I read Brian Greene's The Elegant Universe and Fabric of the Cosmos a while back and am trying to reacquiant myself with physics principles he talks of in there. The one I am specifically thinking of is how relativity explains how/why things would visually "shrink" as they approach the speed of light. Anybody care to help out (I'm too lazy right now to go back and search for the particular section :D)? 'preciate it.

Hi Celpha,
Relativity says that time and space are much the same thing, considered from different perspectives (or 'reference frames').

If you and I move past each other at high speeds, then we have different perspectives of things. For example, if two event happen at the same time from my perspective, they don't necessarily happen at the same time from your perspective. If two events happen at the same time for me, then for you it depends on how far apart they are in our direction of travel.

Let's say we want to measure each others cars as we pass. I set up cameras all along my car, pointing to the side, and I'll set them all off at the same time as you pass. You do the same with cameras on your car.

When my cameras fire, they flash all at the same time from my perspective, but not from your perspective. From your perspective, the camera at the back of my car fires first, then each one in turn with the front camera firing last. This means that my cameras will "measure" your car as being shorter than my car.

The same thing happens in reverse for your cameras. They flash at the same time from your perspective, but not from my perspective, so you measure my car as being shorter than yours.


Now... be careful. This isn't about what we see. I don't see my cameras all flash at the same time, because the light from each flash takes a short time to reach me. If I'm in the middle of the car, I'll see the middle flash first, and the front & back flashes together a little later. If I want to check that they fired all together, I'll have to allow for the time it took for the light to reach me first.


This also means that you don't necessarily see the length contraction visually.
What you see is more complicated! You can read more here if you like: Can You See the Lorentz-Fitzgerald Contraction? (http://math.ucr.edu/home/baez/physics/Relativity/SR/penrose.html)

I thought it had something to do with being immersed in cold water.

:D
Jerry: Do women know about shrinkage?
Elaine: What do you mean like laundry?
Jerry: No, like when a man goes swimming afterwards.
Elaine: It shrinks?
Jerry: Like a frightened turtle!
Elaine: Why does it shrink?
George Costanza: It just does.
Elaine: I don't know how you guys walk around with those things.

Hi Celpha,
Relativity says that time and space are much the same thing, considered from different perspectives (or 'reference frames').

If you and I move past each other at high speeds, then we have different perspectives of things. For example, if two event happen at the same time from my perspective, they don't necessarily happen at the same time from your perspective. If two events happen at the same time for me, then for you it depends on how far apart they are in our direction of travel.

Let's say we want to measure each others cars as we pass. I set up cameras all along my car, pointing to the side, and I'll set them all off at the same time as you pass. You do the same with cameras on your car.

When my cameras fire, they flash all at the same time from my perspective, but not from your perspective. From your perspective, the camera at the back of my car fires first, then each one in turn with the front camera firing last. This means that my cameras will "measure" your car as being shorter than my car.

The same thing happens in reverse for your cameras. They flash at the same time from your perspective, but not from my perspective, so you measure my car as being shorter than yours.


Now... be careful. This isn't about what we see. I don't see my cameras all flash at the same time, because the light from each flash takes a short time to reach me. If I'm in the middle of the car, I'll see the middle flash first, and the front & back flashes together a little later. If I want to check that they fired all together, I'll have to allow for the time it took for the light to reach me first.


This also means that you don't necessarily see the length contraction visually.
What you see is more complicated! You can read more here if you like: Can You See the Lorentz-Fitzgerald Contraction? (http://math.ucr.edu/home/baez/physics/Relativity/SR/penrose.html)

I did not understand your description of the perception of length shrinkage.

Please describe in greater detail what a distant and stationary observer sees as a moving object approaches, passes, and leaves the stationary observer.

Lorentz contraction, time dilation, relativity of simutaneity.... These are about what actually is, in the understanding of a particular observer. Now for a long time it was erroneously thought that these gave an accurate picture of what was visually seen. It wasn't untill the 1950's that a researcher (don't recall his name atm) pointed out that since visual appearance depends on the photons that arrive at the eye at a particular instant and not the photons that were all emmited at a particular instant, the visual appearance is quite different.

>>edit I should have just read the link. It explains better than I do. And it was two researchers, Penrose and Terrell.:p

At the risk of being doubly redundant, I requesrst that Pete please describe in greater detail what a distant and stationary observer sees as a moving object approaches, passes, and leaves the stationary observer.

Perhaps Pete has (conveniently) left the building?

Patience, CANGAS. You've been away for almost 3 months, surely you can wait a couple of hours for a reply.

Please describe in greater detail what a distant and stationary observer sees as a moving object approaches, passes, and leaves the stationary observer.
Relativistic optics at the ANU (http://www.anu.edu.au/Physics/Searle/)

how/why things would visually "shrink" as they approach the speed of light.

Maybe it just gets colder...

It doesn't shrink it spreads but doesn't grow. It occupies more space while containing the same amount of mass of the original area.

Well actually that is shrinking in a way:rolleyes:

But the value is still the same.

Patience, CANGAS. You've been away for almost 3 months, surely you can wait a couple of hours for a reply.


Relativistic optics at the ANU (http://www.anu.edu.au/Physics/Searle/)

eteP, your link was no help at all. Cangas thanks you for trying the best you know how to clearly explain a baffling matter.

Did I write your name backwards just then? Maybe I have aixelsyd. CANGAS says sorry.

I think it would be best if you would explain in your own words, step by step, in exact detail how travel time of photons from an observed object to an observers eye results in the apparition of length contraction. Imagine that I am a small child with no physics education. Imagine that you are trying to explain to my little baby head this baffling thing. Assuming that you actually do understand it yourself, as you seem to claim, it may be wonderfully simple to my little baby head once a real exprt has clearly explained it.

And please be sure to comprehensively explain how the photon travel time causes the visual illusion when the moving object is approaching, is directly in front of the observer, and then is going away.

I trust that you have long ago carefully and accurately drawn diagrams of these things and have precisely determined the relationship between the actual condition of the moving object versus the visual recognition observed.

I am eagerly and thankfully awaiting patiently for your description in your own words of the Einstein Special Relativity length contraction.

CANGAS:

You seem to be conflating two separate effects: the measured length on a moving object and the way that object appears to an observer receiving light from it.

Which effect do you wish to know about? And what are your points of confusion?

CANGAS:

You seem to be conflating two separate effects: the measured length on a moving object and the way that object appears to an observer receiving light from it.

Which effect do you wish to know about? And what are your points of confusion?

James r: When the Pete can't answer the question, you come to the rescue?

My previous post(s) amply covered my dilema after the inadequacy of eteP's ( oops, I did it again; my aixelsyd self is really sorry!).

Read my previous posts and then please try to adequately answer my previously stated questions. (Hopefully assuming that you can. ).

Thanks to your generous self ever so much in advance.

Sorry CANGAS, I have no idea what you're confused about. Can you explain a little more clearly what the problem is?
And please, spell my name properly.

James r: When the Pete can't answer the question, you come to the rescue?

I posted before Pete even attempted to answer the question.

This is the great thing about sciforums: it's democratic. Anybody capable of answering a question can contribute.

If you prefer to talk to just one person, try sending them a personal message, instead of posting in the public forums.

Read my previous posts and then please try to adequately answer my previously stated questions. (Hopefully assuming that you can. ).

Ok. It looks like you're conflating two separate issues, as I said in my previous post. So, now you need to provide me with more information about your understanding, so I can better assist you.

So...?

Can I clear this up?

Objects don't "shrink" when they're travelling very fast. Yes, length contraction will be observed, but it doesn't actually occur. There's a subtle but important difference. The best way I can illustrate this is through a little thought experiment:

Imagine yourself flashing head-first past the earth at .99c. We use √(1-v²/c²) to work out that you experience a sevenfold length contraction - multiply .99 by itself to get .98 and subtract this from one to get a fiftieth, which is roughly a seventh multiplied by a seventh. So when I look at you, you look less than a foot tall. But when you look at me, I look less than a foot tall too. Moreover, when you look at the earth, it looks flattened, something like a frisbee. But that just how it looks to you, not how it is. The earth didn't flatten into a frisbee because you flashed by.

The best way to think of all this is that the length contraction is a "trick of perspective". If you and I are separated by distance, we each look small to one another. But we don't go round thinking that we are smaller. We know it's a trick of perspective. Length contraction is a similar trick of perspective, that applies when we are separated by velocity.

If I can add this: in the end, your observations are the only reality you've got. But that's not to say that your observations are giving you a faithful picture of reality. Things can get somewhat distorted, but that's Relativity for you.

Hi Farsight,
You are welcome to your opinion of things, of course. But you aren't welcome to present yourself as an authority on reality.

Actually, SR says that you would not directly observe length contraction.
A highway scene like this:
http://www.anu.edu.au/Physics/Searle/HWnewton.jpg
Would look this this to something passing by at 0.76c:
http://www.anu.edu.au/Physics/Searle/HWheadli.jpg
You can see a lot of things in that picture, but length contraction doesn't jump out.

What length contraction means in SR is best described by the barn and pole.
If a long rod passes through a short barn at high enough speed, then both doors of the barn (front and rear) can close simultaneously while the pole is inside and open again without touching the pole. Relativity FAQ (http://www.math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html)

Or try it like this: as I pass you, you measure my speed and the time it takes for me to go past. From those measurements, you can figure out my length.
SR says that the length you figure will be contracted from my proper length.

The earth didn't flatten into a frisbee because you flashed by.
SR says The Earth doesn't change because something passes by.
SR says that the Earth is always a flattened frisbee shape in that frame of reference (right know, the Earth is a flattened frisbee shape), just as it is always (close to) spherical in its own frame of reference. The presence or otherwise of some object or observer in a given frame of reference is irrelevant.

Length and shape are frame dependent, just as position is. Your length is always two feet in some reference frame, just as your position is always (0,0,0) in some reference frame.

Did I write your name backwards just then? Maybe I have aixelsyd. CANGAS says sorry.

CANGAS,
Perhaps you are familiar with the tragic story of the dyslexic policeman who spent his entire law enforcement career looking for IUDs.:shrug:

Hi Celpha,
Relativity says that time and space are much the same thing, considered from different perspectives (or 'reference frames').

If you and I move past each other at high speeds, then we have different perspectives of things. For example, if two event happen at the same time from my perspective, they don't necessarily happen at the same time from your perspective. If two events happen at the same time for me, then for you it depends on how far apart they are in our direction of travel.

Let's say we want to measure each others cars as we pass. I set up cameras all along my car, pointing to the side, and I'll set them all off at the same time as you pass. You do the same with cameras on your car.

When my cameras fire, they flash all at the same time from my perspective, but not from your perspective. From your perspective, the camera at the back of my car fires first, then each one in turn with the front camera firing last. This means that my cameras will "measure" your car as being shorter than my car.

The same thing happens in reverse for your cameras. They flash at the same time from your perspective, but not from my perspective, so you measure my car as being shorter than yours.


Now... be careful. This isn't about what we see. I don't see my cameras all flash at the same time, because the light from each flash takes a short time to reach me. If I'm in the middle of the car, I'll see the middle flash first, and the front & back flashes together a little later. If I want to check that they fired all together, I'll have to allow for the time it took for the light to reach me first.


This also means that you don't necessarily see the length contraction visually.
What you see is more complicated! You can read more here if you like: Can You See the Lorentz-Fitzgerald Contraction? (http://math.ucr.edu/home/baez/physics/Relativity/SR/penrose.html)

Thanks to you all for murkying up this issue almost insurmountably.

In a serious physics forum ( or this one ), we would hope to read serious comments and not Petes or Jame Rs jokes.

Is the Special Relativity length contraction a real thing or merely a visual illusion?

The representatives of this physics forum ( Pete ( spelled properly?) and James R ) have been formidably oblique.

We are prompted to conclude that either they do not know or that they deathly afraid to say what they believe.

Can I clear this up?

Objects don't "shrink" when they're travelling very fast. Yes, length contraction will be observed, but it doesn't actually occur. There's a subtle but important difference. The best way I can illustrate this is through a little thought experiment:

Imagine yourself flashing head-first past the earth at .99c. We use √(1-v²/c²) to work out that you experience a sevenfold length contraction - multiply .99 by itself to get .98 and subtract this from one to get a fiftieth, which is roughly a seventh multiplied by a seventh. So when I look at you, you look less than a foot tall. But when you look at me, I look less than a foot tall too. Moreover, when you look at the earth, it looks flattened, something like a frisbee. But that just how it looks to you, not how it is. The earth didn't flatten into a frisbee because you flashed by.

The best way to think of all this is that the length contraction is a "trick of perspective". If you and I are separated by distance, we each look small to one another. But we don't go round thinking that we are smaller. We know it's a trick of perspective. Length contraction is a similar trick of perspective, that applies when we are separated by velocity.

If I can add this: in the end, your observations are the only reality you've got. But that's not to say that your observations are giving you a faithful picture of reality. Things can get somewhat distorted, but that's Relativity for you.

The success of Special Relativity was based upon the alledged failure of MM 1880s and the alledged success of the Einstein Special Relativity length contraction of the arm of the interferometer.

If the length contraction is merely a visual illusion, MM1880s is not satisfactorily explained.

If the length contraction is merely a visual illusion, MM1880s is not satisfactorily explained.
That is correct. Length contraction is real, not merely a visual illusion.

That is correct. Length contraction is real, not merely a visual illusion.

Proof:

The success of Special Relativity was based upon the alledged failure of MM 1880s and the alledged success of the Einstein Special Relativity length contraction of the arm of the interferometer.
That is not correct.
The SR explanation of the Michelson Morley experiment is simply that the speed of light is frame invariant, that it doesn't depend on motion with respect to an ether.

Hi Farsight,
You are welcome to your opinion of things, of course. But you aren't welcome to present yourself as an authority on reality...

I don't present myself as an authority on reality. I just consider myself to be somebody who understands Special Relativity.

...SR says that the Earth is always a flattened frisbee shape in that frame of reference

The Earth is spherical, a little flattened at the poles, but it exhibits a symmetry, and it rotates. The frame of reference where the Earth looks like a frisbee does not allow for this, and has no objective reality. It is not a real thing. It is your, in simple terms, your viewpoint. You must allow for your viewpoint before inferring objective reality from your observations, even though those observations form the basis of your reality. In normal life we know this: when we are separated by distance we do not claim that "you are small", because we understand perspective.

This is a similar situation. Special Relativity does not "say" that the Earth is a flattened frisbee in that frame of reference, it says that's how we will observe it from that frame of reference. The difference is moot. It can be examined by considering relativistic twins, asking if the symmetrical length contractions are objective reality or subjective reality, then considering contradictions. Please check up on this, and don't be tempted to claim that "what we observe is real" because it's too simplistic - we see colour but we know that colour is a quale, and frequency is what's real.

Hi Farsight,
I see you have ignored the main substance of my post. Since your post is simply repeating what you said earlier, I have nothing more to add, except that your assessment of your understanding of SR might be off the mark.

CANGAS:

In a serious physics forum ( or this one ), we would hope to read serious comments and not Petes or Jame Rs jokes.

I don't think I have either posted any substantive comment on the issue in this thread, or made any jokes.

I remind you once again not to insult other members of sciforums, or you may be banned.

Now, to answer your question:

Is the Special Relativity length contraction a real thing or merely a visual illusion?

It's a real thing.

Pete: I didn't intend to ignore your post. I just didn't think it was necessary to respond point by point. But to show willing, I will:

Actually, SR says that you would not directly observe length contraction. A highway scene like this... would look this this to something passing by at 0.76c... You can see a lot of things in that picture, but length contraction doesn't jump out.

Fair enough.

What length contraction means in SR is best described by the barn and pole. If a long rod passes through a short barn at high enough speed, then both doors of the barn (front and rear) can close simultaneously while the pole is inside and open again without touching the pole.

Wrong. Motion is relative. There is no difference between the above and the short barn passing over the long rod at high speed. Then the short barn is even shorter, and you've got yourself a contradiction.

Or try it like this: as I pass you, you measure my speed and the time it takes for me to go past. From those measurements, you can figure out my length. SR says that the length you figure will be contracted from my proper length.

Yes it does. But this is the length I figure. It's the length I observe. You can make the same measurements of me. I say I'm six feet long and you're one feet long. You say you're six feet long and I'm one feet long. We contradict each other. This is why I said the measurements are subjective not objective. I treat my subjective measurement as objective, and you do the same. But when we confer and exchange notes, we realise there's more to it, and we cannot trust our observations to yield objective reality.

SR says The Earth doesn't change because something passes by.
SR says that the Earth is always a flattened frisbee shape in that frame of reference (right now, the Earth is a flattened frisbee shape), just as it is always (close to) spherical in its own frame of reference. The presence or otherwise of some object or observer in a given frame of reference is irrelevant.

I think I've answered this but I will reiterate: reference frames are not real objects that real objects are "in". Reference frames are our observational standpoints. We do not say "the earth is a frisbee here and a pancake there". We understand the perspective effect and learn why our observations are unreliable, and then we work out that the earth is round.

Length and shape are frame dependent, just as position is. Your length is always two feet in some reference frame, just as your position is always (0,0,0) in some reference frame.

No, observed length and shape are frame dependent. Your position is always (0,0,0) in "your" reference frame. You don't move in your reference frame. That's how it works. And that means that "in" your reference frame, the sun goes round the earth. It simply doesn't.

Wrong. Motion is relative. There is no difference between the above and the short barn passing over the long rod at high speed. Then the short barn is even shorter, and you've got yourself a contradiction.


No, it is you who is wrong. Yes, in the rod's frame the barn is shorter than the rod, but the opening and closing of the doors that was simultaneous in the barn's frame, are no longer simultaneous in the rod's frame. There is no contradiction. This is a standard, basic exercise in sophomore physics. You really should try to work through it before pontificating on it.

I am correct. The doors have nothing to do with it. There is no difference between the "length-contracted" long rod fitting within the short barn and the "length-contracted" short barn passing over the rod like a doughnut on a stick. Because motion is relative.

Your use of the terms "in the rod's frame" and "in the barn's frame" merely back up my explanation regarding subjective versus objective measures and perspective.

I am correct.
Well of course! -Why didn't you put it like before!
:rolleyes:

I am correct.


You are wrong.


The doors have nothing to do with it.


The non-simultaneity of the opening and closing of the doors in the rod's frame has everything to do with it. Work it out from the Lorentz transformations yourself to see why. As I said, this is sophomore physics. Go learn it.