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Just to precise the things a little bit: the Lorentz contraction doesn't mean that the material is "compressed" as a string giving an internal tension; it's an effect only due to relativity of simultaneity:by definition, an object's lenght is the difference of the positions of its extremes "measured simultaneously". It's for this simultaneity in the definition, that an object's lenght is frame-dependent.

Quote from: lightarrow on 21/03/2009 16:14:53Just to precise the things a little bit: the Lorentz contraction doesn't mean that the material is "compressed" as a string giving an internal tension; it's an effect only due to relativity of simultaneity:by definition, an object's lenght is the difference of the positions of its extremes "measured simultaneously". It's for this simultaneity in the definition, that an object's lenght is frame-dependent. You're trying to confuse me again. And succeeding.

There's a fun relativistic length-contraction thought experiment concerning a ladder and a two-doored shed.In the thought experiment, a twelve foot long ladder is approaching an eleven foot long shed, which has a door in each end, at relativistic speed. Because of the apparent length contraction, as observed by someone standing in the shed, the observer should be able to close both shed doors while the twelve foot ladder is entirely inside the eleven foot shed. However, from the ladder's point of view, it is the shed that is contracted and it is therefore impossible for both doors to be closed while it is inside it []

Quote from: LeeE on 21/03/2009 20:36:15There's a fun relativistic length-contraction thought experiment concerning a ladder and a two-doored shed.In the thought experiment, a twelve foot long ladder is approaching an eleven foot long shed, which has a door in each end, at relativistic speed. Because of the apparent length contraction, as observed by someone standing in the shed, the observer should be able to close both shed doors while the twelve foot ladder is entirely inside the eleven foot shed. However, from the ladder's point of view, it is the shed that is contracted and it is therefore impossible for both doors to be closed while it is inside it []Sometimes I really hate you!

Quote from: DoctorBeaver on 21/03/2009 21:56:49Quote from: LeeE on 21/03/2009 20:36:15There's a fun relativistic length-contraction thought experiment concerning a ladder and a two-doored shed.In the thought experiment, a twelve foot long ladder is approaching an eleven foot long shed, which has a door in each end, at relativistic speed. Because of the apparent length contraction, as observed by someone standing in the shed, the observer should be able to close both shed doors while the twelve foot ladder is entirely inside the eleven foot shed. However, from the ladder's point of view, it is the shed that is contracted and it is therefore impossible for both doors to be closed while it is inside it []Sometimes I really hate you! Noo! If you think about what I wrote, you should grasp that the paradox comes from the wrong assumption that when you measure the lenght of the ladder (or the shed) the simultaneity is absolute, while instead is frame-dependent.Another name for this paradox is "The barn and the Pole paradox":...sorry, you cannot view external links. To see them, please REGISTER or LOGIN

If 2 objects are following each other at relativistic speed then the distance between the front and back of each object appears to shrink. So what about the distance between the back of the first object and the front of the second? Does that also appear to shrink so that they seem closer together? []

Quote from: DoctorBeaver on 22/03/2009 13:18:53If 2 objects are following each other at relativistic speed then the distance between the front and back of each object appears to shrink. So what about the distance between the back of the first object and the front of the second? Does that also appear to shrink so that they seem closer together? []If they are traveling at a uniform velocity and being 'at rest' when compared to each other they will belong to the same 'frame of reference' and there will be no Lorentz contraction seen between them. But if you are thinking of them accelerating at the same exact velocity? I guess they still could be seen as being 'at rest'? I don't really know, that's seems a tricky one DB.

You've touched on what I was wondering. Would that be a way for us to see what is outside our visible universe?

If we were travelling at relativistic speed, the distance between us and the visible horizon would be less. Does the contraction mean we could see past it or would time dilation rear its ugly head and prevent it?

Thank you, Alberto. Is there a limit to the amount of contraction or is it another 1 of those horrible infinity things? At c, does everything have zero length?

You see, there was another side to this that I was wondering about and that's why I mentioned time dilation. If you could travel at, say, 0.99c, how much contraction would there be between you and the horizon of the visible universe?

Or does contraction only apply when you pass someting?

Here's what I was puzzling over. In our frame of reference here on Earth it has taken light 13.7 billion years to get here. Now, nothing can travel faster than c and in your own frame of reference it doesn't matter how fast you go, time will appear to pass at the normal rate. So, it should take you more than 13.7 billion years to get there by your own timescale.

But, if the distance is greatly contracted then at 0.99c it may take you less than 13.7 billion years to get there. That can't be right.

The only solution I can see is that time dilation must come into it somehow but I can't figure out how because in your own frame of reference there shouldn't be any. Or is there something else that I'm missing completely?

... in your own frame of reference it doesn't matter how fast you go, time will appear to pass at the normal rate.

dlorde - That is precisely my point.Alberto - You said "You get there in 13.7*0.14 = 1.92 billion years.".From the point of view of someone on Earth, it has taken light 13.7 billion years to make the journey. From the perspective of someone travelling at 0.99c light would still be travelling at c relative to him (basic GR). Therefore, to him light would still take 13.7 billion years to travel that same distance. Am I right so far?

So if you were travelling at 0.99c the light from the edge of the visible universe would take considerably less than 13.7 billion years to reach Earth?

The visible edge would no longer be 13.7 billion years * 300,000km/sec distant?

How far does the contraction go? From the photon's perspective, would the distance be zero?

Quote from: DoctorBeaver on 24/03/2009 20:09:22So if you were travelling at 0.99c the light from the edge of the visible universe would take considerably less than 13.7 billion years to reach Earth? No, to reach *you*.

QuoteHow far does the contraction go? From the photon's perspective, would the distance be zero?The photon's perspective doesn't exist. Let's talk about the perspective of a passenger travelling at near c: yes.

Quote from: lightarrow on 24/03/2009 20:37:16No, to reach *you*.But, surely, you would see the distance between the edge and Earth contracted therefore light would travel that distance in less than 13.7 billion years.

No, to reach *you*.

QuoteThe photon's perspective doesn't exist. Let's talk about the perspective of a passenger travelling at near c: yes.My brain is going to hurt again now. I just know it.OK. So at (near)c distance reduces to zero. Therefore it must take zero time to get anywhere as everywhere would be in the same place but of zero length. That sounds like a singularity to me []

The photon's perspective doesn't exist. Let's talk about the perspective of a passenger travelling at near c: yes.

Would you yourself be contracted to zero size?

If not, how could you fit there? And if it is only length that is contracted, does that mean that everything becomes 2-dimensional? I don't like the thought of that.

And it's this last remarkable ability that really freaks me 'off' and on...That they can 'interact' in time I mean, not that we might be able to see one with our eyes.Anyway you look at it, and I have looked at it , I still having trouble reconcile myself with its 'ability' to interact in 'time' while in itself more or less, to my eyes that is, existing 'outside' of it.

The thought I don't like is that dimensions can be reduced to zero size by velocity. What does that say about our concept of dimensions? Or time, for that matter?

QuoteAnd it's this last remarkable ability that really freaks me 'off' and on...That they can 'interact' in time I mean, not that we might be able to see one with our eyes.Anyway you look at it, and I have looked at it , I still having trouble reconcile myself with its 'ability' to interact in 'time' while in itself more or less, to my eyes that is, existing 'outside' of it.Which is something I was getting to. I was taking it 1 step at a time to make sure I was thinking correctly.

Quote from: DoctorBeaver on 24/03/2009 21:59:37The thought I don't like is that dimensions can be reduced to zero size by velocity. What does that say about our concept of dimensions? Or time, for that matter?It's only a mathematical limit, you will never be able to reach exactly c, so why do you worry about it exactly?

OK, forget zero size. Their effective size can be altered by velocity. I find that troublesome.

I just don't like the thought of it. It just seems wrong. I want to punch it.

What is "wrong" it to ascribe essential meanings to the concepts of "space" and "time"; they haven't.(But we were born with them so it's difficult for us humans to get rid of them).

Quote from: lightarrow on 25/03/2009 23:41:54What is "wrong" it to ascribe essential meanings to the concepts of "space" and "time"; they haven't.(But we were born with them so it's difficult for us humans to get rid of them).It's not that it's difficult for me to accept that space & time are not rigid structures. I understand warping & contraction of space, and its implications, due to gravity. My problem is getting to grips with the notion that distances can get shorter as we move faster. I know the difference would be immeasurably small, but if I walk somewhere it will be further than if I drive there at 100mph.I fully accept time dilation and I don't have a problem with that as I more-or-less understand the reasoning behind it. But I don't understand contraction of distance in the same way. Maybe if I did I would feel happier about it.

The lenght contraction means that you could arrive there in a few seconds, and, furthermore, that light from a distant source beyond the limit will have to cover a less distance to reach us, but we couldn't see past it from the beginning, we should wait to meet the light (emitted from the distant source) at ~ half journey.

it's not a "real" contraction in the sense that there is no internal tension; you could think of it as an "artefact" of how we *define* distance between two points