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Anonymous Poster

# Light Speed Thought Experiment...

10/19/2006 2:39 PM

Take a closed and evacuated pipe; put a lamp (and all accomanying power and controls)at one end, and a separate light recieving circuit at the other end, and pipe is accelerated longitudinally to the speed of light, (with the lamp in the trailing end), and the lamp is switched on and off repeatedly , will the reciever ever record a signal of reciept?

Alternately, if the assembly is accelerated to light speed, in the opposite direction longitudinally, (reciever trailing) will the circuit record an event? Alternately, if the assembly is accelerated to light speed sideways, will an event be recorded?

Just curious.

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#1

### Re: Light Speed Thought Experiment...

10/19/2006 4:18 PM

Bit outside my field, but surely lumps of stuff don't get accellerated to the speed of light (even in a thought experiment)?

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#19

### Re: Light Speed Thought Experiment...

10/20/2006 9:03 PM

You don't need to accelerate a pipe to lightspeed to do this thought experiment, in fact, you don't need a pipe or even a thought experiment.

Get in your car tonight and drive with the headlights on, would the photons coming off the heated filiments not be exceeding the speed of light? They are leaving the headlamp at (in vacuum) 300,000 km/s, add the forward velocity of the car......

This is fundamental relativity, and, as I see, has already been adressed in other posts.

To an observer on the side of the road, the headlights would be slightly (imperceptably) shifted to the blue as the car approaches. This explains why tailights are red, the car is receding, ergo, redshift and the new headlights seen on Lexus, Mercedes et al are more blue in colour because these cars are fast!

Sorry.... it is my nature to be a smart _ss, it masks my ignorance.

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#2

### Re: Light Speed Thought Experiment...

10/19/2006 4:55 PM

Jorrie? Jorrie? Where are you?

I think I know what he would say. First, we know that we can not accelerate mass to the speed of light, but let's say we get close. We know from Einstein that the speed of light in a vacuum is constant for any inertial reference system. An inertial reference system is simply a point from which we make a measurement. It can have any relative velocity, but usually we reference an objects speed to some point in time-space. For example, at the airport you are in one inertial reference system. The aircraft landing by you is another. From one point of view the plane is in motion. People in the plane see you approaching them! Both you and the aircraft are moving from the inertial reference point of the Sun, which is also in motion. And so on, and so on. So an inertial refernce plane is just a place to hang our hat.

That means for one observer who is at "rest" watching your thought experiment speeding by, would measure the lamp's photons moving at exactly the speed of light. However, the resting observer would also notice that the frequency of the light would be "blue shifted" as it approaches you and "red shifted" as it recedes. Much like a train whistle does when it passes you by.

Now, for the observer on the inertial reference system that contains your thought experiment, that observer also measures the speed of the photons from the lamp and guess what? They are also moving at the speed of light! Einstein also asked it this way, if Einstein was moving at the speed of light and held a mirror in front of him, would he see his face. The answer is yes!

Now, relativity also tells us that objects moving at speed experience time dilation. Let's take two observers, A and B. A is at rest and B is moving at 90% the speed of light. Both A and B have clocks and let's say B is moving in a giant circle so that at some point he returns to where A is. When B finally returns, B's clock will appear to have lost time compared to observer A's clock. A will complain how long he was waiting for B and that his parking meter has expired. B will complain that it took no time at all!

We experience this same phenomena right here on Earth! Let A be 'you' for example and B will be your 'wife'. You drive your wife to the shopping mall and you remain in the car (inertial reference system A at rest). Now your wife goes inside (inertial reference system B in motion) and proceeds to shop. When your wife returns you are tired, bored, hungry, thirsty, and hot. It has been a long time sitting in that car. However, your wife is shocked to find you in the condition you are in. For her, the experience has only been a few minutes. I have postulated that this is why women live longer than men. Women experience time dilation that forces their clocks to run slower than men.

Now do you see a connection between time and the observed speed of a photon? Curiously, the thought experiment observer measures the photon from the lamp to be at the speed of light. So does the stationary observer see the very same photon moving at the exact same speed. However, the two observers are experiencing different time dilation.

This dilation is very real and even satellites in orbit experience a slight effect from the orbital velocity as predicted by Special Relativity.

Jorrie, I hope I didn't muck this up too badly! I know you would have done a bang up job!

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#3

### Re: Light Speed Thought Experiment...

10/19/2006 7:31 PM

So if Einstein could see his face in a mirror at light speed, then the detector would recieve a signal?

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#4

### Re: Light Speed Thought Experiment...

10/19/2006 8:34 PM

Yes, although you need to concede that no object can actually reach the speed of light, but you could get close, in theory. The speed of light is an asymptote. Additionally, Special Relativity states that the mass of an object increases with velocity toward infinite mass at the speed of light. So you can never accelerate infinite mass, so you can't get to the speed of light. I digress...

Remember, everything is relative and relativity states that the speed of light is constant in a vacuum regardless of what inertial reference system you use (in either case of a still observer or an observer moving with the experiment).

So, the detector will receive the photon regardless of orientation of the experiment with respect to the experiment's inertial reference plane's velocity vector. I know you asked what would be the result if the velocity vector was tangental to the emission of the photon and I am saying the direction doesn't matter.

Fun stuff, no? Does that make sense?

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#7

### Re: Light Speed Thought Experiment...

10/20/2006 2:45 AM

Just a note: As this is a thought experiment, we have a bit more latitude in how we actually perform the experiment and in what properties we ascribe its participants. In this experiment, particularly, we're not constrained by the problems of accelerating real apparatus to the speed of light. We're not accelerating real apparatus; just the concept of it, and so we can dispense with the relativistic contributions of mass. What we cannot dispense with in this particular experiment are time, length, and velocity.

(BTW: Lorentz' equations have a great deal to say about the outcome of this experiment. Also, be careful with what otherwise seem to be normal everyday concepts, such as the idea of two or more events occurring 'simultaneously.' Where such ideas are rather well-behaved at everyday velocities, they can quickly grow venemous fangs near the speed of light. Wanna see my scars?)

--Europium

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#5

### Re: Light Speed Thought Experiment...

10/19/2006 9:55 PM

I don't think Jorrie should be allowed to leave for a few days. Or maybe we should just ban relativity questions till he comes back (no knock on your answer, he just really knows that stuff)

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#6

### Re: Light Speed Thought Experiment...

10/19/2006 11:47 PM

I think anyone would say, "Go ahead and try it. Then come back with your results."

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#14

### Re: Light Speed Thought Experiment...

10/20/2006 12:48 PM

I think you explained it very well, Jorrie would be pleased.

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#24

### Re: Light Speed Thought Experiment...

10/23/2006 2:11 PM

Hi Hero, you wrote: "Jorrie? Jorrie? Where are you?"

I'm still around, just took a long weekend in the subtropical eastern parts of my country... absolutely stunning!

Anyway, I'm too tired (from driving) to read all these posts tonight (and will probably be for a day or 2), but, I'll do so and come back with some vague response, which will probably be what you predicted it will be, but... who knows?

Jorrie

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#25

### Re: Light Speed Thought Experiment...

10/23/2006 2:18 PM

Yeah!!! He's back!!!

Let the Relativity questions commence.

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#26

### Re: Light Speed Thought Experiment...

10/23/2006 2:21 PM

Relativity? You mean like, if a man and woman are married in Arkansas, and move to Califronia, but then get a divorce, are they still brother and sister?

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#30

### Re: Light Speed Thought Experiment...

10/24/2006 1:09 AM

OMG! This is priceless!

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#27

### Re: Light Speed Thought Experiment...

10/23/2006 3:28 PM

Welcome back! Sounds like you enjoyed your trip, but we are happy to hear of your eventual recovery and look forward to more fascinating posts. ;-)

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#28

### Re: Light Speed Thought Experiment...

10/24/2006 12:40 AM

The OP did not make it clear, but I think all assumed that the tube is no longer accelerated when the measurements are made. (BTW if, in a though experiment, the system could achieve "c" relative to some inertial frame, it could still be "accelerated", or at least feel the effect of acceleration, although it will never exceed "c" relative to any inertial reference frame).

If the tube travels inertially, the OP's questions have been answered (and more). The light will always reach the detector, irrespective of direction of movement. The easiest way to get one's head around this is to let the tube drift inertially and then accelerate a set of observers so that they pass the tube at "c" (though experiment freedom) inertially from various directions. To each of them, the tube moves at "c".

Obviously this cannot make any difference to what happens, or is measured, inside the tube! However, it is exactly equivalent to the scenario stated in the OP. If it was not so, the whole of relativity theory would tumble down...

And, as have been stated by others, it has to do with time dilation and Lorentz contraction - i.e., the difficulty of measuring when we have only light to do our measurements/observations with.

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#29

### Re: Light Speed Thought Experiment...

10/24/2006 1:08 AM

Damn, Jorrie! It took you long enough! We was all pining away lookin' for y'all an' jus' barely holdin' on til y'all got back from wherever you was!

Where the hell have you been, anyway? No, don't answer that.

muiporuE--

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#8

### Re: Light Speed Thought Experiment...

10/20/2006 2:53 AM

So what you are asking is that; at the speed of light, when you want to turn the interior lights on, on the enterprise, will you 'see'?

I think that due to time dilation, yes you will!

We await Jorries response with great antici…………………pation…

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#9

### Re: Light Speed Thought Experiment...

10/20/2006 5:04 AM

Before to continue with these ravings, wont us to keep in mind - once and for all - that are 2 (two) definitions of speed ?

1. ONE-way of speed: requires 2 (two) clocks in different places (that, obviously, will be de-synchronized). This was NEVER measured and - may be - will be never.

2. TWO-way of speed: requires only 1 (one) clock. It was many times measured (first Galileo) and now is well-known - 1 part to 10^10.

The second postulate of SRT assumes that the ONE-way speed of light is constant and isotropous for all inertial observers.

But - yet from remote 1935 (Reichenbach) - now one knows that this is a convention -useful to semplify, but not necessary, only a convention.

Now, if you wont, go on.

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#10

### Re: Light Speed Thought Experiment...

10/20/2006 5:19 AM

I understood all of that up to 'before'...

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#11

### Re: Light Speed Thought Experiment...

10/20/2006 11:06 AM

Not to worry, PlbMak. It all becomes much clearer after 30-40 beers.

(bartender, gimme another stout)

--Europium

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#12

### Re: Light Speed Thought Experiment...

10/20/2006 11:33 AM

You're absolutely right, Camillo, as the following example clearly illustrates:

There was once a lady named "Bright"

Who could travel much faster than Light

She set out one day,

In a relative way,

And came back the previous night.

(bartender, please clean up this mess)

--Europium

Anonymous Poster
#13

### Re: Light Speed Thought Experiment...

10/20/2006 12:45 PM

The speed of light would be constant, but I am not sure that the receiver would receive the signal since the electrons in the circuit that convey the signal from the detector through the receiver circuit have mass. What happens at the detector circuit may be the question to address.

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#15

### Re: Light Speed Thought Experiment...

10/20/2006 1:33 PM

I believe this lies in the realm of Einstein's special relativity. However, special relativity only needs to be envoked if the light source is stationary. From what I understand about the theory, the speed of light is not relative to an observer, and that no matter how fast the observer is traveling, he will always observe the speed of light to be speed of light. In other words, the light speed he observes will not be c minus the observer's speed.

If you are speeding away from a light source at 1/2 the speed of light, the light beam is traveling away from you at c, (not c/2)

Even if I misunderstand this theory, it doesn't apply here. The light source is traveling at light speed, so the receiver will indeed see it turn off and on.

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#16

### Re: Light Speed Thought Experiment...

10/20/2006 2:30 PM

Suppose the detector end of this tube is cut off -- nothing to block a photon. Suppose, also, that the tube is being propelled at light speed (ignoring the fact that its mass will not allow that) on a path parallel to the earth's surface. Then, when a photon is launched from the emitter, that photon will be traveling at double light speed (in relationship to the earth's surface) as it leaves the front of the tube. True?

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#17

### Re: Light Speed Thought Experiment...

10/20/2006 3:51 PM

Maybe ... but, since you've cut off the end with the detector, we'll never know!

Hey, Jorrie's away, let's all play !

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#18

### Re: Light Speed Thought Experiment...

10/20/2006 4:58 PM

My......brain......HURTS!

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#20

### Re: Light Speed Thought Experiment...

10/20/2006 9:15 PM

Einstein once said...

"Put your hand on a hot stove for a minute and it seems like an hour. Sit beside a pretty girl for an hour, and it seems like a minute. Thats relativity."

"What time does Munich station stop at this train?"

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#21

### Re: Light Speed Thought Experiment...

10/22/2006 7:36 PM

The light source and receiver are not actually moving with respect to each other. Thus it makes no difference how fast, or in which direction the "pipe" is accelerated.

The receiver will always detect the light.

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#22

### Re: Light Speed Thought Experiment...

10/22/2006 11:38 PM

I haven't read all the responses and I'm too lazy to go look up what I'm thinking of. But this reminds me of an old experiment designed to detect the direction of the "ether wind". It was a rather simple device with a candle as a light source that split the candle light through a series of half silvered mirrors into equal length paths in opposite directions. I forget what the detector was, interference slits maybe. Anyone recal what I am thinking of or who did the experiment? Maxwell or Lorentz comes to mind but I'm not sure that is correct.

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#23

### Re: Light Speed Thought Experiment...

10/23/2006 3:47 AM

... that's the Michelson-Morley experiment.

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