Why No One Has Measured The Speed Of Light

I've heard it claimed that we can only measure the round-trip speed of light, not the one-way speed of light, because the maths says that reality would look identical if it was 0.5c in the x+ direction and ∞ in the x- direction. I find this hard to stomach, but I'm going to trust it also applies to e.g. magnetism being Lorenz transformed electric fields, because relativity violates "common sense" all over the place and reality doesn't care about my stomach. ▶ this video

>The speed of light is obviously the same either way It's not obvious that it is :) We don't have any way to prove that it is the same, because every experiment to measure the speed of light going from A to B requires some light to go from B to A which cancels out any difference. We just assume that it is the same because we don't have any reason to believe it's not. Veritasium video on this topic: ▶ this video

> light always move on the same speed, on any direction and for every observer has been tested again and again I might be wrong or misunderstanding, but I think that hasn't actually been tested ever. Only two-way speed of light is known. [1] [1] ▶ this video

The video in question for reference: link Interestingly, when I saw that I figured this channel for a goner because he was going to turn to the darkside, but quite contrary to expectations I feel like his channel has actually blossomed since then. His older videos are fine, but often just explaining concepts that you can find a lot of other explanations on, but some of his recent videos have rocked the boat even among physicists and IMHO he was ultimately proved to be correct for them. I particularly point at his videos about how nobody has measured the one-way speed of light ( ▶ this video ), which was well-known among people very very deep in the field of relativity but not very well known outside of that (even amongst physicists in other fields, let alone lay people) and the electricity videos he's done in the last year ( link , link ) which attracted a whole whackload of "debunking" videos, which were themselves wrong (the best "bunking" video, if I may call it that, is probably AlphaPheonix's: link , since he actually set up the experiment, albeit on a smaller scale). They aren't all of that caliber, but the knot theory video was very good too and not something I've seen a hundred times elsewhere, and the airships video that preceded that one clearly had a high degree of research put into it beyond just the hype.

There is a cool video by Veritasium about the specifics of how the speed of light is actually measured (1). I wasn’t aware that only the two way speed of light was measured so far, not the one way. Which is probably very irrelevant but still quite interesting. (1) ▶ this video

Fields aren't quantised, that's the problem , that's why gravity can't be quantized. It's a field! Immediately, dozens of people are about hit the reply button and say something like: "But all of modern physics is based on quantization!" or something to that effect. The issue is that there's two ways to think of quantization, and for almost all practical experiments, there's no way to distinguish between the two. Essentially, you can think of either the fields being quantized, or the interactions with fields being quantized. Unfortunately almost all experiments measure only interactions with fields, so there's few practical ways to distinguish between the two. The mathematics is largely equivalent as well, so physicists "picked one" of the two options, and forgot about the other, equally valid option. This is similar to the Veritasium video titled "Why No One Has Measured The Speed Of Light".[1] From inside the universe, it's basically impossible to measure the one-way speed of light, all experiments measure the two-way speed of light. So... we just assume that it's the same speed both ways. Right now, that works well enough. If it stops working , then we need to revisit that assumption instead of devising ever more complex mathematics to explain away our faulty assumption. One difference between the "fields are quantized or not" issue and the "one-way speed of light" issue is that the former is testable. It's just that most experiments don't happen to test it. Any experiment

This video reminds me of the Veritasium video on the impossibility of measuring light in one direction: ▶ this video We don't KNOW for a fact that light travels the same speed in both directions when we measure it, but it seems likely.

Ex-physicist here. This is not true. See ▶ this video for an explanation. You can have a spatially asymmetric speed of light and be perfectly in line with every experiment to date. The speed of light appearing constant in every inertial reference frame is experimentally verified and measured. But it's an axiom that the speed of light has no spatial preference. Each measurement of the speed of light sneaks in this axiom in subtle ways. I think you misinterpreted the parent's point. They weren't saying c being a constant in all reference frames is an axiom. Rather, they were saying that it's convention that c doesn't have a spatial, directional, preference. It's a different claim.

We've never actually measured the speed of light in one direction. It's impossible. Vertasium video on the subject - ▶ this video

I think he is easily one of the smartest people on YouTube and is producing some of the very best videos. He is also a brilliant educator. For examples, his videos on special [1] and general relativity [2] were real eye-openers for me, even though I have studied physics and have seen this stuff explained a million times before. What I really don't like are the new video titles. At some point he started A/B testing them to maximize his revenue. He even made a video about it [3]. Of course, this selects for the most outrageous clickbait in order to get that sweet engagement. I am starting to wonder if he took into account, how much this is starting to alienate long-term followers. [1] ▶ this video [2] link [3] link

I love Veritasium. Derek has done a number of videos that are awesome like this. The videos are great and educational, but he also manages to subtly weave in an almost spiritual subtext to them as well. For example, in this one, you learn about imaginary numbers in some cool ways, history, how awesome e^ix is, etc, but also this yen-yang balance between embracing not only what is real but taking a chance on what is imaginary. Some of my other favorites: - link - link - link - link - ▶ this video I think he’s going to lose the $10,000 bet on the car though.

Not entirely related, but a fun and interesting tangent: There's actually no way that we know of to measure the "one-way" speed of light, as the specific synchronization that you use (and this post uses to do its calculation) assumes that the speed of light is the same in both directions. For all we know, light travels infinitely quickly in one direction and at c/2 in the return direction. link recent-ish video about this: ▶ this video

Relative time (in five minutes) without relativistic speeds is actually uniform. The is no observable difference to either participant. And there is no concept of now in a significantly distant location. Related video: ▶ this video

There is a Veritasium video really fun to watch which exmplains with examples why you cannot measure one way speed of light: ▶ this video

Veritasium recently published a video [1] on why the speed of light can't be measured one way, only two ways (i.e. the speed it travels from point A to point B then back to point A). While I'm not privy to all the physics details of it, I found the video very compelling even as a thought experiment. [1]: ▶ this video

Veritasium just had a good video which discusses this: ▶ this video It turns out there's some circularity in your definitions of "Move clocks at the exact same speed and acceleration in opposite directions" and "synchronized"; that's basically the definition of "synchronized", which is fine from a certain point of view, but from the point of view of examining the nature of space and time themselves, not adequate.

You overstate it by calling it a theory. I'm sure you just watched the Veritasium video [1] on this very subject. The point wasn't to claim that space has a preferred direction (which would contradict known physics) but highlighting that there is no way to prove the speed of light is uniform, only that the round trip speed is 'c'. Second, even if the speed of light is unbounded (or has a much higher bound) in one direction, that doesn't give a free ride for objects with mass to travel any faster than they currently do. It would take an incredible amount of energy to accelerate such rocks to near/above the constant 'c' and then you'd need to explain how it returns to normal asteroid speeds before crashing into Earth. [1] ▶ this video