# Lorentz Contraction: Explaining the Mystery

• Unto
In summary, the Michelson-Morley experiment showed that light does not always travel at the speed of light. emission theory is consistent with this result, but the ballistic theory is not.
Unto
I don't understand why this happens. And it's not enough to tell me 'well it happens because Simultaneity is relative' or 'Because Time dilates between frames'. I can still say I don't understand why it happens because shouldn't 2 frames adjust their measurements so that they get the same proper length?

Can someone explain, as this is the only SR concept that I don't understand very well.

Actually, neither time nor simultaneity have anything to do with it. The real answer is "that is what experiment shows". In fact, Lorentz himself derived that formula by asking himself, "Given that the Michaelson-Morley experiment gives a null result, what must be true?"

Unto said:
I don't understand why this happens. And it's not enough to tell me 'well it happens because Simultaneity is relative' or 'Because Time dilates between frames'. I can still say I don't understand why it happens because shouldn't 2 frames adjust their measurements so that they get the same proper length?
Yes, every frame can calculate an object's proper length and get the same answer. Lorentz contraction means that the coordinate length, not proper length, of an object depends on the coordinate system (frame) used.

For example, if the proper length of an object is 10 meters, then in a reference frame in which the relative velocity is 0.8c, its coordinate length will be 6 meters, while its proper length will be 10 meters as calculated by either frame. Coordinate length equals proper length only for an object at rest in the applicable frame.

Why does the frame moving at velocity 0.8c measure 6 meters in the first place?

Why when you look at objects from distance, they appear to be smaller?

When you move, you look at spacetime from a different angle. So you measure the proper length differently.

Dmitry67 said:
Why when you look at objects from distance, they appear to be smaller?

When you move, you look at spacetime from a different angle. So you measure the proper length differently.

At last, a reasonable explanation as to why this happens. Was fed up with the waffly crap my textbooks kept giving me. This is just like parallax right?

Thanks ;)

Unto said:
At last, a reasonable explanation as to why this happens.
Actually physics never tells you 'why' just 'how much'. What Dmitry67 wrote is just a geometrical interpretation of 'how much'. The object is rotated in space-time and we see only the shortened projection onto space. Here an animation:

Unto said:
At last, a reasonable explanation as to why this happens. Was fed up with the waffly crap my textbooks kept giving me. This is just like parallax right?

Thanks ;)
It's not a visual thing like parallax--when Dmitry67 said "you look at spacetime from a different angle" he presumably wasn't talking about what you see visually, but rather the way the plane of simultaneity in your rest frame is oriented at a different angle from the one in the frame where you're moving. This page has a good primer on spacetime diagrams in SR, and here's a diagram from that page illustrating why different frames measure length differently:

HallsofIvy said:
Actually, neither time nor simultaneity have anything to do with it. The real answer is "that is what experiment shows". In fact, Lorentz himself derived that formula by asking himself, "Given that the Michaelson-Morley experiment gives a null result, what must be true?"

Hopefully, you understand MMX does not prove the light postulate.
This is a common misconception.

MMX is consistent with light emission theory.

One such attempt is known as the Emission Hypothesis (or the ballistic theory of light), and was developed partly by Walther Ritz (C&N p.353). According to this theory, light behaves like bullets shot from a gun, its speed with respect to the source being a universal constant and independent of any ether. This idea is consistent with the null results of the Michelson-Morley experiment and many others.
http://laser.phys.uAlberta.ca/~egerton/specrel3.htm

Problems with emission theory
The simplest form of emission theory says that radiating objects throw off light with a speed of "c" relative to their own state of motion
http://en.wikipedia.org/wiki/Emission_theory

Modern Physics/Michelson-Morley Experiment
Walter Ritz's emitter theory (or ballistic theory), was also consistent with the results of the experiment
http://en.wikibooks.org/wiki/Modern_Physics:Michelson-Morley_Experiment

This paper shows emission theory is consistent with SR.
http://arxiv.org/PS_cache/physics/pdf/0512/0512036v1.pdf

This rules out any conceptually coherent ballistic theory of light propagation, according to which the speed of light is the vector sum of the velocity of the source plus a vector of magnitude c. Ironically, the original Michelson-Morley experiment was consistent with the ballistic theory, but inconsistent with the naïve ether theory, whereas the Sagnac effect is consistent with the naïve ether theory but inconsistent with the ballistic theory. Of course, both results are consistent with fully relativistic theories of Lorentz and Einstein, since according to both theories light is propagated at a speed independent of the state of motion of the source.
http://www.mathpages.com/rr/s2-07/2-07.htm

Last edited by a moderator:
cfrogue said:
Hopefully, you understand MMX does not prove the light postulate.
This is a common misconception.

MMX is consistent with light emission theory.
Other experimental evidence is inconsistent with the emission theory, though.

JesseM said:
Other experimental evidence is inconsistent with the emission theory, though.

Also, Sagnac is inconsistent with emission theory.

This rules out any conceptually coherent ballistic theory of light propagation, according to which the speed of light is the vector sum of the velocity of the source plus a vector of magnitude c. Ironically, the original Michelson-Morley experiment was consistent with the ballistic theory, but inconsistent with the naïve ether theory, whereas the Sagnac effect is consistent with the naïve ether theory but inconsistent with the ballistic theory. Of course, both results are consistent with fully relativistic theories of Lorentz and Einstein, since according to both theories light is propagated at a speed independent of the state of motion of the source.
http://www.mathpages.com/rr/s2-07/2-07.htm

cfrogue said:
This paper shows emission theory is consistent with SR.
http://arxiv.org/PS_cache/physics/pdf/0512/0512036v1.pdf
I didn't try to analyze this paper too carefully but I get a crackpotty feel from it, note on p. 4 where he writes "Einstein’s concept of a clock that one tick of the clock always represents one second in moving inertial frames is wrong and leads to the clock paradox or twin paradox as pointed out in Ref.[9]." (Ref. 9 being another paper by the same author) The standards for getting a paper on arxiv.org aren't so high, there's a lot of stuff on there that would never pass peer review. In any case, on pp. 4-5 the author says that the notion of light's velocity depending on the source only makes sense if you are using the "quasi-velocity" rather than the normal coordinate velocity in a given frame, the quasi-velocity apparently being defined by change in position in the moving frame divided by time in the stationary frame.
Now, we see the light velocity c (m/s), instead of light quasi-velocity (m/tick), is independent of the relative velocity due to the relativity of simultaneity. Since Ritz’s emission theory [8] involves the light quasi-velocity (m/tick) dependent of the source, the theory is consistent with the postulate of constancy of light velocity except the Lorentz factor 1/gamma.

JesseM said:
I didn't try to analyze this paper too carefully but I get a crackpotty feel from it, note on p. 4 where he writes "Einstein’s concept of a clock that one tick of the clock always represents one second in moving inertial frames is wrong and leads to the clock paradox or twin paradox as pointed out in Ref.[9]." (Ref. 9 being another paper by the same author) The standards for getting a paper on arxiv.org aren't so high, there's a lot of stuff on there that would never pass peer review. In any case, on pp. 4-5 the author says that the notion of light's velocity depending on the source only makes sense if you are using the "quasi-velocity" rather than the normal coordinate velocity in a given frame, the quasi-velocity apparently being defined by change in position in the moving frame divided by time in the stationary frame.

Yes, I admit I do not like it either.

But, I supply it for a full comparison.

I believe we need to look at every angle and some are very suspect like this one.

For example, if Ritz's theory is consistent with SR, then why does light from moving light sources show no increase in the speed of light.

But, again, I need to supply what I have found.

JesseM said:
It's not a visual thing like parallax--when Dmitry67 said "you look at spacetime from a different angle" he presumably wasn't talking about what you see visually,
Yes, it is just an geometrical analogy:
- 3D space objects are projected onto our 2D retina
- 4D space-time objects are projected onto 3D space

## 1. What is Lorentz Contraction?

Lorentz Contraction, also known as length contraction, is a phenomenon in Einstein's theory of relativity where an object's length appears to decrease when it is moving at high speeds relative to an observer.

## 2. How does Lorentz Contraction work?

According to Einstein's theory, as an object moves closer to the speed of light, the distance between its front and back ends appears to shorten from the perspective of an observer. This is due to the warping of space and time at high speeds.

## 3. Why is it called "Lorentz Contraction"?

The phenomenon is named after Dutch physicist Hendrik Lorentz, who first proposed the concept of length contraction in 1892 as a way to reconcile the results of the Michelson-Morley experiment with the existing theories of electromagnetism.

## 4. Does Lorentz Contraction only apply to objects moving at the speed of light?

No, Lorentz Contraction is a relative concept and can occur at any speed close to the speed of light. However, the effects become more noticeable at higher speeds.

## 5. How does Lorentz Contraction impact our understanding of space and time?

Lorentz Contraction is one of the key principles of Einstein's theory of special relativity, which revolutionized our understanding of space and time. It shows that the laws of physics are the same for all observers, regardless of their relative motion, and that space and time are not absolute but are interconnected and can change depending on an observer's frame of reference.

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