# Length Contraction

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8 minutes ago, J.C.MacSwell said:

Right, but less time lost on a clock should equate to more time elapsed.

The clock tells you elapsed time. So less time on the clock means less time elapsed, no?

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6 minutes ago, Strange said:

The clock tells you elapsed time. So less time on the clock means less time elapsed, no?

True. So if "time lost" is elapsed time it reads correctly.

But generally speaking time gained or lost on a clock due to relative effects is the discrepancy.

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23 hours ago, J.C.MacSwell said:

Are you sure that is what you meant to say?

No! Obviously I was looking through the wrong end of the telescope.

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• 3 weeks later...
On 21-9-2017 at 8:43 PM, zapatos said:

I think the difficulty arises because of the language around whether or not the object 'contracts'; that is, whether or not an object goes through some sort of metamorphosis simply because someone happens to be viewing it from a new frame.

Keep in mind that in 4D spacetime the 'object' is not a 3D item: a measured 3D object is only part of the full object, being 4D.

An observer doesn't view a 3D object from a new frame. A new (different) frame measures a different 3D section through the 4D object, hence a different 3D unit is the result.

All 3D sections are equally "physically real".

Edited by VandD
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1 hour ago, VandD said:

Keep in mind that in 4D spacetime the 'object' is not a 3D item: a measured 3D object is only part of the full object, being 4D.

An observer doesn't view a 3D object from a new frame. A new (different) frame measures a different 3D section through the 4D object, hence a different 3D unit is the result.

All 3D sections are equally "physically real".

Clearly put sirs. +1

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On 9/19/2017 at 3:59 PM, thomas reid said:

In Relativity when something is moving does it become actually physically shorter or does it only appear shorter in the rest frame?

The answer is that length contraction is real but it depends on the point of view that determines how it's real and to whom it's real.

Assume there is a ship traveling from Alpha Centauri to the Earth at 80% c.

You will often hear it said that the moving frame will be length contracted. This is a confusing statement that can be misleading.

From any inertial reference frame (non accelerating), the observer is considered to be in the stationary frame and the moving frame is the frame that is in motion relative to that observer.

Therefore from the ships PoV, the ship is normal and it is the Earth's frame that is in motion and the Earth, the sun,  the entire solar system and including the orbit of the planets that is length contracted by 60%

But from the Earth's PoV, it is the ship's frame that is length contracted so the Earth is normal and the ship is 60% shorter.

So is it real?

Yes its real and has real consequences such as demonstrated in the Twin Paradox.
( BTW, not all effects are real, but time dilation and length contraction are both real.)

So what is actually being contracted?

It is the dimension of space in the direction of travel for the moving frame that is being contracted. Therefore everything in that dimension that is moving along within the frame is being contracted at the same time.

Note that there is another interpretation of SR that length contraction is an effect of the rotation of spacetime into the complex plane, or a hidden dimension of spacetime that is at 90° phase shifted from our spacetime. It's just an explanation for what is causing the effects but is still consistent with all results.

It makes a certain amount of sense though. i.e. if you view a yard stick from the side it appears to be 3 feet long. as you rotate it,, it appears to be length contracted. The difference being the rotation is into another dimension so the length contraction is real.
Edited by TakenItSeriously
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Due to relativity of simultaneity, relative moving observers don't measure between same set of simultaneous events. Hence different distance, length.

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On 10/22/2017 at 0:34 AM, VandD said:

Keep in mind that in 4D spacetime the 'object' is not a 3D item: a measured 3D object is only part of the full object, being 4D.

An observer doesn't view a 3D object from a new frame. A new (different) frame measures a different 3D section through the 4D object, hence a different 3D unit is the result.

All 3D sections are equally "physically real".

Is it an established fact that objects are 4D?

Can't you interpret Relativity with 3D objects diving in a 4D continuum?

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4D is a model representing/corresponding to physical behavior.

Can you interpret SR in terms of basic physics, matter, light, and motion?

Edited by phyti

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