# The Naked Scientists Forum

### Author Topic: Speed of Light  (Read 14965 times)

#### pat

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##### Speed of Light
« on: 04/11/2002 19:40:56 »
I've always wondered how you actually measure the speed of light ?

Pat

#### chris

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##### Re: Speed of Light
« Reply #1 on: 05/11/2002 22:49:05 »
I actually have to credit 'physlink' for the answer to this. Anyway, here goes...

The Danish astronomer Olaus Roemer first measured the speed of light in 1676 using measurements of the eclipses of Jupiter's moons. He noticed that the eclipse of any given moon was 7 minutes longer when the earth was moving away from Jupiter in its orbit, than when it was moving towards Jupiter. He correctly argued that the time increases (by about 3 and a half minutes) due to the increased distance that the light has to travel when the earth is moving away from Jupiter. Conversely, when the earth is moving towards Jupiter, the time of each eclipse is correspondingly decreased by the same amount (3.5 minutes approx.)

Since the speed of the earth in its orbit was known by this time, the speed of light was calculated using the observed 7 minute variation in eclipse times. With this method Roemer came up with an answer of 140,000 miles per second, which is an excellent approximation of the correct value (186,000 miles per second) given the crudity of the method.

#### Broca

• Sr. Member
• Posts: 162
##### Re: Speed of Light
« Reply #2 on: 29/08/2003 03:47:00 »
No matter how hard you try, you will never be able to travel the speed of light. The faster you go you also increase the mass. We will always be at 99.9% of the speed of light.

#### bezoar

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##### Re: Speed of Light
« Reply #3 on: 30/08/2003 04:10:28 »
So if I slow dowm my exercise walk in the morning, will I decrease my mass?  Damn, and I've been walking briskly to do it!

Bezoar

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##### Re: Speed of Light
« Reply #4 on: 30/08/2003 07:19:14 »
Hehehe....science in action.

#### heythere

• Jr. Member
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##### Re: Speed of Light
« Reply #5 on: 01/09/2003 00:19:55 »
Ahhh, but the faster you walk the thinner you become. Have you not heard of length contraction? The faster an object moves on an axis the smaller it becomes on it. I love relativity!

#### bezoar

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##### Re: Speed of Light
« Reply #6 on: 01/09/2003 03:26:23 »
So, which axis do I have to move fast on?

Bezoar

#### heythere

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##### Re: Speed of Light
« Reply #7 on: 04/09/2003 00:25:08 »
The x axis...well forwards.
But you only appear thinner to things not in your field of perception...to you, you would remain the same and everything else would contract.

#### tom

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##### Re: Speed of Light
« Reply #8 on: 24/09/2003 10:04:01 »
quote:
Originally posted by bezoar

So if I slow dowm my exercise walk in the morning, will I decrease my mass?  Damn, and I've been walking briskly to do it!

Bezoar

True. At zero speed, your mass is let's say 70,000000000000 kg. At 5 km/h your weight raises to 70,00000016 kg (0,16 micrograms more!).

#### tom

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• Posts: 22
##### Re: Speed of Light
« Reply #9 on: 24/09/2003 12:37:54 »

True. At zero speed, your mass is let's say 70,000000000000 kg. At 5 km/h your weight raises to 70,00000016 kg (0,16 micrograms more!).
[/quote]

Sorry, wrong calculations. This mass is achieved at 14 km/sec. At 5 km/h your mass raises for some femto or attograms (it can't be calculated with calculator)

#### chris

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##### Re: Speed of Light
« Reply #10 on: 24/09/2003 14:37:56 »
neat work Tom !

Chris

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- Groucho Marx

#### Pappy

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##### Re: Speed of Light
« Reply #11 on: 19/11/2003 15:31:51 »
Zero speed, is that possible? Isn't everything in motion?

#### Ylide

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##### Re: Speed of Light
« Reply #12 on: 19/11/2003 18:21:16 »
It's relative.  =)

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#### Quantumcat

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##### Re: Speed of Light
« Reply #13 on: 25/11/2003 13:15:05 »
yeah ...

If you're sitting in the car, you're still but you are moving compared to the ground. Some one standing on the ground is moving compared to the sun. Someone on the sun is moving compared to the galaxy, etc etc etc

Am I dead? Am I alive? I'm both!

#### qpan

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##### Re: Speed of Light
« Reply #14 on: 05/12/2003 00:28:04 »
There is no such thing as absolute speed (except speed of light)- where would you measure it from? Everything's speed is relative (except light)- e.g. car's speed relative to road, Earth's speed relative to sun. However at speeds close to the speed of light, something strange happens as according to einstein's theory of relativity, the speed of light is constant for any observer from any system (i.e. travelling at any speed). Therefore, if you were travelling from a star at 50% the speed of light, the light emitted from the star would still travel past you at the speed of light and not 50% the speed of light- you cannot gain speed on light! Instead, time would bend!

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

#### Quantumcat

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##### Re: Speed of Light
« Reply #15 on: 05/12/2003 07:08:31 »
How do you know that, or anyone know that? No one's ever gotten anywhere near the speed of light, then measured the speed of light to see that it's the same again?

Am I dead? Am I alive? I'm both!

#### qpan

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• Posts: 260
##### Re: Speed of Light
« Reply #16 on: 05/12/2003 12:17:25 »
That is the whole basis of Einstein's theory of special relativity! With a name like quantum cat, you should know that fact already! The reason that you travel slower through time the faster you travel is also based on this fact- time needs to distort in order for light to travel at the same speed for everyone! The speed of light is the same for everyone regardless of which reference frame you are in. See below:
http://en.wikipedia.org/wiki/Speed_of_light

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

#### Ylide

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##### Re: Speed of Light
« Reply #17 on: 05/12/2003 18:22:27 »
I think the Doppler Effect helps reinforce what qpan is saying.  The fact that your speed can affect your perception of the wavelength of electromagnetic energy without actually changing the energy is a pretty good example of relativity.  For this to occur, the light would need to propagate at a constant speed but from a moving source and/or observed by a moving observer.

It's the same with sound waves propagating through air...sound doesn't move faster or slower if the object emitting the sound is moving, it just changes the frequency of the sound relative to an observer.

BTW, I got your IM's quantum, but I was sleepin.  =P

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#### OmnipotentOne

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##### Re: Speed of Light
« Reply #18 on: 21/02/2004 01:24:11 »
So obviously the faster you go the larger in mass you get, but what about if you get slower?.......Isnt there some computer that uses light to transmit data or somthing, quite interesting

#### Ylide

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##### Re: Speed of Light
« Reply #19 on: 21/02/2004 06:43:51 »
Actually, the mass of an object is independent of speed. Some textbooks talk about relativistic mass, but what they are really talking about is that the kinetic energy and momentum increase more quickly than 1/2 m v^2 for large velocities, and if you stick with the old classical equations, you put the extra stuff in the mass term. But I prefer to think of the mass as being due to the number of atom the object contains, and use the correct relativistic expression for kinetic energy (gamma - 1)mc^2 or momentum (p = gamma m v), where gamma = 1/sqrt(1 - (v/c)^2).

In summary, this doesn't mean that mass changes with speed, it means that objects with mass cannot achieve the speed c.  There is in theory a maximum velocity (less than c) for any object that is a function of its mass.

Also, it should be noted that the value of c is when it is in a vacuum.  the actual speed of light changes with its medium...recently several labs supposedly broke the speed of light, but all that really happened is they created a medium in which light could travel faster than c.  (just as a medium like water makes light travel slower than c)  Inside this medium, light speed is still the maximum velocity....when those same photons transitioned to another medium, such as air, they would slow down.

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#### bezoar

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##### Re: Speed of Light
« Reply #20 on: 21/02/2004 15:55:56 »
The speed of light was the speed at which my ex boyfriend drove his car.

#### OmnipotentOne

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• Posts: 146
##### Re: Speed of Light
« Reply #21 on: 24/02/2004 19:50:16 »
SO the speed of light is 670million mph, and that is a fixed speed, so unless we somhow go faster that speed will alwasy be fixed.

#### qpan

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• Posts: 260
##### Re: Speed of Light
« Reply #22 on: 24/02/2004 21:33:34 »
I thought the speed of light was 299 792 458 m / s ?

Also, no matter how fast or slow you go, the speed of light still remains constant (it is impossible to catch up with a photon).
If you travel "faster" than light, what actually happens is time majorly dilates to compensate for the fact that you observe light to be travelling at 299 792 458 m / s (so you effectively travel back in time, if it is possible and assuming you could actually travel "faster" than light).
A simple thought experiment to prove this:
Due to relativity, we are entitled to say that we are, at this instant travelling faster than light in some reference frame (and everything around us is also travelling faster than the speed of light at a similar speed to us). Now as we are theoretically travelling "faster" than light relative to a point in space,  we can measure the speed of light in a vacuum, and it is 299 792 458 m / s, as proved by numerous scientists. We are perfectly entitled to do this as there is no such thing as absolute speed -  and this simple experiment shows that the speed of light is the same for everyone, even if they are travelling "faster" than the speed of light relative to some point in space. In our experiment, the reference point would have to actually be a moving point in space travelling faster than the speed of light away from us. We would be travelling backwards in time relative to this point, but as our surroundings are travelling at similar speeds to us, we are NOT travelling backwards in time relative to our surroundings.

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
« Last Edit: 24/02/2004 21:51:39 by qpan »

#### Ylide

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##### Re: Speed of Light
« Reply #23 on: 25/02/2004 01:51:47 »
Just to clarify:

The speed of light is fixed in it's medium.  It's very important to note the distinction.  The speed of light in a vacuum is 2.9979x10^8 m/s only in a vacuum.  It is slower in most other media (gases like atmospheres, liquids, clear solids) and faster in a select few.  (can't remember what media they used to get light going faster than c)

Either way, no matter what the medium, you're still not going to, as gpan put it, catch up with a photon.

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#### qpan

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##### Re: Speed of Light
« Reply #24 on: 25/02/2004 19:22:52 »
I think they used caesium vapor to get light to travel faster than the speed of light in a vacuum.

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

#### The Naked Scientists Forum

##### Re: Speed of Light
« Reply #24 on: 25/02/2004 19:22:52 »