# The Naked Scientists Forum

### Author Topic: What reference point do we use to measure speed?  (Read 3127 times)

#### gorjenk

• Guest
##### What reference point do we use to measure speed?
« on: 16/11/2010 19:30:03 »
Gordon Kubank  asked the Naked Scientists:

I find it hard to believe that nothing can go faster than the speed of light.

Speed is relative so when a rocket is way out past our solar system we still seem to refer to its speed in relation to earth. How do we know how fast it is going relative to a fixed point in space if everything is moving at different speeds, because when it left earth we were travelling through space at a pretty high speed.

Could the whole universe be travelling through space at 10 times the speed of light. If everything is travelling together we have no way of knowing how fast anything is really travelling, or do we.

I have heard of galaxies moving at a high percentage the speed of light.  Where is the reference point to measure the speed?

Red & blue shift is used to measure speeds towards us or away from us. How do you measure speed of a star or galaxy travelling across our path?

Gordon Kubank
Sunshine Coast
Queensland

What do you think?
« Last Edit: 16/11/2010 19:30:03 by _system »

#### QuantumClue

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##### What reference point do we use to measure speed?
« Reply #1 on: 16/11/2010 20:01:59 »
Speed is relative so when a rocket is way out past our solar system we still seem to refer to its speed in relation to earth. How do we know how fast it is going relative to a fixed point in space if everything is moving at different speeds, because when it left earth we were travelling through space at a pretty high speed.

Because an object tends to stay in a fixed motion unless effected by some external force. The is one of Newtons Laws and it defines an inertial body.

Basically, nothing can travel faster than lightspeed, because for massless particles, lightspeed is a constant speed, no particle discovered has broken this law. There are solutions to relativity where an object can travel faster than light, but not accelerate past it. It has an infinite amount of energy at its lowest speed, the speed of light, just as much as a particle with matter would require an infinite amount of energy to reach the speed of light.

Moreover, these exotic hypothetical superluminal particles have now more or less been discredited by maintream because of what is called ''tachyonic condensation''.

#### syhprum

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##### What reference point do we use to measure speed?
« Reply #2 on: 17/11/2010 04:39:53 »
"Could the whole universe be travelling through space at 10 times the speed of light."
This is a meaningless statement, the universe by definition is all there is there is no space within which it can travel!!

#### Geezer

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##### What reference point do we use to measure speed?
« Reply #3 on: 17/11/2010 07:15:16 »
"Could the whole universe be travelling through space at 10 times the speed of light."
This is a meaningless statement, the universe by definition is all there is there is no space within which it can travel!!

Syhprum, I'm sure you've heard of "thinking outside the box". This is an example of "thinking outside the Universe"

#### Soul Surfer

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• keep banging the rocks together
##### What reference point do we use to measure speed?
« Reply #4 on: 17/11/2010 09:06:22 »
gorjenk you have just realised the relativity problem that was first addressed by Galileo back around 1600  refined by Newton and gravitation in the 1700s and put to bed for the moment by Einstein around 1900.  Read and understand these replies and you will be on the way.  any standard physics text book should explain things for you.  You are creating problems in your mind that really are not there.

The measurement of across the path velocities is only possible for objects that can be seen to change position in the sky this means only closer objects and quite high speeds.  Even then there can be problems when what you are seeing is not an object that is actually moving but a bright flash of light that is travelling through a cloud that reflects a bit of it back.
« Last Edit: 17/11/2010 09:12:56 by Soul Surfer »

#### The Naked Scientists Forum

##### What reference point do we use to measure speed?
« Reply #4 on: 17/11/2010 09:06:22 »