Speed measurement -- Limitations to "instantaneous" measurements?

In summary, the concept of instantaneous is something that is quantifiable, but is not always true to reality.
  • #71
russ_watters said:
No, radar guns measure speed from doppler shift of reflected radio waves. They are single-point/instantaneous measurements.
. . . . plus a number of other similar comments. But this is not true.The information that can be gleaned from an observation is always subject to Noise. The more 'instantaneous' the measurement, the wider the bandwidth of the measuring instrument and so the more noise energy will affect the result. So a 'good' instantaneous measurement is progressively harder and harder to achieve.
We initially learned our Differential Calculus in the ivory tower of Pure Mathematics (even if we didn't know it). Later, if we were lucky, we found that the basics of taking limiting cases of smaller and smaller intervals had to involve caveats about a function being continuous and differentiable before the simple dy/dx can be looked on as valid. QM excludes this condition so we are left with a system that just works but don't look too hard at it.

This thread has a parallel in all the other threads that deal with Zeno's Paradox etc.. Only Professors of Very Hard Sums are qualified to deal with the bottom line of this and many of those guys can't find their back pocket with both hands.
 
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  • #72
Paul Colby said:
Most quantities I can think of are actually inferred from measurements.

In this digital age, we could even say that most quantities are inferred from voltages!
 
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  • #73
Stephen Tashi said:
In this digital age, we could even say that most quantities are inferred from voltages!

Well, voltage without some corresponding current can't be measured either. Even an FET has some current flowing into the gate to effect or change the drain-source impedance. If I were making grand generalizations (and I am) signals are power. Taken with the measurement device's integration time this makes a measurement an amount of energy. Nothing in experimental science comes with 0 energy cost ever as far as I can tell.
 
  • #74
sophiecentaur said:
QM excludes this condition so we are left with a system that just works but don't look too hard at it.

I think you'll find that describes classical mechanics rather than QM!
 
  • #75
gmax137 said:
How about when you toss a ball upwards? At the top of it's trajectory the velocity is up, then zero, then down. The time at zero velocity is "instantaneous," that is, the ball has zero velocity but only for a zero duration time interval. I don't think this is an "analysis" artifact or approximation, it is a necessary consequence of an object changing direction. Unless we actually live in a simulator-type universe where there is an imperceptible clock "cycle time."

The ball's velocity passes through 0 in theory. The ball has a position only in some theory. Now, the OPs question used the word 'quantifiable' which I read as measurable. After using the "quantifiable" word the OP went on to ask about counting faster. Sure sounded like they were asking about limitations to the measurement of a theoretical concept.
 
  • #76
Paul Colby said:
The ball's velocity passes through 0 in theory. The ball has a position only in some theory.
I have no idea what this means. The ball exists independent of anyone's theories. I must be missing your point.
 
  • #77
gmax137 said:
I have no idea what this means. The ball exists independent of anyone's theories. I must be missing your point.
Well, a real ball may be treated (modeled) as an ideal rigid solid in classical mechanics, as an elastic solid supporting internal vibrations, or a collection of atoms, QM, LQF etc. The number of atoms in the ball is not really fixed since air molecules might adhere to the surface and then release, so there is a thermodynamic component to the center of mass. The center of mass is certainly a theoretical construct in and of itself. My point is the velocity of the ball and it's position don't exist other than in a model.
 
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  • #78
Paul Colby said:
My point is the velocity of the ball and it's position don't exist other than in a model.
You and I are just going to disagree on that.
 
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  • #79
PeroK said:
I think you'll find that describes classical mechanics rather than QM!
It does but there is a lower scale limit to which Classical Mechanics applies. The word "instant' only really applies to idealised mathematical functions. But we're talking angels on a pinhead here.
 
  • #80
gmax137 said:
You and I are just going to disagree on that.
Fair enough. I will close with my choice of the words "don't exist" #77 is a really poor one. A better choice closer to my intended meaning is "aren't defined."
 
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  • #81
berkeman said:
Can you give an example of something that happens instantly?
an application of mass would create instantaneous universal repercussions...??
 
  • #82
Say this mass "evaporated" from a close dimensional universe
ericlreite said:
Just sayin'

into ours, would the effects be instantaneous?
phinds said:
Speed (relative to something) is quantifiable in math terms. What else is there? If you mean can we measure speed to infinite precision then certainly not.
ok, we can all manipulate mathematical variables, agreed. Does that not mean we can give T a value of plus or minus nothing or infinity?
 
  • #83
ericlreite said:
into ours, would the effects be instantaneous?
Wouldn't that violate conservation of energy in this universe? There is much data and theory in this universe that currently would rule this out. I haven't seen the data from any other universe. Perhaps a reference?
 
  • #84
ericlreite said:
into ours, would the effects be instantaneous?
That would go against the basics of Relativity, surely? I don't see why the idea is attractive enough to pursue.
 
  • #85
Quantum entanglement can cause an event in two places to happen instantaneously. As an example an entangled photon at point (a) gets measured binding its state to the measurement, the same state will be bound to the other entangled photon at point (b).
 
  • #86
PeteLindsey said:
Quantum entanglement can cause an event in two places to happen instantaneously. As an example an entangled photon at point (a) gets measured binding its state to the measurement, the same state will be bound to the other entangled photon at point (b).
I know about QE but trying to apply the OP to this example is problematic. Where would "ds/dt" be quantifiable. I think it's outside the scope of the thread. (Beyond my pay grade at least)
 
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  • #87
PeteLindsey said:
Quantum entanglement can cause an event in two places to happen instantaneously.
No, that is not quite an established fact. What is established is that when entangled systems, e.g. two particles, are measured at two different locations the measurements are shown to have a special, strong correlation when the sequences of measurements afterwards are compared.

Entanglement does not say that the measurement of one entangled particle instantly causes the other entangled particle to acquire a state (e.g. wave function collapse). And the underlying mechanism - if there is any - for entanglement is unknown at this time.
 
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<h2>1. What is meant by "instantaneous" speed measurement?</h2><p>"Instantaneous" speed measurement refers to the speed of an object at a specific moment in time. It is the speed at a particular instant, rather than an average speed over a period of time.</p><h2>2. What are the limitations of instantaneous speed measurement?</h2><p>Instantaneous speed measurements are limited by the precision and accuracy of the measuring instrument. They also do not take into account any changes in speed that may occur between measurements.</p><h2>3. How can environmental factors affect the accuracy of instantaneous speed measurements?</h2><p>Environmental factors such as wind, temperature, and surface conditions can affect the accuracy of instantaneous speed measurements. For example, strong winds can impact the speed of an object, making it difficult to obtain an accurate measurement.</p><h2>4. Is it possible to accurately measure the instantaneous speed of a moving object?</h2><p>While it is possible to obtain a relatively accurate instantaneous speed measurement, it is important to keep in mind that there will always be some level of error due to limitations in the measuring instrument and external factors.</p><h2>5. How can we minimize errors in instantaneous speed measurements?</h2><p>To minimize errors in instantaneous speed measurements, it is important to use high-quality measuring instruments and to take multiple measurements at different points in time. This can help to account for any changes in speed and improve the overall accuracy of the measurement.</p>

1. What is meant by "instantaneous" speed measurement?

"Instantaneous" speed measurement refers to the speed of an object at a specific moment in time. It is the speed at a particular instant, rather than an average speed over a period of time.

2. What are the limitations of instantaneous speed measurement?

Instantaneous speed measurements are limited by the precision and accuracy of the measuring instrument. They also do not take into account any changes in speed that may occur between measurements.

3. How can environmental factors affect the accuracy of instantaneous speed measurements?

Environmental factors such as wind, temperature, and surface conditions can affect the accuracy of instantaneous speed measurements. For example, strong winds can impact the speed of an object, making it difficult to obtain an accurate measurement.

4. Is it possible to accurately measure the instantaneous speed of a moving object?

While it is possible to obtain a relatively accurate instantaneous speed measurement, it is important to keep in mind that there will always be some level of error due to limitations in the measuring instrument and external factors.

5. How can we minimize errors in instantaneous speed measurements?

To minimize errors in instantaneous speed measurements, it is important to use high-quality measuring instruments and to take multiple measurements at different points in time. This can help to account for any changes in speed and improve the overall accuracy of the measurement.

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