How do 'moving rulers shrink'? The strange physics behind special relativity
Special relativity is beyond weird. Among its many statements are that moving clocks run slow and that moving rulers shrink. But how are we supposed to make sense of this?
Special relativity is beyond weird. Among its many statements are that moving clocks run slow and that moving rulers shrink. But how are we supposed to make sense of this? To understand the physics of relativity, we have to dip back in time a bit.
In 1865, James Clerk Maxwell discovered that what we call "light" is really waves of electricity and magnetism. But like all waves, these waves needed to wave through something. Sound waves move through air. Ocean waves move through oceans. So Maxwell believed light waves traveled through a substance known today as the appropriately oldtimey "luminiferous aether."
This "aether" (or "ether") had to have some strange properties. You couldn't feel it, touch it, smell it or otherwise sense it. So it had to be nearly invisible but also had to allow light to wave through it, so it had to exist. In the late 1800s, there were many debates about the nature of the aether, and it turns out, everybody was wrong.
Related: The doubleslit experiment: Is light a wave or a particle?
But they didn't know they were wrong until a pair of scientists decided to measure our movement through the aether in 1887. That pair was Albert Michelson, of the Case School of Applied Science, and Edward Morley, of Western Reserve University, who devised an experiment we now call the MichelsonMorley experiment at a place we now call Case Western Reserve University.
The basic idea is that if the aether exists, we should be swimming through it and we should notice this movement as a change in the speed of light.
The MichelsonMorley experiment tried to measure this and totally failed. So there was a problem. Light is a wave, and it must move through something — the aether. But we can't seem to measure our own motion through the aether. So what's going on?
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Lengthy contractions
Shortly after the MichelsonMorley experiment, physicist Oliver Heaviside noticed something funky: When electric charges are set in motion, their electric fields squish a little bit along the direction of that motion.
Then came Hendrik Lorentz, who had an absolutely wonderful thought: If we're all made of electric charges and the fields shrink when they move, then maybe we shrink when we move. So we can't measure changes in the speed of light because of length contraction — as we move through the aether, the speed of light changes, but so does our measurement apparatus, thus canceling it out.
This was considered a rather successful theory; it worked and explained all of the data. Matter squishes when it moves from some physical interactions, and the aether is there but undetectable.
Then, Einstein showed up and asked a very important question: If this aether is always and forever undetectable, then why do we need it? Why don't we just let things contract on their own — not to explain away some experimental result we don't like, but as a bare fact of the universe?
This is Einstein's big result. Other people were working in the direction of relativity, but nobody made the leap he did. Einstein declared length contraction to be a feature, not a bug, in the universe. No more aether, no more attempts to fit a square electromagnetic peg into a round aether hole. Lengths contract when they move. Period. End of discussion.
Einstein's length contraction was a little different from Lorentz's. For Lorentz, it was a physical effect, stuff smooshing together. For Einstein, it was a feature of space itself, independent of the actual objects. And this realization allowed Einstein to take yet another powerful leap.
The birth of relativity
To make it all work, Einstein realized there had to be some giveandtake. You can't just have length contraction — moving rulers shrink — on its own. You also need time dilation — moving clocks run slow. These always work together to allow all observations and all perspectives to make sense.
For example, take the humble muon, the heavier sibling of the electron. Because the muon is massive, it has a short life span — only 2.2 microseconds. When energetic particles strike air molecules in the upper atmosphere, they generate muons that then come streaking down toward the ground.
These muons travel at nearly the speed of light, but that's still not fast enough for them to reach the ground during their short lives. But relativity teaches us that moving clocks run slow — from our perspective, the muons persist much longer, so they have more than enough time to make the journey.
But the muon has a different perspective. It doesn't experience time dilation from its point of view, from which it will exist for only 2.2 microseconds. So how does the muon have enough time to reach the ground from its perspective? The answer on this side is length contraction — from the perspective of the speedy muon, the distance to the ground is much shorter, so it doesn't have that far to go.
Special relativity is the mathematical machinery we need to switch perspectives and keep everything organized. The universe may be crazy, but at least it follows rules we can understand.
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Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at UrbanaChampaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

Patrice Ayme Einstein published a rebuttal:Reply
The author unjustifiably stated a difference of Lorentz's view and that of mine concerning the physical facts. The question as to whether length contraction really exists or not is misleading. It doesn't "really" exist, in so far as it doesn't exist for a comoving observer; though it "really" exists, i.e. in such a way that it could be demonstrated in principle by physical means by a noncomoving observer.
— Albert Einstein, 1911... "Zum Ehrenfestschen Paradoxon. Eine Bemerkung zu V. Variĉaks Aufsatz". Physikalische Zeitschrift. 12: 509–510.
This being said, the situation is not as simple as it was long thought to be. The socalled "Bell Paradox" of 1976, basically published by Dewan and Beran first in 1959 (American Journal of Physics, 27, p 517) shows that length contraction, like time dilation, has a sort of absolute character which brings deeper questions as it seems to violate Galileo's Principle of Relativity at some extreme speeds (the basis of modern Relativity). 
Sandpoint A nice conventional summary of the ruler shrinking. It however is not correct. Consider 2 points separated in distance by a foot. Now accelerate them at the same rate and for the same time. In the non accelerated frame they are still a foot apart. Now if you ask the two points in their rest frame how far apart they are, and to do this using light, they will tell you they are more than a foot away. Now if those two points were materially connected they would think they had been stretched, and they will shrink until they appear to each other to be separated by a foot. At this point, in the unaccelerated coordinate system, they will appear to have shrunk. And the shrinkage would be precisely the amount that Einstein equations would have predicted.Reply 
Questioner I believe,Reply
Light traversing a mass field has less phase oscillations than it would have without it.
Because time is slower for an external viewer And the space is reduced in a mass field the light gets from point A to point B in the expected extrrnal time (although it may be redirected),
but it's a slightly 'younger', 'fresher' photon than otherwise. 
Questioner When space is relatively contracted or expanded,Reply
time slows or speeds up respectively to make space seem to be uniformly distributed per light's/EM's traversal of it. 
George²
So, in fact, distant galaxies are mistakenly thought to be moving away at enormous speeds. In fact, the light from them is shifted to the red sector due to the fatigue of passing through the gravitational and other fields along the way until it reaches us.Questioner said:I believe,
Light traversing a mass field has less phase oscillations than it would have without it.
Because time is slower for an external viewer And the space is reduced in a mass field the light gets from point A to point B in the expected extrrnal time (although it may be redirected),
but it's a slightly 'younger', 'fresher' photon than otherwise. 
billslugg When these photons get tired and assume lower energy levels, where does the excess energy go?Reply 
AboveAndBeyond
Into the expansion of space?billslugg said:When these photons get tired and assume lower energy levels, where does the excess energy go? 
billslugg There is no preferred direction in space. Such reradiated energy would be in all directions and we would receive some of it. Then we would see these photons in addition to the red shifted tired photons that we see. But we don't see anything in addition to them.Reply
In classical theory, the photons shift downward towards the red, but since space is expanding, there is room for more of them. The total amount of energy does not change. 
Atlan0001 Since when does the man standing next to the railroad track at rest on an Earth spinning at speed, orbiting the sun at speed, the solar system moving through galaxy and universe at speed, establish observations of speed, including observations of length to be all length on the spot at a distance, for all the universe in and at every single point of it?! What he observes at a distance being exactly what is local at the distance . . . the reality on the very spot at the distance , , . and at the very velocity he measures from his stand next to the railroad on the Earth at a distance?!Reply
Throw the principle of uncertainty, the principle of growing uncertainty in a growing complexity of chaos at distance, in a macrocosmic trash can! Throw relativity's prediction of its own breakdown at distance in a growing complexity of chaos in the trash can!
"The map is NOT the territory!" Well, apparently it is the territory according to far too many Earth bound 1dimensional seeing and thinking observers standing at rest by railroad tracks on Earth looking out through telescopes that resolve only the lightinformation in the light that arrives to them! 
william.walker39 The speed of light is not a constant as once thought, and this has now been proved by Electrodynamic theory and by Experiments done by many independent researchers. The results clearly show that light propagates instantaneously when it is created by a source, and reduces to approximately the speed of light in the farfield, about one wavelength from the source, and never becomes equal to exactly c. This corresponds the phase speed, group speed, and information speed. Any theory assuming the speed of light is a constant, such as Special Relativity and General Relativity are wrong, and it has implications to Quantum theories as well. So this fact about the speed of light affects all of Modern Physics. Often it is stated that Relativity has been verified by so many experiments, how can it be wrong. Well no experiment can prove a theory, and can only provide evidence that a theory is correct. But one experiment can absolutely disprove a theory, and the new speed of light experiments proving the speed of light is not a constant is such a proof. So what does it mean? Well a derivation of Relativity using instantaneous nearfield light yields Galilean Relativity. This can easily seen by inserting c=infinity into the Lorentz Transform, yielding the GalileanTransform, where time is the same in all inertial frames. So a moving object observed with instantaneous nearfield light will yield no Relativistic effects, whereas by changing the frequency of the light such that farfield light is used will observe Relativistic effects. But since time and space are real and independent of the frequency of light used to measure its effects, then one must conclude the effects of Relativity are just an optical illusion.Reply
Since General Relativity is based on Special Relativity, then it has the same problem. A better theory of Gravity is Gravitoelectromagnetism which assumes gravity can be mathematically described by 4 Maxwell equations, similar to to those of electromagnetic theory. It is well known that General Relativity reduces to Gravitoelectromagnetism for weak fields, which is all that we observe. Using this theory, analysis of an oscillating mass yields a wave equation set equal to a source term. Analysis of this equation shows that the phase speed, group speed, and information speed are instantaneous in the nearfield and reduce to the speed of light in the farfield. This theory then accounts for all the observed gravitational effects including instantaneous nearfield and the speed of light farfield. The main difference is that this theory is a field theory, and not a geometrical theory like General Relativity. Because it is a field theory, Gravity can be then be quantized as the Graviton.
Lastly it should be mentioned that this research shows that the Pilot Wave interpretation of Quantum Mechanics can no longer be criticized for requiring instantaneous interaction of the pilot wave, thereby violating Relativity. It should also be noted that nearfield electromagnetic fields can be explained by quantum mechanics using the Pilot Wave interpretation of quantum mechanics and the Heisenberg uncertainty principle (HUP), where Δx and Δp are interpreted as averages, and not the uncertainty in the values as in other interpretations of quantum mechanics. So in HUP: Δx Δp = h, where Δp=mΔv, and m is an effective mass due to momentum, thus HUP becomes: Δx Δv = h/m. In the nearfield where the field is created, Δx=0, therefore Δv=infinity. In the farfield, HUP: Δx Δp = h, where p = h/λ. HUP then becomes: Δx h/λ = h, or Δx=λ. Also in the farfield HUP becomes: λmΔv=h, thus Δv=h/(mλ). Since p=h/λ, then Δv=p/m. Also since p=mc, then Δv=c. So in summary, in the nearfield Δv=infinity, and in the farfield Δv=c, where Δv is the average velocity of the photon according to Pilot Wave theory. Consequently the Pilot wave interpretation should become the preferred interpretation of Quantum Mechanics. It should also be noted that this argument can be applied to all fields, including the graviton. Hence all fields should exhibit instantaneous nearfield and speed c farfield behavior, and this can explain the nonlocal effects observed in quantum entangled particles.
*YouTube presentation of above arguments: sePdJ7vSQvQ:0View: https://www.youtube.com/watch?v=sePdJ7vSQvQ&t=0s
*More extensive paper for the above arguments: William D. Walker and Dag Stranneby, A New Interpretation of Relativity, 2023: http://vixra.org/abs/2309.0145
*Electromagnetic pulse experiment paper: https://www.techrxiv.org/doi/full/10.36227/techrxiv.170862178.82175798/v1
Dr. William Walker  PhD in physics from ETH Zurich, 1997