[Dewald Visser]

Dewald Visser  asked the Naked Scientists:
   Hi there you naked scientists, I really love your podcasts, it has transformed my twice-daily commute between Pretoria and Johannesburg to the best time of the day!!!

Einstein stated that an object in space that is at rest or at constant velocity will not be able to determine the difference, right? So then how does a spacecraft measure its own speed, is it even possible?

Am I wrong in assuming it can only be done from outside the spacecraft like earth, or is there some trick that is used?

Please keep up the good work, regards to all from sunny South Africa.

Dewald Visser

What do you think?

[Soul Surfer]

When you are dealing with unmanned spacecraft there is always one or more two way radio or microwave links between the spacecraft and earth.  As well as communicating information and instructions between the earth and the spacecraft they can also function as an extremely precise range measurement device with an accuracy of the order of or better than the wavelength of the radio or microwave signals.  The same would apply to manned spacecraft.  This gives ranging information and radial velocity and acceleration.  Other information from this can be calculated from the known positions and velocities of the earth station and the basic gravitational field calculations as the body moves through space used by astronomers. This may include the effects of several gravitating bodies like the sun, earth and moon or  the sun, earth, jupiter and one of its moons etc .  It is quite a complicated process but very accurate.

[yor_on]

If you're asking if there exist a 'absolute speed' that one can measure inside that ship, I don't think there is. We have a constant called 'lights speed in a vacuum' that represents the 'fastest' anything is expected to go (excluding 'FTL neutrinos' now in main stream science. There are a lot of experimental evidence of this being correct, but you can't measure yourself against that 'speed', as light don't care what you think/define as your speed. That constant will always be a constant 'c' to you in the ship, no matter how fast you measure yourself to go relative Earth, or the solar system.

What you can do is to use the blue shift to get an approximative 'speed', as a guess, when getting to relativistic speeds that is. But there it also has to do with how the stars are 'moving' relative you. Maybe using so called 'fixed stars' very far away can help, but I'm not sure of it.

So the speed 'c' is better described as a constant to me, than as a measure of a 'global speed limit', although all 'relative motion' if taken to its limit should be able to come close to that constant.

[Bill S]

What about speed measured relative to the cosmic microwave background; would that work?

[yor_on]

As a speed measurement relative the CBR it should. But it doesn't guarantee a absolute speed as we have no way of knowing if the CBR is a absolute frame of reference. And I'm not totally sure on how the CBR is thought to work if we use the 'balloon expanding'.

If you think of that 2D example and then consider the visible 'horizon' around Earth the CBR seems more or less isotropic (being the same) for us but if we jump to some other spot on that expanding balloon they should see the same phenomena, although the 'space' is expanding between those two spots. So we may be relativistically moving relative that other spot at the same time as we when measuring ourselves against the CBR find ourselves to have another speed. What I mean is that I doubt that you can use it as a 'global map', also defining a speed relative that 'other spot' on that expanding balloon.

How Fast Are You Moving When You Are Sitting Still?

And we can't really say what is 'still' here, can we? 'Uniform motion' is a definition we have for something being the exact same, no matter your speed relative the Earth, or CBR, inside a 'black box experiment'. Then we have one more, being 'at rest' relative something, as me relative my room, at rest with the ground, at rest with Earth. But that last one is arbitrarily made.

Which makes the idea of calling lights speed in a vacuum a 'constant' preferable to me, before calling it a defined speed. That as when you compare 'frames of reference' conceptually you find yourself able to question that definition, even though it 'locally', using your ruler and clock always will be the same. Speeds are weird, constants too

But constants are at least easier to understand.

[cover it]

Of course spaceships have a speedometer, how else would they know how far they have travelled lol