Ask a physicist anything. (7)

[Zippy the Wonderslug]

Also, since this forum is more active, would someone mind answering this question?

http://www.christianforums.com/t7617902/

Lastly, what all video file formats can be played before the entire video needs to be fully downloaded first?

 

[Chalnoth]

Someone said the neutrinos may have different energy levels (in GEV "gigaelectron volts" sp?) at CERN and from the supernova, which might make the comparison inadequate.

The problem with that idea is that it begs the question as to why the dependence of neutrino speed on energy would be just so as to make the supernova measurement of neutrino speed agree with the speed of light more than a thousand times more accurately. That's just far too great a coincidence.

 

[Chalnoth]

Also, since this forum is more active, would someone mind answering this question?

http://www.christianforums.com/t7617902/

Lastly, what all video file formats can be played before the entire video needs to be fully downloaded first?

Well, the issue there is that Windows has nothing whatsoever to do with adding the ".part" extension. It's the program performing the download that is doing that. You can, if you like, manually copy the .part file to a new file with the destination extension and try playing that. But that has a few problems:
1. You have to manually re-start the video file when you reach the point in the video file that the download was at when you made the copy.
2. You'll have to be sure to save the video file with a different name, as your download program may freak out if it tries to write to a file that's already there when it finishes the download.
3. Windows Media Player and most other movie players lock the file when they're reading from it, preventing any other program from writing to it. This is basically built in to how Windows operates, and it means that there really isn't any way around the file extension issue: you just can't play the file while it is in the process of being downloaded. You can make a copy, as noted above, but you can't just play the file.

So in the end, if you want to watch a movie while it is downloading, you really need something that is actually designed for streaming from the start. That means setting up a server/client model. On a home network, you can actually do this without too much difficulty by directly playing the movie file from the remote hard disk. Unfortunately I'm not aware of how you can do this directly on an X-Box, but it's easy enough to do between computers connected on a LAN.

 

[Zippy the Wonderslug]

^ Wow.

I didn't think it would be that hard.

On my Windows XP, I double click on the .part file, it doesn't know what to do with it, then I get a pop up selection screen on what app I would like to open it with, I select VLC, and it starts playing even while it's still downloading.

The problem is, I don't know how to make Mplayer accept the .part extension on my Xbox.

 

[Chalnoth]

^ Wow.

I didn't think it would be that hard.

On my Windows XP, I double click on the .part file, it doesn't know what to do with it, then I get a pop up selection screen on what app I would like to open it with, I select VLC, and it starts playing even while it's still downloading.

The problem is, I don't know how to make Mplayer accept the .part extension on my Xbox.

Well, on the Xbox, can you simply browse to files you have shared on your PC? If so, you should be able to just play them directly.

 

[Zippy the Wonderslug]

Yeah, they're on a shared network.

When I click on the video .part file in XBMC nothing happens.

I have to wait until it's completely downloaded and has the .part removed from it's name.

 

[Chalnoth]

Yeah, they're on a shared network.

When I click on the video .part file in XBMC nothing happens.

I have to wait until it's completely downloaded and has the .part removed from it's name.

Oh, I see what you're saying. You want to view it on your Xbox while downloading it on your PC. I thought you wanted to view it while transferring it to your Xbox from your PC. Sorry, not sure I can help.

 

[Zippy the Wonderslug]

^ Okay. Thanks anyway.

 

[CTD]

Is the dependence of general relativity upon special relativity mathematical, or purely philosophical?

 

[Wiccan_Child]

Is the dependence of general relativity upon special relativity mathematical, or purely philosophical?

Mathematical. Special Relativity is a special case of General Relativity (just like how Classical Mechanics can now be seen as a special, limited case of the more general Quantum Mechanics). General Relativity is a generalisation of Special Relativity. SR is a 'special case' of GR, while GR is more... general.

In principle, if SR is proven false, then GR must be false also (if GR then SR; ¬SR; therefore, ¬GR). If GR is proven false, SR might still be true.

Einstein formulated SR, then incorporated gravity to create GR. In essence, GR simplifies to SR when spacetime is treated as being uniform and even. When you generalise the curvature of spacetime, you need the more general GR.

If GR is true, then so is SR. If SR is false, then so is GR.

 

[Chalnoth]

Is the dependence of general relativity upon special relativity mathematical, or purely philosophical?

To clarify slightly Wiccan_Child's statement: general relativity reduces to special relativity if there is no space-time curvature. So General Relativity can be thought of as special relativity + curvature.

 

[Wiccan_Child]

To clarify slightly Wiccan_Child's statement: general relativity reduces to special relativity if there is no space-time curvature. So General Relativity can be thought of as special relativity + curvature.

I knew there was a more succinct way of saying it

 

[CTD]

That pretty much works.

 

[Resha Caner]

OK, I need help from math wizards. Is that Wiccan_Child or Acropolis or one of our other resident scientists?

This is what probably appears to be a very trivial question, but it has some fascinating implications IMO.

"Orthogonality" is a very important concept in math and is very useful in mechanics. It means that 2 variables can change independently of each other. In geometry that manifests in the idea of perpendicularity, which is when two curves intersect to form 2 congruent angles.

My question is this: Why is this concept only based on 2's? In other words, could I create a math with some type of orthogonality where 3 curves meet to form 3 congruent angles? Or n curves meet to form n congruent angles?

 

[Chalnoth]

OK, I need help from math wizards. Is that Wiccan_Child or Acropolis or one of our other resident scientists?

This is what probably appears to be a very trivial question, but it has some fascinating implications IMO.

"Orthogonality" is a very important concept in math and is very useful in mechanics. It means that 2 variables can change independently of each other. In geometry that manifests in the idea of perpendicularity, which is when two curves intersect to form 2 congruent angles.

My question is this: Why is this concept only based on 2's? In other words, could I create a math with some type of orthogonality where 3 curves meet to form 3 congruent angles? Or n curves meet to form n congruent angles?

Yes! It's just a factor of the number of dimensions. In two dimensions, a maximum of two lines can meet at right angles. In three dimensions, a maximum of three lines can meet with each at right angles to one another. In four dimensions, a maximum of four lines can meet with each at right angles to one another.

All orthogonal lines intersect at right angles with one another, in all dimensions. So for n=2, you have 1 right angle (only one possible pair of lines). For n=3, you have 3 right angles (can make right angles with lines (1,2), (1,3), and (2,3)). For n=4, you have 7 right angles. And so on.

 

[Resha Caner]

Yes! It's just a factor of the number of dimensions.

Hey, thanks for the answer, but I guess my question was unclear. I realize one can increase the number of dimensions, but there is the age old debate about whether a 4th physical dimension really exists.

Let's say we confine ourselves to a plane ... which traditionally has 2 dimensions. That it has 2, however, is an almost obvious, tautological result of the definition of perpendicular.

So, I'm asking if a different definition could be used that would divide the plane into n independent parts rather than just 2. I think it can be done, but I'm wondering if there is a flaw in my thinking.

Even if it could, the next obvious question would be: why? Even if one could, it would be less elegant, less efficient to do it that way. At least at first glance. However, I have reasons for thinking there may be cases where it would be better to use more than 2 dimensions.

 

[mzungu]

Hey, thanks for the answer, but I guess my question was unclear. I realize one can increase the number of dimensions, but there is the age old debate about whether a 4th physical dimension really exists.

Let's say we confine ourselves to a plane ... which traditionally has 2 dimensions. That it has 2, however, is an almost obvious, tautological result of the definition of perpendicular.

So, I'm asking if a different definition could be used that would divide the plane into n independent parts rather than just 2. I think it can be done, but I'm wondering if there is a flaw in my thinking.

Even if it could, the next obvious question would be: why? Even if one could, it would be less elegant, less efficient to do it that way. At least at first glance. However, I have reasons for thinking there may be cases where it would be better to use more than 2 dimensions.

This guy pretty much explains it very well: Dimensions of Physical Space | Of Particular Significance

 

[Chalnoth]

Hey, thanks for the answer, but I guess my question was unclear. I realize one can increase the number of dimensions, but there is the age old debate about whether a 4th physical dimension really exists.

Let's say we confine ourselves to a plane ... which traditionally has 2 dimensions. That it has 2, however, is an almost obvious, tautological result of the definition of perpendicular.

So, I'm asking if a different definition could be used that would divide the plane into n independent parts rather than just 2. I think it can be done, but I'm wondering if there is a flaw in my thinking.

No, that can't be done. Basically, the third direction on a plane will always be a function of the other two.

You can see this most easily with linear algebra. Basically, each direction on the plane can be represented as a two-dimensional vector, and those vectors can be composed into a matrix. If you ever add a third vector to the matrix, the determinant of the matrix is always equal to zero (because the matrix won't be square, and the determinant of a non-square matrix is always zero), which indicates that the third vector is just a linear sum of the other two.

That explanation might be a bit opaque, but yes, it is absolutely proven that you can't have more than two independent directions in two dimensions.

 

[Resha Caner]

You can see this most easily with linear algebra. Basically, each direction on the plane can be represented as a two-dimensional vector, and those vectors can be composed into a matrix. If you ever add a third vector to the matrix, the determinant of the matrix is always equal to zero (because the matrix won't be square, and the determinant of a non-square matrix is always zero), which indicates that the third vector is just a linear sum of the other two.

Cool. It might seem odd, but my engineering program never required me to take a linear algebra course, so what I know is what I learned on my own. As such, I didn't realize the determinant of a non-square matrix is always zero. Very interesting. I understand you explanation at an abstract level ... but then I get n-dimensions at an abstract level as well. I'm going for something a bit different.

When I try to visualize it, I can also understand that it only takes 2 coordinates to uniquely define the position of any point in a plane. Any other way of doing it would duplicate points.

But ... what if it's not points I'm interested in but paths. In this case I have a planar 3 DOF mechanism in my head. Don't the 3 DOF correspond to dimensions in a way? And if my Range (my "space") is the set of points traced by the mechanism, then I can uniquely define each point on the path in terms of those 3 dimensions (my Domain).

 

[Chalnoth]

But ... what if it's not points I'm interested in but paths. In this case I have a planar 3 DOF mechanism in my head. Don't the 3 DOF correspond to dimensions in a way? And if my Range (my "space") is the set of points traced by the mechanism, then I can uniquely define each point on the path in terms of those 3 dimensions (my Domain).

Well, yeah, paths are a bit more difficult. The problem is that in an abstract sense, a path is an infinite-dimensional object (since you can have an infinite number of locations among the path, each of which is defined by two values in two dimensions). Dealing with generalized paths can be quite a fun mathematical/computational problem, and commonly appears in General Relativity and Quantum Field Theory.