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How does one measure the absolute velocity of an object? 
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#19
Nov2412, 01:25 PM

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It sounds like this discussion is not about science, which is concerned with the objective reality that we perceive and can measure, but is instead a discussion about mysticism. I suggest it be moved to the philosophy section or discontinued altogether.



#20
Nov2412, 01:30 PM

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By definition, science is the description of that which is observed. If you are trying to talk about something that is unobservable, then you're in the wrong place.



#22
Nov2412, 02:14 PM

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I would recommend to read about Scientific method. 


#23
Nov2412, 03:24 PM

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The "observer" in relativity isn't an active participant in the process of "observation", no act of perception is involved, and there is no question that the phenomena being studied exist whether they are observed or not. (It's an amusing irony that even as I write this, another thread on black holes is busily demonstrating that just because something cannot be observed that doesn't mean it's not real). It is true that many explanations of relativistic phenomena are described in terms of what a human observer would see: Beams of light bounce between mirrors on their way to someone's eyes, I observe the readings on a clock moving relative to me and compare them with a clock at rest relative to me, I measure the length of a moving rod, and so forth. But that's just a particular style of description, one that comes naturally to a practicing scientist working with the results of observations. Approaching the problem from a more philosophical stance, you may be more comfortable with a different model of what "observer" and "frame of reference" mean in relativity. So try this one, which I first encountered in Taylor and Wheeler's "Spacetime Physics": Imagine that we fill a large volume of space with a threedimensional grid of meter sticks, all at rest relative to one another, fixed at right angles to one another where their ends meet. At every intersection, we place a machine containing a recording device and a clock; all the clocks are synchronized. (It may be not be practical to construct such a ensemble across an interestingly large volume, but it is clear that I can do this is a small volume and that there is no theoretical objection to expanding it to an arbitrary size). Now each recording device can generate an independent record of events at its location: At 3:00 PM a spaceship flew through this spot at .5c; at 3:38 PM a bomb exploded right here; at 4:07 PM a nuclear decay occured; and so forth. Collectively, these recordings amount to a description of everything that happened within that volume of space, both when and where, to a resolution of one meter (and if we aren't happy with that resolution we could have chosen to build a more closely spaced grid). Now our intrepid scientist can gather all these recordings at his leisure, maybe centuries after the events in question happened, and use them to piece together a complete history of what went on across the volume of space. Or he can choose to burn them... but most of us would agree that the events in question "really" happened, independent of any act of observation, no matter what he does with the recordings. (BTW  don't be fooled into thinking that my lattice of rods and clocks will allow you to define an absolute velocity. I specified that all the rods were at rest relative to one another, but there's nothing to prevent another researcher from building his own lattice of rods and clocks, also at rest with respect to itself, but moving relative to my lattice). 


#24
Nov2412, 03:57 PM

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#25
Nov2412, 04:08 PM

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#26
Nov2412, 04:14 PM

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Regarding the relativistic obsession with observers. Generally it is simply a short hand way of establishing a reference frame. Don't read too much into it. 


#27
Nov2512, 09:23 AM

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So let's do an example to see what we get. I like to use units where c=1 to make the calculations simpler. Let's say an object has traveled x=6 lightseconds in t=10 seconds. First off, we can see that its velocity through space is 0.6c. This is the same as saying beta, β=0.6, We can see that its spacetime interval is √(10^{2}6^{2}) = √(10036) = √64 = 8. Now we want to calculate the Proper Time, τ. It is τ = t/γ. Gamma, γ is equal to 1/√(1β^{2}) = 1/√(10.6^{2}) = 1/√(10.36) = 1/√0.64 = 1/.8 = 1.25. So τ = 10/1.25 = 8. Now we can calculate the velocity of through spacetime as 8/8 = 1 or in my selected units, c. Now it is no coincidence that we ended up with a spacetime velocity of 1 or c. We can do the above calculation symbolically as follows: The spacetime interval is √(t^{2}x^{2}) The Proper Time is τ = t/γ = t√(1β^{2}) = √(t^{2}(tβ)^{2}) Now we note that since β=x/t, then tβ=x so we can substitute this in the above equation and get: τ = √(t^{2}x^{2}) Since the spacetime interval equals the proper time, the spacetime interval will always equal 1 or c. It's nothing more than a mathematical curiosity based on the definitions. Knowing that, I take you back to my first post #9 to answer your original question. 


#28
Nov2612, 12:50 AM

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Since the spacetime interval equals the proper time, the spacetime velocity will always equal 1 or c. 


#29
Nov2612, 10:30 AM

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so then is it possible that if i view one ship moving at the 99.999999% of the speed of light say (in reference to my observations), that the speed of light for them is then much faster than the speed I am viewing, while observing them?



#30
Nov2612, 10:42 AM

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#31
Nov2612, 11:07 AM

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But the ship will have no knowledge or awareness of this. It's simply the coordinates that are used in the frame of reference to describe what is happening. If the ship tried to measure the speed of light, it would have to set up a reflector some measured distance away and time how long it takes for the light to make a round trip. The calculated value would turn out to be exactly c because its clock would be running slow and its ruler would be contracted along the direction of travel. Now if we transform this scenario into the rest frame of the ship, as phinds suggests, then the speed of light in that reference frame will be c. 


#32
Nov2612, 11:47 AM

P: 421

The 'invariant interval' X, between two events is fixed. The events do not move. He can therefore state Xct = 0. He knows that for a moving observer, the outbound and return trips for a reflected signal in the direction of motion have different space and time values, since he uses c±v. He knows an observer can't be at the emission and detection of the same photon. He proceeds to define the path lengths as equal to preserve a constant c, and provide values where measurement cannot provide them. The photon and its path must exist, since it is detected! As of 1800, all particles included in the standard model of quantum physics were unknown because there was no experiment capable of their detection. Looking back, it would be foolish to say they didn't exist. 


#33
Nov2612, 12:35 PM

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#34
Nov2612, 01:24 PM

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#35
Nov2612, 01:26 PM

P: 366

Well you don't, and in your frame you are not moving, to put it more precisely, the sentence "I am moving is meaningless for point particles".



#36
Nov2612, 01:32 PM

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The same way you measure the total weight of angels dancing on the point of a pin!



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