The light year long wall

In summary, the conversation discusses the hypothetical scenario of having a brick wall that is one light year long when at rest, but is then thrust at a person at a speed of .99999c, causing it to contract to only one foot in length. The question is posed as to when the person would be able to stand in front of the wall without being hit by it, and it is debated whether the wall would bunch together in the middle or towards the front. It is also mentioned that if the person were to step aside and look perpendicular to the wall, they would see it passing by for a year. The conversation also briefly touches on the concepts of general relativity and special relativity in relation to the scenario.
  • #1
Wizardsblade
148
0
Ok I have had a course is general relativity, but not on special. I was wondering something though...
If you have a brick wall a light year long (when at rest) and then have it thrusted at you at a speed such that it is only 1ft long, i.e. .99999c, when would you be able to stand in front of it without being hit by the wall?
For example does it bunch together in the middle so you can stand in front of it for 6 months or maybe it bunches towards the front so you can move into its path after just a split second? The weird thing is that if you look parallel to it you should see for the whole year right? Just something I thought was fun to think about, any thoughts?
 
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  • #2
I'm not sure, but it dosn't seem logical that you can wait any longer because of the fact that it "bunches up". Because let's say that we are only looking at the first brick of the wall, since there is no acceleration, the rest of the wall isn't relavent (right?) cause the whole wall is moving at the same speed. The front brick only contracts a little bit so it would have almost no effect on the time. You could reason like this until you're only looking at a tiny part of the front of the wall and then the contraction wouldn't matter. On the other hand I could be totally wrong...:uhh:
 
  • #3
Wizardsblade said:
If you have a brick wall a light year long (when at rest) and then have it thrusted at you at a speed such that it is only 1ft long, i.e. .99999c, when would you be able to stand in front of it without being hit by the wall?
What do you mean by standing "in front" of it? In the path of its motion? Or watching it go by?

Realize that objects are measured to contract along their direction of motion. So I don't know why you'd think you could stand in the path of a moving wall and not be struck. (Are you thinking that it contracts perpendicular to its motion? It doesn't.)
 
  • #4
How can you do GR without SR?
 
  • #5
Wizardsblade said:
Ok I have had a course is general relativity, but not on special. I was wondering something though...
If you have a brick wall a light year long (when at rest) and then have it thrusted at you at a speed such that it is only 1ft long, i.e. .99999c, when would you be able to stand in front of it without being hit by the wall?
For example does it bunch together in the middle so you can stand in front of it for 6 months or maybe it bunches towards the front so you can move into its path after just a split second? The weird thing is that if you look parallel to it you should see for the whole year right? Just something I thought was fun to think about, any thoughts?
I think I understand the question. If so, then I don't think it is worded very well.
A brick wall is at rest with respect to you. The length of the wall is 1 light year, and the close end of the wall is one light year away from you. So the far end of the wall is two light years away from you.

. ......... .WALLWALLWALLWALLWALLWALLWALLWALL.
You ........Near end .........Far end

Forgetting dynamics and rigidity, the entire wall starts moving toward you at such speed that the length of the wall is foreshortened to 1 foot. Will the wall hit you in 1 year, 1.5 years, or 2 years?

Rather than wonder about the dynamics, let the wall remain stationary while you move toward the wall. In order to keep acceleration out of it, assume that you are already traveling at full speed at the time you reach the point labeled "You" in the diagram. If the length of the wall is measured to be 1 foot, then so is the distance from you to the near end. The near end will hit you momentarily, and the far end soon after.
 
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  • #6
Oh sorry I guess I studied special.
I know you will get struck when your standing in front of the wall but when will you be struck? Because the wall is contracted to 1ft, will it be immediately, or in 6 months or 12 maybe?

Thanks jimmysnyder that is a better way to put the question.

So does this mean that you can step aside from the wall for a second then move back and the wall won’t hit you? I am aware that you can't actually move aside or back in front of the wall without hitting it, but if you can get back in its projected path will the whole wall be behind you?

I guess the part that really boggles me is the thought that if you step aside and look perpendicular to the wall you will see it passing you for 1 year.
 
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  • #7
Wizardsblade said:
If you have a brick wall a light year long (when at rest) and then have it thrusted at you at a speed such that it is only 1ft long, i.e. .99999c, when would you be able to stand in front of it without being hit by the wall?...For example does it bunch together in the middle so you can stand in front of it for 6 months...

Because it is being accelerated towards you, I think you would find that it bunched up at your end. It sounds a bit like you are thinking of a giant holding the back end and thrusting it at you like a spear. One problem with this, is that the electromagnetic forces that push the back of the wall forward travel at light speed and take a year to get to the front of the wall! The wall is obviously going to have to compress if the back end is moving forward at nearly light speed toward the front, when the front doesn't know about it yet. :eek:
 
  • #8
Wizardsblade said:
I guess the part that really boggles me is the thought that if you step aside and look perpendicular to the wall you will see it passing you for 1 year.
The length is always contracted in the direction of motion. Whether the wall passes you or heads directly towards you it will be contracted.

The answer to how soon you can step into the wall's path is simple: 1 foot / 0.99999c seconds after the front of the wall reaches you.

I'm not sure how much sense it makes to ask whether the length is contracted to the front, middle or back of the wall in general. It depends how you set up your frame. If the front of the wall is at x = 0 at time t = 0, then the back of the wall is at x = 1 foot in your frame. If you choose a different frame, and so different values of x and/or t to measure the front of the wall, the length will be contracted differently.

It ain't called relativity for nothing! How you transform co-ordinates of one frame to another depends on what t is when x' = x.
 
  • #9
Wizardsblade said:
Ok I have had a course is general relativity, but not on special. I was wondering something though...
If you have a brick wall a light year long (when at rest) and then have it thrusted at you at a speed such that it is only 1ft long, i.e. .99999c, when would you be able to stand in front of it without being hit by the wall?
For example does it bunch together in the middle so you can stand in front of it for 6 months or maybe it bunches towards the front so you can move into its path after just a split second? The weird thing is that if you look parallel to it you should see for the whole year right? Just something I thought was fun to think about, any thoughts?

I think that the question here is very similar to the question in the thread

https://www.physicsforums.com/showthread.php?t=87370"

As I understand it, the problem is that you have a brick wall that's 1 light year long, and you instantaneously accelerate the entire wall up to a relativistic velocity. You then ask what the time to impact is.

The basic issue is that step #1, accelerating the enitre wall instantaneously to a relativistic velocity, is much trickier than it appears.

Infinitely rigid bodies do not exist in relativity, so if you push on one end of the wall, the other end won't start to move immediately. So you can't accelerate the wall by pushing on one end of it.

Now, you could imagine putting rocket moters evenly around the wall, and starting them all upon a signal, the signal being carefully judged so that every bit of the wall starts accelerating towards you "at the same time" and "at the same rate" where "at the same time" means "at the same time in your frame of reference". This leads to "Bell's spaceship paradox". The problem is that in the frame of reference of the wall, there will be internal stresses that break the wall if you try to do this. (It's usually thought of as a string in the Bell's spaceship paradox).

There is a way to accelerate the wall towards you without generating internal stresses in the wall. This corresponds to the notion of "Born rigidity". It's discussed in the sci.physics "Rotating disk" faq

http://math.ucr.edu/home/baez/physics/Relativity/SR/rigid_disk.html

What winds up happening is that the back of the wall must accelerate harder than the front of the wall in order to satisfy the condition of "no internal stresses".
 
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1. What is a light year long wall?

A light year long wall is a theoretical structure that spans a distance of one light year, which is equivalent to about 9.5 trillion kilometers. It is a common concept used in science fiction and hypothetical scenarios, but there is currently no known technology that could construct such a massive wall.

2. How long would it take to travel across a light year long wall?

Assuming the wall is a solid structure, traveling across it would take approximately one light year, or about 9.5 trillion kilometers. This would take an immense amount of time, even at the speed of light, making it practically impossible to cross in a human lifetime.

3. Can a light year long wall be seen from Earth?

No, a light year long wall cannot be seen from Earth. The wall would be located at a distance of one light year away, and even with the most advanced telescopes, it would be impossible to see such a massive structure at that distance.

4. How would a light year long wall impact space travel?

If a light year long wall existed, it would present a significant obstacle for space travel. It would be nearly impossible to navigate around or through it, and it would require a massive amount of energy and resources to overcome it. However, since a light year long wall is currently only a theoretical concept, it does not have any impact on current space travel.

5. Is it possible for humans to build a light year long wall?

Currently, there is no known technology or resources that would allow humans to build a light year long wall. It would require a level of engineering and construction far beyond our current capabilities. However, with advancements in technology, it is possible that one day we may have the ability to build structures of this scale.

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