Comment by DenisM | Hacker News Reader

Comment by DenisM | Hacker News Reader

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There was an experiment where an impulse of light came out the other end of a material faster than light could have traveled through vacuum in the same space. The eventual explanation was as follows - imagine that your outgoing bunch of photons looks like this: :::... The first three photons are "invisible" because there is so few of them that they are below the equipment sensitivity. The second group of six particles is more dense and so they are visible to equipment. Hence, it is deemed that the light has "entered" the material when the second group has entered, long past after the first one actually did. Having entered the material the first group has triggered release of energy from the material, and the second group was partially consumed by the material (energizing it for the next time around). The outgoing bunch looked like this: ...::: And this time it was the first group of photons that was detected. So the apparent speed of the beam was higher than the speed of light. However that's not because the same particles traveled faster than light, but because the peak energy of the entire bunch has shifted forward during travel. If you try hard enough, the light will have "exited" the material before it has "entered".

Similar thing could be happening here.

That is such a well known result in physics that the likelihood of it occurring here is almost zero. I remember first reading about it and the scientists basically said "we broke the speed of light" without looking for further explanations. These guys seem like they generally don't believe that they have broken the light barrier, but they need help figuring out what they actually did do from the broader community. Due to their very humble attitude, I bet they examined the literature and ruled this particular possibility out very early on.

I bet you there is a systematic error, but I don't think it's this one.

My general rule with physics is that if I can think of it, then a real physicist will laugh at it's triviality.

What DenisM is describing is not the result of a systemic error. It is not an error at all. 'Something' is travelling faster than light in that experiment. The problem is that the 'something' isn't a physical object.

The common example is the beam of a lighthouse. Suppose a lighthouse revolves once per minute. At one lightyear distance from the lighthouse, the angular velocity of the beam is 2*Pi lightyears per minute, which is much faster than the speed of light. However, this is not a problem: the beam is not a physical entity. There is no single particle actually travelling faster than the speed of light. It's just some construct in our minds to which we assign that velocity.

Something similar is happening in the experiment described by your parent and possibly in the experiment in the linked article.

Perhaps I misinterpreted, but I think DenisM was talking about something completely different. He mentioned that because of a lack of density in the leading photons, the first piece of equipment missed the measurements while the second one picked it up. That would a problem with the system they're measuring with, not at all to do with treating light as a single entity.

As I interpreted it, the critical part of his explanation is the fact that the higher density part shifted from the back to the front. If that change in density would be due to actual photons shifting that way, then those photons would have traveled faster than light (whereas the entire 'blob', on average, traveled exactly at the speed of light).

Could be explained if the photos leaving the source look like this: :::::..... and arrive like this: :::::.::, where the 'tip' folds back on itself to become visible, still before the original bulk of the photon cluster.

At one lightyear distance from the lighthouse, the angular velocity of the beam is 2Pi lightyears per minute*

Actually, that's the linear velocity. The angular velocity would be 2*pi radians per minute and is independent of distance.

I have used the word "Similar" in the most loose meaning of this word, merely to suggest to the audience that "faster than light thing has been observed" does not have to mean that the existing model is broken.

Phase velocity vs group velocity.

But it's pretty easy to check that everything is alright with group velocity. If there is zero neutrino production at t=0 (in the reactor's frame of reference), and then the production jumps up, there can be no doubt about the group velocity.

Bizarre effects relative to superluminal phase velocity only appear when you start generating particles before t=0 and you mess with the medium in between. Then the "superluminal" illusion is created by quanta created before t=0.

TLDR: I find it bizarre that seasoned professionals could be fooled by phase velocity or a similar effect.

Phase velocity vs group velocity.

Yes, thank you. Why did it take me so many words? :)

I'm glad that it did take you so many words. I remember reading about this a while back, but could never remember what it was called, or why the light particles actually didn't break the speed barrier. I really appreciate your laymen explanation, and now, thanks to you parent, also know what the effect is called.

Think of the wagon wheels, as seen in a western movie, turning back, or moving at bizarre speeds in either direction. It's not a perfect analogy, but it's a related phenomenon.

That's called aliasing.

Well, you gave the layman's explanation. Not everyone is well versed in these specific terms.

I'm not familiar with that experiment, but it doesn't sound to me like information traveled faster than the speed of light, which is really the important part of the speed of "light."

The fun thing is some physicists manipulated group velocities to create time lenses

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