How do you measure speed in space?

[believer_since_1956]

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.

Triangulation will work for interplanetary distances also, please refer to the my previous post to yours (attached above) I have done some editing to my posting and provided references to triangulation utilized by the Deep Space 1 spacecraft. Also Wayne how do you attach a picture to a post. I have and excellent extract from the DS1 navigation document but I can't seem to find a way to import it. Thanks

 

[dryson]

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.

Wrong. Eventually the gravity of the Earth will pull your craft into the atmosphere. Otherwise the shuttle and I.S.S. would not have to use thrusters to keep the orbit of the craft at the correct altitude.

 

[believer_since_1956]

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.

Wrong. Eventually the gravity of the Earth will pull your craft into the atmosphere. Otherwise the shuttle and I.S.S. would not have to use thrusters to keep the orbit of the craft at the correct altitude.

Dryson
The reason ISS and shuttle need additional thrust is there is still atmospheric drag at 230miles. The drag slows the space craft eventually to sub orbital velocities. Example the Moon has been in orbit for a long, long time and is not moving closer to the Earth, and the Moon is way beyond Earth's atmosphere.

 

[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.

Wrong. Eventually the gravity of the Earth will pull your craft into the atmosphere. Otherwise the shuttle and I.S.S. would not have to use thrusters to keep the orbit of the craft at the correct altitude.

One of these days you should start to use the same physics as everyone else on earth. As stated above, of it were not for atmospheric interaction, the ISS could orbit forever at that altitude.

If you are in orbit, unless something slows you down, you will orbit forever.

Too much Star Trek, I think :roll:

 

[unclefred]

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.

DSN can measured velocity quite accurately directly by Doppler. All you need to know is where to point the antenna, you do not heed to know position. Absolute position is much harder to measure and taking the difference between two such positions adds the errors from both and gives an average velocity over the time interval. Since most orbits are curved, and the speed varies along the path, one must be careful using the average.

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.

Close to a planet, measuring its diameter directly gives you the distance, but you have to be extremely close. Measuring a planet with respect to the background stars will help determine your position, provided you have other information. That technique has been the focus of several optical navigation attempts by NASA. It requires lots of information about the planned path and the target planet's orbit. The technique does not directly measure stars, it simply uses them as a background for reference. Within the solar system, star measurements (triangulation) will not give your position because the star patterns do not change. In interstellar space, the only possibility is to somehow know which stars are nearby and to measure them with respect to the other farther away stars. Even measuring the key nearby stars will probably only get your position to within a few light years.

 

[kg]

Correct me if I'm wrong but I thought that reference stars are used by spacecraft for orientation to keep its antenna pointed toward Earth. I also thought that determining the spacecrafts position was done from Earth based observation and calculation and not so much by the spacecraft itself.

 

[believer_since_1956]

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.

DSN can measured velocity quite accurately directly by Doppler. All you need to know is where to point the antenna, you do not heed to know position. Absolute position is much harder to measure and taking the difference between two such positions adds the errors from both and gives an average velocity over the time interval. Since most orbits are curved, and the speed varies along the path, one must be careful using the average.

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.

Close to a planet, measuring its diameter directly gives you the distance, but you have to be extremely close. Measuring a planet with respect to the background stars will help determine your position, provided you have other information. That technique has been the focus of several optical navigation attempts by NASA. It requires lots of information about the planned path and the target planet's orbit. The technique does not directly measure stars, it simply uses them as a background for reference. Within the solar system, star measurements (triangulation) will not give your position because the star patterns do not change. In interstellar space, the only possibility is to somehow know which stars are nearby and to measure them with respect to the other farther away stars. Even measuring the key nearby stars will probably only get your position to within a few light years.

DSN
Yes Doppler works for determining velocity. Keeping in mind the orbital velocity of the Earth needs to calibrated out of the solution.
FYI
Position using a 1.03MHz sub-carrier can be measured to a 0.145km resolution
reference http://deepspace.jpl.nasa.gov/dsndocs/810-005/203/203C.pdf


Triangulation
The fact that the star patterns are not changing is what makes triangulation possible. Your position is changing so the angular measurements to the stars are changing. From then it becomes a matter of Trigonometry. Think about how a star will occupy a different angular position in the sky every night with respect to the Eastern horizon at the same time as the Earth moves in it orbit, keeping in mind the star is not moving from our perspective.

Also remember it is possible to navigate by star sitings, this technique has been done for centuries, when you leave the planet if you navigate by this method it is just and extension of this technique. The fact that the star patterns do not change is what makes it possible. It's trigonometry based.

References
Did you look up the references from JPL I posted earlier about how Deep Space 1 utilized both DSN and Triangulation?

 

[orionrider]

All these methods work around here, not in some other system or galaxy.

There seems to be at least a 'natural' speed range in (local) space. If you check asteroid and planet velocities (all things susceptible to collide), the relative speeds are always measured in km/sec: tens maybe hundreds of km/sec, but no more.
AFAIK, there are no examples of solid macroscopic objects hurtling at relativistic speeds.

Receding galaxies seem to go much faster, but this is caused by expansion of space itself, these objects cannot really crash into something.

 

[believer_since_1956]

All these methods work around here, not in some other system or galaxy.

There seems to be at least a 'natural' speed range in (local) space. If you check asteroid and planet velocities (all things susceptible to collide), the relative speeds are always measured in km/sec: tens maybe hundreds of km/sec, but no more.
AFAIK, there are no examples of solid macroscopic objects hurtling at relativistic speeds.

Receding galaxies seem to go much faster, but this is caused by expansion of space itself, these objects cannot really crash into something.

Actually the point I am trying to make with Triangulation is it will work anywhere in the Universe provide you have 3 relative unchanging reference points. By relative unchanging reference points I mean with respect to a long time duration the reference points (stars, galaxies, quasars, black holes, etc) do not move from your perspective. In other words you are the object that is moving. I realize if you are taking a long duration voyage (multiple lifetimes) problems will develop, however that to can be compensated for by knowing the paths of the reference points through space.

I realize I have digressed from the original thread, however I am a former artillery officer, an engineer, and an amateur astronomer needless to say I love maps and navigation discussions.

 

[MeteorWayne]

The reason stuff in the solar system is measured in tens of km/sec is because there's a maximum speed something can be moving and still be part of the solar system. At 1 AU from the sun, the max speed is 41 km/sec and that leads to a very elliptical orbit (earth's close to round orbital speed is ~ 30 km/sec). Anything moving faster is leaving and won't be back. Most asteroids orbit the sun in the same direction as the earth, so the approach speed to us is less than 30 kim/s most of the time. Comets, many of which have very elliptical orbits can be traveling at ~ 41 hm/s. Ones orbiting the sun in the other direction (retrograde) can approach us close to 71 km/s such as Halley's comet, Tempel-Tuttle or Swift-Tuttle. That's the max speed of approach to earth at 1 AU. Two comets orbiting the sun in opposite directions could hit at ~ 82 km/s head on at 1 AU. Of course the speed limit is faster closer to the sun, and slower further away.

Faster objects (at 1 AU) would have to come from outside the solar system, and there's a whole lot of empty out there...

 

[unclefred]

Actually the point I am trying to make with Triangulation is it will work anywhere in the Universe provide you have 3 relative unchanging reference points. By relative unchanging reference points I mean with respect to a long time duration the reference points (stars, galaxies, quasars, black holes, etc) do not move from your perspective. In other words you are the object that is moving. I realize if you are taking a long duration voyage (multiple lifetimes) problems will develop, however that to can be compensated for by knowing the paths of the reference points through space.

I realize I have digressed from the original thread, however I am a former artillery officer, an engineer, and an amateur astronomer needless to say I love maps and navigation discussions.

We may be saying the same thing depending on what the words "triangulate" and "position" mean.

(1) To paraphrase Wikipedia: Triangulation is the process of determining the location of a point by measuring angles to (or from) known points. I think that is what you are doing when you measuring "the position in space of 3 stars from your viewpoint".

(2) Position is defined as a specific the place at a specific time in 3 dimensional space. To define the a position one must establish some sort of coordinate measuring system, which then means one must establish a reference point or zero point in the system.

(3) I need to introduce the term "attitude". The spacecraft attitude is its angular orientation in space. To define attitude one must establish some sort of angular coordinate measuring system, which then means one must establish a reference point or zero point in the system. The most recognizable such system is the astronomy system of Right Ascension and Declination.

The Phoenix spacecraft (since you mentioned it earlier) had 2 star cameras on it, one active and one backup. They were used to determine the spacecraft attitude (not the position). As far as I know, all spacecraft have something similar to measure star positions, determine the attitude, and thus allow the spacecraft to be rotated and pointed to a specific direction. At any time the spacecraft can be rotated to any attitude, totally independent of its position in its orbit. The star cameras give absolutely no information on the spacecraft's position.

The triangulation discussion has me a bit puzzled. The stars look the same from anywhere in the solar system. Measuring them (3 stars, 4 stars, or a million of them) will tell you absolutely nothing about your position, no matter how accurately you measure them. However if you measure something nearby and use the stars as a background reference, then you can determine position relative to the nearby object. For example: two astronomers can measure all the stars they want and there is absolutely no way they can tell if their observatories are both at Kitt Peak or if one is somewhere in Italy. However, as soon as they take a measurement to something nearby (it could be the moon, the horizon, or the window sill) they can immediately determine their relative positions. That appears to be what you are calling triangulation.

The NASA article you referenced does not use triangulation as a position finding technique. In the referenced article (http://nmp-techval-reports.jpl.nasa.gov ... port_A.pdf) the word "triangulation appears only once (first paragraph on page 60). In this instance, it is referring to a one time process used only for 3 hours before fly-by. They plan to use a "combination of simple triangulation and area analysis" to ensure the safety of the spacecraft and not hit the asteroid. They do not define what they mean by either technique.

The Optical navigation concept is summarized in the second paragraph of the abstract (page vi). it says "The theoretical basis of AutoNav is a process in which images of asteroids (typically main-belt) are taken against the distant stars and, through the measured parallax, geometric information is inferred." The technique is also described in the introduction (page 1) and says:
"Optical Navigation, as it is currently being applied by the deep-space probes of JPL/NASA, is a technique by which the position of a spacecraft is determined through astrometric observations of targets against a background field of stars. The stars and target positions are known by ground or other observations, independently, or concurrently made, and the position of the spacecraft taking the image is inferred from the error in the position of the near-field object against the far-field (i.e. the parallax)."

The technique is easy to describe, but complex to do. By measuring the asteroid against the stars, one can establish a line from the star (actually a Right Ascension and Declination point) through the asteroid. The asteroid's absolute position is established by ground orbit determination before the mission starts. With the asteroid position known, the line is then fixed in space and the spacecraft must be on that line. We don't know exactly where on the line, but we can make a reasonable initial guess. Multiple measurements (days, weeks or months apart) of this kind establish multiple lines and for each we have a reasonable initial guess where on the line the spacecraft is. As more measurements are collected, the position guesses on the line can be refined so that the individual positions all lie along a physically possible orbit.

 

[believer_since_1956]

Actually the point I am trying to make with Triangulation is it will work anywhere in the Universe provide you have 3 relative unchanging reference points. By relative unchanging reference points I mean with respect to a long time duration the reference points (stars, galaxies, quasars, black holes, etc) do not move from your perspective. In other words you are the object that is moving. I realize if you are taking a long duration voyage (multiple lifetimes) problems will develop, however that to can be compensated for by knowing the paths of the reference points through space.

I realize I have digressed from the original thread, however I am a former artillery officer, an engineer, and an amateur astronomer needless to say I love maps and navigation discussions.

We may be saying the same thing depending on what the words "triangulate" and "position" mean.

(1) To paraphrase Wikipedia: Triangulation is the process of determining the location of a point by measuring angles to (or from) known points. I think that is what you are doing when you measuring "the position in space of 3 stars from your viewpoint".

(2) Position is defined as a specific the place at a specific time in 3 dimensional space. To define the a position one must establish some sort of coordinate measuring system, which then means one must establish a reference point or zero point in the system.

(3) I need to introduce the term "attitude". The spacecraft attitude is its angular orientation in space. To define attitude one must establish some sort of angular coordinate measuring system, which then means one must establish a reference point or zero point in the system. The most recognizable such system is the astronomy system of Right Ascension and Declination.

The Phoenix spacecraft (since you mentioned it earlier) had 2 star cameras on it, one active and one backup. They were used to determine the spacecraft attitude (not the position). As far as I know, all spacecraft have something similar to measure star positions, determine the attitude, and thus allow the spacecraft to be rotated and pointed to a specific direction. At any time the spacecraft can be rotated to any attitude, totally independent of its position in its orbit. The star cameras give absolutely no information on the spacecraft's position.

The triangulation discussion has me a bit puzzled. The stars look the same from anywhere in the solar system. Measuring them (3 stars, 4 stars, or a million of them) will tell you absolutely nothing about your position, no matter how accurately you measure them. However if you measure something nearby and use the stars as a background reference, then you can determine position relative to the nearby object. For example: two astronomers can measure all the stars they want and there is absolutely no way they can tell if their observatories are both at Kitt Peak or if one is somewhere in Italy. However, as soon as they take a measurement to something nearby (it could be the moon, the horizon, or the window sill) they can immediately determine their relative positions. That appears to be what you are calling triangulation.

The NASA article you referenced does not use triangulation as a position finding technique. In the referenced article (http://nmp-techval-reports.jpl.nasa.gov ... port_A.pdf) the word "triangulation appears only once (first paragraph on page 60). In this instance, it is referring to a one time process used only for 3 hours before fly-by. They plan to use a "combination of simple triangulation and area analysis" to ensure the safety of the spacecraft and not hit the asteroid. They do not define what they mean by either technique.

The Optical navigation concept is summarized in the second paragraph of the abstract (page vi). it says "The theoretical basis of AutoNav is a process in which images of asteroids (typically main-belt) are taken against the distant stars and, through the measured parallax, geometric information is inferred." The technique is also described in the introduction (page 1) and says:
"Optical Navigation, as it is currently being applied by the deep-space probes of JPL/NASA, is a technique by which the position of a spacecraft is determined through astrometric observations of targets against a background field of stars. The stars and target positions are known by ground or other observations, independently, or concurrently made, and the position of the spacecraft taking the image is inferred from the error in the position of the near-field object against the far-field (i.e. the parallax)."

The technique is easy to describe, but complex to do. By measuring the asteroid against the stars, one can establish a line from the star (actually a Right Ascension and Declination point) through the asteroid. The asteroid's absolute position is established by ground orbit determination before the mission starts. With the asteroid position known, the line is then fixed in space and the spacecraft must be on that line. We don't know exactly where on the line, but we can make a reasonable initial guess. Multiple measurements (days, weeks or months apart) of this kind establish multiple lines and for each we have a reasonable initial guess where on the line the spacecraft is. As more measurements are collected, the position guesses on the line can be refined so that the individual positions all lie along a physically possible orbit.

I do believe we are on the sheet of music so to speak. Our dialog has been most informative, I had do some research to verify my claims it was fun. I'm looking forward to future discoures.

 

[brellis]

Star-pointing on spacecraft traveling for a long time may encounter some shifting, eh? How're the V-gers doing in that regard?

 

[MeteorWayne]

Star-pointing on spacecraft traveling for a long time may encounter some shifting, eh? How're the V-gers doing in that regard?

I don't believe they have imaging capacity any more.... :ugeek:

 

[dilligaff01]

I have a couple of questions but I have no doubt I'm way off base, so I will rely on you guys and gals to figure out what I'm getting at and set me straight :lol: . I beilieve that Einstein's theory of relativity said that time and speed are tied together so someone approaching light speed wouldn't notice their clocks running slower but an outside observer would. If time slows at faster velocity, wouldn't the opposite be true for slower speeds?

If so, wouldn't that affect our perception of the speed of light and throw off our measurements?

I'm probably way off base but this is the line of thought that brought about the questions. Even without the expansion of the universe, our planet, solar system and galaxy are all still moving through the universe. Without an absolute reference point we can only estimate our speed relative to other planets, galaxies, etc. but the way I understand it is that we are in motion as is everything around us. My basic grasp (not understanding, just grasp) of this concept leads me to think that we have time as we know it due to this movement. If we were moving faster or slower than light speed our clocks would run slower or faster but we wouldn't notice.

This is an idea that I have been knocking around for awhile but I haven't seen a mention of the opposite effect of velocity/time anywhere so I'm curious about it. Hopefully all this makes some kind of sense

 

[csmyth3025]

I have a couple of questions but I have no doubt I'm way off base, so I will rely on you guys and gals to figure out what I'm getting at and set me straight :lol: . I beilieve that Einstein's theory of relativity said that time and speed are tied together so someone approaching light speed wouldn't notice their clocks running slower but an outside observer would. If time slows at faster velocity, wouldn't the opposite be true for slower speeds?

If so, wouldn't that affect our perception of the speed of light and throw off our measurements?

"Our" time is always the same no matter how we're moving. If an alien spacecraft came zooming past us at 0.999 c, and if we could somehow see his (or her - or its) clock, we would say that the spaceship's clock is running slower than our Earth-based clock. The funny thing about Relativity is that, if the alien could somehow look out the window and see our Earth-based clock, he/she/it would say that our clock is running slow.

Just to add to your bewilderment - both we and the alien would be absolutely right! The alien's clock is running slower from our point of view in our frame of reference and our clock is running slower from the alien's point of view from his/her's/its frame of reference.

The real icing on this layer-cake of confusion, though, is the fact that if we shine a flashlight at the alien and he shines a flashlight at us, we will both measure the speed of light to be exactly the same - regardless of whether we both measure the speed of light when the alien's spaceship is approaching us or going away from us!

It's all relative, you see - except the speed of light

Chris

 

[SpeedFreek]

In short, there is no absolute frame that light propagates relative to.

In a vacuum, light always propagates 300,000 km/s faster than whoever is measuring it, however fast they are travelling.

 

[believer_since_1956]

In short, there is no absolute frame that light propagates relative to.

In a vacuum, light always propagates 300,000 km/s faster than whoever is measuring it, however fast they are travelling.

If you are traveling at 299,999 km/s and you turn on a flash light the light will only travel at 300,000 km/s not at 599,999km/s. This is what relativity indicates and has been proven by many experiments in particle colliders.

 

[dilligaff01]

I have a couple of questions but I have no doubt I'm way off base, so I will rely on you guys and gals to figure out what I'm getting at and set me straight :lol: . I beilieve that Einstein's theory of relativity said that time and speed are tied together so someone approaching light speed wouldn't notice their clocks running slower but an outside observer would. If time slows at faster velocity, wouldn't the opposite be true for slower speeds?

If so, wouldn't that affect our perception of the speed of light and throw off our measurements?

"Our" time is always the same no matter how we're moving. If an alien spacecraft came zooming past us at 0.999 c, and if we could somehow see his (or her - or its) clock, we would say that the spaceship's clock is running slower than our Earth-based clock. The funny thing about Relativity is that, if the alien could somehow look out the window and see our Earth-based clock, he/she/it would say that our clock is running slow.

Just to add to your bewilderment - both we and the alien would be absolutely right! The alien's clock is running slower from our point of view in our frame of reference and our clock is running slower from the alien's point of view from his/her's/its frame of reference.

Ok so our perception of our own time doesn't change presumably because our clock is moving at the same speed we are. I kinda get that. I don't quite understand why our earth based clock would seem to be running slower from the perspective of the alien if they are the one moving. Would that be because with no absolute reference point it would appear to the alien that everything else was moving past them at 0.999 c instead of them moving? Would an absolute reference point change that? Would an absolute reference point make it appear that our clock was running faster? Could we use that absolute reference to gauge our actual speed through the universe and determine if our perception of time was actually faster or slower than some other planet somewhere moving at a different absolute speed?

I understand (as much as I understand any of this) that there is no absolute reference point which is why everything is measured in relative speed. I guess my ultimate question is why does everything make other clocks appear to run slower, never faster, from one perspective to another?

Sorry to throw all these questions at you all but I find the subject fascinating even though, or maybe because, I'm not that knowledgeable about it. I've been bouncing these questions around in my head for awhile now trying to figure out how to word them and that's after reading through the message board and doing some internet research trying to find the answer myself. I appreciate the time and patience it takes for you all to educate us

 

[csmyth3025]

A good place to start if you're trying to understand the basic ideas about the speed of light is the "Introduction to Special Relstivity" article in Wikipedia, which can be found here:
http://en.wikipedia.org/wiki/Introduction_to_special_relativity

Chris

 

[SpeedFreek]

In short, there is no absolute frame that light propagates relative to.

In a vacuum, light always propagates 300,000 km/s faster than whoever is measuring it, however fast they are travelling.

If you are traveling at 299,999 km/s and you turn on a flash light the light will only travel at 300,000 km/s not at 599,999km/s. This is what relativity indicates and has been proven by many experiments in particle colliders.

Yes, that is what I said! Light always travels at 300,000 km/s in a vacuum.

Nobody (in an inertial frame, in a vacuum) will ever measure light to be travelling at anything but 300,000 km/s. They will always measure light to be travelling at 300,000 km/s.

However fast they are moving, relative to anyone else.

Relativity indicates that the speed of light is always measured as c, regardless of the relative inertial velocity of the measurer. It is the reason for time-dilation and length contraction - the constancy of c to all inertial frames, regardless of their motion. It is the reason that events that are simultaneous in one frame may not be simultaneous from another frame, after the light-travel time has been accounted for, if the frames have relative motion. There is no absolute time.

It is the reason the Special Relativity rid us of the need of a "luminiferous ether", a medium relative to which light propagates, an absolute frame of reference. There is no absolute space.

But it is a bit weird, I grant you! The speed of light is a constant, regardless of your relative motion. We have tested this experimentally.

In the scenario you mention, if I am travelling at 299,999 km/s and the speed of light is 300,000 km/s, then when I shine my flashlight, you see the light propagating from the front of my spaceship, moving 1 km/s faster than my spaceship. True.

But on board my spaceship, if I have a long pole with a mirror on the end sticking out the front and I bounce my light signal from the flashlight off it, so I can measure the speed of that light directly, I find it travels at 300,000 km/s to me! It is as if I am still at rest, as far as the speed of light is concerned!

The answer to this conundrum is found through the notion that space and time are part of the same thing and your motion through one changes your motion through the other, but only when you measure your motion relative to something else in space, as you cannot measure your motion relative to space itself, as there is no absolute "rest frame" for space. Or something like that!

Chris's link above explains all a whole lot more thoroughly than I can here.

 

[unclefred]

In the scenario you mention, if I am travelling at 299,999 km/s and the speed of light is 300,000 km/s, then when I shine my flashlight, you see the light propagating from the front of my spaceship, moving 1 km/s faster than my spaceship. True.

But on board my spaceship, if I have a long pole with a mirror on the end sticking out the front and I bounce my light signal from the flashlight off it, so I can measure the speed of that light directly, I find it travels at 300,000 km/s to me! It is as if I am still at rest, as far as the speed of light is concerned!

Don't come to the conclusion that the spacecraft speed has no affect on the light. The light is greatly affeccted by the speed. The guy on the spaceship and the guy outside will see the light completely different. The guy on the outside will see the light drastically red shifted (or blue shifted depending on direction). The guy on the spaceship will see no shift in the light bouncing off his mirror on the end of the stick.

 

[SpeedFreek]

Don't come to the conclusion that the spacecraft speed has no affect on the light. The light is greatly affeccted by the speed. The guy on the spaceship and the guy outside will see the light completely different. The guy on the outside will see the light drastically red shifted (or blue shifted depending on direction). The guy on the spaceship will see no shift in the light bouncing off his mirror on the end of the stick.

Yes of course, rather than a difference in the speed of light, an observer would measure a Doppler shift of that light instead.

 

[csmyth3025]

...I guess my ultimate question is why does everything make other clocks appear to run slower, never faster, from one perspective to another?...

To be exact, everything doesn't make other clocks appear to run slower. One example is the clock on a GPS satelite (excerpt from the Wikipedia article on the Global Positioning System, which can be found here: http://en.wikipedia.org/wiki/Global_Positioning_System#Correcting_a_GPS_receiver.27s_clock):

. Special relativity predicts that the frequency of atomic clocks moving at orbital speeds tick more slowly than stationary ground clocks by a factor of (v^2/c^2) or about 10^-10 , a delay of about 7 μs/day, where the orbital velocity is v = 4 km/s, and c = the speed of light. The time dilation effect has been measured and verified using GPS.

The gravitational frequency shift effect on GPS due to general relativity is that a clock closer to a massive object runs slower than a clock farther away. Applied to GPS, the receivers are much closer to Earth than the satellites, causing GPS clocks to be faster by a factor of 5×10^−10, or about 45.9 μs/day.

So, in this case the weaker gravitational field at the altitude of the satelite causes the clock on the GPS satelite to run slightly faster than the clock on the ground. In fact, before they are launched, the clocks on GPS satelites are intentionally set to run slightly "slow" (on the ground) in order to offset the combination of these two effects.

This reply raises a question in my mind to which, perhaps, someone may be able to respond: If the orbital speed of the GPS clock causes it to run slower than the clock on the ground, doesn't the speed of the clock on the ground (relative to the GPS satelite) also cause the Earth-bound clock to run slower than the GPS clock (to someone riding in the satelite)?

Chris

 

[EarthlingX]

I just got this wild flash :
- since the speed is always relative to something, one is always at rest relative to himself
- relative to what does matter need to approach the speed of light to convert to energy ? Does it convert at all, relative to itself ?