The Speed of Light is the same for all observers.

Pants

Pants

Pants

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I'm just trying to get my head around this. How is the speed of light constant for all observers? Can someone help me out using this realistic example?

Say that someone has a torch (a bright one) on earth. We have two observers, Ship 1 and Ship 2.

Ship 1 blasts off at 0.8c... A few hours later:
Ship 2 blasts off at 0.9c in the same direction.

At the same time Ship 2 is passing Ship 1, the torch is flashed. For both ships to observe the same speed of light, surely the light would have to reach them at the same time?

But it wouldn't, ship 1 would see the light sooner, as by this time Ship 2 would be long gone...?

It's constant if we're observing from earth, and it makes sense that it would reach Ship 1 first. But how is it constant from the perspective of the ships?
 
Pants said:
For both ships to observe the same speed of light, surely the light would have to reach them at the same time?
Click to expand...

Why?
If two cars were driving away, one at 40mph and one at 50mph, and both were shot at from the same distance by two identical bullets going at 500mph, the the car travelling at 50mph would be hit marginally later. However, the distance the bullet travelled would also be greater (having travelled slightly further from the gun), so (ignoring resistance and all that) the bullet would have still travelled at the same average speed.
I think.
 
Pants said:
I'm just trying to get my head around this. How is the speed of light constant for all observers? Can someone help me out using this realistic example?

Say that someone has a torch (a bright one) on earth. We have two observers, Ship 1 and Ship 2.

Ship 1 blasts off at 0.8c... A few hours later:
Ship 2 blasts off at 0.9c in the same direction.

At the same time Ship 2 is passing Ship 1, the torch is flashed. For both ships to observe the same speed of light, surely the light would have to reach them at the same time?

But it wouldn't, ship 1 would see the light sooner, as by this time Ship 2 would be long gone...?

It's constant if we're observing from earth, and it makes sense that it would reach Ship 1 first. But how is it constant from the perspective of the ships?
Click to expand...

To put it simply it's to do with time dilation. Time goes at different speeds for each observer, so as you get quicker the time slows more and more.
 
Antelopethy said:
Why?
If two cars were driving away, one at 40mph and one at 50mph, and both were shot at from the same distance by two identical bullets going at 500mph, the the car travelling at 50mph would be hit marginally later. However, the distance the bullet travelled would also be greater (having travelled slightly further from the gun), so (ignoring resistance and all that) the bullet would have still travelled at the same average speed.
I think.
Click to expand...

In your scenario the 40mph car would see the bullet gaining on it at 460mph, and the 50mph would see the bullet gaining on it at 450mph. However light always appears to be travelling at 300,000km/s no matter how fast you are going, and this is what the op is trying to understand. Both cars would see a beam of light gaining on them at 300,000km/s no matter how fast they are going.
 
as ship 2 is going faster, the passage of time for him is slower relative to ship 1

So both ships perceive the same passage of time at the instant of the torch flash.
 
Judgeneo said:
as ship 2 is going faster, the passage of time for him is slower relative to ship 1

So both ships perceive the same passage of time at the instant of the torch flash.
Click to expand...

Christ that's going to be even harder for me to get my head around!

Edit: So if you actually had a stopwatch on both ships, they would record the same amount of time has passed before they saw the torch light. But if you had a stopwatch recording the same thing from earth you would record that the light got the Ship 1 first, then record a later time for Ship 2. That makes my head hurt.
 
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Yup, its pretty confusing to say the least. Especially the first time you meet it.


My advice would be to immerse yourself in the mathematics, as rigorously as your teacher/textbooks allow. It gave me a much better understanding of how things work
 
Light travels at different speeds through different materials - fastest through a vacuum, slowest (without stopping altogether) i believe is through diamond?
 
Pants said:
Christ that's going to be even harder for me to get my head around!

Edit: So if you actually had a stopwatch on both ships, they would record the same amount of time has passed before they saw the torch light. But if you had a stopwatch recording the same thing from earth you would record that the light got the Ship 1 first, then record a later time for Ship 2. That makes my head hurt.
Click to expand...

It's easy to understand if you draw a diagram of it. Imagine a light source 3million km away from both ships and then plot how far the light and the ships will have travelled after each second from an observers point of view while calculating how much time has passed for each of the ships, (easier if you consider one of the ships just to be stationary).
 
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PermaBanned said:
Light travels at different speeds through different materials - fastest through a vacuum, slowest (without stopping altogether) i believe is through diamond?
Click to expand...

current record is 8m/s apparently.

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

You can use the same technique to stop light entirely as well. Or in theory you could stop it using nano rings and the Aharonov-Bohm effect

http://en.wikipedia.org/wiki/Aharonov–Bohm_effect


Cool stuff
 
Using the same principal you can in theory travel into the future this way (time travel into the past is unlikely to ever be possible), if you orbit a super massive object at near light speed you could end up years into the future in very little time due to time dilation. The speed of light does change around the Universe though. Close to a black hole light cannot even escape and at the centre all time stops.

Just need some kind of near light speed propulsion as we already have a super massive Gilly :p




Hehehe!
 
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aardvark said:
to be fair he's right - only the 'apparent' speed of light changes.
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No no, light actually does travel slower through a medium. Its actually possible to break the speed of light in water for instance, it happens in nuclear reactors sometimes, and a pulse of light is emitted by the speeding object, similar to how you create a shockwave when you break the speed of sound.

The speed of light in a vacuum however, is constant, and can never be reached.
 
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