First of all let me say Hello! as this is my first post. I am a biologist, and have been reading over the very basics of SR for a little while and have a lot of questions on the implications of the theory. I had been asking my questions primarily at a different forum, one that wasn't directed towards physics so I figured this would be a better place for me to ask my questions and hopefully receive responses that I can comprehend. This was my last post at the forum, with a lot of the questions I had. We had previously been discussing an example of I was on an airplane traveling at 300mph, and then we talked about if I was on an airplane traveling .865c. In the first paragraph, if it is not clear, I am wanting to understand what exactly is meant by an "inertial reference frame." -- Hmm. I think the key to me understanding this is realizing what an "inertial reference frame" is. I know inertia, in the most basic sense, is an objects resistance to movement. So is the inertial reference frame the observer that isn't moving? I mean, the one that isn't traveling at speeds a fraction of c? The one on earth. I realize the person on earth is the inertial reference frame, but in the advanced calculations, since the earth is rotating around the sun, it is undergoing constant acceleration. Is this ever a factor? In addition to the above paragraph, could this be what makes us "realize" time as we do? Obviously, a second is a unit of time that we arbitrarily invented, and now it "is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. (some website from Google)". But with us rotating around the sun, it seems the sun would be the inertial reference frame (ignoring the wobble from planets) and that we on earth would be undergoing (I guess) constant acceleration. The speed of the earth around the sun is very fast, I'm sure. I'm sure over 30,000 mph. I don't have the data to do the division to see what factor of c that is, but I'm sure it's a decent factor (I'm sure less than .05c though), but much greater than the example of the airplane. Using what I've learned from you guys here and probably poor logic of my own, I would say the entire earth, and presumably everything on it has aged less than the sun (I know the sun existed before earth). Since the earth (and everything on it) is undergoing the constant acceleration, and the sun, relative to us, is static, the earth would have aged slower than the sun. Would this be a factor in calculating the age of the sun? Or is the age of the sun calculated by other means (fuel left in it)? But if any of the above has merit, then wouldn't we have to take it back a step further and look at the earth revolving around the sun, which is moving around in our galaxy? And even further, assuming a singularity-type-entity that the big bang came from, our entire galaxy is moving away from the point of the big bang at probably a very fast speed, so we'd age less than the point from which we all came from. Which I guess makes sense, that would make us younger than the universe's existence? If the Milky Way ages less than the singularity from which we came from, and, uh oh, my mind is locking up I can't see the answer here, but, oh wait no thats wrong. If it were the opposite, that would mean we've been around longer than the universe existed. Hmm. Wait, this last part about our galaxy and the singularity-type-entity we came from really isn't an issue because the singularity exploded, (I'm frowning now), and doesn't exist anymore for us to use as an inertial reference frame. But as I asked above with more complicated calculations with Einstein's SR, since all the galaxies in the universe are moving away from a central point (or each other? but couldn't you have two moving in somewhat parallel?) could the age differences be calculated using different reference frames? I'm sure they (you guys, the physicists who know this stuff) have methods for calculating the ages of galaxies, etc. Or for knowing that something is x light years away. I guess my question is if SR theory is used in addition to conventional methods, like to check for differences in ages. I guess this is also all hinged on whether or not the galaxies, our sun, the earth, etc, is moving at speeds which are large enough of a fraction of c to make a difference when calculating ages. It seems to me that even if it's a small part of c, in a time frame of 11 billion years, that it could have an effect. Even my original question/example with a 300 mph airplane would show differences over 11 billion years (although everyone would die from the airplane's disgusting food -- my apologies if anyone is an airplane food chef!) This is amazing stuff. Really, congratulations to you guys for devoting so much time to studying and understanding this stuff. Thank you anyone in advance for reading this, and for any reply you may give me. I am enjoying learning the basics of this subject. I know the time you put in a reply may seem to be worthless, but the knowledge that I and others gain is priceless. --Aychamo NOTE: I found a figure that says the earth moves about 66,000 mile per hour around the sun. That is 66,000 mi/hr / 60 min/hr / 60 sec/min = 18.33 miles per second. The speed of light is 186,000 miles/second. So the 18.33 (repeating) / 186,000 means the earth is moving at 9.85 * 10^-5c. Pretty insignificant, but still 220 times more significant than an airplane moving at 300 mph. I don't know the math, but over 4.5 billion years that is bound to have some effect. I cannot find figures for the sun's speed in revolving about in the Milky Way (I actually don't like the candy bar).