losfomot

The velocities of these objects are not relativistic. That is to say that SR does not apply to these velocities. If an object was to give off ripples in spacetime due to SR, it would NOT be an object whose velocity is due to the expansion of space.

A person in a ship flying past the moon at 99% of c will experience the universe (space and time (or spacetime)) differently than a person on Earth watching him fly by the moon. But the person on Earth will not notice anything unusual about the gravity of the moon aside from factoring in the gravity that the mass of the spaceship contributes. Of course, the mass of the spaceship will be larger for the observer on Earth, which would mean that the ship would give off a stronger gravitational field then it would have at non-relativistic speeds... I suppose, in that sense, there is a mixing or overlapping of SR and GR effects. I believe the OP is thinking along the lines that the universe is expanding, so a lot of it is moving at relativistic speed relative to us, the further an object is from us, the faster it is moving away, and the more massive it must be to us because of SR effects (Hence, more gravity). This probably WOULD be considered an addition of SR and GR effects if it were true. But SR effects do not apply to motion due to the expansion of the universe because this motion is not motion within spacetime, but rather motion due to the expanding of spacetime itself. Of course, as I always like to state lately, I am far from an expert. Rereading the OP, maybe I'm wrong... I'm not entirely sure what he is trying to say.

First off, I am not an expert... take what I say with a grain of salt... and maybe some lemon. Suppose for a minute that a spaceship is flying at 150,000 Km/s past Earth. An Earth observer watches the ship go by and measures the ships velocity as 1/2 the speed of light. Suddenly the ship emits a flash of red light. 10 seconds later a beam of light bursts forth from the front of the spaceship. Earth observer sees the lightbeam moving forward at (obviously) the speed of light. The Earth observer says: 'The beam of light is moving at 1/2 the speed of light relative to the spaceship' This is an example of erroneously mixing frames. A cannot decide how B is moving relative to C (at least not by observation and measurement alone) The reason for this is that the two observers (A and B) see the universe differently. Lets go back to our event, but this time from the Spaceships point of view... A spaceship is traveling past the Earth at 1/2 the speed of light. As he goes past, he decides to turn on his headlights. He flicks a switch and a red flash of light is given off... 8.5 seconds later a bright beam of light (the headlight) shoots out in front of the ship moving away from the ship at (obviously) the speed of light. The observer on the spaceship thinks about it and decides: 'I'm moving at 150,000 Km/s relative to Earth, and my headlight is moving away from me at an additional 300,000 Km/s, so the beam of light must be moving at about 450,000 Km/s relative to the Earth' Again, incorrectly mixing frames. But You will notice that the time between the flash and the headlight turning on was 10 seconds for the Earth observer, but only 8.5 seconds for the Spaceship occupant. This is because of time dilation. Time dilation is a contributing factor as to why you cannot mix frames... different amounts of time have passed for the two observers over the course of the same event. Lets pick another event to analyze... The Earth observer watches as the spaceship headlights are turned on. Ten seconds later, the beam of light has traveled a total of 3,000,000 Km, and the space ship has traveled 1,500,000 Km in the same direction, so the Earth observer sees that, in 10 seconds, the beam of light has traveled only 1,500,000 Km relative to the Spaceship. Now we know that the beam of light is actually moving at the speed of light relative to the spaceship, so in those 10 seconds the light beam should have traveled 3,000,000 Km ahead of the Spaceship, but Earth sees the light beam only 1,500,000 Km ahead... what explains this discrepancy? First of all, remember that, although 10 seconds have gone by on Earth, only 8.5 seconds have gone by for the spaceship, so 8.5 seconds X the speed of light = about 2,550,000 Km. That gets us part of the way to the 1,500,000 Km that Earth sees, but its not nearly enough... so we guess that time dilation cannot be solely responsible for the difference... and we're right. Another consequence of relativistic speed is length contraction. The faster you go in a given direction, the shorter the distance gets in that direction. What this means for our spaceship guy is... hmmm Lets say that, as the ship passes by Earth, it is heading directly toward the Sun. From Earth's point of view, the spaceship is 150,000,000 Km from the Sun. From the Spaceship's POV, the Spaceship is only about 130,000,000 Km from the Sun. This is not an illusion, the spaceship exists in a universe where the distance really is 20,000,000 Km less than it is in the Earth observer's universe. It is a combination of time dilation and length contraction that explains the apparent contradiction between what each observer sees. For the Earth observer, X amount of time has gone by, and Y amount of distance has been covered for the spaceship to get from Point A to Point B. For the Spaceship observer, not only has less time gone by during the same event, but Point A and Point B are closer together, so less distance was traveled. The speed of light is constant in any frame. Einstein realized this and tried to think of a set of rules the universe must have that would allow this to be true. Time dilation, length contraction, and other things collectively known as Special Relativity was the explanation he came up with. It works very well in allowing the speed of light to be constant in all frames, and has other consequences, many of which have been experimentally verified. I hope I haven't mucked things up too much. relative to it's previous speed relative to what? But that's just it, there is no direction that everything is moving away from more prominently than any other direction. We have looked at the universe, and 'taking into account they speed direction and mass', we find everything is moving away from everything else equally. (not counting the Great Attractor which I believe, relative to the size of the universe, is a local phenomenon.)

You cannot 'pass' light in the sense that you can outrun it because you CANNOT GO FASTER THAN LIGHT. You CANNOT GO FASTER THAN LIGHT What you're describing is the Doppler effect, and it has nothing to do with going faster than light.

I am loving this thread. Dark energy was always a pain in the ass. At first glance I would tend to agree with Martin that the effect would be too small to account for the acceleration that we see, but what do I know? Maybe a big crunch is still possible, though we've been fooled into deducing the opposite?

I wish you were right. Here is a thread I started a couple of years ago... unfortunately it was chopped up by the forum moderators because it was getting off-topic, they were cracking down on that sort of thing at that time. A lot of good stuff got trashed... (sigh) This quote is also from a couple of years ago: Here is the thread I took it from. Also a good read.

So you just started this thread to make the statement 'darkness is not light's opposite' ... and... that's it?... ok... good for you. Nobody is arguing with you about that because you go on to explain that, by light, you mean photon, and by opposite, you mean some sort of anti-photon, which darkness obviously is not. But then you go on to say that the photon's opposite (or 'dark light') would cancel out the photon leaving darkness. And that is what people are trying to set you straight on. Your scenario does not work because the photon has no opposite.

I answer hesitantly, as I am no expert... The mass of an object determines it's gravity (field). However, all we have right now is a description of how gravity acts. In other words, we have a set of equations that we can use to accurately calculate orbits and predict the existence of things like black holes. The equations we have are built around the idea that space is generally flat, except around mass. Mass curves space just like a bowling ball curves the surface of a trampoline. This curvature then tells mass how to move (orbits and such). Just like a marble tossed alongside the bowling ball will circle the bowling ball because of the curved surface of the trampoline. The marble spirals closer and closer until it hits the bowling ball, but this is because friction is slowing the marble down, without friction, the marble would continue circling the bowling ball just like the Earth circles the Sun. I was happy when I first heard this idea because I thought it was a solid mechanism for gravity, the curvature of space. I was a little disheartened to be made to understand later that the curvature of space is merely a convenient mental picture that we use to understand the effects of gravity. It is not necessarily what is actually happening. ie There is not a physical tangible thing called space or spacetime that actually curves in the presence of mass. So, in conclusion, I can't really help you... my understanding is that we have 'laws' and equations that tell us, to a high degree of accuracy, how mass moves in the presence of other mass. And that's as far as I go.

It has nothing to do with 'curved space', this would imply that it is not an illusion at all. The truth is that when you take a picture of the moon in the sky and again (with the same camera on the same settings) on the horizon, and then measure the size of the moon in those pictures, they are identical in size. Even in the picture, the moon on the horizon might seem larger... but it is just your perception. If it were anything other than your perception, then the size difference would show up when you measure the diameter of the moons in the pictures. No size difference. So the explanation for the illusion lies with how your brain interprets the image. ie: perception!

Here, you are definitely talking about the planets rotation on its own axis. And it is easy to see why you think the situation is strange here... Mars does have about twice the mass that Mercury has... so why is the surface gravity so similar? The answer has nothing to do with rotation. The surface gravity is determined by the mass of the object, as well as your distance from the center of mass. Mars has more mass but it is less dense and quite a bit larger than Mercury, so you are farther from the center of mass. Mercury is compact and smaller than Mars, so you are much closer to the center of mass. If Mars's mass were compacted until both planets were the same size, then you would be much heavier on Mars than on Mercury (twice as heavy, I think)

It seems to me that the OP meant that the planets all ORBIT THE SUN in the same direction

Yes, you could cover a ridiculous distance in a human lifetime. I don't know about 'traverse the entire universe', because you have to remember that the universe is expanding, most of it is moving away from us faster than the speed of light, and it has had quite a head start... how about this... If you were to accelerate at about 10 m/s/s (roughly earth's gravity) and continue that thrust for about 30 years, you would see the end of the universe as we know it. IE you would be somewhere in deep space, and you would likely not see a single speck of light because the universe will be very old , cold and dark. HERE is a great device for calculating distance covered at relativistic speeds. Just change the 'time relative to rocket' (how long you want to be in the ship accelerating) and hit compute.

You should be able to find everything you need through these links However, you can pick up a 4.5" reflector telescope for under $300 new, I picked one up at a garage sale for $60.00 before. They are great for beginners, and you can clearly see Saturn's rings on a clear night (you have to get away from light pollution though... take a drive to the country) If you have never had a scope before, I recommend you buy one of these first, get to know how they are designed and how they work, and then decide if you want to build a better one for yourself. Check on EBay, there are some good deals on 4.5" and 6" telescopes, if you go for a 4.5", try to stay away from the shorter tube (they call them 'fast' telescopes) they look nicer but the quality suffers.

I see how you got those numbers, and the picture they paint looks about how it should, but I wonder if it's legitimate? I tried a much simpler method of measuring the acceleration with Morgans Calculator data, and I got negative results... I made a graph showing 'Distance now' against 'Speed now' z=00.074 - Distance=01BLY - Speed=0.07c z=00.150 - Distance=02BLY - Speed=0.14c z=00.229 - Distance=03BLY - Speed=0.21c z=00.312 - Distance=04BLY - Speed=0.29c z=00.398 - Distance=05BLY - Speed=0.36c z=00.488 - Distance=06BLY - Speed=0.43c z=00.583 - Distance=07BLY - Speed=0.50c z=00.683 - Distance=08BLY - Speed=0.58c ......... z=568 - Distance=45BLY - Speed=3.26c As you can see, the resulting graph is virtually a straight line graph, not showing acceleration or deceleration over distance.

The Hubble parameter would be 'getting smaller all the time' whether the expansion is accelerating or decelerating. As long as the universe is expanding at all, the Hubble parameter would be getting smaller with time. With a constant expansion (no acceleration either way) I believe the Hubble parameter would halve everytime the universe doubled in age (or, going back in time, the Hubble parameter would double everytime you halved the age of the universe) They say the universal expansion is accelerating because they looked billions of years into the past and the expansion is faster now than it was back then... so I'm not sure what you mean by 'only recently the acceleration has started'

And it seems to make sense... Of course it means that the Hubble constant should actually be referred to as the Hubble parameter (as Martin refers to it, and as I will from now on) because, it must be getting smaller all the time (despite the fact that the universe seems to be accelerating), and it must have been a pretty huge number for the first little while. This does not seem right. "The expansion have (has) been decelerating"?

Thank you for that... Now let's see if I have this right... A Hubble CONSTANT would show the Hubble Sphere to be receding at c. And an accelerating expansion (a hubble not so constant) would cause the Hubble Sphere to continue receding but at a speed less than c... the amount less than c would depend on the value of acceleration. This would mean that the Sphere is getting larger in terms of distance, but smaller in terms of the amount of spacetime it encompasses. Further, a decelerating universe would have a Hubble Sphere that recedes faster than c. is this right?

When looking at the universe on a large scale, we see galaxies moving away from us proportional to distance. This is due to expansion. What I'm wondering is... disregarding expansion, is there a lot of motion in the universe? Or is everything pretty static.

Thank you for that link, I'm still trying to wrap my head around the article, there is a lot of information there. Of course, by definition it must. During inflation the Hubble Constant was higher which made our Hubble Sphere smaller. In a universe that is currently accelerating, I don't see how this is possible. But if you look at their diagrams depicting 'co-moving' motion, the Hubble Sphere appears to be getting smaller on the 'now' timeline. We can't see it yet because our past light cone hasn't caught up yet, our past light cone touches the Hubble Sphere at a point where the radius of the Hubble Sphere is still increasing with time. The size of our universe and the size of the Hubble Sphere are two very different things. And the only way I could see the Hubble Sphere expanding at c is if the Universe was collapsing... which doesn't appear to be the case. Back to my Question... It does seem clear that as you move away from the Earth, your Hubble Sphere changes from Earth's... You see more of the universe in your direction of travel all the time... and you see less of the universe in the direction you are leaving. So my question (rephrased) is: If you travel at a constant acceleration away from the Earth, will you eventually have a Hubble Sphere in which the Earth is not a part of?

I, in my ship, would feel a constant acceleration of 20 m/s/s for the entire trip. Earth, watching my ship, would see me initially moving away at 20 m/s/s... but, as I pick up speed, Earth would see my acceleration slowing down to a greater and greater degree. Relative to Earth, I will not accelerate to or beyond the speed of light. (although this is not necessarily true, hence my original post) The point is, I can accelerate at a constant value for as long as I want, there is no 'law of physics' that says I can't and it will not take any 'extra' energy to do so. My acceleration will simply be interpreted differently depending on the observers reference frame.

Obviously I would need an imaginary never ending fuel source for this to be possible, but as far as relativity is concerned I don't see the problem.

I hate arguing with a physics expert, because I will probably put my foot in my mouth... on that note, I have to disagree with you. The hubble constant is somewhere between 50 and 80 km/s/Mpc... lets use an average of 65km/s/Mpc. 1 Mpc = 3,262,000 LY So, for every 3,262,000 LY of distance, an objects velocity of recession increases by 65km/s c = 300,000 km/s 300,000 km/s divided by 65 km/s is 4615.384615 (Mpc) 4615.384615Mpc x 3,262,000 LY per Mpc = 15.055 Billion LY So at about 15 billion LY objects are receding at the speed of light. This is a set distance... in 5 billion years from now, the point where objects recede at c will still be 15 billion light years away as long as the hubble constant remains constant. In fact, it has (not so) recently been thought that the recession velocity (ie the expansion of space) actually INCREASES over time... that its speeding up. If this is true then our OBSERVABLE universe is actually SHRINKING, not expanding at c. OK.. I'm ready for that foot.

I don't think the 'Hubble Sphere' itself expands at c... I believe that the edge of the hubble sphere is the point where galaxies and such are receding from us at c (due to the expansion of space)... the point where the observable universe ends. However, each point in the universe has it's own, different, hubble sphere. So the more distance and velocity you put between yourself and the Earth, the bigger the difference in your Hubble Spheres. So what I am wondering is if you accelerate for long enough, will you ever put enough velocity and spacial distance between yourself and Earth such that you will leave Earth's Hubble Sphere... effectively traveling away from Earth faster than light?

OK.. let's say I'm in a ship that accelerates away from Earth at an acceleration of 20m/s/s and then maintained that same thrust forever... At some point, wouldn't I have traveled far enough and fast enough, that I would have left Earth's hubble sphere, and therefore be traveling away from Earth at a velocity faster than light?

Sort of but not really... I guess I could be more technically specific... how about this... If I were in a ship and I left Earth (toward Vega) with a thrust that allowed me to move away from Earth at an acceleration of 20 m/s/s... and then I continued that same thrust for the entire trip to Vega... how long would it take me to get there? This scenario is NOT fantasy. You can accelerate forever and still never reach the speed of light relative to any point in space. (actually I am not sure that this statement is completely true, in fact I think I will have to ask another question related to this statement) Thank you, that sounds about right.