physics question... speed of light
Wed, 2008-06-11 17:49
#1
physics question... speed of light
Wed, 2008-06-11 17:49
#2
dinner table question. Imagine a 10000 mile long 2x4 floating in space. If we push one end of it forward will the other end move instantly (ie: faster than the speed of light?)
Wed, 2008-06-11 17:57
#3
Depends on who you include in the word "we", but if it doesn't include me, then no.
Mind you don't get a splinter.
Wed, 2008-06-11 18:04
#4
quote:
Originally posted by Ward:
[b]dinner table question. Imagine a 10000 mile long 2x4 floating in space. If we push one end of it forward will the other end move instantly (ie: faster than the speed of light?)[/b]
A dinner table question? What did you have for dinner?
Wed, 2008-06-11 18:23
#5
Well, we had food for dinner. But non of us are quantum physicists so the question couldn't be answered.
Wed, 2008-06-11 18:39
#6
Well I think the answer to your question is that a 2X4 doesn't come in that length hence the name 2X4. [img]biggrin.gif" border="0[/img]
Wed, 2008-06-11 18:43
#7
Boom Boom
What kind of 2x4 was it? A piece of wood, or a truck? [img]wink.gif" border="0[/img]
Wed, 2008-06-11 18:47
#8
Boom Boom you are thinking of a 4x4. They drink a lot of gas.
Wed, 2008-06-11 18:54
#9
Boom Boom
My Mazda truck is a 2x4. [img]tongue.gif" border="0[/img]
ETA: oops, maybe I should have said 4x2. [img]redface.gif" border="0[/img]
[ 11 June 2008: Message edited by: Boom Boom ]
Wed, 2008-06-11 19:01
#10
Pogo
My quick answer would be no because of the 'give' in the wood which would use up time as it transferred the pressure along the length of the 2x4.
Wed, 2008-06-11 19:02
#11
M. Spector
quote:
If you have a long rigid stick and you hit one end, wouldn't the other end have to move immediately? Would this not provide a means of FTL [faster than light] communication?Well, it would if there were such things as perfectly rigid bodies. In practice the effect of hitting one end of the stick propagates along it at the speed of sound in the material; this speed depends on the stick's elasticity and density. Relativity places an absolute limit on material rigidity in such a way that the speed of sound in the material will not be greater than c.
[url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#4... explanation continues here...[/url]
Wed, 2008-06-11 20:08
#12
quote:
Originally posted by Ward:
[b]dinner table question. Imagine a 10000 mile long 2x4 floating in space. If we push one end of it forward will the other end move instantly (ie: faster than the speed of light?)[/b]
No, it won't. Motion can only propagate through the material at a speed less than the speed of light. You forget that solid objectsaren't really so solid.
Wed, 2008-06-11 20:21
#13
Sven
quote:
Originally posted by Ward:
[b]dinner table question. Imagine a 10000 mile long 2x4 floating in space. If we push one end of it forward will the other end move instantly (ie: faster than the speed of light?)[/b]
Another "dinner table question": What's on the other side of the edge of the universe?
Wed, 2008-06-11 20:31
#14
Probably someone named [b]nevS[/b] asking "?esrevinu eht fo edis rehto eht no s'tahW"
Wed, 2008-06-11 20:56
#15
Sven
quote:
Originally posted by martin dufresne:
[b]Probably someone named [b]nevS[/b] asking "?esrevinu eht fo edis rehto eht no s'tahW"[/b]
[img]biggrin.gif" border="0[/img]
Thu, 2008-06-12 00:23
#16
quote:
Originally posted by Sven:
[b]Another "dinner table question": What's on the other side of the edge of the universe?[/b]
There is no "edge" of the universe. The common analogy is to say that the universe is like the surface of a balloon. It has a finite width around, but there's no edge.
Thu, 2008-06-12 01:18
#17
There are theorists who suggest that black holes must contain a single point of infinite density where known laws of physics no longer apply, an "edge" of the space-time continuum. An object entering the black hole would be squeezed out of existence. Some say that a spaceship's free-fall trajectory toward the black hole, but not close enough to be drawn into it, could be a one-way time machine.
I think it was Einstein who suggested that black holes could be contortions of the space-time continuum, sort of like a picnic table cover being space-time and but with raised folds in the sheet pulling distance-time together. If we could push a sharp pencil through the fold, the tip might come out at the far end of the universe, or some such.
Thu, 2008-06-12 06:53
#18
Erstwhile
quote:
Originally posted by PB66:
[b]There is no "edge" of the universe. The common analogy is to say that the universe is like the surface of a balloon. It has a finite width around, but there's no edge. [/b]
quote:
Originally posted by Fidel:
[b]If we could push a sharp pencil through the fold, the tip might come out at the far end of the universe, or some such.[/b]
....or pop the balloon! [img]eek.gif" border="0[/img]
You mad fools! You'll kill us all, with your pointy sticks pokin' at the space-time continuum!
Thu, 2008-06-12 13:24
#19
No because there is no such thing as a rigid body in relativity.
The most common form of this is the detonator paradox: [url=http://www.madsci.org/posts/archives/2001-09/1001434136.Ph.q.html]http:/...
It was on a final exam I took in relativity, four years ago. I got that one wrong.
Fri, 2008-06-13 00:20
#20
quote:
Originally posted by Fidel:
[b]There are theorists who suggest that black holes must contain a single point of infinite density where known laws of physics no longer apply, an "edge" of the space-time continuum. An object entering the black hole would be squeezed out of existence. Some say that a spaceship's free-fall trajectory toward the black hole, but not close enough to be drawn into it, could be a one-way time machine. [/b]
Oops, yes, Fidel is quite right. Deep within a black hole, it is expected there will be singularities, according to classical relativity. Either space-time will simply end, or quantum effects will become important, which might remove the singularities. It's not really known what happens with the singularities in relativity, and, more generally, properly combining quantum effects and relativity remains one of the biggest problems in physics. There was a lot of research done on this sort of thing in the '70's, but not much progress was made, and researchers have put this aside to work on problems which can be solved.
I was thinking the question was about what it meant for the universe to be expanding. In that case, we imagine the universe to be some-what like a three-dimensional version of the surface of a balloon. It has a total length around, but no edge. If the balloon is inflated, that length around increases, but for imaginary two-dimensional people living on the balloon (or for us in the three-dimensional universe), it cannot be seen where that expansion is occuring.
Sat, 2008-06-14 22:30
#21
quote:
Originally posted by Sven:
[b]Another "dinner table question": What's on the other side of the edge of the universe?[/b]
If there is an edge, we can't see it or get to it and it can't affect us. The observable universe - that is, those parts of the universe from which light has been emitted that can reach us since the beginning of the universe - is [url=http://en.wikipedia.org/wiki/Observable_universe]92 billion light years across[/url]. But there is no reason to believe that if you could somehow be teleported past that you would see a universe that's any different in general. The universe certainly doesn't end there.