Hi

I'm pretty new to this kind of thing but I have a question about the actual speed of spacecraft.

Is all speed of spacecraft measured in space relative to the things we consider to be "still" (i.e the solarsystem) or is it measured to absolute "stillness"? If so, how do you stay completly still in space if its even possible.

Another question is, since the galaxy is moving and spinning, would it be possible to "break" a spacecraft in order for stars etc come to you? That would mean to decelerate instead of accelerate which in my mind anyways seems more efficient but I'm probably wrong

Many thanks!

There is no such thing as absolute rest. All motion is measured relative to some convenient reference system. In the case of space craft, the motion is usually measured with respect to Earth (for near-earth orbital missions), the Sun (for interplanetary missions), or the target planet (for orbiter missions, once orbit of the target is achieved).

I think you mean "brake", not "break". It is possible, and is done in some limited cases (for example the Messenger mission), but the cases where it is used generally rely on gravitational assists for the lion's share of the breaking. Adjusting speed in space by any significant amount using strictly propulsion requires rediculous amounts of fuel.

Physically speaking, there is no difference between acceleration and deceleration - they're both just a change in speed. To "brake" a spacecraft so that a star behind us in galactic rotation catches up with it would cost exactly the same amount of fuel and time as "speeding" it up so that it catches up with an equidistant star ahead of us.

Our everyday experience that deceleration is easier than acceleration comes from the fact that we're surrounded by the air, friction with with tends to "pull" all velocities towards its own - to increase your speed you have to work against the friction force, to decrease your speed it works with you (assuming no winds!). Out in interplanetary or interstellar space, the medium is much too tenuous to exert much friction, and it participitates in galactic rotation anyway, so what tiny friction force there is will work against you whether you want to go faster or slower than the galactic rotation.

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The problem is, the OP meant to ask, is it possible to "brake" by using some mechanism to slow the spacecraft to the "absolute rest." He or she is imagining that there is an immovable ether, and wondering if it's possible to have some kind of device like a tire that would slow down the spaceship. And the answer is that it is not possible, because there is nothing fixed to "brake" to.

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As above, so below

I think all of you guys understood my question and thank you for your replys, very helpfull indeed!

Well, I've been thinking for awhile now (not my strong side)

If you are inside a spacecraft moving in some direction and observing light inside that spacecraft "flowing" from the back of the craft to the front. Would that light from another "perspective", say earth move faster than the speed of light since the spacecraft itself is moving in some additional speed to the speed of light.

Tanks.

It's a very common question here. I think there are many threads dedicated to that question. But very simply, the answer is no. One of the features of the speed of light is that is doesn't change depending on your movement. If you are speeding toward one lamp and moving away from another, the light from the two lamps will still come to you at the same speed, c. If you read about Einstein you'll quickly come across that.

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As above, so below

I'm actually quite familiar with einstein and his work, thats why I asked. I somehow managed not to come across any information regarding moving objects emitting light.

I do have another question though, as always its probably just my mind playing tricks on me. The hubble deep field image shows galaxies in the begining of the universe 14 or so billion years ago. As I understand it the light that is arriving to us now would have been emitted when we were also part of that same primeval soup of galaxy birth since the universe was denser, not as we are now ofcourse but some of the particles. Therefor that same light should have already hit us back then in the early universe unless we somehow managed to out run it?

I realize I'm fumbling in the dark here I'm just looking for some answers

Thanks again.

Yes, we outran the light coming from the galaxy earlier on as the expansion was quicker then and we were carried along with it.
No, it doesn't violate Relativity since our coordinate speed with respect to space locally was always zero. It wasn't us moving, it was the space between us and them that was expanding.

Oh and Einstein's earlier work is all about the implications of light always moving at the speed of light regardless of the speed of the observer.

It always come back to Albert Einstein, the Boris Johnson of the Physics world.

All attempts to measure the speed of light have found that no matter how you measure, where you measure, why you measure it, who is measuring, you get the idea, it is always the same.

If we start from that premise, indeed it is premise number 1 of the Theory of Special Relativity, then we get to all the business of length contraction, time dilation etc. If a spacecraft shoots away from us at speed v and we fire a beam of light at it. Does the spacecraft observe the beam of light as travelling at c-v? No. It observes it travelling at v also. What happens is that time dilates and space contracts to make that happen.

One thing the spacecraft would observe differently though is the wavelength of the light. Because it is moving away, it would be redshifted.

http://www.fourmilab.ch/cship/cship.html

Check this out. It's great for learning how the relativistic speeds affects things.

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The only way x > c is if you use Newtonian mathematics, which special relativity replace... in order to fully understand it you need to familiarize yourself with the mathematics of SR

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