he measures only 600 km/sec relative to the CMB. This is because his space - and the CMB - is generally co-moving with him, and his whole region is sharing the Hubble expansion of space away from our region of space.

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