[Causality1]

What I've always found wild about gravity and photons is that gravity produces the same proportionate effect, i.e., acceleration, on them as it does normal matter and the only reason the sun and planets don't render light nearly unusable for sensing by utterly distorting the light's path is that it travels so quickly it doesn't spend any significant amount of time in a gravity well.

I don't think I'll ever fully wrap my head around the kind of numbers that implies, that when light is at approximately sea level it is accelerating at 9.8m/s^2 toward the core of the earth and the only reason it doesn't all get dragged out of the air and to the ground is that it moves too fast to notice.

[DebtDeflation]

That's the fundamental idea behind a black hole. The concept of escape velocity is based on the curvature of spacetime and is represented by √2GM/R where M is mass and R is radius. Light travels along the curvature of spacetime. At the event horizon of a black hole, escape velocity equals c, so from that point down to the singularity, spacetime is curved so much that light simply isn't traveling fast enough to escape.

[danbruc]

That is an interesting thought that never occured to me. It would render the headlights of your car pretty much useless if the light just fell to the ground a few meters in front of the car. I wonder if one has or could measure the effect in the lab - it's about 0.5 μm over a distance of 100 km, so it seems not totally out of reach.

[kgwgk]

Your comment reminded me of Gamow's Mr. Tompkins in Wonderland. I don't know if he mentioned that in particular, but he discussed what relativistic effects would look like if the speed of light was 10 MPH.

http://www.arvindguptatoys.com/arvindgupta/tompkins.pdf

[danbruc]

There is also a free game, A Slower Speed of Light. [1]

[1] http://gamelab.mit.edu/games/a-slower-speed-of-light/

[Causality1]

It's a bit tricky because it isn't light taking a curved path through your lab; it's light taking a straight path through the curved spacetime in your lab.

[danbruc]

The elevator thought experiment suggests that the light will indeed also take a curved path through my lab, but I am way out of the territory where I have any good intuition. And I have no intuition at all what would happen to the matter the elevator is made of if one accelerated it hard enough to make the effect noticable, nor do I have any idea what Earth's gravity does to matter in my lab. Can I have a trully straight ruler out of some common material? Or at least not as bend as the light beam? And now I am not even sure anymore what straight would mean.

[Causality1]

If you had a 100km long rod that was perfectly, impossibly straight and infinitely strong, placing it in a gravity well equal to that at the earth's surface would still make it look bent by half a micrometer to an outside observer, in the same way you can draw a triangle on a sphere using nothing but three right angles and three straight lines.

If that's not interesting enough, do some reading on the relative nature of spontaneity. Two things that happen at exactly the same time in one reference frame don't happen at the same time in another. Crazy.

[danbruc]

I guess this is an autocorrect issue, but you certainly mean simultaneity. Anyway, I have been through several university courses on special and general relativity with all the math and have at least some basic understanding of the thing. However, as said before, I have no real intuition for it.

So the only thing I can work from right now is the elevator thought experiment and it seems to me that I can have a perfectly straight rod in a 100 km wide elevator accelerating at 9.81 m/s² while a light ray would bend downwards by 0.5 µm. So ignoring the fact that Earth's gravitational field is not uniform, this seems to suggest that even a rod made out of some common material would not be bend or appear bend or whatever in Earth's gravitational field. Maybe this analogy just leaves the realm of validity of the thought experiment, but I honestly don't know.

I am also not convinced by your analogy with the triangle on a sphere, gravity is curvature of spacetime but the analogy involves only curvature of space. If you tell me that you are a physicist specialized in general relativity, I will take your word for it, otherwise I will keep some doubt about getting my nice rod bend in the same way the light path gets bend, after all they are two quite different things, an extended object versus a path through spacetime.

[liberte82]

This ties in well with the revelation many high school physics students have that a bullet fired from a gun parallel to the ground also falls to the earth at the same rate as any other object.

[100ideas]

A handy rule of... thumb I like to keep in mind is that it takes about 1 ns for light to travel from your hand to your eye...

(... and then 10,000,000 ns for your brain to process it...)

[Causality1]

If you haven't seen the brief lecture from Admiral Grace Hopper regarding nanoseconds it's well worth a watch. https://youtu.be/JEpsKnWZrJ8