[irish del (OP)]

hi all
I'm new to this site ,having read a briefer history in time I've been left with loads of questions that i cant get my head around....
if i took of in a space ship would i be moving in space or would space be moving outside the ship?
how can i measure speed..OK i could be moving away from the earth at speed but you could argue that it is the earth that is moving away from me...
and i could be moving at an even greater speed in relation to an object that's allready moving at a terrific speed away from the earth and me ....
if my speed cant be measured then am i going at the speed of light to a photon of light moving in the opposite direction
am i moving???

[RD]

i could be moving away from the earth at speed but you could argue that it is the earth that is moving away from me...

You in the spacecraft would experience force due to the acceleration as you gained speed, we on Earth would not: not symmetrical.

[irish del (OP)]

but what am i gaining speed in relation to the earth.
i could be moving towards another object that is moving faster then me in the same direction so i would be in fact slowing down to that object .
if i am slowing down to that object then how can i be acceleration...

what speed am i going?

[lyner]

The present thought (relativity) is that there is no fundamental 'grid' through which we move. Any motion is relative to something else.
Get in a spaceship and blast away in it for an hour at maximum thrust. Once the engine has stopped accelerating you, you can look at another ship and it will be going at a speed and in a direction relative to you. He will measure the opposite about you.

To say which of you is moving and which is stationary is meaningless.
Any change of speed or direction of motion is called acceleration. What you refer to as deceleration is much better to call negative acceleration.

[irish del (OP)]

when Einstein talks about an object traveling towards the speed of light how is the speed being measured ,against which object in the universe is it being compared too....
don't stars and planets all move in different direction to each other and at different speeds,how can you tell the speed of any object without comparing it to all objects in the universe......
or is speed only relative to the objects original position over a given time..
thanks for your replies so far

[RD]

As we now know, the fixed stars are not fixed. Those that reside in the Milky Way turn with the galaxy, exhibiting proper motions. Those that are outside our galaxy (such as nebulae once mistaken to be stars) participate in their own motion as well, partly due to expansion of the universe, and partly due to peculiar velocities.[3] (The Andromeda galaxy is on collision course with the Milky Way at a speed of 117 km/s.[4]) The concept of inertial frames of reference is no longer tied to either the fixed stars or to absolute space. Rather, the identification of an inertial frame is based upon the simplicity of the laws of physics in the frame. In particular, the absence of fictitious forces is their identifying property.

http://en.wikipedia.org/wiki/Inertial


http://en.wikipedia.org/wiki/Absolute_space_and_time

[lyner]

when Einstein talks about an object traveling towards the speed of light how is the speed being measured ,against which object in the universe is it being compared too....

The speed is measured relative to the observer.

[Flyberius]

The speed is measured relative to the observer.

Indeed.  Once you swallow that pill it all begins to make sense.

Like with the two spacemen floating past each other, and each one seeing the other in slow motion.  I never understood that till I went back and read it all over.

The speed of light is relative to the observer and the rest all flows from that.  Still thats about as far as my knowledge on the subject extends.

[irish del (OP)]

If theres no one to observe except myself on the craft how can the speed be measured.........should i also take into account the original speed of the earth(from which i took of from) as it moves through the solar system,galaxies,universe if i'm going in the same direction as the earth.
 

[lyner]

NO. You just see what you see. That is the only information you need.

[irish del (OP)]

NO. You just see what you see. That is the only information you need.