phyti

This is in response to a recent thread on closing speed, which in my opinion was not totally resolved, primarily due to an inadequate definition of 'closing speed' and the improper application of the concept. The original example is shown in fig. 1, with A and B moving at .7c toward M which is at rest relative to E. The question is : What is the closing speed of A and B relative to E? Typically closing speed is the rate of decreasing distance between two objects moving toward each other, but not necessarily in opposite directions. That would be a special 1-dimensional case and since this example is 2-dimensional, we consider all angles. We define 'closing speed' as the rate of change of radial distance between two objects. In fig. 2 the d-axis shows the radial distance from A (or B) to E, plotted as the hyperbola. It's obvious as A approaches M, the distance decreases to a minimum at M, then increases beyond M. The red curve crossing the 0-axis of E indicates the 'closing speed' of A relative to E. It approaches the limit .7c at extreme distances, but equals zero at M. Its value is negative for decreasing distance and positive for increasing distance. The dashed segments indicate a continuous path without collision at M. Only if there is no offset distance EM, will closing speed equal linear speed. Relative to A, closing speeds are -.7c for M, and -.94c for B, per SR. The collision of A and B separated by 1.4 ls happens in 1 sec because of simultaneous motions, not because of 'something' moving at 1.4c. The linear and closing speeds of all objects involved have been calculated.

I saw his animation. It's demonstrating that light moves independently of its source. Light does not acquire the speed of the emitter. A good analogy is a boat moving in water, and a stone dropped from the boat into the water. The speed of the waves from the stone is determined by the properties of the water, and is independent of the boat speed. The counterparts for the water, stone, and waves are space, ship, and light. You have simultaneous motion of light and two ships, and a space-time drawing is very helpful in sorting it all out. From E's perspective, what distance did the light travel, and how much time did it take? The answer is understanding that the 'closing speed' is not strictly a speed, but a rate of contraction or expansion of a spatial interval, which itself is NOT moving. Close a pair of scissors. The point where they cross moves much faster than the tips, BUT that point is not an object, but the positional relation of one blade to the other.

yes, if .2 sec is not enough time for an evasive maneuver! no, at c. Light speed is constant in space, independent of the motion of any object, thus it only has one speed, c. E sees the signal leave A at (t=0,x=-.7) and meet B at (t=.8,x=.1). Light traveled at .8 ls/.8 s = 1c. no, as explained in 2. If there was no time dilation for A and B, they would calculate the 1.4c closing speed relative to each other. For E it's a comparison of A-speed to B-speed, but that's not the same as calculating the speed of an object,since there is no object moving at 1.4c. The only reason particles are directed in head-on collisions is because it's not possible to accelerate them in one direction to c.

You answered your own question here, but are confusing closing speed with absolute propagation speed of light. The speed of an object is the rate of change of position relative to a reference point. The rate of change of the space between A and B by E is not the same thing as the speed of A or B by E. The space is not an object moving relative to E, but is the rate of A moving relative to B as measured by E. For consistency, all measurements should have a common ref. point, which in this case is E. Light at c, A at .7c, B at -.7c. If A sends a warning signal to B, it requires: distance/speed = 1.4 light sec/ 1.7c = approx .8 sec, giving a warning of approx .2 sec. The warning signal still moves at c in space relative to E, but B is simultaneously moving toward the signal at .7c in the same space, i.e., time is a function of both speeds.

I still agree with Farsight's reasoning about time, and his bean counting example he posted a few years back. He sees it for what it is, a measuring process, no different than measuring spatial distances, or a correlation reference to study various physical relations, like the motion of a stone in a gravitational field. The sleep/coma/death example shows that if you aren't counting time, you aren't aware of its passage. The simple fact that motion alters the observers sense of time (per SR) is sufficient to indicate that it is not a common or universal shared experience, like so many people floating down a river on a boat. SR measures distances with clocks, thus the space-time connection, but that is no mystery either. Time travel provides more drama, mystery, and adventure than bland science, and sells more books. Who has the greater fan base, Einstein, or Star Trek? I'm not anti science fiction, but like to maintain a clear distinction, and not mix the two.

The definition of absolute says "independent of anything", therefore if light speed is determined by the structure of space, then it isn't absolute by that definition. Constant is also defined as 'not varying with respect to other things', so I see little or no difference between the two tems. Considering the formation of language through definitions based on other definitions, with no 'fundamental' defintions, I see no significant difference.

Case 1 is an apparent process involving perception. Case 2 is a physical process that happens independently of an observer. Merged post follows: Consecutive posts merged If space has properties such as permittivity, permeability, and gravitational curvature, it can't be nothing. If light speed was not constant, there would be no time dilation.

The propagation speed of light can be constant due to the structure of space, in an absolute sense, and the observers measured speed be constant, in a relative sense.

The same number of universal events are occuring for you and the earth. Your clock is just slicing time into longer intervals, and you (like the clock) age slower. You're still observed from earth for the duration of your motion, i.e., you never leave the earth's 'now'.

I think this is a question of semantics. I'm not using that scientific definition. No one has ever asked me for the displacement of a particular place, and if I just tell them how far away it is without a direction what help is that? I would prefer 'length' as a scalar, independent of orientation, and distance a vector (from a to b, or its reverse, b to a, implying an order, or signed value or direction). This provides a more fundamental explanation for using negative numbers than the 'signs rule', i.e., negative numbers are actually vectors, a length rotated 180 °, a two component concept. You mentioned polar coordinates, and they require direction (2 angles) and a length. The laser beam to the moon doesn't work without a direction. Most science involving motion involves vector notation. The arguments I present are intended to declassify time as a dimension in the sense of spatial dimensions. To me time is just a diary, describing the events of the day. No one moves through time because as someone else said, nobody leaves the present. Thanks to the Greeks for moussaka.

You have to be cautious when thinking 'ideal' symmetry. Science has been surprised many times when discovering that the 'real' universe doesn't require it. Per GR, mass produces curves in space, or directs objects to move in a manner that makes it appear that way. It's gravity in a different dress! Here's a little more to consider. Distance has two components, magnitude and direction, but time has only magnitude, thus they are not equal. Mathematically distance is a vector, and time is a scalar/number. Time is also a process with an ordered sequence of events that continuously grows in one direction, and is equivalent to counting. Distance has extent and can be measured repeatedly resulting in the same values, time has no extent and its values are not reused but increase. The reference to histories, graphs, Smolin, Minkowski, etc. makes another point. All mathematical constructions do not have to correspond to something physically real. The world line of an object is a superposition of its states at different times, a history, and is not observed by anyone unless you have some form of memory.

michel; post 19 This may be new to you, but not to everyone. Science already measures spatial distance using a clock, eg., reflecting laser beams from the moon. The time is scaled by c to equal distance. We commonly use time and distance interchangably for travel purposes. A destination 60 miles distant is 1 hr away (with an assumed speed of 60mph). SR however demonstrates that time depends on the motion of the observer, which alters his external spatial measurements. Distance is a (unknown) property of space. Space must have a structure if it is curved by mass, i.e., you can't curve nothing. Time is a measure of activity. An example that is never or rarely used is the metronome. It establishs a beat (frequency) to play a given number of notes per measure. It's just a standard and nothing more, as are all clocks. Clocks just slice the common time of the universe into different length intervals, depending on your motion. With SR's world of events, when we measure time, we match some event in space with the closest clock event. Any good text on SR will show how the invariant interval, an equality with 3 independent variables, is converted to a 4 dimensional statement. It's just convenient math. Merged post follows: Consecutive posts merged It would have zero momentum, the same as the universe. The objects would have a range of velocities, thus it would not be still relative to them. It could only be motionless relative to all things if they too were motionless.

The perceived events are real for the observer, who is coincident (in space and time) with the photon detection (or emission). His data is already historical, as is all his data. He can only know where the objects were. His calculations for distant objects by tracing light paths only provides apparent positions, i.e. where he thinks they are. Time dilation due to his motion has altered his perception.

I was always impressed by the simplicity of this geometric demonstration seen in a text book years ago. The algebraic interpretation: (a+b)^2 - 2ab = c^2.

Clock synchronization is relative to each frame using 2-way light signals. Clock frequencies are a function of speed. Light is the universal measuring rod. You are actually measuring distance using a clock!

refer to drawing O' can only speculate or rely on the info from O with this incomplete scenario. If O actually measures the distance to the target with a return signal, which could be relayed to O', then O' would assign event 2 to his clock event 2', and calculate the distance as D(1+v/c)/g, in agreement with your answer, but occurring after passing M. Using SR transform, x' = g(x-vt) = gx(1+v/c)

Other considerations: A life long study of all non-black objects will not allow you to state the color of crows, a study of crows is required. But that study would not include all crows. There may have been a blue crow that lived and died before your study but was never recorded. These type of absolute 'true' statements are not possible due to lack of knowledge. The best you can do is a statement with a high degree of confidence. The history of science demonstrates that knowledge is always in a state of refinement.

The number of occurences does not increase the truthfulness of a statement. It only requires one sighting of an albino crow to falsify the 1st statement. Many species have been declared extinct only to be found later. The color of crows is not determined by the color of other objects, so there is no relationship of the 1st to the 2nd. I don't see any paradox or logic here!

If no movement, how do you get all that red shift?

It seems you are saying events happening in the universe are grouped differently within a time unit defined by the motion of the observer. Example: a distant star pulses (event-e) at a uniform rate. all motion is perpendicular to the direction of the star. at 0c, a records 12 per sec. at .6c, b records 15 per sec. at .8c, c records 20 per sec. eeeeeeeeeeeeeeeeeeeee a___________a________ b______________b_____ c___________________c The same number of events happens for all observers (the constant time idea), the observers clocks slice them into different length sequences, depending on their motion. If this is not a correct interpretation, please clarify.

liam, post 3 There are no points in time, and nothing passes, unless you're a poet. Your comment is a common misconception trying to objectify time, but it's not a thing, it's a relationship, just like a spatial measurement. Length is expressed in terms of a unit of measure, i.e. a number. You are measuring activity (events) using a standard rate of activity (clock). A clock has to produce a uniform periodic event. The precision of the uniformity depends on the purpose, from the sun for daily social activities, to atomic vibrations for scientific experiments. Once you record the times, you have an ordered sequence of events for historical study or prediction purposes. The only motion is from A to B in space. Of course you can't go back because the dynamic universe has changed to a different configuration, and will not repeat itself.

gamma is approx 10 doppler is approx. 20 he hears 2 yr + 1 day in approx 36 days

To assume time as a dimension, or a progression within a dimension is in contradiction to observation. As mentioned, parts of the universe do not disappear or reappear. If an object speeds up and it's clock slows, does it get behind in time? If so, how would it catch up? We see the universe of objects, all with various velocities, persisting simultaneously. Because of spatial extent, we also know our simultaneous perception of the objects does not imply simultaneous states for those objects. The analysis of subjective time keeping shows we are only measuring the rate of activity, by matching external events to a standard event (clock). It seems asprung is searching for an objective time, a causal action (like a synch pulse) that transforms all parts of the universe from one state to another simultaneously, i.e. a universal now. This would allow a common now for all observers, and provide a universal frame for building theories. The subjective time would still have to be local because it depends on the speed of the observer relative to other nearby objects.

1. You don't move through time, you and your clock run at a slower rate. Eg: you leave earth and return after a trip at .6 light speed. While you are away earth flashes a beacon once a month. You return 6 months later earth time. Your clock records 4.8 months, but your log records 6 flashes. Time did not slow down, the same number of events happen in the world, but your clock just sliced the time into longer intervals. 2. The light clock records the minimum interval between ticks at rest, thus any motion will only lengthen the intervals.

Faye: You might want to review SR. 1. Events are in essence photon emissions and absorptions, and because light speed is independent of the source, the interval between events is invariant/fixed. Objects (excepting structureless particles) would be multiple events. 2. The imaginary "i" is used as a mathematical device for purposes of symmetry. This supposedly makes time an orthogonal and thus independent dimension, but, with four variables, only three can be independent and will decide the fourth. It's a only a model for the universe. 3. The idea of all things moving through time has a serious problem. Light/photons are timeless, i.e., experience no time, call it maximum time dilation, so they can only move in a spatial dimension, within the framework of this type of theory. The problem then is, how do they communicate between objects that are moving through time? That's it for now It's wise to ask questions.