What is the speed of "motion"?

[BFEENEY]

TL;DR Summary

When do you see someone wave at you?

Motion is probably not the right word, maybe perception. I was thinking one day about the speed of things and came upon a question I couldn't answer. So you have two people on either side of a valley, person #1 claps their hands. That sound reaches person #2 in X amount of time. Person #1 then flashes a light and person #2 sees it in Y amount of time. What about when person #1 waves at person #2? When does person #2 see #1 wave? That is how I came to the question 'what is the speed of motion?'. I know the distance/time formula but that's a different (I think). Would that speed be closer to the speed of light? Thanks everyone!

-Brian

 

[kuruman]

The speed of light is the distance light travels from point A to point B divided by the time it takes to make the trip. The same applies for a runner, a car, sound, etc.
Does motion, or whatever, travel from point A to point B?

 

[Ibix]

When does person #2 see #1 wave?

When the light reflecting off person 1's hand reaches person 2.

 

[jbriggs444]

You seem to be talking about the speed of causal influence or the speed of information transfer.

If the special theory of relativity is correct, this can be no faster than the speed of light. Otherwise, causality paradoxes arise.

Signalling using light is not only possible, but easy. So the maximum speed of information transfer is $c$.

 

[Baluncore]

Don't forget, that the light entering your eye, or the sound entering your ear, must be processed and classified by your neural networks, before it becomes a perception in your world view.

 

[jbriggs444]

Don't forget, that the light entering your eye, or the sound entering your ear, must be processed and classified by your neural networks, before it becomes a perception in your world view.

Do you know whether we have any good measurements of the latency between sensory stimulus and perception?

I know that we can put an upper bound on the number by measuring the latency between stimulus and response -- around 250 ms.

I like to muse that our "now" lives somewhere inside the 250 ms gap. Exactly where is somewhat irrelevant.

 

[Baluncore]

Do you know whether we have any good measurements of the latency between sensory stimulus and perception?

For vision, it is of the order of 250 ms, but it comes down to training and the restriction of alternative distractions.

If the event has not happened before, you will not respond, as you are still learning and developing the "concept node" that feeds your perception. You will then do a "double take", are you really sure, before recognising it. With training, you may get it down to 50 ms. The quickest handgun draw, survives to play again.

Ears take longer than eyes to recognise things. That is because the ear does a physical spectrum analysis in the cochlea, that includes low frequencies, and must then use language recognition to separate signal from noise.

The slowest thing is changing your mind and cancelling a muscular action you have just then commanded. Once you start to pull the trigger, if someone unexpectedly steps into the line of fire, that muscle movement cannot be overtaken by a command not to. It is then safer to pull a shot sideways, than not fire. That deliberate waste of the shot must be a learned response in close combat.

 

[BFEENEY]

Thanks everyone for the responses. I was leaning toward the speed of light thinking that it was a reflection of light we see.

 

[BFEENEY]

The speed of light is the distance light travels from point A to point B divided by the time it takes to make the trip. The same applies for a runner, a car, sound, etc.
Does motion, or whatever, travel from point A to point B?

I was thinking of things being more stationary I guess. A person standing still waving his hand side to side has no forward motion, so no real distance covered, just time

 

[kuruman]

Thanks everyone for the responses. I was leaning toward the speed of light thinking that it was a reflection of light we see.

An obvious "proof" of that is that if the person waves a hand in a completely dark room, that motion will not be detected by another person at some distance. However, it might be detected by a bat as a Doppler-modulated echo of the cry it sends out (https://c21.phas.ubc.ca/article/bats-and-the-doppler-shift/)

Thus, the waving motion travels at the speed of light when light is available. When light is not available, it cannot be detected by a human, but it can be detected by a bat in which case the information travels at the speed of sound.

 

[BFEENEY]

Thus, the waving motion travels at the speed of light when light is available. When light is not available, it cannot be detected by a human, but it can be detected by a bat in which case the information travels at the speed of sound.

That makes sense

 

[sophiecentaur]

thinking that it was a reflection of light we see.

That's pretty much always - except when but, for hundreds (thousands) of years it was realised that the sound of an event lags behind what we see and they soon found that sound travels at around 300m per second. The speed of light is so much greater that vision was assumed to be instantaneous. It wasn't till 1850 ish that Fizeau found a pretty accurate value using a rapidly spinning toothed wheel. That's less than 200 years ago for an accurate value.

Ears take longer than eyes to recognise things.

I don'think the actual response time for a visible event is much better than hearing an event. The delay is largely to do with our slow brains but often we cleverly compensate for in sports and fighting.

 

[jbriggs444]

often we cleverly compensate for in sports and fighting.

A half second (for instance) delay in perception and planning is countered by a half second of prediction so that the fact of the event and fact of our action coincide. As with a judge clicking his stop watch exactly when the runner crosses the finish line.

 

[sophiecentaur]

A half second (for instance) delay in perception and planning is countered by a half second of prediction so that the fact of the event and fact of our action coincide. As with a judge clicking his stop watch exactly when the runner crosses the finish line.

Those little details must have been the ruin of many a poor experimenter in history.

 

[Baluncore]

Those little details must have been the ruin of many a poor experimenter in history.

And the death of every second gunman in the West.

In a quick draw contest, the draw becomes faster than is possible based on seeing the start of the physical draw. Consistently impossible times between -5 ms and +5 ms begin to appear with practice. The poker-faced winner, must be recognising an involuntary change in the body or face of the opposition, then outdrawing them using a better-trained muscle memory.

I once repaired the digital timer used by a quick-draw showman. During testing, I was surprised by how much of a part was played by subconscious anticipation.

The quicker-draw showman bought the timer from its slower-draw owner, with his winnings, to settle the debt. And no, the digital timer had not been shot.