# Is there anywhere where we can \*see\* light travelling at c? **URL:** \ **Category:** Factual Questions\ **Created:** [June 2, 2023, 5:01pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924 "2023-06-02T17:01:42Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [June 2, 2023, 5:01pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/1 "2023-06-02T17:01:42Z")
Before you reply with one of the obvious answers, let me explain what I mean: - I know you can slow light in various media, that doesn’t count for the purposes of this thread - I don’t mean detecting a photon and knowing it travelled at c (which much of our technology relies upon…it’s not like it’s in doubt). I mean…let’s say you have an extremely powerful pulse of EM radiation (e.g. the radio waves of a pulsar) go through a very thin cloud of material that scatters that radiation. Then we would see a “wake”, travelling across the cloud at c. So we need a combination of a very powerful EM source, that’s far enough away, that pulses in some way, near to a medium that scatters it an appropriate amount. Is there anywhere like that? Sorry if it’s a dumb question, it’s strangely hard to google. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [June 2, 2023, 5:19pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/2 "2023-06-02T17:19:06Z")
I don’t know of any specific examples of being able to observe the propagation of light at a vast distance away in space, although the right circumstances for observing such a thing must surely have occurred at some point. However, here is a different approach that really does show the propagation of a pulse of laser light. It doesn’t involve any significant slowing, though technically the light traveling through the milky water in the Coke bottle has been slowed a bit, but the reflections that can be seen emerging from the bottle are pretty much back to traveling at _c_ again. It’s a fascinating apparatus that, through repeated scans, effectively has a frame rate of a trillion FPS: > **[Visualizing video at the speed of light — one trillion frames per second](https://www.youtube.com/watch?v=EtsXgODHMWk%2F)** > > MIT Media Lab researchers have created a new imaging system that can acquire visual data at a rate of one trillion frames per second. That's fast enough to ... ---
**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:** [June 2, 2023, 9:08pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/3 "2023-06-02T21:08:01Z")
That’s the one I thought of, too. It’s really not what it looks like, though: It’s not a bunch of frames of a single pulse; each frame is of a different pulse of light, and the timing of the camera is just precisely adjusted such that each frame captures the next pulse at a point slightly further on. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 2, 2023, 9:17pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/4 "2023-06-02T21:17:29Z")
One example, taken by Hubble: [![](https://img.youtube.com/vi/CJiqjppxToI/maxresdefault.jpg "Image Sequence of V838 Monocerotis Epochs") ](https://www.youtube.com/watch?v=CJiqjppxToI) The gas is not moving–what you’re seeing is the pulse of light illuminating an expanding shell around the star. ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [June 2, 2023, 9:23pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/5 "2023-06-02T21:23:49Z")
> [@Chronos](#): > > That’s the one I thought of, too. It’s really not what it looks like, though: It’s not a bunch of frames of a single pulse; each frame is of a different pulse of light, and the timing of the camera is just precisely adjusted such that each frame captures the next pulse at a point slightly further on. - A single pulse can be filmed in real time now. > **[Filming the Speed of Light at 10 Trillion FPS](https://www.youtube.com/watch?v=7Ys_yKGNFRQ%2F)** > > What is the fastest thing we as the human race know of? Gav and Dan try and film that. > **[At 10 trillion frames per second, this camera captures light in slow motion |...](https://techcrunch.com/2018/10/12/at-10-trillion-frames-per-second-this-camera-captures-light-in-slow-motion/)** > > Light is the fastest thing in the universe, so trying to catch it on the move is necessarily something of a challenge. We've had some success, but a new rig built by Caltech scientists pulls down a mind-boggling 10 trillion frames per second, meaning... ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 2, 2023, 9:27pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/6 "2023-06-02T21:27:02Z")
> [@Mijin](#): > > Sorry if it’s a dumb question, it’s strangely hard to google. “Light echo” seems to be the term of art in astronomy. There are several examples from Hubble. Another one (a bit more subtle than the other I mentioned): [https://hubblesite.org/contents/media/videos/2017/42/998-Video.html?keyword=Supernovae](https://hubblesite.org/contents/media/videos/2017/42/998-Video.html?keyword=Supernovae) ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 3, 2023, 2:17am UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/7 "2023-06-03T02:17:06Z")
Incidentally, it’s possible for a light echo to appear to travel _faster_ than c. Imagine a pulse of light far away from some source, so that it’s nearly planar. And imagine that it intersects a slab of gas at a shallow angle relative to the light. You’ll see the line of intersection illuminate, but it’ll move faster than c. Of course, this is no different than how the spot of a laser beam, or even just a shadow of something, can move faster than light. There’s no physical object moving, and so no top speed. It just means you have to be careful with your conclusions. This can also happen when things are illuminated at the same time, but you see things close to you first (since the signal also goes at c) and you make invalid conclusions about the speed of things. That can happen to physical objects, but again there’s no violation since you’re really seeing events that happened at different times. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [June 3, 2023, 3:13pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/8 "2023-06-03T15:13:53Z")
Thanks, these were just the kind of thing I was looking for 🙂 And “light echo” has now entered my lexicon. ---
**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:** [June 3, 2023, 6:21pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/9 "2023-06-03T18:21:43Z")
And now I’m smacking myself for not thinking of the astronomical examples. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [June 3, 2023, 6:29pm UTC](https://boards.straightdope.com/t/is-there-anywhere-where-we-can-see-light-travelling-at-c/984924/10 "2023-06-03T18:29:21Z")
My favourite light echo example is **Hubble’s Variable Nebula**. It has shadows moving faster-than-light, because of the effect mentioned by Dr Strangelove above. [![](https://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/NGC2261_LRGB_season_2021-22_18fps.gif/640px-NGC2261_LRGB_season_2021-22_18fps.gif) ](https://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/NGC2261_LRGB_season_2021-22_18fps.gif/640px-NGC2261_LRGB_season_2021-22_18fps.gif)