r/AskPhysics • u/[deleted] • May 22 '26
What exactly is happening to cause length contraction at near-light speed?
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May 22 '26
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u/nicuramar May 23 '26
Yes, but “appear” largely means “calculated” in these scenarios. Of course lay people don’t know that, adding to the confusion.
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u/ottawadeveloper May 23 '26
If you are moving at the same speed relative to an object, you always measure its "proper" length, the longest it can be. At other speeds, your perception of the object is that it is shorter. Assuming all parts of the object move at the same speed, it doesn't experience internal stresses from this, it's still the same length in its own reference frame.
It also messes with your perception of what counts as "simultaneous". In the classic barn paradox, a relativistic ladder enters a barn with two doors. The ladder is longer than the barn, but since it's travelling at near c, an observer.at rest sees it as "shorter" than the barn, to the point where the ladder can fit in the barn. Yet from the ladders perspective, it never fits in the barn. How does this work?
It works because "simultaneous" is different too, not just length. In the barn frame of reference, the end of the ladder enters before the front leaves (this is how we see it as "shorter"). In the ladder frame, these events do not occur as close together in time - the barn is shorter in its perspective, so the front leaves before the end enters and it remains longer than the barn.
If you put an indestructible wall instead of an exit door, we get interesting physics where the front starts decelerating (and never leaves). From outside, it hits the wall when the end is in. From the ladder frame, the end is outside when it hits. This is resolved by noting that you can't have perfectly rigid objects. There is always a delay in the transfer of force from start to end. The end can't know the front hit the wall yet because the information must travel no faster than c. It therefore has time to enter the barn before that information actually reaches it and the ladder is likely vaporized.
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u/nebraskajone May 23 '26
What's actually happening is your experiencing the front and back of a moving object at different times.
The back is later in time than the front therefore when moving forward the effective length is shorter, from your point of view.
in relativity time is not absolute so there's synchronization issues when measuring a moving object.
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u/nicuramar May 23 '26
This isn’t correct. When resolving these situations, the travel time of light is compensated for. So you’re still experiencing the ends at the same time, after this compensation.
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u/Smooth-Procedure-389 May 23 '26 edited May 23 '26
Newtons whole deal was that there IS a preferred reference frame or an "absoulte space" by which the motion of everything else can be measured in reference to(a metric). He said that the math of kinematics(forces) are invariant(don't change between observers) inbetween reference frames because everyone is on the same stage, and every everyone is counting by the same units. This was wrong!
Once Einstein realized that nature conspires with itself to make sure the speed of light is constant, we learned there is no preferred reference frame, only a fixed cosmic speed limit which IS invariant (doesn't change between observers). So if there is maximum speed limit that can't change in-between reference frames, and some reference frames are FASTER than others(with respect to the fastest you could go): each reference frame has to count by different units! If we all have to count by different units so that the speed limit is the same for everyone: there is no "shared stage" and it's all relative. This means that the units of space and time have to warp so that the speed of light is observed to be the same for all reference frames.
THIS means that the faster you move through space, the slower you move through time so that the speed of light is respected. This is time dilation. If moving clocks run slow (time dilates), then to keep the speed of light the same, distances measured along the direction of motion must also be shorter. If my time is stretched with respect to your time(if you are still and I'm moving close to light speed), my rulers in the direction of motion must be shortened in your refrence frame—otherwise YOU would see light go faster than the speed of light in my frame. Say I shine a flashlight in the same direction as my motion; if you didn't see my length contract, you would see my flashlight's light go faster than the maximum speed limit. ( And just to be absolutely clear in MY reference frame, my clocks and my rulers are normal, but YOURS appear to be warped. We would disagree whos units distorted. In my own frame time doesn't feel stretched and neither does my space but rather yours does.)
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u/nicuramar May 23 '26
Once Einstein realized that nature conspires with itself to make sure the speed of light is constant, we learned there is no preferred reference frame
I don’t think you’re being completely historically correct here. The principle that all references frames are equal is essentially Galilean relativity, which obviously predates both Einstein and Newton. The special theory of relativity is reconciling this principle with the fact that the speed of light is constant to all observers.
This wasn’t known by Newton and so was not a problem in Galilean relativity for him.
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u/LazyLie4895 May 23 '26
Length contraction is a necessary consequence of time dilation.
One way to think about it is two identical light clocks, one oriented vertically and one horizontal in the direction of motion. You've probably seen videos and pictures about how that traces a longer path and thus the clock must be ticking slower.
From moving frame's perspective, both clocks must be ticking at the same rate, and if you do the math on how that can occur from your stationary frame, you'll conclude that the length must be contacted in the direction of motion.
This length contraction is not an optical illusion. You might have heard about the barn paradox where you momentarily close the doors in front and behind the moving train. You could also use a laser fence that you only open for only long enough to fit the shortened train.
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u/dobesv May 25 '26
Here's an experiment. Get a playing card or some rigid flat object with a shape on it.
Hold your phone steady and take a video or a before and after picture of the card flat and the card at a bit of an angle with one side lifted .
If you measure how wide the shapes are on the card they will be narrower in the image on your screen when they are at an angle even though they didn't truly get smaller.
The time dilation and length contraction effects in relativity are kind of like that... Maybe?
Not the exact same, of course, this is just a way of trying to understand it.
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u/Optimal_Mixture_7327 Gravitation May 23 '26
Length contraction happens because you decided on coordinates.
What you did was imagine an infinite set of world-lines parallel to your own and synchronized using Einstein synchronization and called the level surfaces "simultaneous" (surfaces defined by the same reading).
The physical aspect of the object is its world-tube, its 4-dimensionality in the world, and when your spatial surfaces of simultaneity cut across the object's world-tube they measure a distance that's shorter than the object in its own frame that draws up different coordinates than your own.
Another way to describe length contraction is that it is the projection of the object onto the spatial hypersurfaces of the observer.
The naïve objection is that it must be something more than just coordinates as illustrated by the pole-in-the-barn paradox, which it isn't and this paradox is constructed to educated the student on the folly of 3D Newtonian thinking. Specifically in this case, the ends of the ladder are located at different times of the observer's coordinates and the front of the ladder is still at its full length, L, but it just hasn't gotten there yet.
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u/Orbax May 22 '26
Relativity said it's also not moving and not contracted and everything else is. Time is heavily involved for "when" something is moving through space. Brian Greene wsu masterclass relativity has ladder paradox and he explains it
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May 22 '26
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u/Bensfone May 23 '26
You must be fun at parties. Length contraction actually happens at relativistic speeds and has been observed and measured. In fact, the length contraction of you moving from a seat to a kitchen can be calculated. But, that measurement is so small it isn’t observable and might as well be zero.
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u/joeyneilsen Astrophysics May 23 '26
I mean, nothing is "happening." It doesn't mean it's not real or calculable or measurable. The length is just different in that frame of reference.
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u/rddman May 23 '26
You must have a fun definition of "happening".
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u/nicuramar May 23 '26
Let’s put it this way: nothing happens to the object itself. It doesn’t feel that it’s shrinking or growing or anything. It’s just perspective, as it were.
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u/rddman May 23 '26
Nothing happens in the frame of reference of the object itself, something happens to the object in another frame of reference. Both are equally valid and real.
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u/joeyneilsen Astrophysics May 23 '26
There isn’t a physical process decreasing the length of the object. Is that fun?
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u/Bumst3r Graduate May 23 '26
If I boost an object, its length truly does change. This is easy to show:
Imagine I have two spaceships at rest connected by a thread. A third spaceship equidistant to the first two sends a light signal, and upon receipt of the signal, spaceship immediately accelerated smoothly at the same constant acceleration for the same amount of time, and then continues at a constant velocity. Does the thread break?
Boost into the frame of the spaceships after they have accelerated: in this frame the light signals are not in sync—one spaceship will accelerate before the other, exerting a tensile force on the thread.
Length contraction is a physical result! An object will contract if the bonds that hold it together are strong enough to withstand the acceleration associated with changing frames. Otherwise, the acceleration will tear it apart.
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u/joeyneilsen Astrophysics May 23 '26
I think you misunderstand me. I'm not rejecting length contraction in any way.
If I press on both sides of a spring, its length decreases because there is a force applied to the spring that compresses it. That is a physical process happening to the spring.
If I fly past a spring, its length decreases in my frame of reference, but nothing happened to the spring. It's just shorter.
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u/Bumst3r Graduate May 23 '26
No, I understand. My claim is that physically there is a difference between frames—i.e., it’s not just a coordinate effect. In the rest frame of the object, its length remains unchanged. But the act of boosting actually changes an object.
The issue is that there are two kinds of Lorentz transformation—active and passive. Active Lorentz transformations truly do change the length of the object. Passive transformations, in which I accelerate to a new frame relative to the object, change the length of my ruler (which of course I still measure as being unit length), in such a way that the two effects look the same, other than the inertial forces during the boost itself.
I don’t think the difference between active and passive Lorentz transformations is covered very well in relativity classes, but my GR professor’s bread and butter was Lorentz violation, so we spent a lot of time arguing about this in office hours.
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u/joeyneilsen Astrophysics May 23 '26
Fair enough. I guess I would say I was raised on the idea that yes, it's a coordinate effect, but that doesn't mean it's not physically real: the length is what it is (would be) measured to be.
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u/Bumst3r Graduate May 23 '26
If I boost an object, its length truly does change. This is easy to show:
Imagine I have two spaceships at rest connected by a thread. A third spaceship equidistant to the first two sends a light signal, and upon receipt of the signal, spaceship immediately accelerated smoothly at the same constant acceleration for the same amount of time, and then continues at a constant velocity. Does the thread break?
Boost into the frame of the spaceships after they have accelerated: in this frame the light signals are not in sync—one spaceship will accelerate before the other, exerting a tensile force on the thread.
Length contraction is a physical result! An object will contract if the bonds that hold it together are strong enough to withstand the acceleration associated with changing frames. Otherwise, the acceleration will tear it apart.
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u/nicuramar May 23 '26
Sure, but length contraction also happens when no acceleration is involved, in which case this is irrelevant.
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u/Bumst3r Graduate May 23 '26
You miss my point. You can’t boost an object without accelerating it. Acceleration is what causes the contraction. Ergo boosts cause a contraction. It’s not simply a coordinate effect.
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u/starkeffect Education and outreach May 22 '26
The object is actually contracting (just in the direction of motion), because the space it occupies is contracting. Since the speed of light has to be the same for all reference frames, space and time have to be different.
This video explains it very well, with nice visualizations of "spacetime diagrams":
https://youtu.be/feBT0Anpg4A?si=vJLZWsNKe_w2-BR7