# The Naked Scientists Forum

### Author Topic: Is speed relative?  (Read 1364 times)

#### thedoc

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##### Is speed relative?
« on: 17/07/2012 07:30:02 »
Luke Bizeray  asked the Naked Scientists:

If a ship accelerates to say, two thirds the speed of light, while another ship accelerates to the same speed from the opposite direction, are they actually moving towards each other at more than the speed of light? What would a person on each ship see as they approached and as they passed (ignoring the obvious limitations of human sight)? How is this different from a ship moving away from the earth at 1.33... times the speed of light?

Regards,
Luke

What do you think?
« Last Edit: 17/07/2012 07:30:02 by _system »

#### Soul Surfer

• Neilep Level Member
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• keep banging the rocks together
##### Re: Is speed relative?
« Reply #1 on: 17/07/2012 08:32:38 »
This is the classic relativity paradox and it leads to the reasons why when you are moving very fast or watching a very fast moving object time flow changes and dilates.

You would not and could not see each other moving faster than the speed of light.  You would see the other ship moving towards you faster than you would see stationary objects in the same direction moving towards you but the ship would be going slower than the speed of light and would approach the speed of light as you both approached the speed of light if you both accelerated.

There is a classic equation that describes the speeds that you would measure but frankly it does not mean much to most people.  All you need to understand is that because of the time effects you appear to be approaching each other slower than you really are.

Of course a stationary observer standing somewhere between you would see you and the other ship moving towards him at above half the speed of light in different directions but never see anything that exceeds the speed of light.  it is just that at these very high speeds the perception of space and time becomes distorted in a non linear way.

#### why

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##### Re: Is speed relative?
« Reply #2 on: 18/07/2012 00:25:29 »
does this mean that the two beams of particles in the LHC travelling at close to speed of light in opposite directions collide in a different time speed than outside of the collider?

#### Soul Surfer

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##### Re: Is speed relative?
« Reply #3 on: 18/07/2012 08:55:45 »
I do not understand what you are trying to say when you say "time speed".

Please remember that time too is relative and there is no absolute time reference.   We can only measure time differences between two events happening as seen by a single observer. What two or three observers moving differently measure as the time difference between events happening is their own business.

An "observer" riding on the particles in the LHC would measure time passing perfectly normally but the image of say the joints in the pipe it is travelling through would look very distorted compared with when it was moving slowly.  There are some interesting videos showing how the image of a "normal" world distorts as one moves very fast.

watch
http://io9.com/5850799/what-would-the-world-look-like-if-you-approached-the-speed-of-light
To see a video of the effects of travelling very fast on your image of your surroundings.

When the LHC is being used with lead nuclei we "see" the colliding nominally spherical nucleii flattened out and colliding like face on coins.  Each nucleus "sees" itself as perfectly spherical but "sees"  the approaching nucleus as a flat disc too.
« Last Edit: 18/07/2012 09:16:40 by Soul Surfer »

#### The Naked Scientists Forum

##### Re: Is speed relative?
« Reply #3 on: 18/07/2012 08:55:45 »