A Question about Space-Time

[etcetcetc00]

I'm looking for an explanation of what four dimentional space-time means in terms of understanding the nature of time. Admittedly, I cannot really conceive of what a four dimenional structure would look like. I've heard all the analogies of imagining a two dimensional space with time as the third dimension, so I'm not looking for anything like that.

Calling time a fourth dimension and weaving a "fabric" of space-time seems to give time some spatial qualities that I'm having trouble reconciling. If all points in a 1 dimensional line are present within a 2 dimensional plane, all ponts on a 2 dimensional plane are present within a 3 dimensional cube, all points in a 3 dimensional cube are present at a given moment in 4 dimensional time, is it that all moments are present along a hypothetical 5th dimension?

Forgive me for asking against dimensionary reduction in the explanation while using it to ask my question, but it's the best way I can think to ask it. Imagine the whole universe from the moment of the big bang to the ultimate end of the universe are put into an elevator. At the bottom floor is the big bang, and at the tip floor is the end of the univese. Imagine this building is astronomically tall. Each minute between the big bang and the end of the universe is represented by one floor on this building, and at each floor the contents of the elevator represent the universe at those moments. As the elevator (space) travels up the elevator shaft (time) it changes accordingly. Does relativity mean that all these floors exist? The elevator moves as it does, and I am in no way trying to suggest the possibility of time travel because it's not like a normal person can drive a normal elevator in a normal building. I just don't understand what the fabric of time means.

[north]

I'm looking for an explanation of what four dimentional space-time means in terms of understanding the nature of time. Admittedly, I cannot really conceive of what a four dimenional structure would look like. I've heard all the analogies of imagining a two dimensional space with time as the third dimension, so I'm not looking for anything like that.

Calling time a fourth dimension and weaving a "fabric" of space-time seems to give time some spatial qualities that I'm having trouble reconciling. If all points in a 1 dimensional line are present within a 2 dimensional plane, all ponts on a 2 dimensional plane are present within a 3 dimensional cube, all points in a 3 dimensional cube are present at a given moment in 4 dimensional time, is it that all moments are present along a hypothetical 5th dimension?

Forgive me for asking against dimensionary reduction in the explanation while using it to ask my question, but it's the best way I can think to ask it. Imagine the whole universe from the moment of the big bang to the ultimate end of the universe are put into an elevator. At the bottom floor is the big bang, and at the tip floor is the end of the univese. Imagine this building is astronomically tall. Each minute between the big bang and the end of the universe is represented by one floor on this building, and at each floor the contents of the elevator represent the universe at those moments. As the elevator (space) travels up the elevator shaft (time) it changes accordingly. Does relativity mean that all these floors exist? The elevator moves as it does, and I am in no way trying to suggest the possibility of time travel because it's not like a normal person can drive a normal elevator in a normal building. I just don't understand what the fabric of time means.

 

time has no " real fabric "

 

the concept of the fabric of time is a mathematical concept , " fabric" is a mathematical expression only , it has no physical reality at all

 

inotherwords by applying time only into the dynamics between two objects makes no difference upon the dynamics between the two objects

Edited April 22, 2009 by swansont
fix quote tag

[NowThatWeKnow]

I'm looking for an explanation of what four dimentional space-time means in terms of understanding the nature of time....

 

I have been asking about spacetime in another thread so I am also looking for answers. At this point I understand that space and time was connected by man because the math worked out and it was easier then dealing with an extra dimension of time. Most do not think time is the fabric of space but that space is just a metric. However, it seems the jury is still out as far as some type of ether.

 

In some ways it seems that the speed of light and time are similar because they are both constant, but only relative to the frame they are in. You have "t", proper time, that you experience in your frame. Then you have "T", the time in a different frame then yours. Some will say you have universal time, a clock at rest with the universe CMB and (I think) in the absence of gravity.

[north]

I have been asking about spacetime in another thread so I am also looking for answers. At this point I understand that space and time was connected by man because the math worked out and it was easier then dealing with an extra dimension of time. Most do not think time is the fabric of space but that space is just a metric. However, it seems the jury is still out as far as some type of ether.

 

In some ways it seems that the speed of light and time are similar because they are both constant, but only relative to the frame they are in. You have "t", proper time, that you experience in your frame. Then you have "T", the time in a different frame then yours. Some will say you have universal time, a clock at rest with the universe CMB and (I think) in the absence of gravity.

 

space is room , elbow room so to speak

 

time is the mathematical following of the change in position of an object too another and by doing so can lead to knowledge gained by inference as to why

[etcetcetc00]

I'm just so confused by this concept of relativity. I watched people on TV like Michio Kaku talk about theoretical time travel well beyond our technology, but the possibility alone is what's strange. How could we even consider time travel if there weren't different points in time for us to travel to? What's more, how can there only be one present time, when we can't even establish an objective present?

 

I hope nobody on this forum feels I'm wasting their time with these basic conceptual questions, but I want to learn about this stuff because it's how the world works. If anybody has a good book reccomendatio they can give me that will help explain this, that would be great.

[swansont]

It turns out from relativity that time and distance vary in opposite ways for something that is moving. So a certain 4-D vector that measures distance and time will always have an invariant length — if it moves the time part dilates but the length contracts, so it's the same length in all frames.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/vec4.html

[etcetcetc00]

Ok. If an object travels at the speed of light, it's time slows down severely relative to ours as a result of time dilation. This might make me think that it should go much faster than the speed of light because it travels at the speed of light and its time slows down, but it still moves at the speed of light relative to me because the length it travels becomes longer?

[Sisyphus]

Ok. If an object travels at the speed of light, it's time slows down severely relative to ours as a result of time dilation. This might make me think that it should go much faster than the speed of light because it travels at the speed of light and its time slows down, but it still moves at the speed of light relative to me because the length it travels becomes longer?

 

Don't talk about "moving at the speed of light." That's impossible. Also don't talk about moving at some velocity without specifying what it's moving relative to. That's meaningless. What you mean is moving at a significant fraction of the speed of light relative to something else, like the Earth.

 

And finally, always keep in mind what reference frame you're talking about. From your own perspective, your own "clock" always runs at the same speed. It's only for observers in a different frame of reference that your time is moving slower. It is true, however, that from your perspective, the time your journey takes will be shorter than would seem possible for someone in another reference frame, for example only having 1 year pass (for you) while traveling 10 light years. But you are still not traveling faster than the speed of light, from either perspective. From the outside, you're traveling less than the speed of light and taking longer than 10 years (and your time appears to be moving slower). From your perspective, you're traveling less than the speed of light, taking less than a year, and only traveling less than one light year. Actually, to be more accurate, from your perspective you aren't traveling at all, and your destination is approaching you at some speed.

[etcetcetc00]

Don't talk about "moving at the speed of light." That's impossible. Also don't talk about moving at some velocity without specifying what it's moving relative to. That's meaningless. What you mean is moving at a significant fraction of the speed of light relative to something else, like the Earth.

 

First, I know light speed travel is impossible. It's a hypothetical question. Second, I chose something relative to the speed of light. Me. Presumably on earth where I've always been. Every detail need not be explicitly stated. I'm asking to try to gain some perspective on these principles. If an object (Let's go with a photon) travels at the speed of light, there is time dilation. It's time runs slower relative to me (On Earth). The fact that it's traveling this fast while experiencing slower time than me would make me think it would seem to go much faster than the speed of light. From what I gather, though, the reative length the photon travels is longer than the length I would witness it traveling. This would give it symmetry. Am I getting this?

[Sisyphus]

It matters because traveling at light speed is fundamentally different, hypothetical or no. It's not just a matter of being impossible. It's not described in the same way. For example, as you approach light speed relative to me, I'll observe your time compression approaching infinity. The model breaks down. Time is not part of what a photon "experiences." So, once again, your question is relevant to observers moving at significant fractions of C relative to one another.

 

Anyway, neither you nor I will perceive that either one of us is moving faster than light. Always specify reference frame, and it will make more sense. Say you travel to a star 10 lightyears away, and undergo enough acceleration that due to relativistic time compression, you only experience 1 year for the journey. In my reference frame, you spent somewhat more than 10 years to travel 10 light years, and your time appeared to slow down quite a bit. In your reference frame, however, you only spent 1 year making the journey, but the journey was a much shorter distance. At all times, the speed of light relative to each of us was observed to be equal in every direction: C.

[NowThatWeKnow]

I'm just so confused by this concept of relativity. I watched people on TV like Michio Kaku talk about theoretical time travel well beyond our technology, but the possibility alone is what's strange. How could we even consider time travel if there weren't different points in time for us to travel to? What's more, how can there only be one present time, when we can't even establish an objective present?...

 

There is only one present time in the universe. No one is living in the past and the future simultaneously just because time may pass at different rates in different frames. No one is living in 2007 and 2010 while we are experiencing our "now".

 

It is quite possible to travel into the future using speed or gravity. You slow your time down and then return to Earth's frame at the desired year. You can not go back in time since you can not go slower the 0, only forward. At this point we have only accomplished traveling seconds into the future, not years

[north]

relativity is really about perspective only

 

from ones perspective , an object seems to go at a certain speed

 

but from the perspective of the object its self , it goes with a certain speed

 

time-dilation is again about the perspective of the observer , but to the object its self , it does not dilate