#### Confused by Brian Cox and special relativity

Alexis Davis

Alexis Davis
Total Posts:  124
Joined  19-05-2011

05 September 2012 03:36

I read a book a while back by Brian Cox called Why Does E=MCsquared and, first, let me say that I enjoyed it tremendously and would recommend it to anybody like me that is utterly lacking in a science background. It was informative and a lot of fun. But, I had this uneasy feeling of confusion that I get when there is something that doesn’t quite add up to me, but I can’t put my finger on it at the time. It finally dawned on me what my problem was. The book starts with establishing that the notion of absolute motion and absolute time must be jettisoned, since there is no experiment that will show if I am the one moving in a straight line at a constant speed. The motion must always be thought of as relative to something else. He goes on to show how time ticks at different rates for things moving at different velocities with some very simple math, and how experiments have established that the predictions of this math is as accurate as we are able to determine. He goes on to show how everything moves through space/time at the same velocity, C, with more or less of an objects momentum being used in either the motion through the space vector or the time vector, and how massless objects, like photons, use all of their momentum going through the space vector, while objects having mass use a combination of some through space and the rest through time. So far, so good.

My problem is that, if you graph this in two dimensions, like on a piece of paper, with, say, the Y axis representing movement through space and the X axis representing motion through time, all of a photon’s motion would be along the Y axis. This opens the possibility of the entirety of an objects momentum being along the X axis, meaning all of its motion is through time. That would mean that it is truly at rest in an absolute sense as far as motion through space goes, and, this could be determined in an experiment measuring how fast time is ticking for the object compared to how fast time is ticking on a theoretical construction traveling at C. It seems that this would be a pretty challenging experiment to conduct, but, it also seems possible. I am pretty confident that my problem rests solely with my extremely limited understanding of the subject, so, a little help would be appreciated.

burt

burt
Total Posts:  14640
Joined  17-12-2006

05 September 2012 14:18

VeronicaS - 05 September 2012 01:36 AM

I read a book a while back by Brian Cox called Why Does E=MCsquared and, first, let me say that I enjoyed it tremendously and would recommend it to anybody like me that is utterly lacking in a science background. It was informative and a lot of fun. But, I had this uneasy feeling of confusion that I get when there is something that doesn’t quite add up to me, but I can’t put my finger on it at the time. It finally dawned on me what my problem was. The book starts with establishing that the notion of absolute motion and absolute time must be jettisoned, since there is no experiment that will show if I am the one moving in a straight line at a constant speed. The motion must always be thought of as relative to something else. He goes on to show how time ticks at different rates for things moving at different velocities with some very simple math, and how experiments have established that the predictions of this math is as accurate as we are able to determine. He goes on to show how everything moves through space/time at the same velocity, C, with more or less of an objects momentum being used in either the motion through the space vector or the time vector, and how massless objects, like photons, use all of their momentum going through the space vector, while objects having mass use a combination of some through space and the rest through time. So far, so good.

My problem is that, if you graph this in two dimensions, like on a piece of paper, with, say, the Y axis representing movement through space and the X axis representing motion through time, all of a photon’s motion would be along the Y axis. This opens the possibility of the entirety of an objects momentum being along the X axis, meaning all of its motion is through time. That would mean that it is truly at rest in an absolute sense as far as motion through space goes, and, this could be determined in an experiment measuring how fast time is ticking for the object compared to how fast time is ticking on a theoretical construction traveling at C. It seems that this would be a pretty challenging experiment to conduct, but, it also seems possible. I am pretty confident that my problem rests solely with my extremely limited understanding of the subject, so, a little help would be appreciated.

Hope this doesn’t add to the confusion.  In relativity you are working in 4 dimensions, one of time and the other three of space.  But the time you actually measure yourself is the 4-dimensional distance you travel along your world line.  Light travels at speed c (speed is how fast it is traveling, velocity is how fast and what direction).  In relativity this means that there actually is no time for light because the 4 dimensional distance along light paths is zero.

eudemonia

eudemonia
Total Posts:  9031
Joined  05-04-2008

05 September 2012 17:04

Inertial frame of reference is the concept to be studied concerning special relativity.

Alexis Davis

Alexis Davis
Total Posts:  124
Joined  19-05-2011

05 September 2012 23:20

burt - 05 September 2012 12:18 PM

Hope this doesn’t add to the confusion.  In relativity you are working in 4 dimensions, one of time and the other three of space.  But the time you actually measure yourself is the 4-dimensional distance you travel along your world line.  Light travels at speed c (speed is how fast it is traveling, velocity is how fast and what direction).  In relativity this means that there actually is no time for light because the 4 dimensional distance along light paths is zero.

No, actually that was kind of my starting point, albeit poorly formulated. I understand that space/time is four dimensional, I just used a two dimensional graph for simplification, with one axis representing three dimensional space, and the other axis representing time. I also get it that from a photons frame of reference time is not moving because all of its motion is spent traveling through space. My question was why could something not do basically the same thing, except for all of its motion is involved in going through time, and, if that is indeed possible, that would seem to me to bring back the idea of absolute motion which was abandoned to derive special relativity in the first place, and that it would, in theory, be possible to perform an experiment that would show in an absolute sense that an object was not moving through space, and that it would therefore be possible to deduce absolute motion quantitatively based on how fast time was ticking relative to nothing other than C.

burt

burt
Total Posts:  14640
Joined  17-12-2006

06 September 2012 07:04

VeronicaS - 05 September 2012 09:20 PM
burt - 05 September 2012 12:18 PM

Hope this doesn’t add to the confusion.  In relativity you are working in 4 dimensions, one of time and the other three of space.  But the time you actually measure yourself is the 4-dimensional distance you travel along your world line.  Light travels at speed c (speed is how fast it is traveling, velocity is how fast and what direction).  In relativity this means that there actually is no time for light because the 4 dimensional distance along light paths is zero.

No, actually that was kind of my starting point, albeit poorly formulated. I understand that space/time is four dimensional, I just used a two dimensional graph for simplification, with one axis representing three dimensional space, and the other axis representing time. I also get it that from a photons frame of reference time is not moving because all of its motion is spent traveling through space. My question was why could something not do basically the same thing, except for all of its motion is involved in going through time, and, if that is indeed possible, that would seem to me to bring back the idea of absolute motion which was abandoned to derive special relativity in the first place, and that it would, in theory, be possible to perform an experiment that would show in an absolute sense that an object was not moving through space, and that it would therefore be possible to deduce absolute motion quantitatively based on how fast time was ticking relative to nothing other than C.

But time doesn’t “tick” relative to C, C is a speed so is measured as distance/time.  The catch in what you’re suggesting is that there are no preferred directions in space.  Every observer travels along their world line and, if they are not accelerating (for which you go over to general relativity) they are, in their own frame of reference, at rest.  They may experience a sense of motion relative to other things they see, but in their frame they are at rest.  Think if it this way, a single observer in an otherwise completely empty space.  How would they ever know they were moving?

Alexis Davis

Alexis Davis
Total Posts:  124
Joined  19-05-2011

08 September 2012 17:12

burt - 06 September 2012 05:04 AM

Think if it this way, a single observer in an otherwise completely empty space.  How would they ever know they were moving?

That’s exactly the part that I am confused on. It seems to me that it might be possible to quantitatively determine the time dilation experienced in that frame of reference, and, if there is any, you are moving.

burt

burt
Total Posts:  14640
Joined  17-12-2006

08 September 2012 18:21

VeronicaS - 08 September 2012 03:12 PM
burt - 06 September 2012 05:04 AM

Think if it this way, a single observer in an otherwise completely empty space.  How would they ever know they were moving?

That’s exactly the part that I am confused on. It seems to me that it might be possible to quantitatively determine the time dilation experienced in that frame of reference, and, if there is any, you are moving.

No, with no other frame of reference to compare to, how could you determine time dilation?  You would only have your own frame since the universe is otherwise empty.  If you were in an accelerating frame that brings in general relativity, so we’ll ignore that.  And in the regular universe, every reference frame is equivalent so you couldn’t establish any particular one as preferred (they’re all relative, hence the term relativity which tells you how to express the laws of nature in different frames).