Constant speed of light (3 Viewers)

[Xayma]

Originally posted by Constip8edSkunk
yeah to the guy on the train the tunnel wont b sealed when he enters the tunnel and when it is it will pass through (partially?) and crash .. right?

No what I think happens, is that relative to the outside observer the train is in the tunnel sealed off.

Releative to the train, hes not in the tunnel.

 

[abdooooo!!!]

whatever... im out of here.

the nerd training thing is starting again... i'll be back some time after the holidays to argue with you people.

 

[Xayma]

nerd training thing???? residentials?

 

[Wohzazz]

Originally posted by abdooooo!!!
all Wohzazz wanted to know is whether this length contraction is actually physical or just a percieved thing... if you know what i mean.

exactly! I still have doubts about this. So has all the length contraction of train due to relevistic effects happened in one frame but not the other (namely, from train's frame)?

Originally posted by Xayma
Constip8edSkunk I really think we have completley confused Wohazz

Yeah i'm really confused ATM, i'm not mature enough to deal with philsophical + realivistic effects at the same time. Let me age a few more months first

Originally posted by Xayma
No what I think happens, is that relative to the outside observer the train is in the tunnel sealed off.

Releative to the train, hes not in the tunnel.

This is how I interpretted it. So you cannot really say whether what happens to the train, it depends on the frame. This is like parallel existence like Xayma's gun situation. One of himself has died being shot, the other is alive. Isn't like time travel based on this concept..like switching between frames

And I think my question on stopping the train and whether it reverts to its original length, with reference to one frame (the onlooker), has been answered. Or maybe i missed reading some of the posts like i have a 'tendency' to.

 

[Wohzazz]

edit/ sorry posted twice

 

[Xayma]

OK first of all dont worry about the gun situation, that was just a hypothetical situation where time dilation comes into affect.

Remember the following.

Length contraction is present in both frames of refrence.

From the outside observer perspective the train is travelling at near the speed of light so it appears to be contracted and will fit in the tunnel (you dont need to worry about this for the HSC).

From the trains perspective, everything else is travelling "backwards" at near the speed of light, making them appear contracted, hence the train wont fit in the tunnel.

For the HSC at most you will have to understand the first situation and that if the walls were sealed off, relative to the train they werent done simultaneously.

 

[Wohzazz]

Originally posted by Xayma

For the HSC at most you will have to understand the first situation and that if the walls were sealed off, relative to the train they werent done simultaneously.

because light reaches the train from the forwards end faster than the backward end?

 

[Xayma]

Light reaches the train from the closest end first.

 

[BlackJack]

From what I've seen of the situation... Xayma should be right.

I think this happens:

At the instant you (the evil person trying to crash the train by sealing the tunnel) see the train going in, whereas the train is actually in front of where you think it is.
Because:
The light coming from the train took time in getting to your 'eyes'. In that time, the train moves forward as usual in its 0.9c supper-fast-speediness.
Think of two cars travelling from the same spot. One labelled 'photon' ,the other the train.
Therefore, like light coming from alpha centauri, you see a picture on the train in the past.

(And not only that, the light coming from each part of the train you see started from a different point in time. But that's not part of the question. This is apparent length contraction.)

Hence, you seal the tunnel, thinking that your machiavellian plan has succeeded. You see nothing happen in the same instant as light has just started travelling from the tunnel.

What you see afterwards is the train moving out of the tunnel effortlessly. Then you see the tunnel being sealed.

 

[wogboy]

Here's another interesting paradox to solve, it's called the Bug paradox. It goes like this:

There's a small bug stuck in a hole in the ground which is 5 cm deep. Somebody tries to kill the bug by getting a rivet and hitting it with a hammer into that hole where the bug is at a very high speed (say 0.8c). The natural length of the rivet is 5 cm (I drew a quick picture attached to this post)

The bug sees the rivet coming, but it doesn't worry about being killed since it sees that the length of the rivet is shorter than the depth of the hole. But in the frame of reference of the rivet, the bug hole is contracted, so it's depth is shorter than the length of the rivet, so somebody who is in the same frame of reference as the rivet will think that the bug will be easily squished by the rivet.

Who is correct, will the bug get killed or not? Why?

Hint: The solution to this paradox is similar to the solution to that paradox involving that train going thru the tunnel.

 

[zeropoint]

Regarding your bug paradox,

When the rivet head hits the wall its relative velocity drops to zero and consequentially the rivet extends to the same length as the hole. Therefore the bug loses out from both perspectives.

 

[wogboy]

Yep, this bug paradox (like the train one) illustrates that just because two events are seen as "simultaneous" in one frame of reference, it won't necessarily be seen as "simultaneous" in another.

For the person in the frame of reference of the moving rivet, the head of the rivet will hit the top of the hole at the same time the bug is squashed by the base of the rivet. In the bug's frame of reference though the head of the rivet will hit the top of the hole before the base of the rivet rushes out at him. Either way he's dead

 

[zeropoint]

Originally posted by BlackJack
From what I've seen of the situation... Xayma should be right.

I think this happens:

(snip)

I'm sorry but you are mistaken.

Although the door operator does experience ``light-lag'' due to the closeness of the train's velocity to light speed, this is merely an engineering problem that can be resolved by having the train emit pulses of light at regular time intervals as measured onboard the train. The door operator may then coordinate to have the doors shut simultaneously at the instant in time when the train is completely inside the tunnel and then re-open the doors just before the train hits the door. Due to the relativity of simultaneity, the train driver will measure the far door to close prior to the near door, and likewise the far door will open prior to the near door. Therefore, the train passes through the tunnel safely in both frames.

 

[Xayma]

Zeropoint, theres nothing about opening the doors in this situation, once the doors are shut they are shut wondering if it is indeed possible to trap the train in the tunnel (before it hits the wall)

 

[zeropoint]

Xayma,

If the doors are kept shut, the train will be trapped inside the tunnel in a compressed state in both frames.

 

[BlackJack]

Hmmm... supposing that happens, and the door operator correctly predicts when the train would be inside the tunnel.

(The original q circumstance.)

Originally posted by Wohzazz

Say that we have some tunnel that is 80m long. If you travel near the speed of light in a 120m train, your train gets contracted to a length that will fit in the tunnel from an onlooker outside's frame of reference.

One of the questions was whether the train sees everything else contracted.

Which (in my opinion) is yes, as they are moving past the train at high speed.

Well, supposing the train also fits inside from the train's frame (length dilation,) we'd be able to discern that our own inertial frame is moving at high velocity. This violates the inability to differentiate inertial frames thing.

What happens now?
If length dilation is indeed a physical phenomenon, then to the outsider, the train is neatly trapped. However, to the train things aren't so fortuneate.

 

[zeropoint]

Originally posted by BlackJack
One of the questions was whether the train sees everything else contracted.

Which (in my opinion) is yes, as they are moving past the train at high speed.

Correct.

Well, supposing the train also fits inside from the train's frame (length dilation,) we'd be able to discern that our own inertial frame is moving at high velocity. This violates the inability to differentiate inertial frames thing.

No. The train observes the tunnel approaching at high speed, therefore the tunnel experiences length contraction in the train's frame.

What happens now?

Suppose each end of the train is defined by the points A and B, and the ends of the tunnel are defined by the points C and D. The corresponding lengths in the rest frame are thus AB and CD with AB > CD. In the train frame, the length of the train is AB and the length of the tunnel is C'D' with C'D' < CD. I have made a crude ASCII diagram to illustrate this.

 

[Xayma]

What if this train can't be compressed except by length contraction (ie if you try to compress it, it deforms), would the train be deformed or just contracted in length.

 

[zeropoint]

Either the train is compressed to the length of the tunnel or the tunnel is stretched to the length of the train. I suspect in reality the train/tunnel will explode due to structural stress.

 

[Xayma]

It isn't likely to explode just more likely to be severly deformed. More likely the train will barrell out as it is forced under compressive loads.