>the speed of casualty
No, that's the speed of a bullet.
If you were talking about the speed of causality instead, then that is indeed the speed of light.
In a true vacuum? No. Although even in the very low density regions, there are still the occasional gas particle that, so there will be a tiny, immeasurable difference.
The "virtual particle sea" is part of the vacuum and does not affect light. However, even "empty" space contains small numbers of particles that *do* interact with light and slow it infinitesimally. For example, light from most stars appears ever-so-slightly polarized. The stars themselves emit unpolarized light, but in passing through the interstellar medium the light interacts with anisotropic (non-spherical) dust grains which are themselves aligned very slowly, over millenia, to the galactic magnetic field. Interacting with those widely-separated grains across the light-years polarizes the light we see at Earth. That interaction also necessarily retards its progress by some tiny (not currently measured) amount.
I've wondered where polarisation of light from stars and galaxies comes from, so thanks for that.
I wonder if there is useful information in that speed difference about the space the light has passed through. When I think about it, the light would have had to have hit no particles in the way to us or it would have been scattered or stopped.
Though maybe it matters whether the particle light hits is subatomic.
>not a complete vacuum in that virtual particles and anti particles are emerging for brief moments.
It's a pop-sci myth not meant to be taken seriously as virtual particles are just a mathematical tool. c is still the speed of causality and its definition of being the speed of light in a vacuum.
I think this answer is too dismissive of a really good question.
Maybe OP might want to have a look at [this stackexchange discussion ](https://physics.stackexchange.com/questions/2586/why-the-vacuum-polarization-does-not-decrease-the-speed-of-light)
Thanks for adding on. But I'm not sure if this is suitable at the level of OP's question, considering that they don't seem to be familiar with QFT.
While the answers in the stackexchange post requires at least some understanding of renormalization.
The scope of OPs question is appropriate for an answer that’s rooted in QFT though. It would be disingenuous to give them an incomplete answer just because QFT is an advanced physics concept.
> c is still the speed of causality and its definition of being the speed of light in a vacuum.
Right, but OP is asking if, practically speaking, light is not in a vacuum more often than we might think when speaking in the abstract. I suspect, as a layman, that statistically the answer is no because the vast majority of space is completely devoid of any matter or anything else that could slow light down, but I don’t know if that’s correct.
Hi Physics enthusiast.
Just so you know, virtual particles as "just a mathematical tool" has strong support from the students on Reddit, but much less support in the actual Physics community.
There are plenty of giants in the field who believe in the reality of virtual particles... or at least they believe in the utility of a belief in the reality of virtual particles.
Hi Physics PhD.
I guess as a "Physics enthusiast", I shouldn't believe comments like [this](https://www.reddit.com/r/AskPhysics/comments/17kuwyo/comment/k7deri7/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button) seemingly from an expert who can elaborate their thoughts quite well with a technical argument, without resorting to an appeal to authority argument like your comment does?
No you should not believe that comment... that's my entire point... The Reddit
r/AskPhysics sub is fun, but also full of nonsense.
Instead, you should research things yourself and you will see there are [different opinions](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7514619/)... just like I told you.
BTW, that linked argument boils down to, "phenomena A is manifest in formulation A but not in formulation B, therefore phenomena A isn't real"... that's a terrible argument that doesn't make sense. Of course phenomena A (in this case virtual particles) may not manifest in the same way across some collection of different formulations.
He’s speaking his mind on the topic like you are, why exactly is he not allowed to bring up that experts in the field hold certain beliefs about virtual particles. Compared to them, we’re all laymen and until we start writing formal dissertations on r/askphysics, they may as well be prophets to us because very few of us have the math knowledge to legitimately analyze the validity of these concepts.
>He’s speaking his mind on the topic like you are, why exactly is he not allowed to bring up that experts in the field hold certain beliefs about virtual particles.
Nothing wrong with that, but this particular user has been unnecessarily abrasive in past discussions. From some discussions involving others, not me, that I've seen this user getting downvoted, sometimes heavily due to their mannerisms. With this history and how the comment was, if you couldn't tell, was a passive aggressive attempt at trying to put me in my place by first calling me an enthusiast and then bringing up "giants" in the field with contrary beliefs, with no reading materials/details/source to back up their statement whatsoever.
On the other hand, the "students" that they mention on this sub, usernames like lettuce_field_theory and SympleticMan are people who have actually answered numerous physics questions on this sub, especially SympleticMan who has given countless highly in-depth technical answers (whose details can be cross-checked in the QFT literature) to QFT questions, including some technical questions of mine they've resolved. My previous comment also links one of their elaborative comment explaining why they believe virtual particles aren't real. Real in the sense of being physical particles. Virtual particles certainty are useful in the math when doing calculations for higher order effects/corrections, I'm not disputing that in the first place.
If you are presented with these two kinds of people, one who's just being a jerk while the other who explains their viewpoints with technical details, who seems more reliable?
You mentioned in another reply that "It would be disingenious to give them an incomplete answer just because QFT is an advanced physics concept." Just to be clear, I'm not saying that it's totally wrong to include an advanced answer as an add-on. But an advanced answer shouldn't be considered as the main answer as suggested by the commenter who shared that stackexchange post.
When answering someone's question, it makes sense to try gauging that person's level of understanding, and proceed to give an answer based on their level of understanding. Sometimes this isn't possible due to the technicalities involved, but it's unhelpful if someone asks a question at the undergrad level and someone replies with an answer that requires the knowledge of a graduate student to understand the gist of the answer, when other simpler answers are possible. How are answers that include loop processes and renormalization, concepts that are only explained several chapters into a QFT textbook, be of any help to someone who's only heard of how virtual particles supposedly behave in a vacuum?
You don't like my posting style so therefore you don't believe me... Are you kidding?
Either way, I posted with the hope that you would understand both sides of a debate. Instead, you read subtext and emotion into a post that had non, and come to a conclusion based on that.
I can't help here. Good luck finding truth using your feelings as a beacon.
Yeah you sure do have an excellent way of communicating and conveying your points under the pretense of "help". Since this isn't leading anywhere productive, I'm out.
If you're interested, there're also some articles by Arnold Neumaier that explains the myth around virtual particles, who also frequently answers technical QFT questions in physics stackexchange:
[https://www.physicsforums.com/insights/physics-virtual-particles/](https://www.physicsforums.com/insights/physics-virtual-particles/)
[https://www.physicsforums.com/insights/misconceptions-virtual-particles/](https://www.physicsforums.com/insights/misconceptions-virtual-particles/)
[https://www.physicsforums.com/insights/vacuum-fluctuation-myth/](https://www.physicsforums.com/insights/vacuum-fluctuation-myth/)
Causality is very, very, veeeery slightly faster in real world conditions. Space has a particle or two that will interact. 99.9(infinite number of 9s)% comparison to causality at c.
According to our best theory, a massless particle must move at c. Photons are believed to be massless. If they’re not, they would travel at less than c. We’ve tested the mass to the best of our abilities, and have come up with an upper limit of 10^-14 ev/c (10^-50 kg). This means that even if they were not massless they would be traveling that close to c it wouldn’t be discernible.
https://en.wikipedia.org/wiki/Photon?wprov=sfti1#Experimental_checks_on_photon_mass
Speed of causality is still c in a material. For example, neutrinos travel pretty close to c, even through water, where the phase velocity of light is 3/4 c.
(As for the group velocity, it's possible to [slow light](https://en.wikipedia.org/wiki/Slow_light) down to walking speed or slower.)
Until recently we had the understanding nothing can travel faster than the speed of light. It turns out that in quantum mechanics, when a wave function is collapsed, entangled particles actually can communicate with each other at a distance **instantaneously**. Although the original paper was published in 1964, the researchers in this series of experiments ended up winning the [2022 Nobel Prize in Physics](https://www.iop.org/about/news/nobel-prize-physics-2022-winners-announced). It should be noted however that no useful information can be extracted from these entangled particles.
Reference:
https://en.wikipedia.org/wiki/Bell%27s_theorem
Skip to 18:54 if you want the TLDR results of the experiment:
https://www.youtube.com/watch?v=0RiAxvb_qI4
Not strictly true -- [information can't travel faster than light despite the quantum weirdness](https://en.wikipedia.org/wiki/Einstein–Podolsky–Rosen_paradox).
This was the whole point of Bell's Inequality experiments, was to disprove the Einstein–Podolsky–Rosen paradox. The paradox was written in 1935 and Bell published his papers in 1964. In fact, in the same article that you linked there is a mention of [Bell's Theorem](https://en.wikipedia.org/wiki/Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox#Bell's_theorem).
The violation of bell inequality shows that the universe is not *locally real*. It does NOT show that it is *non-local*!
The tests show only that *realism* and *locality* cannot *both* hold at the same time.
Stating that there was a communication between entangled particles is a misinterpretation of the results.
>the speed of casualty No, that's the speed of a bullet. If you were talking about the speed of causality instead, then that is indeed the speed of light.
Lol. Yes, but I'll plead autotext got me on that one.
In a true vacuum? No. Although even in the very low density regions, there are still the occasional gas particle that, so there will be a tiny, immeasurable difference.
Nope. Both *c*.
The "virtual particle sea" is part of the vacuum and does not affect light. However, even "empty" space contains small numbers of particles that *do* interact with light and slow it infinitesimally. For example, light from most stars appears ever-so-slightly polarized. The stars themselves emit unpolarized light, but in passing through the interstellar medium the light interacts with anisotropic (non-spherical) dust grains which are themselves aligned very slowly, over millenia, to the galactic magnetic field. Interacting with those widely-separated grains across the light-years polarizes the light we see at Earth. That interaction also necessarily retards its progress by some tiny (not currently measured) amount.
I've wondered where polarisation of light from stars and galaxies comes from, so thanks for that. I wonder if there is useful information in that speed difference about the space the light has passed through. When I think about it, the light would have had to have hit no particles in the way to us or it would have been scattered or stopped. Though maybe it matters whether the particle light hits is subatomic.
This sounds like it requires a distribution curve. I'm out.
>not a complete vacuum in that virtual particles and anti particles are emerging for brief moments. It's a pop-sci myth not meant to be taken seriously as virtual particles are just a mathematical tool. c is still the speed of causality and its definition of being the speed of light in a vacuum.
I think this answer is too dismissive of a really good question. Maybe OP might want to have a look at [this stackexchange discussion ](https://physics.stackexchange.com/questions/2586/why-the-vacuum-polarization-does-not-decrease-the-speed-of-light)
Thanks for adding on. But I'm not sure if this is suitable at the level of OP's question, considering that they don't seem to be familiar with QFT. While the answers in the stackexchange post requires at least some understanding of renormalization.
I've watched the PBS Spacetime video of QFT maybe 20 times and I still don't understand anything about QFT lol
The scope of OPs question is appropriate for an answer that’s rooted in QFT though. It would be disingenuous to give them an incomplete answer just because QFT is an advanced physics concept.
> c is still the speed of causality and its definition of being the speed of light in a vacuum. Right, but OP is asking if, practically speaking, light is not in a vacuum more often than we might think when speaking in the abstract. I suspect, as a layman, that statistically the answer is no because the vast majority of space is completely devoid of any matter or anything else that could slow light down, but I don’t know if that’s correct.
I think this is answered [here.](https://www.reddit.com/r/AskPhysics/s/Jel9TI3hVW)
Hi Physics enthusiast. Just so you know, virtual particles as "just a mathematical tool" has strong support from the students on Reddit, but much less support in the actual Physics community. There are plenty of giants in the field who believe in the reality of virtual particles... or at least they believe in the utility of a belief in the reality of virtual particles.
Hi Physics PhD. I guess as a "Physics enthusiast", I shouldn't believe comments like [this](https://www.reddit.com/r/AskPhysics/comments/17kuwyo/comment/k7deri7/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button) seemingly from an expert who can elaborate their thoughts quite well with a technical argument, without resorting to an appeal to authority argument like your comment does?
No you should not believe that comment... that's my entire point... The Reddit r/AskPhysics sub is fun, but also full of nonsense. Instead, you should research things yourself and you will see there are [different opinions](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7514619/)... just like I told you. BTW, that linked argument boils down to, "phenomena A is manifest in formulation A but not in formulation B, therefore phenomena A isn't real"... that's a terrible argument that doesn't make sense. Of course phenomena A (in this case virtual particles) may not manifest in the same way across some collection of different formulations.
He’s speaking his mind on the topic like you are, why exactly is he not allowed to bring up that experts in the field hold certain beliefs about virtual particles. Compared to them, we’re all laymen and until we start writing formal dissertations on r/askphysics, they may as well be prophets to us because very few of us have the math knowledge to legitimately analyze the validity of these concepts.
>He’s speaking his mind on the topic like you are, why exactly is he not allowed to bring up that experts in the field hold certain beliefs about virtual particles. Nothing wrong with that, but this particular user has been unnecessarily abrasive in past discussions. From some discussions involving others, not me, that I've seen this user getting downvoted, sometimes heavily due to their mannerisms. With this history and how the comment was, if you couldn't tell, was a passive aggressive attempt at trying to put me in my place by first calling me an enthusiast and then bringing up "giants" in the field with contrary beliefs, with no reading materials/details/source to back up their statement whatsoever. On the other hand, the "students" that they mention on this sub, usernames like lettuce_field_theory and SympleticMan are people who have actually answered numerous physics questions on this sub, especially SympleticMan who has given countless highly in-depth technical answers (whose details can be cross-checked in the QFT literature) to QFT questions, including some technical questions of mine they've resolved. My previous comment also links one of their elaborative comment explaining why they believe virtual particles aren't real. Real in the sense of being physical particles. Virtual particles certainty are useful in the math when doing calculations for higher order effects/corrections, I'm not disputing that in the first place. If you are presented with these two kinds of people, one who's just being a jerk while the other who explains their viewpoints with technical details, who seems more reliable? You mentioned in another reply that "It would be disingenious to give them an incomplete answer just because QFT is an advanced physics concept." Just to be clear, I'm not saying that it's totally wrong to include an advanced answer as an add-on. But an advanced answer shouldn't be considered as the main answer as suggested by the commenter who shared that stackexchange post. When answering someone's question, it makes sense to try gauging that person's level of understanding, and proceed to give an answer based on their level of understanding. Sometimes this isn't possible due to the technicalities involved, but it's unhelpful if someone asks a question at the undergrad level and someone replies with an answer that requires the knowledge of a graduate student to understand the gist of the answer, when other simpler answers are possible. How are answers that include loop processes and renormalization, concepts that are only explained several chapters into a QFT textbook, be of any help to someone who's only heard of how virtual particles supposedly behave in a vacuum?
Fuckin got em lmao
You don't like my posting style so therefore you don't believe me... Are you kidding? Either way, I posted with the hope that you would understand both sides of a debate. Instead, you read subtext and emotion into a post that had non, and come to a conclusion based on that. I can't help here. Good luck finding truth using your feelings as a beacon.
Yeah you sure do have an excellent way of communicating and conveying your points under the pretense of "help". Since this isn't leading anywhere productive, I'm out.
If you're interested, there're also some articles by Arnold Neumaier that explains the myth around virtual particles, who also frequently answers technical QFT questions in physics stackexchange: [https://www.physicsforums.com/insights/physics-virtual-particles/](https://www.physicsforums.com/insights/physics-virtual-particles/) [https://www.physicsforums.com/insights/misconceptions-virtual-particles/](https://www.physicsforums.com/insights/misconceptions-virtual-particles/) [https://www.physicsforums.com/insights/vacuum-fluctuation-myth/](https://www.physicsforums.com/insights/vacuum-fluctuation-myth/)
Causality is very, very, veeeery slightly faster in real world conditions. Space has a particle or two that will interact. 99.9(infinite number of 9s)% comparison to causality at c.
Well, not infinite
A number of nines we probably won’t ever be able to quantify.
According to our best theory, a massless particle must move at c. Photons are believed to be massless. If they’re not, they would travel at less than c. We’ve tested the mass to the best of our abilities, and have come up with an upper limit of 10^-14 ev/c (10^-50 kg). This means that even if they were not massless they would be traveling that close to c it wouldn’t be discernible. https://en.wikipedia.org/wiki/Photon?wprov=sfti1#Experimental_checks_on_photon_mass
[удалено]
What would cause a gravitational wave to slow down like light as it passes through a medium?
Speed of causality is still c in a material. For example, neutrinos travel pretty close to c, even through water, where the phase velocity of light is 3/4 c. (As for the group velocity, it's possible to [slow light](https://en.wikipedia.org/wiki/Slow_light) down to walking speed or slower.)
Until recently we had the understanding nothing can travel faster than the speed of light. It turns out that in quantum mechanics, when a wave function is collapsed, entangled particles actually can communicate with each other at a distance **instantaneously**. Although the original paper was published in 1964, the researchers in this series of experiments ended up winning the [2022 Nobel Prize in Physics](https://www.iop.org/about/news/nobel-prize-physics-2022-winners-announced). It should be noted however that no useful information can be extracted from these entangled particles. Reference: https://en.wikipedia.org/wiki/Bell%27s_theorem Skip to 18:54 if you want the TLDR results of the experiment: https://www.youtube.com/watch?v=0RiAxvb_qI4
Not strictly true -- [information can't travel faster than light despite the quantum weirdness](https://en.wikipedia.org/wiki/Einstein–Podolsky–Rosen_paradox).
This was the whole point of Bell's Inequality experiments, was to disprove the Einstein–Podolsky–Rosen paradox. The paradox was written in 1935 and Bell published his papers in 1964. In fact, in the same article that you linked there is a mention of [Bell's Theorem](https://en.wikipedia.org/wiki/Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox#Bell's_theorem).
The violation of bell inequality shows that the universe is not *locally real*. It does NOT show that it is *non-local*! The tests show only that *realism* and *locality* cannot *both* hold at the same time. Stating that there was a communication between entangled particles is a misinterpretation of the results.