# Perception of Velocity in Special Relativity

• B
In summary: I don't know what is.Can you define percived velocityHow fast something looks like it’s going.Based on my understanding (and I’m probably wrong) From the viewpoint of the fast moving object, it measures as fast. And from the person watching views it as going slow.If the above is not... clear enough, I don't know what is.
When something is traveling near the speed of light
Relative to what? I can assume that you mean relative to what you call a "stationary frame of reference", and I will make that assumption in my answer below, but you would be better served by not making people have to assume such things. Whenever you describe a scenario in relativity, you should always specify speeds, "moving", "stationary", etc., relative to something. If you're not sure what something to use, just adopt the common default of "the observer".

due to time dilation, would that make it look like it’s going slower according to a stationary frame of reference?
If "going slower" refers to velocity or speed, as I defined those terms in post #26, then no.

PeterDonis said:
Relative to what? I can assume that you mean relative to what you call a "stationary frame of reference", and I will make that assumption in my answer below, but you would be better served by not making people have to assume such things. Whenever you describe a scenario in relativity, you should always specify speeds, "moving", "stationary", etc., relative to something. If you're not sure what something to use, just adopt the common default of "the observer".If "going slower" refers to velocity or speed, as I defined those terms in post #26, then no.
This helps me. Thank you this is the answer I was looking for.

I hope this is more clear.

When something is traveling near the speed of light, due to time dilation, would that make it look like it’s going slower according to a stationary frame of reference?
No. When things go faster [relative to a chosen reference] they go faster [relative to that reference].

Once you correct for speed of light delays in the observations you make, you may notice that clocks on the moving object are advancing slowly. But that does not mean that the object is moving slowly.

Nor does the fact that you measure the object's clocks to be advancing slowly mean that the object itself is moving slowly in some hypothetical, unspecified, "stationary" frame.

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jbriggs444 said:
No. When things go faster [relative to a chosen reference] they go faster [relative to that reference].

Once you correct for speed of light delays in the observations you make, you may notice that clocks on the moving object are advancing slowly. But that does not mean that the object is moving slowly.

Nor does the fact that you measure the object's clocks to be advancing slowly that the object itself is moving slowly in some hypothetical, unspecified, "stationary" frame.
These are the answers I’m looking for. Thank you.

russ_watters, jbriggs444 and berkeman
My apologies to @BadgerBadger92 @malawi_glenn and @Ibix but we had to delete your posts due to referencing a facebook screenshot which gave away personal information of a couple of users.

If you would like to repost your content without the screenshot go ahead and do so.

Jedi

jbriggs444 said:
Once you correct for speed of light delays in the observations you make, you may notice that clocks on the moving object are advancing slowly. But that does not mean that the object is moving slowly.
Just to add to this, @BadgerBadger92, the point is that you use your own clocks and your own rulers to measure the speed of the object. So relativistic effects are irrelevant to your measures because you use clocks and rulers that are stationary relative to you. An observer riding on the object, of course, uses his own clocks and rulers to measure your speed, and comes up with the same speed you measure for him but in the opposite direction.

russ_watters
malawi_glenn said:
what is the difference in seeing how fast it moves than measure how fast it moves?
That's a very important difference (at least for extended objects). What you see is the light reflected from the body, and that's different from measuring distances and thus also velocities of the different point of the extended object.

The most important effect is that you don't "see" length contraction but rather a rotated body, when the latter is moving fast relative to you (see "Terrell effect").

vanhees71 said:
That's a very important difference (at least for extended objects). What you see is the light reflected from the body, and that's different from measuring distances and thus also velocities of the different point of the extended object.

The most important effect is that you don't "see" length contraction but rather a rotated body, when the latter is moving fast relative to you (see "Terrell effect").
I know, was wondering if OP had thought about it ;) Socrative method

vanhees71, PhDeezNutz and phinds

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