# B Is this an adequate thought experiment?

#### Shay Katzir

I've been struggling to develop an intuition about Einstein’s relativity, without luck, until i invented my own thought experiment, to help me. Now i feel i have the intuition, but it leads me to a new misunderstanding, i hope someone here might help me clarify.

My thought experiment involves a dancer (x) & two viewers (y & z), both watching him dance, from a close distance. Few seconds into the dance, y begin to run away from x, in light speed (maybe because he does not dance so well), while still watching him. so as the light of x, is spreading in light speed also, which is the exact same speed y is running away from him, what y perceive, is the the exact same state of x, he saw when he started running. Therefore, he might say, time froze upon x, & he is not dancing at all. While z would not agree with y, because he still sees x continue to dance normally. Btw, if y would double his running speed, he might see x dancing in reverse, like he’s dancing back in time..

Now, this thought experiment, supposed y & z, are using there eyes to watch x & judge the time that pass upon him, according to visual input. But what if, they would use their ears instead, and also, y would run away from x, in sound speed, instead of light speed? In this case, the results would be the same: y will say, the music x were dancing to, stopped exactly when he started to run, and he is still hearing the same note, therefore, again, he might conclude, time stood still upon x. And again, z would not agree, cause he is hearing the music proceeds normally.

So, as my thought experiment, does not present the light, in a special light at all, so should i conclude, it does not reflect Einstein’s relativity correct, & it leads me towards wrong intuition?

Also, this experiment reflect the fact, that time is not only relative to the observer, but also to the observer’s type of sensor, that determine the x time measure. Does this fact also wrong?

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#### phinds

Gold Member
Few seconds into the dance, y begin to run away from x, in light speed
Well, there's your problem. Y has mass and so cannot move away from x at light speed and certainly not, as you later suggest, at TWICE light speed. Things with mass can only move slower than c.

Also, this experiment reflect the fact, that time is not only relative to the observer, but also to the observer’s type of sensor, that determine the x time measure. Does this fact also wrong?
Yes, this is wrong. Time does not depend on the type of sensor used to measure it.

Your comparison of something involving sound vs something involving light is invalid because you CAN move at the speed of sound, and faster, but that's not true with light.

I think you better stick with published experiments regarding relativity, not try to make up your own.

#### A.T.

should i conclude, it does not reflect Einstein’s relativity correct, & it leads me towards wrong intuition?
Yes. You confuse signal delay effects with relativistic effects, which are what is left after you accounted for signal delay.

#### Shay Katzir

Thanks, i now understand its a signal delay effect, but just to clarify: if i start running away, in spead of sound, from a talking clock, that is saying 'five', so i still hear the 'fa' of the five, as long as im running, wouldnt i be right saying, that for me, the clock is stuck in time - not moving at all, & therfore conclude time is relative, because someone, who stays near the clock, was hearing it continue counting, even while for me it was stuck?
Can't signal delay effect, demonstrate in this way, the relativity of time (without the additional effects of close to light speeds)?

#### Nugatory

Mentor
wouldnt i be right saying, that for me, the clock is stuck in time
No. Time is passing for you and time is passing for the sound wave that’s moving alongside you, and time is also passing for the clock as sound waves are coming from it. You might as well argue that time stops if you put earplugs in your ears so that you can’t hear the sounds from the clock.
Can't signal delay effect, demonstrate in this way, the relativity of time (without the additional effects of close to light speeds)?
No. The signal delay effects are the same as in classical physics and don’t tell us anything we didn’t already know. To see relativistic effects you have to consider the implications of the speed of light being the same for all observers - a big difference from the behavior of sound waves.

#### Ibix

if i start running away, in spead of sound, from a talking clock, that is saying 'five', so i still hear the 'fa' of the five, as long as im running, wouldnt i be right saying, that for me, the clock is stuck in time - not moving at all
But if you look at the clock you'll be able to see it ticking normally.
Can't signal delay effect, demonstrate in this way, the relativity of time (without the additional effects of close to light speeds)?
No. The point is that relativistic effects are what is left over after you correct for signal delay. It's actually got nothing to do with the propagation of light, but we often use light signals in explanations because the "always travels at the same speed" behaviour makes the maths easy.

#### Mister T

Gold Member
f i start running away, in spead of sound, from a talking clock, that is saying 'five', so i still hear the 'fa' of the five, as long as im running, wouldnt i be right saying, that for me, the clock is stuck in time - not moving at all, & therfore conclude time is relative, because someone, who stays near the clock, was hearing it continue counting, even while for me it was stuck?
But the clock is still ticking away, as witnessed by the observer who stays near the clock. All you're experiencing is a signal delay effect because you and the signal are moving at the same speed. The amount of dilation depends on the factor $\gamma=\frac{1}{\sqrt{1-v^2/c^2}}$. If $v$ is the speed of sound and $c$ is the speed of light, then $\frac{v}{c}\approx 10^{-6}$. At this speed the amount of time dilation is less than one part in $10^{12}$.

"Is this an adequate thought experiment?"

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