# losfomot

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323

1. ## Does gravity have an effective range?

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.
2. ## Does gravity have an effective range?

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.

4. ## Time Travel Question (Twins Paradox)

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.
5. ## Universal expansion acceleration

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?
6. ## The mechanics of gravity.

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.
7. ## Dark Light - Light's counter-part?

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.
8. ## The mechanics of gravity.

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.
9. ## The mechanics of gravity.

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!
10. ## The mechanics of gravity.

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)
11. ## The mechanics of gravity.

It seems to me that the OP meant that the planets all ORBIT THE SUN in the same direction
12. ## time dilation question

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.
13. ## Telescope for hobby

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.
14. ## Constant Acceleration 2

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.
15. ## Constant Acceleration 2

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'
16. ## Constant Acceleration 2

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"?
17. ## Constant Acceleration 2

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?
18. ## Proper motion in the universe

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.
19. ## Constant Acceleration 2

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?
20. ## Constant Acceleration 2

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.
21. ## Constant Acceleration 2

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.
22. ## Constant Acceleration 2

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.
23. ## Constant Acceleration 2

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?
24. ## Constant Acceleration 2

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?
25. ## constant acceleration

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.
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