Perceive Length Contraction VS Terrell Penrose

[Albertgauss]

TL;DR Summary

I visited a number of websites on what you would see if you travel close to light speed. Trying to figure out what length contraction phenomenon could be experienced/perceived verses Terrell Penrose phenomenon.

This is a more of a "can you confirm" what I am thinking here; I think I understand what is going on, but just want to make sure.

At the end of this post, I reference the two selected websites at the times mentioned that compare "Length Contraction" verses "Terrell Penrose Rotation."

My question: Even though the visual description of an object approaching me would be dominated by the Terrell-Penrose rotated effect of that object--and there would be no obvious manifestation of the length contraction for that object's appearance----am I correct in asserting that I would still perceive length contraction in relativity because length contraction would still bring the approaching object physically closer to me? For example, the sphere appears to be the example of a Terrell Penrose rotated objects but, even though it would not appear to contracted, I would still measure it to be closer to me, which would be a telltale sign of me experiencing length contraction; is this correct?

ScienceClic Alessandro Roussel What would we see at the speed of light? ~ 9 mins 0 seconds in
https://www.bing.com/videos/rivervi...E198C7F4F9D4EE240C52E198C7F4F9D4EE2&FORM=VIRE

Arvin What You Would SEE if You Traveled Near the SPEED of LIGHT ~ 9 min 20 seconds in
https://www.bing.com/videos/rivervi...D922D0454EF8A715B22ED922D0454EF8&&FORM=VRDGAR

 

[Ibix]

Depends a lot on what you mean by "bring it closer" and how you are planning on measuring that.

The conceptually easiest test is to do something like the rod-and-barn experiment and show that a 10m rod fitted (briefly) inside a 5m barn. That seems like a fairly straightforward check to me.

 

[A.T.]

... because length contraction would still bring the approaching object physically closer to me?

If you are looking for a demonstration of the "physicality" of length contraction see:
https://en.wikipedia.org/wiki/Bell's_spaceship_paradox

 

[Albertgauss]

I started thinking about the article of A.T. but I can go back to Ibix rpely for a moment. To better clarify my question, suppose an object in my rest frame is one kilometer away from me. Suppose in another situation, I come up to the point that the object used to be one kilometer away from me and find it is now 100 meters (0.1 kilometer) from me. My velocity is relativistic and constant. Would this be a manifestation of length contraction that could be experienced?

 

[Ibix]

Would this be a manifestation of length contraction that could be experienced?

You need markers at rest at both points in your original frame so that you can recognise them and some way (probably a ruler) at rest in your second rest frame so you can measure the distance.

The ruler is equivalent to the barn; the markers are equivalent to the ends of the rod. So this is simply the rod-and-barn scenario a little bit disguised. Yes, you can (in principle) detect length contraction like that. In practice, we've never got macroscopic objects up to the required speeds.

 

[PeterDonis]

I come up to the point that the object used to be one kilometer away from me and find it is now 100 meters (0.1 kilometer) from me

What does this mean?

 

[Albertgauss]

@PeterDonis ,

Let's try this.

In Frame number 1 I am rest and there are two coordinates, the first coordinate I call my origin and the second coordinate where the object is, make it be a spot, or a point of light only--nothing with any structure. In this frame I measure the spot to be one kilometer way from me.

Frame number 2 is my relativistic frame and now I am a ship moving at a constant, but relativistic light speed. The spot remains at rest with Frame 1. In my ship, I happen to notice that the origin of my spaceship coincides with the origin of my old rest frame 1 just for a moment. Now, at this moment, say that I measure the spot to be 100 meters away from me (say 0.1 kilometer). The spot is physically closer to me now than it was in frame number 1. In this way I would experience or know that length contraction occurs. The spot is physically closer to me in my spaceship frame 2 than in frame number 1 because its distance to me got length contracted.

That is, I am trying to confirm (or reject) that I understand that length contraction manifests itself in the shortened distance an object is in a relativistic frame but not in the description/appearance of that object. (For example, elongated ships would not flatten into silver dollars). You don't have to travel as far to get to an object because it is closer to you in the relativistic frame. Length contraction does not appear easily or readily in the actual description/appearance of the approaching object because then its effect is masked /hidden/incorporated by Terrell Penrose rotations.

If you all think I have explained this correctly, I would be good to go.

 

[PeterDonis]

Frame number 2 is my relativistic frame and now I am a ship moving at a constant, but relativistic light speed.

Does this mean you are at rest in Frame 2 and moving at a constant relativistic speed relative to Frame 1? There is no such thing as "speed" without specifying what frame the speed is relative to.

In what follows I'll assume that the above is what you meant.

In my ship, I happen to notice that the origin of my spaceship coincides with the origin of my old rest frame 1 just for a moment. Now, at this moment, say that I measure the spot to be 100 meters away from me (say 0.1 kilometer).

There are still complexities lurking here. You can't instantaneously "measure" a distance. So how do you know, at the instant your spaceship is co-located with the spatial origin of Frame 1, that the "spot" in Frame 2 is only 100 meters away?

I am trying to confirm (or reject) that I understand that length contraction manifests itself in the shortened distance an object is in a relativistic frame but not in the description/appearance of that object. (For example, elongated ships would not flatten into silver dollars).

This is not correct. Length contraction is a geometric effect of changing frames, similar to the geometric effect of the apparent size of an object changing when you change the angle from which you view it. As such, it affects everything, including both distances to objects and lengths of objects along the direction of relative motion.

 

[Albertgauss]

"Does this mean you are at rest in Frame 2 and moving at a constant relativistic speed relative to Frame 1? There is no such thing as "speed" without specifying what frame the speed is relative to."

Yes, that is correct. But the important thing here is that in frame 1 the spot is at rest, in frame 2 the spot now moves towards me.

"You can't instantaneously "measure" a distance."

Yes, I used the wrong choice of words here. "Instantaneously" is not the best word to use in relativity.

So how do you know, at the instant your spaceship is co-located with the spatial origin of Frame 1, that the "spot" in Frame 2 is only 100 meters away?

What I'm trying to say here is that in frame 2 is that I measure the spot to be 100 meters away. The spot is moving with respect to me so I'm not quite sure how to say I measure a different distance to the spot than when I was in frame 1. If I put a ruler down in frame 2 and the ruler is at rest in frame 2, the ruler in frame 2 will report that the spot is closer to me than when I put a ruler in frame 1. I think that would be the length contraction on the spot's coordinate (or spatial distance) away from me, is this correct? How could this be phrased better?

"As such, it affects everything, including both distances to objects and lengths of objects along the direction of relative motion."

I agree length contraction affects everything but would an object's length contraction itself be observable? If a long rod passes by me at light speed, will we see it actually flatten into a silver dollar? The websites I mentioned above say that we won't be able to, that the length contraction would not be directly observable, because the Terrell Penrose rotation completely dominates the appearance of the object. Now, I'm not sure which effect wins; I'm not really sure if these two websites are accurate about what they say about length contraction present but not observable.

So, now the larger question, will an object display obvious length contraction when it passes by at relative light speed or will it get washed out by the Terrell Penrose rotation? Which effect governs the appearance of the object?

Quite a lot of relativity information on the internet talks about that, due to length contraction, places in the universe are shorter to get to. This would include my spot in this discussion, that the actual distance to far-away galaxies suddenly ends up being a few meters (physically) if the frame 2 boosts to high enough light speed. How do I properly phrase such a statement?

 

[PeterDonis]

@Albertgauss please use the PF quote feature to quote things from other people's posts that you are responding to.

 

[PeterDonis]

would an object's length contraction itself be observable? If a long rod passes by me at light speed, will we see it actually flatten into a silver dollar?

What you actually see is more a matter of Penrose-Terrell rotation. But you can set up a measurement of the rod's length to confirm that yes, its measured length is shorter than it would be if the rod were at rest relative to you.

Quite a lot of relativity information on the internet

Please give some specific references.

 

[PeterDonis]

I'm not sure which effect wins

There aren't two different "effects" here. What you have here are two different questions you can ask.

You can ask what you see, as in, what image the light rays arriving at your eye at a given instant show you. Penrose-Terrell rotation answers that question.

Or you can ask what you measure, as in, if you set up a measurement of the rod's length when it is moving relative to you, what result you will get. Length contraction answers that question.

These are different questions whose answers both depend on the same underlying physics. They are not different "effects" that have to compete to see which one wins.

 

[Ibix]

If I put a ruler down in frame 2 and the ruler is at rest in frame 2, the ruler in frame 2 will report that the spot is closer to me than when I put a ruler in frame 1.

Assuming that "put ruler in frame 1" you mean "put a ruler at rest in frame 1", this is fine.

I think that would be the length contraction on the spot's coordinate (or spatial distance) away from me, is this correct?

Coordinates are just labels for events, so they don't have a length to contract. It is an example of the contraction of the distance between the origin of frame 1 and the spot at rest in frame 1, yes.

I agree length contraction affects everything but would an object's length contraction itself be observable? If a long rod passes by me at light speed, will we see it actually flatten into a silver dollar?

Sort of to the first, no to the second.

Fly the rod through a narrow gap between a strip of lamps and some photo paper. Flash all the lamps simultaneously in their rest frame while the rod is in the gap and the shadow you record on the paper will be the contracted length of the rod. That's length contraction. There are possible arguments about whether that's "observing length contraction", which is why I said "sort of", but it's a measure of the phenomenon.

You will not directly see it, however. Your eye isn't a long strip of photo paper with carefully arranged perpendicular illumination. It's more or less a point detector. Since light is going barely faster than the rod, as the tip of the rod passes your nose, light emitted at the tail of the rod simultaneously hasn't had time yo reach your eye. Instead, what you see of the tail of the rod is due to light emitted in the past when the tail of the rod was further away. So the optical image of the rod is stretched due to the difference in lightspeed delay. We factored this out in the lamp/photo experiment by using light sources and detectors local to each point of the rod, so the delay was always the same along the whole length of the rod.

 

[Ibix]

This would include my spot in this discussion, that the actual distance to far-away galaxies suddenly ends up being a few meters (physically) if the frame 2 boosts to high enough light speed. How do I properly phrase such a statement?

That's fine. My explanation for how you could get to a galaxy millions of light years away is that your clocks tick slowly at high speed relative to me. Your explanation is that the distance between the galaxies was length contracted so they were never millions of light years apart.

(Note that this is an oversimplification, assuming that you start at rest in one galaxy and finish at rest in the other - non-inertial frames are more complicated to describe. What you are actually describing is the frame of someone who was always travelling at very high speed relative to the galaxies and just happened to pass close to us.)

 

[Albertgauss]

I think I'm good here and my question is answered pretty well. Other comments are certainly welcome, but I feel I understand what all has been said here and I appreciate everyone's efforts. All good on my end.