"Speed of Light"

Bullgrit said:

Wonder if anyone has studied images of the universe for duplicate galaxies on opposite sides of us, to determine if we're seeing wrap-around.

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Of course! The Hubble telescope does that sort of thing all day every day. You should check out some of the Hubble Deep Field images.

And they didn't see that. However, we can't see all that far - we can only see about 14 billion light years (the age of the universe is 14 billion years, so at the speed of light which is one light year per year any light originating further than 14 billion light years from us hasn't had time to reach us yet). That's what's called the "observable universe" - the small portion of it we can see.

Morrus said:

Of course! The Hubble telescope does that sort of thing all day every day. You should check out some of the Hubble Deep Field images.

And they didn't see that. However, we can't see all that far - we can only see about 14 billion light years (the age of the universe is 14 billion years, so at the speed of light which is one light year per year any light originating further than 14 billion light years from us hasn't had time to reach us yet). That's what's called the "observable universe" - the small portion of it we can see.

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I know. I've seen the Deep Field images -- in fact, my wall calendar over my desk is images of Hubble pics. They're mesmerizing!

But I haven't seen any references to anyone doing a wrap-around check.

Bullgrit

Bullgrit said:

I know. I've seen the Deep Field images -- in fact, my wall calendar over my desk is images of Hubble pics. They're mesmerizing!

But I haven't seen any references to anyone doing a wrap-around check.

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Well, you can only see 14b light years, so it's not possible to do that (at least - that isn't the case within 14b light years each way; if it is further out, we can't see it).

There's a bunch of theories on the shape of the universe and lots of research into it, but I'll leave an actual physicist to go into that; I get a bit lost when they start talking about "saddle shaped" universes and the like.

Janx said:

Is it possible that things like Photons exist because they artifacts of a rendering (aka display) proecess rather than actual matter?

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Is it possible? Well, sure, it is possible. But note that the simulation posit is what we'd call "non-falsifiable", meaning that there is no test we could do, no information would could ever have, even in theory, that would prove it *wasn't* a simulation. Any evidence we had could be countered with, "Well, the simulation is written to give you that result." So, the simulation idea is ultimately not a scientific question, but a philosophic one.

But, that something so central to how the Universe works is just an artifact? In videogames, programmers go to great lengths to reduce the impact of rendering artifacts on the experience. But photons are friggin' *everywhere*, and most definitely impact the simulation experience. So, if they are artifacts, our programmers aren't very good at their job, now are they?

Bullgrit said:

Shoot a beam of light (like in the camera demonstration above), and you can see the beam from the side (90 degrees off its line of fire) -- that means photons are coming off the beam to the camera lens. I presume photons are being deflected off molecules toward the camera, and the beam is not *producing* more photons. Yes?

But the TED video, when showing the around the corner viewing potential, seems to show photons hitting a surface and "exploding" into more photons.

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Scott DeWar said:

I am guessing the sight of the 'photon bullet' in the coke bottle was diffused light from being scattered by the water. I am further guessing that a light beam in a vacuum would not have anything diffusing it and therefore no viewing from the side, as noticed when you look perpendicular to a line of sight between a star and earth, you can't see the beam or as long as it is not shining through an exploded star, 'dust motes'.

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Right. While there are processes that can create or destroy photons (like those in the laser!), the light propagation you see in that video is all just scattering. Each pulse has many, many photons, and they are bouncing off the water (in the coke bottle), or, in daily experience, dust particles. The "exploding" photons in the cartoon about looking around the corner is their way of illustrating that the single "light bullet" has tons of photons in it. Each photon will hit a slightly different part of the wall and therefore bounce in different directions. But anyway, you only see photons that go directly into your eye, so seeing a light beam "from the side" really means that some of the light is being scattered out of the beam toward you.

By the way, that video is really cool, but I have to point out a bit of "false advertising" or something in it that really bugs me. Notice that the camera takes a frame every 1 trillionth of a second. That's a picosecond, not a femtosecond, which is 1000 times smaller than a picosecond. So calling it "femto-photography" is, well, misleading. I mean, it's hard enough to explain science in a simple but reasonably correct way, so someone intentionally goofing up scientific notation is hard to take. (And I'll eat my hat if no one in his lab knows the difference between "pico" and "femto.")

I'll make a separate post in a bit about the "wrap-around."

Bullgrit said:

Wonder if anyone has studied images of the universe for duplicate galaxies on opposite sides of us, to determine if we're seeing wrap-around.

Bullgrit

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Morrus said:

Of course! The Hubble telescope does that sort of thing all day every day. You should check out some of the Hubble Deep Field images.

And they didn't see that. However, we can't see all that far - we can only see about 14 billion light years (the age of the universe is 14 billion years, so at the speed of light which is one light year per year any light originating further than 14 billion light years from us hasn't had time to reach us yet). That's what's called the "observable universe" - the small portion of it we can see.

Click to expand...

Bullgrit said:

I know. I've seen the Deep Field images -- in fact, my wall calendar over my desk is images of Hubble pics. They're mesmerizing!

But I haven't seen any references to anyone doing a wrap-around check.

Bullgrit

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Hubble isn't really the right instrument to do a search for a finite size universe/wrap-around effect. The main problem is that Hubble only looks very far away in a very small area of the sky, and you've got to look in pretty much every direction if you want to see a wrap-around effect. Think of it this way, if there is one galaxy we are seeing duplicated, we should see it both to the right and to the left. You wouldn't see both duplicates in the same spot to the right, for example.

But you can look for wrap-around in the cosmic microwave background, which is the light left over from the end of the Big Bang and is the farthest away thing we can see (or ever will see, as the universe was opaque at earlier times). This light is like a picture of the universe almost 14 billion years ago, and it tells us the universe was not quite the same temperature or density at every point. So what people have done is look for evidence of repeating patterns in the hot and cold spots which could be explained by a wrap-around effect. There's no credible evidence of any, which means the universe can't be smaller than 14 billion lightyears across (actually even a bit larger if I understand the results correctly). I can't remember if there have been checks done for every possible shape (see below), so I suppose there might be some wiggle room, but I don't think so. If you think about the usual theories of the early universe, it would be kind of weird for the universe to be just the size we see today anyway.

Morrus said:

Well, you can only see 14b light years, so it's not possible to do that (at least - that isn't the case within 14b light years each way; if it is further out, we can't see it).

There's a bunch of theories on the shape of the universe and lots of research into it, but I'll leave an actual physicist to go into that; I get a bit lost when they start talking about "saddle shaped" universes and the like.

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There are two sorts of shape issues here. One has to do with whether space is curved. In 2D, you can think of the three basic curvatures as being flat like a piece of paper, curved like a sphere, or curved like a Pringle chip. Measurements (again from the cosmic microwave background, among others) tell us that our universe is flat (within error). But the other question is if the universe is infinite or finite. In 2D again, the old video game Asteroids is a good example of a flat but finite universe with wrap-around. In fact, there aren't many other possibilities in 2D. In 3D, there are a lot more choices. I'm not a geometer, but my understanding is that the options more or less correspond to how you can take a regular polyhedron and glue the faces together. What I'm not sure about in the cosmological searches if they've got a search algorithm that can handle all possibilities at once of if they've only been able to check a few. I just don't remember, as it's not something I work on actively.

PigKnight said:

...

But, for realsies, why is it called c? Latin? Kicks-n-giggles? Discovering scientist?

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I believe c comes from the Latin celer, meaning fast. This is also the origin of the words accelerate & celerity (like the spell).

Ruzak said:

I believe c comes from the Latin celer, meaning fast. This is also the origin of the words accelerate & celerity (like the spell).

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The first known reference to c standing for "celeritas" is a 1959 essay by Isaac Asimov, and he cited no evidence to support the claim. So, I'd take that with a grain of salt.

I believe it was c for "constant". While physicists do get fanciful with naming sometimes, we do tend to keep it simple when it gets down to writing down the math.

Umbran said:

The first known reference to c standing for "celeritas" is a 1959 essay by Isaac Asimov, and he cited no evidence to support the claim. So, I'd take that with a grain of salt.

I believe it was c for "constant". While physicists do get fanciful with naming sometimes, we do tend to keep it simple when it gets down to writing down the math.

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You do NOT get fanciful about naming. You've spent half a century congratulating yourselves for "strange" quarks like that's some genius display of eccentric creativity. It's not.

You all collectively need to read a comic book someday!

(Which is weird, given that every physicist I know loves comic books. The - arguably - two most famous current popular physicists aside from Hawking - Neil deGrass Tyson and Prof Brian Cox - are both woefully ignorant of both).

Umbran said:

The first known reference to c standing for "celeritas" is a 1959 essay by Isaac Asimov, and he cited no evidence to support the claim. So, I'd take that with a grain of salt.

I believe it was c for "constant". While physicists do get fanciful with naming sometimes, we do tend to keep it simple when it gets down to writing down the math.

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I stand corrected. That's what I get for taking information from a fluffy not-for-science-majors textbook.
Thanks for the correction Umbran. I've been telling it wrong for years.