Immeadiacy of sensory experience

[dingansich]

It doesn't. What makes you think it does?

this...

Because, in one case, it is relevant, in the other it is not.

If you are not grasping why the rays of light emitted from the surface of the train are not relevant to the experiment, I'm afraid we're going to have a lot of trouble helping you further.

 

[Jimmy Snyder]

Actually from another post a while back, it doesn't actually travel slower, but it will seem to have a slower speed in a medium becausethe photon is absorbed and re-emitted by atoms in that medium.

That sounds suspiciously like a trajectory. I think the path is not mechanical like that.

 

[khemist]

That sounds suspiciously like a trajectory. I think the path is not mechanical like that.

I believe DaveC was the one to post in my thread a while back so he would probably be a better person to elaborate.

Say a photon moves through a dense cloud of gas. From what I understand, the time it takes the photon to traverse through the cloud will be less than the time it would take the photon to travel the same distance, because the photon is constantly being absorbed by the electrons and re-emitted, which takes a finite amount of time. This delay will slow the "speed" of light down, though the velocity it has in between the atoms is c.

 

[dingansich]

It doesn't. What makes you think it does?

or maybe i should have said why is the light from the surface of the train "not relevant?"

 

[DaveC426913]

this...

Perhaps you don't understand what a thought experiment is.

In reality, the light delay is there in all cases. In a thought experiment, we eliminate these realities if
1] they do not help explain the point being made and also
2] if they do not hinder the point in principle.

For example, when doing relativistic rocket thought experiments, we don't worry about how much fuel will be used to get up to .99c, or how long it takes to turn around and fly back home. These are things that, while they are quite real, do not help illustrate the point being made.

It is assumed that the teller and the recipient of the story are able to understand that the thought experiment only needs to address factors that will have an impact on the experiment.Thought experiments only apply in a carefully limited situation. In your case, the delay of light from source to destination may not hinder the illustration of the point - but if it did - if you could show that it's relevant - then it would indeed have to be accounted for. The thought experiment would have to be modified.

 

[Jimmy Snyder]

I believe DaveC was the one to post in my thread a while back so he would probably be a better person to elaborate.

Say a photon moves through a dense cloud of gas. From what I understand, the time it takes the photon to traverse through the cloud will be less than the time it would take the photon to travel the same distance, because the photon is constantly being absorbed by the electrons and re-emitted, which takes a finite amount of time. This delay will slow the "speed" of light down, though the velocity it has in between the atoms is c.

If it worked that way, then some photons would arrive ahead of others.

 

[DaveC426913]

Say a photon moves through a dense cloud of gas. From what I understand, the time it takes the photon to traverse through the cloud will be less than the time it would take the photon to travel the same distance,

I think you wrote that wrong. But I get the idea.

because the photon is constantly being absorbed by the electrons and re-emitted, which takes a finite amount of time. This delay will slow the "speed" of light down, though the velocity it has in between the atoms is c.

Well, it doesn't really apply effectively to rarified gas clouds. The photons that make it through a gas cloud is space tend to have passed unmolested though the cloud.

 

[khemist]

Then some clarification is needed...

https://www.physicsforums.com/showthread.php?t=541212 was the post I was referring to.

 

[dingansich]

It is assumed that the teller and the recipient of the story are able to understand that the thought experiment only needs to address factors that will have an impact on the experiment.
[/B]

what about if they have an impact on the logical conclusions/interpretations of the experimental results, like length contraction? If I am not wrong doesn't the moving observer in the train have to come to the conclusion that the train's length has contracted in the direction of motion?

 

[Waterfox]

what about if they have an impact on the logical conclusions/interpretations of the experimental results, like length contraction? If I am not wrong doesn't the moving observer in the train have to come to the conclusion that the train's length has contracted in the direction of motion?

An observer on the train would not measure the train's length has contracted, but the land in front of it contracted. I believe going into special relativity is going off track from your original questions though.

When you see something in reality, light is emitted from a source such as a light bulb, the light will travel at the speed of light in air, "bounce" off an object and some of the rays of light will go into your eye. Small rods and cones in the back of your eye will convert the information of the light rays into signals that are sent to your brain to interpret and this is what you "see". The time it takes for your eyes to convert the light information to electrochemical signals, for it to be sent to the brain and the brain to process them is a lot longer than the time it took for the light to travel across the room (although much quicker for seeing objects large distances away).

In thought experiments we forget about all of this to make the purpose of the thought experiment clear. We imagine that we can instantly "see" this object without the delay of light or brain processing. If you desire to know what you would see in reality for a special relativity thought experiment you could factor in the time delay, although the point of the thought experiment will be the same e.g. you will still measure time dilation/length contraction of an object moving at .9c, in reality it will just take you longer to make these measurements as the speed of information is limited to c.

 

[DaveC426913]

what about if they have an impact on the logical conclusions/interpretations of the experimental results, like length contraction? If I am not wrong doesn't the moving observer in the train have to come to the conclusion that the train's length has contracted in the direction of motion?

That may be relevant to the experiment at-hand. If so, it certainly must be accounted for.

You'd have to get specific about what thought experiment you want to talk about for us to determine what's relevant. For example, would you agree that the increasing mass of the train is not relevant?