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Re: Physics In the Playground
Air doesn't really conduct electricity *at all*. The only way to get it to do so is to use stupidly high voltages, and even then the charge will probably only travel a short distance in the form of a spark.
This is why lightning exists--if air conducted electricity then the built-up charge in a cloud could harmlessly conduct to earth without having to create an ionisation path and temperatures similar to the surface of the Sun!
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Re: Physics In the Playground
Pure distilled water is also a very poor conductor, but most water on Earth is far from pure. Salt water is an excellent conductor, and that's what fills the oceans.
Fresh water is somewhere in the middle. Outside of a chemistry lab, almost all water has something dissolved in it in at least trace amounts, and it doesn't take all that much to make water conductive. More dissolved ions makes it more conductive, of course, and fresh water is much less conductive than salt water, but "more conductive than air" is a point that is passed at quite low concentrations.
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Re: Physics In the Playground
On gravity and tidal forces, is there time dilation between the center and the event horizon?
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Re: Physics In the Playground
Long story short, we have no idea. No information can leave the event horizon (Hawking Radiation doesn't count), so we don't know what's going on there.
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Re: Physics In the Playground
What do you all think time is?
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Re: Physics In the Playground
Quote:
Originally Posted by
Scotchland
What do you all think time is?
Time is a dimension of spacetime, much like length and width.
That's like asking us what we think the x-coridinate is.
Now, our perception of time being different than our perception of space is the interesting question, but that probably has everything to do with universal entropy. It's probably easier to perceive things as going in the same direction as entropy than not. But physics is still the same when time is reversed.
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Re: Physics In the Playground
Quote:
Originally Posted by
The Extinguisher
Time is a dimension of spacetime, much like length and width.
That's like asking us what we think the x-coridinate is.
That's what time describes.
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Re: Physics In the Playground
Quote:
Originally Posted by
Gwyn chan 'r Gwyll
So I'm reading through the thread and I come across this little link. I got to it and am thoroughly amused when my Google translate app for chrome tells me this page is in Romanian and asks if I want it translated. Funniest thing I've seen all day.
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Re: Physics In the Playground
Quote:
Originally Posted by
Scotchland
That's what time describes.
Again, you're asking what width is. It's a dimension of spacetime. That's all it is.
Time, just like every other dimension of spacetime is distorted by motion and gravity. Time, just like every other dimension of spacetime, between two points is irrelevant when they aren't in each others light cones. It's not anything different.
The only reason we think of it as such is because we're accustomed to looking at time as only increasing, because of entropy, so it's easier for us. But time works the same way no matter what, just like length works the same no matter if it's increasing or decreasing.
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Re: Physics In the Playground
Quote:
Originally Posted by
The Extinguisher
Again, you're asking what width is. It's a dimension of spacetime. That's all it is.
Time, just like every other dimension of spacetime is distorted by motion and gravity. Time, just like every other dimension of spacetime, between two points is irrelevant when they aren't in each others light cones. It's not anything different.
The only reason we think of it as such is because we're accustomed to looking at time as only increasing, because of entropy, so it's easier for us. But time works the same way no matter what, just like length works the same no matter if it's increasing or decreasing.
I think we have a misunderstanding. I had been asking whether you all personally thought that time has an actual substance or is just a supposition of the human mind.
I get the impression that you are firmly in the latter camp.
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Re: Physics In the Playground
Quote:
Originally Posted by
Scotchland
I think we have a misunderstanding. I had been asking whether you all personally thought that time has an actual substance or is just a supposition of the human mind.
I get the impression that you are firmly in the latter camp.
It does have an actual substance. It's part of spacetime, no different than length or width. Physics still works when time is reversed, so the fact that we only perceive time going in one direction is one thing, but it's the same effect as the direction up. We tend to use up to describe the opposite direction of a gravitational field, just like we tend to describe time going forward in relation to entropy.
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Re: Physics In the Playground
Quote:
Originally Posted by
The Extinguisher
It does have an actual substance. It's part of spacetime, no different than length or width. Physics still works when time is reversed, so the fact that we only perceive time going in one direction is one thing, but it's the same effect as the direction up. We tend to use up to describe the opposite direction of a gravitational field, just like we tend to describe time going forward in relation to entropy.
And why do you think this?
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Re: Physics In the Playground
Space elves? Because that's the way the universe is made? The same reason physics still works when you flip the x-axis. It's just the way spacetime is. Sure, you can ask why the speed of a photon in a vacuum is constant across all reference frames, but it's hard to ask why things have a speed at all. It's going to get you nowhere.
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Re: Physics In the Playground
What makes you think time has a physical substance? What makes you think it can be manipulated? If you can't defend your answer or even just cite a text that will, then you might as well call your line of reasoning a religion. :smallsigh:
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Re: Physics In the Playground
Quote:
Originally Posted by
Scotchland
What makes you think time has a physical substance? What makes you think it can be manipulated? If you can't defend your answer or even just cite a text that will, then you might as well call your line of reasoning a religion. :smallsigh:
I'm too tired to enter this discussion this late, but if I remember correctly there was a good explanation of the theories of special and general relativity in the introductory chapters of Brian Greene's The Elegant Universe, with intuition and a discussion of the experiments that confirmed it. The rest of the book deals with string theory, but that intro is the best general introduction to relativity I know of, and you need to understand relativity to understand current theories of time. Also, if I recall correctly, discussion of the nature of spacetime and why we perceive time as flowing in one direction made up a large part of Stephan Hawking's A Brief History of Time, or perhaps it was the updated and condensed version A Briefer History of Time. These aren't exactly what you are asking for, but they should help.
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Re: Physics In the Playground
Quote:
Originally Posted by
Drolyt
I'm too tired to enter this discussion this late, but if I remember correctly there was a good explanation of the theories of special and general relativity in the introductory chapters of Brian Greene's The Elegant Universe, with intuition and a discussion of the experiments that confirmed it. The rest of the book deals with string theory, but that intro is the best general introduction to relativity I know of, and you need to understand relativity to understand current theories of time. Also, if I recall correctly, discussion of the nature of spacetime and why we perceive time as flowing in one direction made up a large part of Stephan Hawking's A Brief History of Time, or perhaps it was the updated and condensed version A Briefer History of Time. These aren't exactly what you are asking for, but they should help.
Cool. I'll look for them in the library.
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Re: Physics In the Playground
Quote:
Originally Posted by
Scotchland
What do you all think time is?
Time is a mathematical illusion. It does not really exist. Time gets it dimensionality from space, by mapping the cyclic movement of a body to time. (We call these bodies clocks.)
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Re: Physics In the Playground
Quote:
Originally Posted by
Yora
I finally finished watching. This is the most fascinating thing I've seen in a while.
As for the issue of time, I'm a believer in time but I don't think you can explain it away as simply being a dimension. My first reaction is "time exists so the universe doesn't happen all at once, because that would be silly". But then I consider that space exists as well, and in more than one dimension. So to say it's just a dimension a) does not explain the why the concept exists to begin with, since there's another kind of dimension that also exists, and b) does not explain why there is only one dimension of time and multiple dimensions of space (there are some models with multiple time dimensions, but most theoretical physicists believe in one temporal dimension, with the addition of the 3 spatial ones we can see and up to 7 we can't as of yet).
So the best I can think of, the only justification for time's existence, is entropy. Never mind the laws of thermodynamics though; entropy simply represents change. The fact that the universe can and does change means that time must exist. Moments exist. The universe as it is right now is different from how it was when you were reading the last sentence, and how it was when you started this sentence is different from how it is now that you're reading the end of it. And how it is right now is very different from how it was at the beginning of the Big Bang. That's what time is to me. I don't know why it is that way, but it is.
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Re: Physics In the Playground
Quote:
Originally Posted by
The Extinguisher
It does have an actual substance. It's part of spacetime, no different than length or width. Physics still works when time is reversed, so the fact that we only perceive time going in one direction is one thing, but it's the same effect as the direction up. We tend to use up to describe the opposite direction of a gravitational field, just like we tend to describe time going forward in relation to entropy.
Actually, although special relativity unifies the treatment of space and time, it doesn't quite say that they're interchangeable. In particular, the time coordinate has a different sign from the spatial coordinates in the metric, which means it isn't quite on the same footing. In fact, if it wasn't for time being "the same, but different", special relativity wouldn't be that interesting, since the Lorentz transformation would have the same (hyper)spherical geometry as normal Euclidean rotations, instead of its actual hyperbolic geometry within SR.
One other point is that although there hasn't been experimental detection of time-reversal asymmetry, a lot of physicists think there have to be some processes which don't run the same backward as forward. Their reasoning is based on something called the TCP theorem, which says that for pretty much any allowable quantum field theory, the combined symmetry of the product of time-reversal, charge and parity symmetries has to be exact (i.e. non-violated). Experiments with neutrinos have already shown that CP symmetry is violated separately in at least some decay processes, so for TCP to remain a perfect symmetry, time-reversal invariance would also need to be broken (the reasoning for this is loosely because -1 * -1 = +1, so two violations together make for an unviolated product).
I hope that didn't come off like I'm trying to call you out on time stuff; I just thought I'd clarify a few points and throw out some new topics for discussion.
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Re: Physics In the Playground
Quote:
Originally Posted by
the_druid_droid
Actually, although special relativity unifies the treatment of space and time, it doesn't quite say that they're interchangeable. In particular, the time coordinate has a different sign from the spatial coordinates in the metric, which means it isn't quite on the same footing. In fact, if it wasn't for time being "the same, but different", special relativity wouldn't be that interesting, since the Lorentz transformation would have the same (hyper)spherical geometry as normal Euclidean rotations, instead of its actual hyperbolic geometry within SR.
It's the square of the time coordinate that's subtracted from the sum of squares of the space coordinates. So it uses imaginary numbers. :smalltongue:
Quote:
Originally Posted by
the_druid_droid
One other point is that although there hasn't been experimental detection of time-reversal asymmetry, a lot of physicists think there have to be some processes which don't run the same backward as forward. Their reasoning is based on something called the TCP theorem, which says that for pretty much any allowable quantum field theory, the combined symmetry of the product of time-reversal, charge and parity symmetries has to be exact (i.e. non-violated). Experiments with neutrinos have already shown that CP symmetry is violated separately in at least some decay processes, so for TCP to remain a perfect symmetry, time-reversal invariance would also need to be broken (the reasoning for this is loosely because -1 * -1 = +1, so two violations together make for an unviolated product).
I hope that didn't come off like I'm trying to call you out on time stuff; I just thought I'd clarify a few points and throw out some new topics for discussion.
All the equations of quantum-particle physics are reversible. That means you can reverse them locally creating a small pocket where time appears to run backward. All the equations of quantum-wave physics are not reversible. A wave is an expanding sphere so creating a shrinking sphere that perfectly converges at a point is impossible.
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Re: Physics In the Playground
Quote:
Originally Posted by
shawnhcorey
It's the square of the time coordinate that's subtracted from the sum of squares of the space coordinates. So it uses imaginary numbers. :smalltongue:
It's true that some books use this convention, and then the metric is just the unit matrix, however the point remains that the time coordinate is distinguished from the others; in this case by the presence of a factor of i. Also, most modern books avoid introducing the factor of i, because it isn't really necessary and more advanced work depends on the language of vectors, one-forms and metric tensors anyway. Using these tools, there is no need for the imaginary factor, since the negative sign is transferred to the metric and no square roots appear.
Quote:
All the equations of quantum-particle physics are reversible. That means you can reverse them locally creating a small pocket where time appears to run backward. All the equations of quantum-wave physics are not reversible. A wave is an expanding sphere so creating a shrinking sphere that perfectly converges at a point is impossible.
This distinction is not actually a thing.
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Re: Physics In the Playground
Quote:
Originally Posted by
shawnhcorey
All the equations of quantum-particle physics are reversible. That means you can reverse them locally creating a small pocket where time appears to run backward. All the equations of quantum-wave physics are not reversible. A wave is an expanding sphere so creating a shrinking sphere that perfectly converges at a point is impossible.
Quote:
Originally Posted by
the_druid_droid
This distinction is not actually a thing.
I'm not sure what you mean by "a thing" but the distinction is real. Waves do not behave like particles. Consider this diagram of the double-slit experiment. Notice that the path of the particle bends when it passes thru one of the slits. But there is absolutely no reason for it to do so. A particle in motion remains in motion unless acted on by an outside force. There is no outside force here. The particle should continue in a straight path. Waves behave differently than particle and you cannot explain why the double-slit experiment behaves the way it does without them. You must make the distinction.
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Re: Physics In the Playground
Quote:
Originally Posted by
shawnhcorey
I'm not sure what you mean by "a thing" but the distinction is real. Waves do not behave like particles. Consider
this diagram of the double-slit experiment. Notice that the path of the particle bends when it passes thru one of the slits. But there is absolutely no reason for it to do so. A particle in motion remains in motion unless acted on by an outside force. There is no outside force here. The particle should continue in a straight path. Waves behave differently than particle and you cannot explain why the double-slit experiment behaves the way it does without them. You must make the distinction.
My point is that quantum mechanics applies equally to all physical phenomena, and it is the equations of the theory that must be invariant under TCP symmetry. The distinction between particle-like and wave-like behavior isn't an issue for the TCP theorem, because it deals with the structure of the theory itself, rather than a particular manifestation.
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Re: Physics In the Playground
Quote:
Originally Posted by
the_druid_droid
Actually, although special relativity unifies the treatment of space and time, it doesn't quite say that they're interchangeable. In particular, the time coordinate has a different sign from the spatial coordinates in the metric, which means it isn't quite on the same footing. In fact, if it wasn't for time being "the same, but different", special relativity wouldn't be that interesting, since the Lorentz transformation would have the same (hyper)spherical geometry as normal Euclidean rotations, instead of its actual hyperbolic geometry within SR.
One other point is that although there hasn't been experimental detection of time-reversal asymmetry, a lot of physicists think there have to be some processes which don't run the same backward as forward. Their reasoning is based on something called the TCP theorem, which says that for pretty much any allowable quantum field theory, the combined symmetry of the product of time-reversal, charge and parity symmetries has to be exact (i.e. non-violated). Experiments with neutrinos have already shown that CP symmetry is violated separately in at least some decay processes, so for TCP to remain a perfect symmetry, time-reversal invariance would also need to be broken (the reasoning for this is loosely because -1 * -1 = +1, so two violations together make for an unviolated product).
I hope that didn't come off like I'm trying to call you out on time stuff; I just thought I'd clarify a few points and throw out some new topics for discussion.
That's interesting. I'll admit I've never gone too in depth with relativity (I decided to focus more on math than physics), but that's cool to know. Although, I wonder if it's possible to set special relativity up in such as way that, say, the x-dimension dilated, while the others contracted in motion. Is our choice of time as the separate dimension mostly arbitrary (I think it would be)
As for TCP symmetry, I didn't know CP symmetry had been violated. That's really cool.
Quote:
Originally Posted by
shawnhcorey
I'm not sure what you mean by "a thing" but the distinction is real. Waves do not behave like particles. Consider
this diagram of the double-slit experiment. Notice that the path of the particle bends when it passes thru one of the slits. But there is absolutely no reason for it to do so. A particle in motion remains in motion unless acted on by an outside force. There is no outside force here. The particle should continue in a straight path. Waves behave differently than particle and you cannot explain why the double-slit experiment behaves the way it does without them. You must make the distinction.
Yes, but in quantum physics, nothing is a wave or a particle, but a combination of both. The particle is a wave as well. That's why it works.
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Re: Physics In the Playground
Quote:
Originally Posted by
The Extinguisher
That's interesting. I'll admit I've never gone too in depth with relativity (I decided to focus more on math than physics), but that's cool to know. Although, I wonder if it's possible to set special relativity up in such as way that, say, the x-dimension dilated, while the others contracted in motion. Is our choice of time as the separate dimension mostly arbitrary (I think it would be)
It is possible to set things up so that the space-like coordinates have negative signs in the interval expression (and the metric) and the time coordinate has a positive sign. However, the time and space coordinates are grouped separately, from what I understand, because the speed of light (being an invariant velocity) relates space and time in a specific way. However, it doesn't link two different spatial coordinates in the same way, so you couldn't have arbitrary components dilate and contract independently; they'd have to be linked by the underlying structure based on the invariance of light speed.
Quote:
As for TCP symmetry, I didn't know CP symmetry had been violated. That's really cool.
Re-reading my sources suggests that I misspoke about neutrinos. The neutrino experiment showed that P symmetry alone was violated. It was actually experiments with kaon decays that showed CP was violated (B mesons have a similar effect). The point remains, but I thought I should mention it.
Interestingly, one of the proposed experiments for detecting T violation is to actually try and run certain decay processes 'backward', by firing the products together to re-form the starting particles. The problem is that only neutrino reactions have low enough background noise to be viable, but neutrinos are tricky to work with since they interact so weakly. There are a few indirect methods available, but so far nothing experimentally exists to prove that T is in fact a broken symmetry.
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Re: Physics In the Playground
Quote:
Originally Posted by
The Extinguisher
Yes, but in quantum physics, nothing is a wave or a particle, but a combination of both. The particle is a wave as well. That's why it works.
But the whole point of my argument is that because it is a wave, time is irreversible. If you replaced the detector with a source and the source with a detector and send a quantum backward thru the experiment, the chances of it hitting the detector are slight. The wave properties of quanta make time irreversible.
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Re: Physics In the Playground
Does anyone of you know what causes a column to buckle under compression? I used my google-fu to the point of exhaustion, but all I can find is demonstrations of how buckling is affected by different cross sections, the length of the column, and how the ends of the column is mounted, or people explaining the equation to calculate at what force buckling does occur.
But what causes it? If I have a perfectly shaped column and there is the same compression force at any point of a given cross section, I've seen the claim buckling would still occur. If there's a slight bend in the column, buckling would occur at a lower compression force, but it would still occur if it's perfectly straight and of homogenous composition.
Then what causes the mass to move sideways when the only forces acting on it are up and down?
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Re: Physics In the Playground
Quote:
Originally Posted by
Yora
Does anyone of you know what causes a column to buckle under compression? I used my google-fu to the point of exhaustion, but all I can find is demonstrations of how buckling is affected by different cross sections, the length of the column, and how the ends of the column is mounted, or people explaining the equation to calculate at what force buckling does occur.
But what causes it? If I have a perfectly shaped column and there is the same compression force at any point of a given cross section, I've seen the claim buckling would still occur. If there's a slight bend in the column, buckling would occur at a lower compression force, but it would still occur if it's perfectly straight and of homogenous composition.
Then what causes the mass to move sideways when the only forces acting on it are up and down?
None of the videos you posted earlier (see below) helped?
Quote:
Originally Posted by
Yora
Looking on information how to make fascinating and realistic fantasy castles, I found
this series on engineering on youtube.
It's on a very basic level and rather brief on most topics, but if you don't have any previous knowledge except for what you figured out by playing with wood blocks, sticks, and mud, I find it a very easy introduction. Didn't think it's possible to understand the basic principle of buckling without an advanced degree in physics. :smallbiggrin:
But all those questions like "How do towers topple without being pushed over", "How tall can you stack bricks until the lowest brick at the bottom gets crushed by the weight", or "could you make multi-story buildings with floors made from stone beams without them snapping" (to which the answer is "not in a practical way") are all really not that difficult when someone explains to you the very basics of structural engineering.
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Re: Physics In the Playground
No, it just explains the math at what point buckling occurs and what to do to make it less likely to happen. No explaination what causes it.
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Re: Physics In the Playground
Quote:
Originally Posted by
Yora
... but it would still occur if it's perfectly straight and of homogenous composition.
Then what causes the mass to move sideways when the only forces acting on it are up and down?
But in real life, nothing is perfect. There are always tiny flaws in the structure where failure can start.
The mass moves sideways because the flaws introduce shear forces at an angle. The shear forces can be composed into two components, one vertical, one horizontal. It's the horizontal one that causes sideways movement.