World Science Scholars

16.1 Motion’s Effect on Space

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    • I’m really enjoying this course, great presentation. Stretches the imagination.

    • With space contraction and time dilation, how the velocity of different objects is determined and agreed upon among different observers?

    • I presume it’s like this: the velocity for both is the same, only the direction is opposite. The rocketobserver will say that the earth is moving away at speed v while the observer on earth will say that the rocket is moving away in opposite direction at speed v. However there also is a difference: the rocketobserver feels a force (accelleration) and the observer on earth doesnot.

    • But if you will take GR in account, then none can cliam that they are in state of rest? I wonder does the ‘net’ time dilation incorporate both the effects for so called”Stationary objects”?! What ya think?

      • There is no question that if Gracie uses the measurement method in question – which involves time – she will measure a different (shorter) length than George. Greene goes on to state that space itself is contracting in the direction of motion “space is adjusting for time” as Greene puts it. The (undiscussed) assumption here is is that a time-dependent measurement method continues to remain valid when relative motion is involved. It is by no means the case that Gracie has to use a time-dependent measurement method. For example, she could have set up a series of light beams projecting over the track, one every 10 cm for the length of the platform, as the train passes by a counter records the number of light beams being blocked by the train. This will initially be zero and then start climbing as the train blocks more and more light beams reaching some maximum value and then decreasing as the train starts pulling past the station (I am assuming here that the train is shorter than the station platform) finally falling back to zero as the train leaves the station completely. Gracie can simply read off the maximum value to determine the length of the train.

        The length value Gracie measures will be the same as George’s. No Lorentz contraction takes place and space does not have “to adjust to time”. Just as with the time-dependent measurement method, the measurement is made from the platform. So if Gracie has two measurement methods available which give different results (outside of experimental error) can we consider both methods to be measuring the same thing if they are applied under the same circumstances? The time independent method described above will measure what is called the “proper” or “rest” length but what is the time dependent measurement actually measuring?

        • Simultaneity plays no role in this measurement method.

          • Well known physicists such as Roger Penrose and James Terrell have shown that moving objects generally do not appear length contracted on a photograph. Note that a video is nothing more than a series of photographs played in quick succession. The video showing a taxi cab with length contraction as it passes by is thus incorrect (see page on Length Contraction on Wikipaedia for sources).

    • does contraction of fan stripes when it is revolving is an example of lorentz contraction?

    • does contraction of fan stripes when it is revolving is an example of lorentz contraction?

    • Why do you say they agree on the speed of the train?
      George and Gracie know their notion of time is different. They can both agree the train’s length is 210m.
      Now George gets on the train and speeds up to 30m/s towards Gracie (who moved some distance away to give the train time to speed up).
      Gracie then measures how much time it took for the train to pass her (5.9s), so she concludes the train’s speed is NOT 30m/s like George says, but 210/5.9≈35.6m/s.

    • The train should run at 1.6*10^8 to shrink into 177 meters .

    • The train should run at 1.6*10^8 to shrink into 177 meters .

    • Why Volume and Area don’t shrink ?

    • There is a lot of discussion about what Loentz contraction really is, with a significant group claiming that actual contraction of distance takes place (it is a space-like phenonemon) and another significant group stating that what is really happening (and being described) is that time dilation is taking place (it is a time-like phenomenon).

      Speed = Distance/Time is something everyone agrees on. What is apparently up for grabs is whether it is valid to invert the expression in order to define distance as Distance = Speed x Time or whether this inversion breaks down in a relativistic context where time is dilating.

      If it really is distance then one would need to account for apparent size. Apparent size is a property of distance: when you get closer to an object that object will appear larger. If Lorentz contraction really shortens distance (that is, it is a space like phenomenon and not a time-like phenomenon) then as Lorentz contraction takes place we would get physically closer to the object. By travelling ever closer to the speed of light we could get arbitrarily close. We should then see the object at the distance dictated by the Lorentz contraction. If Lorentz contraction really equates to shorter distances then theoretically, we could even observe a star as being a mile away or even closer (with all its detail!) that is a billion years distant in the rest frame. I do not accept that this is possible so I do not accept that length contraction equates to physical distance shortening.

      There are also other properties affected by (true) distance. Radiation intensity and gravitational pull to name two. If we are truly at just 1 mile distance from a star if we accelerate to the appropriate velocity then if we believe the space-like interpretation of Lorentz contraction we will experience the star just as if it were 1 mile away in a non-accelerated situation. We would be both fried and experience an extremely strongly gravitational pull.

      Note that the apparent size phenomenon is not the same as magnification. A space-like interpretation of the Lorentz contraction postulates that at the right velocity (very close to light) the distance will really be one mile and the observer (in the rocket presumably) will observe exactly the same thing as we would here on Earth if the star were 1 mile away. Magnification presents resolution issues which do not play a role in the apparent size case. If resolution issues are raised to prevent being able to see the star up close then the Lorentz contraction cannot be true distance shortening.

      In my view, Lorentz contraction is a (highly) useful mathematical method for manipulating time dilation (which is a real physical phenomenon) but the Lorentz contraction does not result in actual shortening of the physical distance. Objects and distances do not physically shrink (in the direction of motion or any other direction), a photon is not “everywhere in the universe” and we cannot define Distance to be Speed x Time in a relativistic context.

    • There is a lot of discussion about what Loentz contraction really is, with a significant group claiming that actual contraction of distance takes place (it is a space-like phenonemon) and another significant group stating that what is really happening (and being described) is that time dilation is taking place (it is a time-like phenomenon).

      Speed = Distance/Time is something everyone agrees on. What is apparently up for grabs is whether it is valid to invert the expression in order to define distance as Distance = Speed x Time or whether this inversion breaks down in a relativistic context where time is dilating.

      If it really is distance then one would need to account for apparent size. Apparent size is a property of distance: when you get closer to an object that object will appear larger. If Lorentz contraction really shortens distance (that is, it is a space like phenomenon and not a time-like phenomenon) then as Lorentz contraction takes place we would get physically closer to the object. By travelling ever closer to the speed of light we could get arbitrarily close. We should then see the object at the distance dictated by the Lorentz contraction. If Lorentz contraction really equates to shorter distances then theoretically, we could even observe a star as being a mile away or even closer (with all its detail!) that is a billion years distant in the rest frame. I do not accept that this is possible so I do not accept that length contraction equates to physical distance shortening.

      There are also other properties affected by (true) distance. Radiation intensity and gravitational pull to name two. If we are truly at just 1 mile distance from a star if we accelerate to the appropriate velocity then if we believe the space-like interpretation of Lorentz contraction we will experience the star just as if it were 1 mile away in a non-accelerated situation. We would be both fried and experience an extremely strongly gravitational pull.

      Note that the apparent size phenomenon is not the same as magnification. A space-like interpretation of the Lorentz contraction postulates that at the right velocity (very close to light) the distance will really be one mile and the observer (in the rocket presumably) will observe exactly the same thing as we would here on Earth if the star were 1 mile away. Magnification presents resolution issues which do not play a role in the apparent size case. If resolution issues are raised to prevent being able to see the star up close then the Lorentz contraction cannot be true distance shortening.

      In my view, Lorentz contraction is a (highly) useful mathematical method for manipulating time dilation (which is a real physical phenomenon) but the Lorentz contraction does not result in actual shortening of the physical distance. Objects and distances do not physically shrink (in the direction of motion or any other direction), a photon is not “everywhere in the universe” and we cannot define Distance to be Speed x Time in a relativistic context.

    • Moving objects appear or are measured to be shorten along the. direction of motion only if a time dependent measurement technique is used.

    • Moving objects appear or are measured to be shorter along the direction of motion only if a time dependent measurement technique is used.

    • Moving objects appear or are measured to be shorter along the direction of motion only if a time dependent measurement technique is used.
      “The notion of length itself requires simultaneity.” I disagree with this. You can measure length using other time independent methods. These do not require simultaneity.

    • Here is the point: we have discovered that simultaneity differs for people depending on their relative movement and the next thing we do is to go on to use a measurement technique that precisely makes use of the concept of simultaneity?. Worse yet, we even stop using the technique that we were already using to measure length which involved no simultaneity for the stationary train even though it can be perfectly validly extended to moving trains. In the stationary case, George walks past the front (or the back) of the train and as he passes the front (or back) he attaches the tape measure to the front (or back) of the train and reads of the value as he passes the back (or front) of the train. If the train is moving (assume towards him) he can carry out the identical measurement as he walks past the front (or the back) of train as he passes the front (or back) he attaches the tape measure to the front (or back) of the train and reads of the value as he passes the back (or front) of the train. Does anyone doubt that the value he reads off his identical whether the train is moving or not?

      From a physics viewpoint it simply does not make sense to replace what we believe is a perfectly good measurement technique which gives the same value whether the train is moving or not with respect to the measurer with a different technique that does not. There is no reason, as a thought experiment , that if we have clocks and rods that we cannot also have measuring tapes (even if they are hundreds of light years long).

      That measurement techniques involving relativity of simultaneity gives weird results is clear but it is nonsensical to ascribe these results to actual properties of space: simply use a measurement technique that is invariant to movement such as the measuring tape.

    • “Speed is distance divided by duration which is space divided by time”. So if we learn that the speed of light is constant we also learn that time is not constant so that means that space must in some way compensate for the non-constant aspects of time in order that the ratio stays the same allowing the speed of light stays unchanged. So, in order to ensure, that the speed of light space must adjust itself in tandem with time so that the ratio for light stays fixed. If we consider that time is not constant therefor space must change too in relation to motion so that the ratio of space over time is such that the speed of light remains unchanged.”

      In my view, this is NOT what is going on. Space is NOT “adjusting itself”. Distances are NOT shortening, objects do NOT get shorter. Yet we DO measure shorter distances and we DO measure objects to be shorter. What is changing is not the physical distance but the metric we are using to measure that distance. When our time “slows down” what this means is that the amount of time between two clock ticks increases and as light has a constant speed in space so too does the distance covered between two ticks. Since a meter is defined as the distance light travels in 1/299 792 458 of a second then if that second is longer clearly the meter will end up being longer as light will travel further in the additional time between the two clock ticks. If meters become longer then less meters will fit into any given physical distance and so the measurement of the distance (using meters) will end up giving a smaller value for the measured distance. This is no different to the case of changing from a millimetre to a centimetre metric or from grams to kilograms. In both cases we will measure different values for the same distance or mass.

      In short, it is not space that is adjusting itself but the metric is (by definition) adjusting itself because it is based on a time unit that is variable. As time slows down the meter by its very definition becomes longer. Again, it is simply the metric that is changing, nothing is happening to physical space. Because the metric changes we measure different values for the same physical entities.

      This interpretation immediately resolves a number of difficulties that would need to be explained if true distance shortening (Lorentz contraction) took place. If it really is physical distance that is shortened then one would need to account for apparent size. Apparent size is a property of distance: when you get closer to an object that object will appear larger. If Lorentz contraction really shortens the physical distance then as Lorentz contraction takes place we would get physically closer to the object. By travelling ever closer to the speed of light we could get arbitrarily close. We should then see the object at the distance dictated by the Lorentz contraction. If Lorentz contraction really equates to shorter distances then theoretically, we could even observe a star as being a mile away or even closer (with all its detail!) that is a billion years distant in the rest frame. Something that is very difficult to accept.

      There are also other properties affected by (true) distance. Radiation intensity and gravitational pull to name two. If we are truly at just 1 mile distance from a star if we accelerate to the appropriate velocity then if we believe in true space Lorentz contraction we will experience the star just as if it were 1 mile away in a non-accelerated situation. We would be both fried and experience an extremely strongly gravitational pull.

    • Consider this thought experiment. A rocket is launched. Attached the rocket is a the end of a tape measure. The tape measure roll remain on Earth. On the rocket is another tape measure roll which is attached to the Earth. As the rocket travels into space both tape measures unroll. At any point in time the same physical amount of tape is unrolled: both tapes are stretched taut and are adjacent to each other so it is clear that the same physical amount of tape has been unrolled on each tape measure so the physical distance between Earth and the rocket must be the same whether you are on the rocket or on Earth. So how do you account for Lorentz contraction? The answer is that as the tape measure unrolls on the rocket, observers on the rocket will see more metres (as measured on the tape) go by per second on the tape measure because their second has become longer. From their viewpoint the metres on the tape measure measure a distance that is too short.

    • While I do understand that it would have no effect on Lorentz Contraction or the math at all to this point, I still can’t intuit that height is unaffected, when I look left and right along a highway with telephone poles along side. The farther they are from my perspective the smaller they appear. Now I know it won’t effect Lorentz contraction, but it seems to still be valid. I count proof in astronomical observations of red shift, let me explain. Go back to the light clock, up down up down, now imagine the red shift. The farther away the source photons originate the longer the frequency and the lower the amplitude as well. So in my mind that photon train not only slowed time it also shrunk in height. Maybe even to the point of no longer being detectable (Dark Energy?).

      In the example that vexed me was a third perspective in the triangle. the other ones were linear, while this introduced to me another point that could be anywhere in space. and my mind went to all positions, towards train, behind me, farther down the track, so I could almost visualize a spot and single moment on the train when the slopes could be equal.

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