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Light in a vacuum


quiet

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I will explain the doubt with the following example.

I go to the cinema and the film tells the experiences of some scientists. In one scene, it shows a laboratory capable of operating in a vacuum, where there is no gravity or other fields. There are only the fields that scientists can create when they do an experiment.

The shape of the laboratory resembles a large tube with a rectangular section. The work rooms are on one of the walls of the tube. The inside of the big tube is hollow.

The media available in the laboratory does not detect surrounding objects. Within the scope of those media, the laboratory is alone in an empty space.

Inside, near one end of the tube, a device is mounted that will activate a laser when it receives the order.

The laser points in a direction perpendicular to the wall where the device is mounted. The wall parallel to that, is prepared so that all points are sensitive to light. If the laser pulse hits some point of the second wall, it will be detected.

The separation between both walls is equal to [math] 100 \ \mathbf{m} [/math]

In that scene, the director of the film decides to show the following.

The laser receives the command and is activated. A time equal to [math] 10^{-8} \ \mathbf{s} [/math] is activated. Scientists see that the laser emits light, but nothing is detected in the second wall. They repeat the action and nothing. They do many repetitions and nothing. They check everything and do not find faults or breakdowns.

Now two questions.

1. Does that scene violate special relativity?

2. Does that scene violate Maxwellian electrodynamics?

I suppose that answering the first question is an easier task than answering the second one, when we decide to think everything carefully and in detail.

Edited by quiet
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I Think that it would be well served if you gave your thoughts on the matter and what your reasoning is for it.   You went to the trouble of giving the distance between the walls and the activation period for the laser, so I 'm assuming that you think this is important to the answer. 

Assuming that the whole set up isn't under some extreme acceleration which either severely Doppler shifts the laser or causes its path to bend enough that it hits one of the other surfaces in the room rather than the one opposite it,  There is no reason for the laser not to strike the sensitive wall. 

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Sorry. In first post I have not say that the laboratory isn't accelerated.

Now I can't edit the first post. I beg your pardon.

The distance between walls and the time while laser is emmiting have less importance than the concept of the scene. You can to think in a tiny time and a grand distance between walls.

Edited by quiet
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With respect to the first question, the lab is not accelerated. Then SR applies. Einstein's postulate states that all is like if the lab were in absolute rest, because all systems without acceleration complies the same laws. Then, the director of the movie has violate SR, because in absolute rest the laser pulse reaches the second wall and is detected.

 

More difficult for me is the second question. Maxwell's Electrodynamics implies that once emmited, the EM wave is idependent of the source. I can't understand if this independence stablishes a difference between the lab supossed in absolute rest, and the lab supossed in straight and uniform movement. Without to know in detail all that Maxwellian ED implies, I cant to know if the scene of the movie violates or not this theory.

Edited by quiet
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3 hours ago, quiet said:

The distance between walls and the time while laser is emmiting have less importance than the concept of the scene. You can to think in a tiny time and a grand distance between walls.

Why is that relevant?

13 hours ago, quiet said:

Scientists see that the laser emits light, but nothing is detected in the second wall. They repeat the action and nothing. They do many repetitions and nothing. They check everything and do not find faults or breakdowns.

As you have invented this scenario, why is the light not detected?

13 hours ago, quiet said:

Now two questions.

1. Does that scene violate special relativity?

2. Does that scene violate Maxwellian electrodynamics?

Nothing you have said suggests that it would. But you seem to have omitted a lot of information. Is that deliberate?

35 minutes ago, quiet said:

With respect to the first question, the lab is not accelerated. Then SR applies. Einstein's postulate states that all is like if the lab were in absolute rest, because all systems without acceleration complies the same laws. Then, the director of the movie has violate SR, because in absolute rest the laser pulse reaches the second wall and is detected.

You don’t even need Einstein for this. It goes back to Galileo (he used a ship not a lab).

 

36 minutes ago, quiet said:

I can't understand if this independence stablishes a difference between the lab supossed in absolute rest, and the lab supossed in straight and uniform movement.

There is no difference between these. Again, Galileo not Maxwell said this first. 

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4 hours ago, Strange said:

you seem to have omitted a lot of information. Is that deliberate?

I am keeping many details that, according to the rules of the mainstream forums, do not constitute information. Why these details are not information in the forum? Because they do not comply with the rules of publication due. So, officially, I'm not keeping information.

I must admit that, without the details left fierce of the forum, I would never have had doubts about this case that I presented as a scene from a movie. Frequently we find in the forums threads dedicated to analyze to what extent this or that scene of a film meets or violates the accepted physical laws. I have planted the scene in the simplest way I could. There are no tricks, no traps, no cunning details as we see in some exams.

I'm not ashamed to confess that a simple scene has caused me doubts. I would really like such an experiment to be done in practice. I would like to see that light always reaches the second wall, regardless of what happened before the experiment. For example, before doing the experiment, activate the engine of the lab ship and spend a lot of time accelerating. I'm thinking of an acceleration that if it were done with respect to a planet, it would achieve that the ship reached 99% of C with respect to the planet. After that turn off the engine and allow straight and uniform movement. The next thing is to do the experiment. Relativity, Galilean, Einsteinian, however, expects light to reach the second wall. The details I have keeped does not expect the same. Therefore, in the scene of the imaginary film director, the light is not detected in the second wall. Filmmakers love scenes that challenge established physics.

Edited by quiet
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6 hours ago, quiet said:

With respect to the first question, the lab is not accelerated. Then SR applies. Einstein's postulate states that all is like if the lab were in absolute rest, because all systems without acceleration complies the same laws. Then, the director of the movie has violate SR, because in absolute rest the laser pulse reaches the second wall and is detected.

 

More difficult for me is the second question. Maxwell's Electrodynamics implies that once emmited, the EM wave is idependent of the source. I can't understand if this independence stablishes a difference between the lab supossed in absolute rest, and the lab supossed in straight and uniform movement. Without to know in detail all that Maxwellian ED implies, I cant to know if the scene of the movie violates or not this theory.

Maxwell’s equations are the laws the the light has to follow. So if the observation is that the scenario violates SR, it’s because Maxwell hasn’t been followed.

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1 hour ago, quiet said:

I'm not ashamed to confess that a simple scene has caused me doubts. I would really like such an experiment to be done in practice. I would like to see that light always reaches the second wall, regardless of what happened before the experiment

I can't imagine why you think it wouldn't. As we are always in motion (relative to something, but also stationary relative to something else) the experiment is done every time light is used. 

It seems you think there is some sense in which something can be "really" or absolutely stationary. Galileo showed this I is wrong. Experiments like Michelson-Morley show it is wrong. Mathematics like Noetger's theorem show it is wrong. Thousands of experiments have attempted to detect this sort of Lorentz violation, to incredible levels of accuracy. 

1 hour ago, quiet said:

The details I have keeped does not expect the same.

Then your details are wrong. Why don't you tell us what they are and we may be able to explain where you are going wrong. 

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I will try to express my doubt as best I can.

- Similar experiment done with a gun that shoots a bullet, instead of a laser that shoots a pulse of light. The bullet has mass and when it leaves the pistol it does not become independent of the linear momentum conferred by the pistol. In a vacuum it retains that linear momentum. If you imagine the laboratory at absolute rest, then the velocity of the bullet has a component equal to zero in the longitudinal direction of the laboratory. If you imagine the laboratory moving in a straight and uniform way with respect to some reference that the laboratory does not detect with the available media, then the velocity of the bullet and the speed of the laboratory have equal components in the longitudinal direction of the laboratory. That is inevitable because the bullet has mass and retains the longitudinal component of the impulse. The laboratory will always see that the bullet travels a line perpendicular to the wall where the gun is placed and reaches the second wall.

- Now the experiment with the laser. The light does not take linear moment of the laser device. It takes energy that is used to create fields E and B. These fields operate together and trace their own intrinsically straight path, because electromagnetic induction operates with intrinsically right angles. The source only provides energy to the EM field, it does not provide linear momentum that must be conserved. Then the light pulse that comes out of the laser does not have in its speed a longitudinal component equal to the longitudinal component of the speed of the laboratory. In the speed of that pulse, all the components are equal to zero, minus the only component that exists, which is perpendicular to the wall where the laser device is mounted. If the speed of the laboratory has a sufficiently large component in the longitudinal direction, the following may happen. While the pulse travels, the laboratory leaves the region where the pulse is spreading. In that case, the pulse reaches a region that the second wall has left and, for that reason, does not hit the wall. The source can not confer linear momentum to fields that have no mass.

- The details omitted for not being duly published constitute a book of 130 pages, which does not allow to obviate the problem of linear momentum. The linear momentum of EM radiation is given by the theorem that Maxwell demonstrated, which establishes the following expression.

[math]p = \dfrac{E}{C}[/math]

That linear momentum does not come from the source of radiation. It comes from a process wich is independent of the source. For that reason I doubt that in all cases, regardless of what happens before the experiment, the laser pulse reaches the second wall.

P.S.
It could happen that Special Relativity applies to objects with mass and not to electromagnetic waves in a vacuum.

Edited by quiet
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57 minutes ago, quiet said:


- Now the experiment with the laser. The light does not take linear moment of the laser device. It takes energy that is used to create fields E and B. These fields operate together and trace their own intrinsically straight path, because electromagnetic induction operates with intrinsically right angles. The source only provides energy to the EM field, it does not provide linear momentum that must be conserved. Then the light pulse that comes out of the laser does not have in its speed a longitudinal component equal to the longitudinal component of the speed of the laboratory.

Huh?

if you shine a laser on a target on the other side of the room, the light will not miss the target if the room moves laterally at constant velocity. The system will behave exactly as it does at rest.

Light does have linear momentum, imparted by whatever emitted it (or imparted to whatever absorbs or reflects it) 

 

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Question: According to you (or the film maker) the laser does not hit the second wall; where does the photons emitted by the laser go?

11 hours ago, quiet said:

I'm thinking of an acceleration that if it were done with respect to a planet, it would achieve that the ship reached 99% of C with respect to the planet. After that turn off the engine and allow straight and uniform movement.

 @swansont and @Strange already gave good explanations. But I'll try to address this from another point of view within the context of the setup. You seem expect that experiments run before and after acceleration have different outcome. To me that seems to imply that the light source (laser) must have some kind of "memory" of beeing accelerated. But if nature would behave like that it would have strange consequences. Think of what happens if the space lab in the movie receives a spare laser from a service spaceship? If that spaceship was starting from another location and accelerated differently, the spare laser would "remember" a different acceleration than the the original laser and therefore behave in a different way. 
It seems like the movie maker have a serious plot hole? 

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On 22/11/2018 at 1:02 AM, quiet said:

Scientists see that the laser emits light,

How ?

 

On 22/11/2018 at 1:02 AM, quiet said:

where there is no gravity or other fields. There are only the fields that scientists can create when they do an experiment

Again how?

The mere fact you have an enclosed laboratory space creates capacitance and inductance.
Tinkerer has just fixed his microwave and offers a pictorial explanation of just how easy this is to occur in his last post ehre.

https://www.scienceforums.net/topic/117119-gotta-love-microwave-ovens/?tab=comments#comment-1082143

On 22/11/2018 at 1:02 AM, quiet said:

The laser receives the command and is activated. A time equal to 108 s is activated

The equipment (including the wiring) involved in very action of activating and driving the laser will generate fields.

On 22/11/2018 at 1:02 AM, quiet said:

The separation between both walls is equal to 100 m

In that scene, the director of the film decides to show the following.

The laser receives the command and is activated. A time equal to 108 s is activated.

I think you have not correctly said what you mean here.

The velocity of light is 3 x 108 m/s in vacuo.
Are you trying to arrange a time between emission and detection by trying to move the walls away faster than light?

Is this the reason for the 100metre gap and is the time 10-8 seconds meant to be achieve this?

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2 hours ago, studiot said:

Are you trying to arrange a time between emission and detection by trying to move the walls away faster than light?

That is a good question. One of the many details omitted in this (impossible) scenario is the direction of movement (relative to what?) that causes Quiet's confusion. 

Is the lab supposed to moving in the same direction as the laser light? And hence "outrunning" it so it never reaches the far wall?

On 22/11/2018 at 1:02 AM, quiet said:

1. Does that scene violate special relativity?

2. Does that scene violate Maxwellian electrodynamics?

Special relativity assumes the invariant speed of light because that is what Maxwell's equations show.

So your thought experiment violates pretty much all known physics. But apart from that ...

22 hours ago, quiet said:

Then, the director of the movie has violate SR

I think this is slightly, uhm... "dishonest"?. There is no film director. This is your idea. Why not just admit it?

Either say, "I don't understand this aspect of SR" or "I have this alternative theory that contradicts SR".

Trying to push the blame on to someone who doesn't exist (and therefore can't answer back) is not reasonable.

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On 11/23/2018 at 12:18 PM, quiet said:

We have seen the movie and commented it while we drink a coffee. Now, someone wait me at home.

Since you are back again on the forum, what’s the answer to my question? What is the name of the movie? 

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