AaS! 201: How Does Light Slow Down?
Ask a Spaceman!
Paul M. Sutter
4.8 • 853 Ratings
🗓️ 6 June 2023
⏱️ 40 minutes
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Summary
If light always travels at the speed of light, how does it slow down when passing through air or water? Does it matter if light is made of particles or waves? What’s the difference between phase velocity and group velocity, and how does that all play into this? I discuss these questions and more in today’s Ask a Spaceman!
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Thanks to Cathy Rinella for editing.
Hosted by Paul M. Sutter, astrophysicist and the one and only Agent to the Stars (http://www.pmsutter.com).
Transcript
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| 0:00.0 | One of my favorite aspects of all of science, and especially physics, and let's face it, |
| 0:13.2 | physics is the best of the sciences, is that it offers multiple ways to look at the same problem. |
| 0:20.1 | Now, a less charitable view of that would be to say that |
| 0:24.0 | we have no idea what's going on, which is, well, kind of true. Do we ever really understand nature? |
| 0:29.9 | But it also means that we can deploy different arguments, different mathematical tools, and different |
| 0:35.2 | pictures to tackle the same underlying scenario. |
| 0:39.0 | This may seem like a disadvantage, as in we have no idea what's going on, but in reality, |
| 0:44.1 | this is an asset. Our knowledge of physics is flexible and adaptable, offering us many different |
| 0:50.2 | insights and ways to solve challenging problems. As an example, let's talk about gravity. |
| 0:56.9 | You know, you often say, you often hear that Einstein's general theory of relativity is an |
| 1:01.5 | improvement or an update or a replacement to Newton's model of gravity. And that's true-ish. |
| 1:09.8 | It's only half-true. |
| 1:15.7 | GR is an improvement to Newton's gravity, but only in certain situations. |
| 1:20.6 | Let's say I have, I don't know, a pie launching device, |
| 1:25.6 | and I want to calculate how far my pie will travel for a given level of launch power. |
| 1:29.0 | Now, ignoring the details of aerodynamics of launched pies, which I admit are critically important in reality, but not for the point I'm trying |
| 1:33.5 | to make. Ultimately, I have to solve a problem involving gravity. The pie launcher launches a pie |
| 1:38.7 | at a particular angle and speed. The Earth's gravity is pulling downwards on the pie. Eventually, |
| 1:44.0 | the pie will hit the |
| 1:44.7 | ground. Now, I have a choice to calculate how far this pie will go. I can use Newton's formulation |
| 1:51.7 | of gravity or Einstein's. And these two formulations of gravity give radically different views |
| 1:58.0 | of what gravity is and how it operates. Newton says it's this invisible force that operates instantaneously. |
... |
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