Development of Physics in a Relativistic Frame

[Mr Shifty]

Caveat: I'm not a physicist, and I have only a popular knowledge of cosmology and relativity. Those that do, please correct any misconceptions I labor under.

It strikes me that the development of modern physics was assisted greatly by the apparent truth of the Cosmological and Copernican Principles: the ideas that space is isoptropic and homogenous and that there is no special location in space. These principles were axiomatic to the development of modern cosmology for a couple of hundred years, but have only been verified observationally in the last decade or so. The apparent near-uniform distribution of stars in the night sky, however, does suggest them. But a major reason that the sky 'looks' uniform is that the peculiar velocity of visible cosmological artifacts (e.g. the cosmic microwave background and visible stars and galaxies) relative to our own is several orders of magnitude less than the speed of light.

So, I'm curious. Imagine our sun was an intergalactic star, far from any gravitational influence, travelling at 0.9c relative to the cosmic microwave background. For half of the year, the night sky would be generally pretty dark except for the motion of planets and other local phenomenon (asteroids, comets, etc) and during the other half of the year, the night sky would be (I think) pretty bright and blue, but not uniformly. There would be a bright spot (rivalling the moon?) and stars would be clustered toward that spot, getting sparser and dimmer out to 30° or so from the center. How would this situation have affected the development of cosmological physics? Note that humans on this Earth would have no notion that they're moving at any special velocity; this would be just how the universe looks.

[problemecium]

o_O My exploded just brain.

I don't have a lot to say on this, but here's what I do have: for one thing, if we were in intergalactic space, there wouldn't be any stars to see (except maybe one once in a blue moon). We would be able to spot galaxies, mostly with a telescope, if we looked in the right direction at the right time. Otherwise I'd expect the sky to be pretty much all black throughout the year - the CMB wouldn't really turn into visible light unless we were going pretty dang close to lightspeed.

Regarding Newtonian-ish physics on the planet surface, it shouldn't be all that different, as whether the speed of the Sun is high or not, the speed of us humans walking around, or even flying in fighter jets, is miniscule next to the speed of light, so we wouldn't recognize any relativistic effects amongst ourselves. Long story short, I don't see Physics developing much differently, but I imagine astronomy wouldn't be anything like what we have.

[rtxoff]

Somehow i can't really befriend with such an scenario. A sky without stars, desolate.

[Mr Shifty]

I don't have a lot to say on this, but here's what I do have: for one thing, if we were in intergalactic space, there wouldn't be any stars to see (except maybe one once in a blue moon). We would be able to spot galaxies, mostly with a telescope, if we looked in the right direction at the right time. Otherwise I'd expect the sky to be pretty much all black throughout the year - the CMB wouldn't really turn into visible light unless we were going pretty dang close to lightspeed.

I'd think Lorentz contraction would bring more galaxies into view, plus Doppler shift and beaming would make them brighter. I'd think at least a few would be visible all the time. (There are 10 or so visible from Earth with the naked eye.) And you'd be able to see the planets and the moon pretty clearly. (All of the planets, except Neptune can be seen unaided.) But think of that; since planets would be the most visible things in the sky, it seems likely that heliocentric cosmology would have been discovered earlier. That's the type of thing I'm thinking out; how physics might have developed differently.

Regarding Newtonian-ish physics on the planet surface, it shouldn't be all that different, as whether the speed of the Sun is high or not, the speed of us humans walking around, or even flying in fighter jets, is miniscule next to the speed of light, so we wouldn't recognize any relativistic effects amongst ourselves. Long story short, I don't see Physics developing much differently, but I imagine astronomy wouldn't be anything like what we have.

Well yes, cosmology is the question. Would we have discovered relativity, even Galilean relativity? To someone on Fast-Earth, it definitely looks like the universe has a polarity. I'd think there would be considerable effort trying to figure out why the sky doesn't have uniform luminosity. Why, once telescopes are discovered, it seems like Fast Earth is off in the distance, far away from everything else in the universe, which is clustered in a corner of the sky. How utterly counter-intuitive and mystifying it would be that if you were to leave in a spaceship and travel at near light-speed away from the heart of the universe, that the universe would actually suddenly spring into homogeneity all around you.

[Mr Shifty]

Another speculation: It might be difficult, on Fast-Earth, to come up with the idea for an expansionary universe. I'm not sure what degree the relativistic motion of distant galaxies toward Fast-Earth would be balanced out by the expansion of space-time, but it's possible that all the galaxies visible from Fast-Earth would actually be approaching, not receding away. They might have a blue-shift that is significant compared to the sun's color spectrum. Fast-earthlings would have every reason to believe that their place in the universe is special.

I guess the point of this exercise is to perhaps sort of elucidate that kinds of suppositions that might underlie our own understanding of cosmological reality. Fast-Earth's perspective would be entirely valid, and the laws of physics would function exactly the same in their frame, but it's hard to imagine that they would come to the same conclusions about the cosmos that we do. And it's entirely possible that they'd come up with a simple and self-consistent physical framework, different than ours, that matches their observations.

[WestAir]

I think you've encountered the part of physics that makes it so difficult for the common person to understand.

[Z-Man]

In order to get the yearly variations you describe, Earth's orbit velocity around the Sun would also need to be close to c.

[Mr Shifty]

In order to get the yearly variations you describe, Earth's orbit velocity around the Sun would also need to be close to c.

I'm pretty sure that's not the case. The visible universe has a rest frame: the frame of the cosmic microwave background. (Note that relativity states that the laws of physics are the same in all inertial frames, not that there are no special frames. Earth travels at something like 600 km/s relative to the CMB, which manifests itself as a polarity in the radiation due to Doppler effects; it's hotter in one half of the sky.) Nearly all visible matter is travelling at speeds several orders of magnitude less than the speed of light relative to the CMB (plasma jets erupting from quasars and probably a few other phenomenon excepted.) If our sun had a velocity 90% of the speed of light relative to the CMB, it would also have near speed-of-light velocity relative to everything visible (except local solar system objects like other planets and comets.) Thus, from any point in the solar system, aberration effects would pull all the visible cosmic phenomenon (i.e. visible galaxies and the CMB) 'forward' into the direction of the sun's travel. From Earth, for half of the year, these phenomena would be in the direction of the sun: only within our horizon during the day, so swamped by the sun's light.

Edited October 13, 2014 by Mr Shifty

[Acemcbean]

I... er... What? You call that a "popular knowledge of physics"?

[Z-Man]

From Earth, for half of the year, these phenomenon would be in the direction of the sun: only within our horizon during the day, so swamped by the sun's light.

Oh, right. That variation would be observable. It sounded differently in your first post, like the structure of the entire sky would change due to velocity addition. Still, we would just perceive and describe it as a seasonal change in relative position of the sun and the "mysterious blob of stuff". At least on one hemisphere, the blob would still be visible through most of the year, just sometimes only in the morning or evening.

[Mr Shifty]

I... er... What? You call that a "popular knowledge of physics"?

I guess what I mean by that is that it's knowledge that anyone can get by reading books about cosmology written for a popular audience. Here are a few I'd suggest:

• From Eternity to Here by Sean Carroll (It's interesting that there are popular science writers named Sean Carroll in both physics and biology. Biologist Sean Carroll's Endless Forms Most Beautiful is probably the best popular book on developmental evolutionary biology (aka evo-devo) out there.)
  
• The End of Time by Julian Barbour
  
• The Elegant Universe by Brian Greene
  
• and of course A Brief History of Time by Stephen Hawking
  

These books are all accessible and readable by anyone with a basic understanding of algebra. I, for instance, don't know the calculus of geometry, Reimann surfaces, manifolds, tensors, fibre bundles, symmetry groups, etc necessary to have a full understanding of general relativity and quantum physics. But I can read and get the gist of basic concepts.

Edited October 13, 2014 by Mr Shifty