Register to reply 
Riding on a beam of light  time and space 
Share this thread: 
#1
Feb2007, 12:09 AM

P: 298

something is confusing to me. lets say i am in an inertial unaccellerated frame, at some sort of relativistic speed. and lets take another inertial frame, an observer on earth.
here is what i have read. the observer on earth measures the relativistic frame's ruler as being less than his own, and measures the relativistic frame's clock ticking slower than his own. but this is also true of the relativistic frame, measuring the ruler and clock of the earth observer's frame as being smaller and ticking more slowly than his relativistic frame's ruler and clock. i have read that if one was travelling on a beam of light, that one would measure no elapse of time and no distance covered, no matter how far the observer on earth measured the light beam to go. but if the guy on the light beam measures no time elapsed and no distance travelled, how could he at the same time, measure the earth observer's clock to be going more slowly than his own, which has measured no time at all ? ditto for distance. 


#2
Feb2007, 05:40 AM

P: 372

ok.Now when the observer is on the light beam,he would see that the earth is moving past him at the speed of light.Therefore,in his frame the earth's clock would not tick.THat is the time dilation from his view for earth would be infinite.



#3
Feb2007, 08:52 AM

Sci Advisor
P: 1,741

How can the observer on the light beam see the earth move at the speed of light, when time dialated for him is infinite? For the beam it would take 0 seconds to travel from A to B. Of course it is physically impossible to observe from a beam of light.



#4
Feb2007, 09:42 AM

Mentor
P: 8,305

Riding on a beam of light  time and space



#5
Feb2007, 10:02 AM

P: 298

one sees this situation mentioned all the time (riding on a light beam).
so, if time does not exist for a photon, does space exist ? according to the argument, the observer on the light beam registers no distance travelled. lets also take a look at another example  instead of being on a light beam, one is travelling close to the speed of light. he notes that the ruler on earth is very small, and the clock on the earth is moving very slowly. what do his rulers and clocks mention ? what seems to be inconsistent with me is that as you go faster, you measure the other guy's ruler as smaller and his time ticking slower  but your ruler and clock does not change. just like the guy on earth  his ruler and clock havent changed to him. he sees the guy travelling very fast as having changed. so if there is no time for a photon, then up to the smallest increment less than the speed of light, one measures his own rulers and clocks as being normal. and then somehow it disappears altogether if it could reach the speed of light ? 


#6
Feb2007, 10:35 AM

Mentor
P: 8,305




#7
Feb2007, 10:52 AM

P: 298

okay, so in your opinion, a photon has no concept of space or time. but this must occur only when the photon is travelling at the speed of light. this occurs in a vacuum. the photon does not always travel at that speed. physics tells us that it slows down in other mediums. so obviously, we must be talking not so much about the photon, but rather "travelling at the speed of light".
i understand that according to einstein, nothing can be accellerated to that speed. yes, i am talking about his proper time. it does seem strange to me that up to the most minute fraction below that speed, an inertial frame measures his own clocks and rulers as not having changed, but at the speed of light  there are no rulers or clocks at all. 


#8
Feb2007, 11:13 AM

Mentor
P: 8,305




#9
Feb2007, 11:22 AM

Sci Advisor
P: 8,470

It is impossible to define a "rest frame" for something traveling at the speed of light (in a vacuum) without violating the postulate of relativity which says the laws of physics should work the same way in all inertial reference frames (and you also get nonsense answers if you plug c into the 'Lorentz transformation' which transforms between the coordinate systems of different frames in motion with respect to one another). If you're interested, there was a long discussion of this issue on this thread. But it might help to think about what happens in the limit as your speed relative to the galaxy (or some other convenient landmark) approaches c. In this limit, you will measure the galaxy's length in the direction you're traveling to approach zero, so the time it takes on your clock to move from one end to the other approaches zero too, which is consistent with the galaxy's perspective where your clock approaches being frozen in the limit as you approach c. But another important issue has to do with the relativity of simultaneity, which refers to the fact that clocks synchronized in one reference frame will be measured to be outofsync in another frame, and this outofsync effect will get larger as the relative velocity increases. In the galaxy's frame, its own length is around 100,000 light years, so in the limit as you approach c the time you are measured to cross it will approach 100,000 years. If there are clocks on either end of the galaxy which are synchronized in the galaxy's frame, then by the time you pass the second clock, its date will be about 100,000 years beyond what the date was on the first clock as you passed it. However, in your frame both clocks are ticking slower than yours (in the limit both clocks will approach being frozen in your frame), and you pass the second almost immediately after passing the first (in the limit, the time between these events approaches zero); the only reason the second clock reads a much higher time is that it was outofsync with the first by about 100,000 years to begin with in your frame.
By the way, on an older thread I came up with an illustration of how the difference in simultaneity insures that two rulers and clocks moving at a significant fraction of c could each see the other one shrunk and each see the other's clocks ticking slow without it leading to a physical contradictionthat's here if you want to look at it. 


#10
Feb2007, 11:32 AM

P: 298

hi cristo,
i read the discussion. while it seems plausible, i hope it is meant as a possibility, and not a sure thing. i have read that the guy on the light beam measures no distance or time, but not that distance and time do not exist. that is not to say that i dont believe that the theory says this  just that i have not interpreted them to say that. i guess what i find lacking about the 2 theories on relativity is that they give us explanantions about our measurements, but they dont have anything to do with the actual reality of what is, since every frame of reference comes up with different answers. all these frames come up with the speed of light being the same. i think the main problem we have is that we cant get measurements instantaneously. and when objects are moving at near the speed at which we can receive information, it really skews our perception of reality, whatever that may be. i am beginning to think that the arguments about time not being a real thing, but simply being a result of motion  is becoming more likely. 


#11
Feb2007, 11:35 AM

P: 298

thanks jesse,
i will read them later, when i have more time. 


#12
Feb2007, 11:46 AM

Sci Advisor
P: 8,470




#13
Feb2007, 11:59 AM

P: 298

i wanted to reply to this statement now. isnt this a bit of circular logic ? i understand that these quantities vary in the theory of relativity. but the theory of relativity does not give us answers as to "what is", only as to "what i measure them to be". if one has 2 answers, then both answers can only be right in its inertial frame. the object itself does not actually exists in 2 different places at the same time. it is not that i am disagreeing with the theories of relativity  just that they do not explain to me what the universe is  they merely give me predictions based on my measurements. just as in your example about the galaxy shrinking to zero in the direction of my motion. the galaxy did not actually shrink. it only seemed to do so, based upon the fact that i am travelling very fast, and my measurements tell me that it shrank. i do think there is an absoluteness to reality. i suspect that we will never be able to figure out what reality actually is  only what we perceive it to be. 


#14
Feb2007, 12:07 PM

P: 298




#15
Feb2007, 01:03 PM

Sci Advisor
P: 8,470

my ruler's left end passing next to galaxy's left end my clock sitting on left end of my ruler reads 3:00 PM, Jan 1, 2000 AD galaxy's clock sitting on left end of galaxy reads 3:00 PM, Jan 1, 2000 AD And likewise, all frames would agree these three events all happened right next to each other in space and time: my ruler's right end passing next to galaxy's right end my clock sitting on right end of my ruler reads 3:00 PM, Jan 1, 2000 AD galaxy's clock stting on right end of galaxy reads 3:00 PM, Jan 1, 102000 AD In my frame, the left end measurement happened simultaneously with the right end measurement, so in my coordinate system that means the galaxy is 1 meter long. But in the galaxy's frame, the right end measurement happened 100,000 years after the left end measurement, so it doesn't have anything to do with the length of the galaxy, it just means the ruler has had time to cover a very large distance between these two events. So the question is, why do you think there has to be any "real truth" about whether different events which do not occur in the same local region of space and time happened "simultaneously" or not? Why can't simultaneity be entirely coordinatedependent, just like most people would assume an object's xcoordinate depends on where you happen to choose to place the origin of your xyz coordinate axes, and what direction you choose to orient the axes? Unless you think there must be an "objective truth" about whether two objects share the same xcoordinate, I don't see why you'd find it necessary to believe there must be an objective truth about whether two events happened at the same timecoordinate. We can just say there's an objective truth about which events happened next to each other in the same local region and leave it at that? 


#16
Feb2007, 01:48 PM

P: 298

hi jesse,
thanks for the very long post. i have no problem with your x being 2 and mine being 4. but when we both measure the size of something and come up with different answers, both of them are not correct. i am interested in what is, not what we perceive things to be. we can only imagine in 3 dimensions. so lets think about an expanding ball. the surface area is an ever expanding 2 dimensional sphere. everyone on that sphere sees himself in the middle of the sphere, when in reality, the middle of the sphere does not even exist in their universe, since the middle is the core, and they are on the surface. however, they are all discussing 2dimensional relativity, and how each of them sees things as they move along the sphere. but us smart 3 dimensional beings can view the entire sphere at once, and understand its true reality. i suspect that we are part of a real 4th dimension, something other than time. and we have no more ability to understand our 3dimensional reality any more than the people on the sphere have of understanding their universe like we do. the only difference is that i dont think most scientists today understand this. 


#17
Feb2007, 01:50 PM

P: 298

when i say "sphere", i should have said surface area on the sphere, as sphere is usually thought of as a ball.



#18
Feb2007, 02:23 PM

Sci Advisor
P: 8,470

Similarly, if we have a 4D minkowski spacetime with different 4D tubes corresponding to the worldlines of objects in this spacetime, and we both choose a different set of xyzt axes, we will both agree on the spacetime interval between any two events when we calculate [tex]c^2*(t_2  t_1)^2  (x_2  x_1)^2  (y_2  y_1)^2  (z_2  z_1)^2[/tex] in terms of our own coordinate system. However, when we talk about the spatial "length" of an object, we are really talking about the size of the crosssection of one of the 4D tubes in our own 3D xyz plane, and since our xyz planes are oriented differently we'll get different answers. Why do you think there should be an "objective truth" about the 3D crosssectional area of 4D tubes in 4D spacetime, when you presumably don't think there is an objective truth about the 2D crosssectional area of 3D tubes in ordinary 3dimensional euclidean space? 


Register to reply 
Related Discussions  
Space like time like light like separations?  Special & General Relativity  6  
Visualizations without spacetime or light  General Discussion  35  
Time Travel using the gravitational field of a circulating light beam  Special & General Relativity  28  
Spacetime and the speed of light  Special & General Relativity  11  
Riding on a beam of light?  Special & General Relativity  9 