How do you measure speed in space?

[csmyth3025]

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 ?

The short answer is that, relative to itself, matter always has the same mass.

If you're going through the solar system at 0.9999 c, your mass is the same (to you) as it has always been. All the planets and moons that you zoom past will seem more massive to you, though.

Something going past you a very nearly the speed of light doesn't have any of it's mass converted to energy as far as I know. But, because of the equivalence of energy and mass, the enormous kinetic energy it has (relative to you) results in it having more mass than if it was just slowly drifting by.

Chris

 

[neomaine]

...
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.
...
Chris

Someone please help me (us) verify: Its not the weaker gravitional field that the sat is passing through its the speed at which its traveling which causes the relative time difference.

 

[SpeedFreek]

Someone please help me (us) verify: Its not the weaker gravitional field that the sat is passing through its the speed at which its traveling which causes the relative time difference.

The satellites relative speed causes a relative time difference of -7 microseconds a day. The weaker gravitational field out there causes a relative time difference of +45 microseconds a day. The net difference is +38 microseconds a day. Gravity wins! The clocks on a GPS satellite "run faster" than a clock at sea level, due to the difference in gravitational potential - gravitational time-dilation beats kinematic time dilation in this case.

 

[neomaine]

Learning something every day, thanks!

 

[dilligaff01]

Someone please help me (us) verify: Its not the weaker gravitional field that the sat is passing through its the speed at which its traveling which causes the relative time difference.

The satellites relative speed causes a relative time difference of -7 microseconds a day. The weaker gravitational field out there causes a relative time difference of +45 microseconds a day. The net difference is +38 microseconds a day. Gravity wins! The clocks on a GPS satellite "run faster" than a clock at sea level, due to the difference in gravitational potential - gravitational time-dilation beats kinematic time dilation in this case.

For every small step forward I take here I end up taking two steps back in my limited understanding lol. If the gravity of the earth actually speeds up the internal clocks, why have I read that the gravity of a black hole is so intense that even time would slow down? Am I getting my science fiction and science fact mixed up?

 

[SpeedFreek]

Someone please help me (us) verify: Its not the weaker gravitional field that the sat is passing through its the speed at which its traveling which causes the relative time difference.

The satellites relative speed causes a relative time difference of -7 microseconds a day. The weaker gravitational field out there causes a relative time difference of +45 microseconds a day. The net difference is +38 microseconds a day. Gravity wins! The clocks on a GPS satellite "run faster" than a clock at sea level, due to the difference in gravitational potential - gravitational time-dilation beats kinematic time dilation in this case.

For every small step forward I take here I end up taking two steps back in my limited understanding lol. If the gravity of the earth actually speeds up the internal clocks, why have I read that the gravity of a black hole is so intense that even time would slow down? Am I getting my science fiction and science fact mixed up?

The GPS satellite's internal clock runs faster than a clock on the surface of the Earth, due to the weaker gravitational field out where the GPS satellite is. So time on Earth runs slower, due to the stronger gravitational field down here, when compared to time on a distant GPS satellite. The stronger the gravity, the slower the clock, when compared to a clock in weaker gravity. In the most extreme case, a distant observer will see a clock stop completely as it crosses the event horizon of a black hole.

But of course, all those clocks are actually running at the same speed. Time always passes at 1 second per second for you, whether you are on a GPS satellite, on the surface of the Earth, or crossing the event horizon of a black hole. This is because time is not absolute, but is relative.

 

[dilligaff01]

The satellites relative speed causes a relative time difference of -7 microseconds a day. The weaker gravitational field out there causes a relative time difference of +45 microseconds a day. The net difference is +38 microseconds a day. Gravity wins! The clocks on a GPS satellite "run faster" than a clock at sea level, due to the difference in gravitational potential - gravitational time-dilation beats kinematic time dilation in this case.

For every small step forward I take here I end up taking two steps back in my limited understanding lol. If the gravity of the earth actually speeds up the internal clocks, why have I read that the gravity of a black hole is so intense that even time would slow down? Am I getting my science fiction and science fact mixed up?

The GPS satellite's internal clock runs faster than a clock on the surface of the Earth, due to the weaker gravitational field out where the GPS satellite is. So time on Earth runs slower, due to the stronger gravitational field down here, when compared to time on a distant GPS satellite. The stronger the gravity, the slower the clock, when compared to a clock in weaker gravity. In the most extreme case, a distant observer will see a clock stop completely as it crosses the event horizon of a black hole.

But of course, all those clocks are actually running at the same speed. Time always passes at 1 second per second for you, whether you are on a GPS satellite, on the surface of the Earth, or crossing the event horizon of a black hole. This is because time is not absolute, but is relative.

So the lesser gravity and limited speed affect the clocks and make them run faster than they should from our perspective .... and increasing either one will make the clocks appear to run slower, I'm assuming incrementally since the clocks are pre-adjusted for this. That tells me someone has measured this effect to counter it. If we bring the satelites to a slightly lower altitude and speed them up or a higher altitude and slow them down we can calculate the effect it has. Couldn't this be used to give us some kind of absolute reference for speed in the universe? couldn't we hypothetically send a satelite out into space to some point with little gravity and calculate the precise speed and direction it would need to travel to speed up it's clocks to the highest point? Basically it would be like a boat holding position in a river with just enough propulsion to counter the current. What would that tell us?

Most of you probably cringe when you see a post from me by now because of questions like this but I appreciate your time and patience with me

 

[csmyth3025]

...So the lesser gravity and limited speed affect the clocks and make them run faster than they should from our perspective .... and increasing either one will make the clocks appear to run slower, I'm assuming incrementally since the clocks are pre-adjusted for this. That tells me someone has measured this effect to counter it. If we bring the satelites to a slightly lower altitude and speed them up or a higher altitude and slow them down we can calculate the effect it has. Couldn't this be used to give us some kind of absolute reference for speed in the universe? couldn't we hypothetically send a satelite out into space to some point with little gravity and calculate the precise speed and direction it would need to travel to speed up it's clocks to the highest point? Basically it would be like a boat holding position in a river with just enough propulsion to counter the current. What would that tell us?

The effects of gravity and (relative) speed on time are taken into account in the way you're thinking of it by "Barycentric Coordinate Time". The Wikipedia article on it can be found here: http://en.wikipedia.org/wiki/Barycentric_Coordinate_Time. The first paragraph of that article explains:

Barycentric Coordinate Time (TCB) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the Solar system. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the barycenter of the Solar system: that is, a clock that performs exactly the same movements as the Solar system but is outside the system's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Sun and the rest of the system.

You should take note, however, that there is no such thing as an "absolute reference for speed in the universe". The speed of anything is always relative to something else. The universe has no "stationary" center.

Chris

 

[molecsur]

Of course you can determine your speed in space, and you don't have to talk about relativity or Doppler effect or CMB or any of that stuff. Measuring it directly is a challenge, but that's what math is for!

If you are in space, you are in orbit. Period. No exceptions. In may be an elliptical orbit or a parabolic orbit or a hyperbolic orbit, but there is some mass out there that is dominating your spacecraft because you are within its gravitational field. Within its sphere of influence. There are other masses out there perturbing your orbit but for purposes of determining your speed, just ignore them.

Given that one focus of your orbit is the thing you are orbiting, and that you are in the known solar system where we know the mass of everything there is to orbit, all you need to know are two orbital parameters, such as period and eccentricity and one position parameter, such as true anomaly. Many other combinations work as well. Then you apply the appropriate equations and you can find your instantaneous velocity. This is a vector, but by definition 'Speed' is the magnitude of that vector.

This velocity is constantly changing unless you are in a perfectly circular orbit, and it is well defined by the polar coordinate system and easily converted to rectangular coordinates or whatever.

As far as directly measuring your speed, if you can bounce a signal off the thing you are orbiting, and you know its mass (which you can always look up), and accurately measure the time for that signal to return (e.g. radar) you can use related equations to determine your speed from multiple measurements with the time between them known.

 

[csmyth3025]

Of course you can determine your speed in space, and you don't have to talk about relativity or Doppler effect or CMB or any of that stuff. Measuring it directly is a challenge, but that's what math is for!

If you are in space, you are in orbit. Period. No exceptions...

I'm not sure such a blanket statement is enirely accurate. It depends on where in space you happen to be. There are vast stretches of space between galactic clusters that would be hard to characterize as being meaningfully influenced by the gravitation of a particular object.

Chris

 

[molecsur]

Of course you can determine your speed in space, and you don't have to talk about relativity or Doppler effect or CMB or any of that stuff. Measuring it directly is a challenge, but that's what math is for!

If you are in space, you are in orbit. Period. No exceptions...

I'm not sure such a blanket statement is enirely accurate. It depends on where in space you happen to be. There are vast stretches of space between galactic clusters that would be hard to characterize as being meaningfully influenced by the gravitation of a particular object.

Chris

In intergalactic space, you would be orbiting the barycenter of the universe. When you get there, let me know and I'll expand the answer.

Even in interstellar space you will be orbiting the barycenter of the galaxy, which is not a particular object, so you got me there too.

My post is still true for anywhere in the solar system, so I was thinking that would count for something.

Maybe people are over-thinking this a bit?

 

[csmyth3025]

...If you are in space, you are in orbit. Period. No exceptions...

...My post is still true for anywhere in the solar system, so I was thinking that would count for something....

Your post is very good and it does remain true for the solar system, in general. When you include a phrase like "no exceptions", however, your inviting all the nit-pickers to come out of the woodwork.

Chris