<blockquote>
<p>If light moved slower in this dense plasma, would it appear like events in other solar systems played out slower than they actually do?</p>
</blockquote>
<p>First, the <a href="https://en.wikipedia.org/wiki/Interstellar_medium" rel="nofollow noreferrer">interstellar medium</a> is not a &quot;dense&quot; <a href="https://en.wikipedia.org/wiki/Plasma_(physics)" rel="nofollow noreferrer">plasma</a> like the Sun's <a href="https://en.wikipedia.org/wiki/Photosphere" rel="nofollow noreferrer">photosphere</a> (which is also extremely tenuous compared to, say, water).</p>
<p>Let's ignore that for now and assume the plasma density was high enough to change the <a href="https://en.wikipedia.org/wiki/Permittivity" rel="nofollow noreferrer">permittivity</a> (thus, the <a href="https://en.wikipedia.org/wiki/Refractive_index" rel="nofollow noreferrer">index of refraction</a> too) such that electromagnetic waves moved at a speed well below the <a href="https://en.wikipedia.org/wiki/Speed_of_light" rel="nofollow noreferrer">speed of light</a> in vacuum.  This would not affect time, i.e., there would be no <a href="https://en.wikipedia.org/wiki/Time_dilation" rel="nofollow noreferrer">time dilation</a> due to this propagation speed difference.  The only difference is that it would take longer for the same electromagnetic wave to move through the dense plasma than a vacuum.</p>

CC BY-SA 4.0
2021-10-27T15:12:05.050Z
2021-10-27T15:12:05.050Z
honeste_vivere
<p>Voyager 2 recently left the solar system and registered an increase in plasma density. If light moved slower in this dense plasma, would it appear like events in other solar systems played out slower than they actually do?</p>

CC BY-SA 4.0
2019-12-14T08:27:48.297Z
"dense" is a relative word here.  The interplanetary plasma is still a very very good vacuum, compared to most vacuums created on Earth.
James K
<light><speed><density><plasma-physics>
If interstellar medium was dense plasma and light slowed down, would things appear to move slower far away?
chron0
# If interstellar medium was dense plasma and light slowed down, would things appear to move slower far away?

(Asked by: username_0 on 2019-12-13)

Voyager 2 recently left the solar system and registered an increase in plasma density. If light moved slower in this dense plasma, would it appear like events in other solar systems played out slower than they actually do?

**Tags:** light, speed, density, plasma-physics

> **Comments:**
> **username_1**: "dense" is a relative word here.  The interplanetary plasma is still a very very good vacuum, compared to most vacuums created on Earth.  

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## 1 answers

### Answer by username_2 on 2021-10-27. Score: 2

> If light moved slower in this dense plasma, would it appear like events in other solar systems played out slower than they actually do?

First, the [interstellar medium](https://en.wikipedia.org/wiki/Interstellar_medium) is not a "dense" [plasma](https://en.wikipedia.org/wiki/Plasma_(physics)) like the Sun's [photosphere](https://en.wikipedia.org/wiki/Photosphere) (which is also extremely tenuous compared to, say, water).

Let's ignore that for now and assume the plasma density was high enough to change the [permittivity](https://en.wikipedia.org/wiki/Permittivity) (thus, the [index of refraction](https://en.wikipedia.org/wiki/Refractive_index) too) such that electromagnetic waves moved at a speed well below the [speed of light](https://en.wikipedia.org/wiki/Speed_of_light) in vacuum. This would not affect time, i.e., there would be no [time dilation](https://en.wikipedia.org/wiki/Time_dilation) due to this propagation speed difference. The only difference is that it would take longer for the same electromagnetic wave to move through the dense plasma than a vacuum.


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