# The Naked Scientists Forum

### Author Topic: Exceeding the speed of light?  (Read 4013 times)

#### wanhafizi

• Sr. Member
• Posts: 106
##### Exceeding the speed of light?
« on: 15/08/2009 09:48:12 »
Hi guys!

They said that speed of light cannot be exceeded by any means...

They said that speed is a measurement of relative rate of motion between two object, yes?

Consider this;

Two sources of photon (flashlights A and B) are directed toward each other and turned on...

Our theory said that both photons coming out of the flashlights are speeding at the speed of light relative to their sources, yes?

Question: What is the relative speed of a photon coming out of flash light A to a photon coming out of flashlight B?

Isn't it TWICE the speed of light?

#### John Chapman

• Hero Member
• Posts: 602
##### Exceeding the speed of light?
« Reply #1 on: 15/08/2009 10:28:14 »
No. The speed of light is ALWAYS constant, regardless of your perspective.

Apparently if you were in a spaceship travelling incredibly fast and you were passed by a ship travelling at a similar speed in the opposite direction and at the same time you both passed a third stationary ship then the light from your headlights would be travelling at exactly the same speed relative to all three ships. Because speed is equal to distance divided by time then, to make this impossible thing happen, time and space will both become distorted to accommodate it. So, the speed of light is ALWAYS constant.

#### Stefanb

• Full Member
• Posts: 75
##### Exceeding the speed of light?
« Reply #2 on: 15/08/2009 15:45:43 »
Like John said, the speed of light is always relative. Meaning if you are going 1 mph less than the speed of light, other light would not only seem to be going 1 mph faster. Instead it would still seem to be going (and it would be) the speed of light. In my mind it is easiest to understand this by thinking about how speed affects time. The faster you go, the slower time moves for you relative to an observer. If you are going at the speed of light, time would not be moving for you. So time does not pass for light. Because light is in this different realm of time and does not age, to me it separates light speed from other speed. Imagine comparing meters to meters per second. No matter what speed you go, a meter stick will never actually change size. And because we can never reach the speed of light (any object approaching the speed will start to gain mass to an infinite amount so we can never get to light speed) think of it as a separate unit. No matter what speed we go, light speed will never change.

Sorry if that way of thinking doesn't help for you :)

#### lightarrow

• Neilep Level Member
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##### Exceeding the speed of light?
« Reply #3 on: 15/08/2009 17:00:45 »
Hi guys!

They said that speed of light cannot be exceeded by any means...

They said that speed is a measurement of relative rate of motion between two object, yes?
No, not in the sense you are meaning.

Quote
Consider this;

Two sources of photon (flashlights A and B) are directed toward each other and turned on...

Our theory said that both photons coming out of the flashlights are speeding at the speed of light relative to their sources, yes?

Question: What is the relative speed of a photon coming out of flash light A to a photon coming out of flashlight B?

Isn't it TWICE the speed of light?

Yes, it is, but only if you mean this: you are at the centre, in an inertial frame of reference, you measure the distance between the two flashes of light (let's not talk of photons because it would introduce complications) and you divide by the time.

Unfortunately this is not a *physical* concept of velocity. For such a physical concept to exist you should put a frame of refence in at least one of the two flashes of light and this is impossible; alternatively, you can have a very fast starship instead of the flash of light; then you put a frame of ref. there and you measure the speed of the other flash of light with respect to the first. In this case you'll find it cannot exceed c. This is because the relative speed is not the difference of the speeds, or, said in another way, the relative speed is not their relative distance divided by the time.
If you want to find the correct law of velocity composition, it is:

Vrelative = (V1 - V2)/(1 - V1*V2/c2)

V1 = velocity of starship
V2 = velocity of flash of light

Example:

V1 = +200,000 km/s
V2 = -300,000 km/s (it's negative because it goes the opposite direction)
--> Vrelative = 500,000 /(1 + 2/3) = 300,000 km/s.
Try with other values and enjoy the result.
« Last Edit: 15/08/2009 17:03:59 by lightarrow »

#### wanhafizi

• Sr. Member
• Posts: 106
##### Exceeding the speed of light?
« Reply #4 on: 17/08/2009 01:41:49 »

I'm still tying to digest what you guys been saying.

Is it possible to explain in a layman terms?

tq

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Exceeding the speed of light?
« Reply #5 on: 17/08/2009 03:07:49 »
Quote from: wanhafizi
They said that speed is a measurement of relative rate of motion between two object, yes?
Only when one of the objects is at rest.

#### wanhafizi

• Sr. Member
• Posts: 106
##### Exceeding the speed of light?
« Reply #6 on: 17/08/2009 03:24:31 »
Quote from: wanhafizi
They said that speed is a measurement of relative rate of motion between two object, yes?
Only when one of the objects is at rest.

What is "at rest"? Everything is moving. Nothing stop...

#### lightarrow

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##### Exceeding the speed of light?
« Reply #7 on: 18/08/2009 08:40:24 »

I'm still tying to digest what you guys been saying.

Is it possible to explain in a layman terms?

tq
Ok, but first define "relative velocity" between two bodies.

#### Farsight

• Sr. Member
• Posts: 396
##### Exceeding the speed of light?
« Reply #8 on: 18/08/2009 14:15:02 »
I'm still tying to digest what you guys been saying. Is it possible to explain in a layman terms?
I think so. Think of the speed of light as something rather special. It isn't really a speed, it's a conversion factor between the measure you call distance and the measure you call time. Check out the two definitions below:

Under the International System of Units, the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom....

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second...

Now think about "a period of radiation". That's electromagnetic radiation. Electromagnetic radiation is light, thus the thing we call a second is being defined in terms of the motion of light. So is the thing we call a metre. So if you were moving very fast with respect to me, the thing you called a second would be different to the thing I called a second, and the thing you called a metre would be different to the thing I called a metre. This affects everything electromagnetic and more besides, including every atom of your body and your rulers and clocks. Things are very different between us, but you're still defining your seconds and metres using the motion of light the way it looks to you, so you'd still be measuring the speed of light to be 299,792,458 m/s.

If you and I were each moving towards one another at some very fast speed such as 0.9c, you could assert that you weren't moving, and it was me moving instead. If you measured how fast I went by, the result you get isn't 1.8c. It's rather like the way you still measure the speed of light to be 299,792,458 m/s. Here's the important point: you won't see me overtaking a light beam, so you still see me as moving slower than light. Hence we use the velocity addition formula like lightarrow was saying. In simple terms it's V1+V2 divided by 1+V1*V2, or 1.8/1.81. You'd see me moving at .994c.

We use the same rules when we're talking about two passing photons. Yes, it takes two years to illuminate a planet midway between two stars four light years apart. But nothing ever "sees" anything flashing by at more than the speed of light.

« Last Edit: 18/08/2009 14:17:50 by Farsight »

#### PhysBang

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##### Exceeding the speed of light?
« Reply #9 on: 20/08/2009 14:03:46 »
Check out the two definitions below:

Under the International System of Units, the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom....

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second...

Now think about "a period of radiation". That's electromagnetic radiation. Electromagnetic radiation is light, thus the thing we call a second is being defined in terms of the motion of light.
In what you have correctly written here, the second is being defined by a duration that we have a means of finding through an operation. There is no motion associated with a transition between hyperfine levels.
Quote
So is the thing we call a metre.
In what you have written, the metre is defined in terms of a distance that we can determine operationally.
Quote
So if you were moving very fast with respect to me, the thing you called a second would be different to the thing I called a second, and the thing you called a metre would be different to the thing I called a metre. This affects everything electromagnetic and more besides, including every atom of your body and your rulers and clocks. Things are very different between us, but you're still defining your seconds and metres using the motion of light the way it looks to you, so you'd still be measuring the speed of light to be 299,792,458 m/s.
While the results are correct, merely defining the second with regards to light does not guarantee the transition from one system of coordinates to another as given by relativity theory. One can construct a wholly Newtonian universe, with Galilean relativity, with these same definitions. Such a model would fail to match the available evidence, but it would still be logically consistent.

#### Vern

• Neilep Level Member
• Posts: 2072
##### Exceeding the speed of light?
« Reply #10 on: 20/08/2009 14:45:17 »
Quote from: PhysBang
One can construct a wholly Newtonian universe, with Galilean relativity, with these same definitions. Such a model would fail to match the available evidence, but it would still be logically consistent.
I don't understand two things here. What is a wholly Newtonian universe with Galilean relativity? And if one were so devised, how would it fail to match what available evidence?

#### The Naked Scientists Forum

##### Exceeding the speed of light?
« Reply #10 on: 20/08/2009 14:45:17 »