h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

OK, that's a physical problem.
If you want to find out how fast an object is moving,
you have to measure two things.
The time and the distance that the object has moved in this time.
To get the time you only have to take a clock and watch the seconds running away.
So far so good.
To measure the distance sounds easy as well, but it isn't.
A car for example driving from Los Angeles to San Fransisco.
You could measure the distance between this to cities but its not
the real distance the car has moved.
Meanwhile the earth on which the car is driving has moved either.
And our sun system, our galaxy and so on are also always on the move.
The result is you cannot really calculate the velocity using the distance.
There must be some further possibility which I don't know.

Aren't you making life a little complicated for yourself? When most people want to know how fast they travelled between Los Angeles and San Francisco, they're only interested in the distance travelled over the surface of the Earth and the time taken to do it. OK, so the Earth has moved, and the Solar System has moved and the whole galaxy has moved. Big deal, so what?

The real problem with measuring distance is that none of our measuring devices are accurate enough over such distances. OK, there's probably some computerised means using GPS, but that will only give you the distance 'as the crow flies'. The real distance depends on the route you take, which is harder to measure.

Besides, measuring the time is also dependent on the accuracy of the clock you use. Consider the sources of such large scale errors of distance and time before considering the minutiae of the movement of astral bodies.

That thing about L. A. and San Fransico is only an example,
maybe it wasn't a good one.
Take two space shuttles instead.
You are right if you are saying that satellites are stationary in relationship to the Earth.
My problem is that thing called time dilatation.
Someone has proven that thing within an experiment.
They have put an very exact clock into an satellite
and the same one was left on earth.
After some circles around our planet the satellite's clock was slow.
And it was the expected value of time dilatation.
To calculate that value, you have to include the movement of our sun system and so on,
because its movement is very fast more than the car which drives from L.A. to where ever you want.
I don't know how fast.
So I think you need some other way of measure the velocity without time or distances.
And this way I wanna to know.
Or you need an fixed point in our universe.
Maybe there is one.
Who knows ?

Since velocity = distance / time, and you regard time as relative (OK, so it is), no one will ever be able to calculate velocity to your satisfaction. Generally, though, people take where they are as stationary and measure distance, time and velocity relative to that point. Unless you ignore time dilation, such calculations have no solution.

Hmm, you want to measure velocity without bringing distance and time into it? That´s as stupid as that guy who claimed he could prove an arrow didn´t move. You should look into what the whole definition of velocity is. And for the rest of the stuff look into Einstein´s theories, they´ll explain it all to you.

I don't meant without distance AND time, but without distance.
How about the doppler frequency shift ?? (Thanks to Bossel for the hint)
Provided that you have a lamp in front of your spaceship
(like the carrot in front of the donkey to encourage him to walk)
and you know the frequency of the emitted light you could always calculate
your real speed. Because the light will be emitted always with light speed
(I hope Einstein was right) and the spaceship will fly faster through the (light-)wave
and the frequency will be higher for the observer in the spaceship.
In this case you also need the moving object relative to you (the lamp) but you don't care
how fast the lamp is moving because the emitted light's velocity is always the same.
This could work ???!
Or if you have some of that lamps hanging around in space
that will be better, I think. So you could calculate every vector of your velocity ...

Mostly folks don't bother to compute absolute velocity for a terrestrial journey because factors like doppler shift and the speed moved over the earth's surface are negligible compared with the velocity of earth in space. Which means that, to a fair approximation, all objects on the planet at the same latitude are moving at pretty much the same speed. Which is pointless.

Measuring velocity without distance is just as impossible. You need both of them to do anything. Besides, the doppler effect IS determined by distance, the distance you travel per time, which is velocity.

And about hanging lamps in front of the spaceships(Enterprise wouldn´t be as cool if it had bicycle lamps), I don´t think that would work. You would probably just see one shift, and those standing next to the spaceship would see another. But I´ll think more about it.

Anothe thing however, is that somebody mentioned the speed of light might be slowing down. And I came to think about how you would actually measure that. If light suddenly slowed down to 200.000 then wouldn´t time also slow down a bit? And then the proportions would still be equal. Is that just my brain malfunctioning, or is there something wrong with that.

Now measuring the speed of the galaxy and such just for going from LA to San Francisco wouldn´t make any sense. The two places would be at the same distance to each other now matter how fast the Earth was speeding, so you don´t notice anything. And if you reach lightspeed and the Earth starts to shrink because of that, you´d shrink at the same level, so you wouldn´t notice it.

I don't believe that time will slow down if the speed of light slows down. But it will make it very difficult to nip down to the off licence if the kids have all grown up and left by the time I get back!

Funny, I know of a chap who went to the corner shop for a bottle of milk and got home three months later. I think the reason had something to do with a visit to the pub, a young blonde female and a holiday in the south of France. When he got back his wife had grown up and left.