# Is there a theoretical minimum to the Phase velocity of light?

**URL:** <https://boards.straightdope.com/t/is-there-a-theoretical-minimum-to-the-phase-velocity-of-light/953106>\
**Category:** Factual Questions\
**Created:** [October 24, 2021, 3:45pm UTC](https://boards.straightdope.com/t/is-there-a-theoretical-minimum-to-the-phase-velocity-of-light/953106 "2021-10-24T15:45:09Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\
**Post date:** [October 24, 2021, 3:45pm UTC](https://boards.straightdope.com/t/is-there-a-theoretical-minimum-to-the-phase-velocity-of-light/953106/1 "2021-10-24T15:45:09Z")

</div>

So diamond has the highest refractive index and presumably the phase velocity of light is reduced by a factor (about 2.4) of the refractive index of diamond.

Is there a theoretical minimum that a media can slow down light (phase velocity ) whether visible light or any other EM ?

Stated another way, Can some material exist in the universe that has a refractive index of 10 or 100 or 1000 … for any of the wavelengths of the EM spectrum ?

---

<div class="post-metadata">

**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [October 24, 2021, 5:35pm UTC](https://boards.straightdope.com/t/is-there-a-theoretical-minimum-to-the-phase-velocity-of-light/953106/2 "2021-10-24T17:35:44Z")

</div>

Diamond is pretty high, as ordinary materials go (though I think there are a few others that are higher). But there are some weird materials that manage indices of refraction up around 10^8. There’s probably no fundamental limit.

---

<div class="post-metadata">

**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\
**Post date:** [October 25, 2021, 7:36pm UTC](https://boards.straightdope.com/t/is-there-a-theoretical-minimum-to-the-phase-velocity-of-light/953106/3 "2021-10-25T19:36:00Z")

</div>

> [@Chronos](#):
>
> But there are some weird materials that manage indices of refraction up around 10^8.

Weird as in highly complex molecule manufactured in a lab, or weird as in Dwarf star alloy?

---

<div class="post-metadata">

**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [October 25, 2021, 8:25pm UTC](https://boards.straightdope.com/t/is-there-a-theoretical-minimum-to-the-phase-velocity-of-light/953106/4 "2021-10-25T20:25:23Z")

</div>

Weird as in made in a lab, though I don’t recall any details of how they do it.

Dwarf-star materials also tend to be very weird, but usually in the opposite direction, with all other speeds being much higher (like a speed of sound a third as large as the speed of light).

---

<div class="post-metadata">

**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\
**Post date:** [October 25, 2021, 8:38pm UTC](https://boards.straightdope.com/t/is-there-a-theoretical-minimum-to-the-phase-velocity-of-light/953106/5 "2021-10-25T20:38:02Z")

</div>

There are [ways to reduce](https://en.wikipedia.org/wiki/Slow_light) the _group velocity_ of light far below c in a material. One experiment slowed it down to 17 meters per second. I’m not sure if that’s what you’re thinking of. But, as I understand it, the group velocity is not the same as the phase velocity.