# NowThatWeKnow

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## Posts posted by NowThatWeKnow

### 1G Relativistic Rocket Ride, Wheeee

that was all what I imagine of the movement in the space .so it just theories in my head .

As long as there is thrust pushing the rocket it will accelerate in the vacuum of space. Inertia says a body in motion will stay in motion in a straight line unless acted on by an outside force. So if there is thrust (outside force), the rocket will continue accelerating towards the speed of light but never make it. There will be no forces acting on the rocket except for the thrust (discounting space dust and gravity). So newton's theory of action and reaction fits well.

Where are the experts??

### 1G Relativistic Rocket Ride, Wheeee

you really hit something at that speed and make another big bang on our universe .

...at 1G of acceleration and every point of it you need more and more energy to keep you go that acceleration.

I would think in the vacuum of space a constant thrust could provide a constant 1G. Could someone elaborate on that please?

I totally agree if you hit something at relativistic speeds there will be a lot of energy created.

### centrifugal force

I understood that centrifugal force is an "apparent" force that is directed away from the center, but it is actually inertia telling a body in motion to stay in motion in a straight line.

### General Relativity stress-energy tensor

I knew it could be put into English! Thank you for coming out of your rubber room (compliment) to help me. It is Saturday night and time to chase a little skirt so the "cosmic vacuum" will have to wait.

Thanks Norman

### General Relativity stress-energy tensor

'v' is velocity of the mass distribution, 'rho', at some point in the field. We start only accounting for identifiable masses. Then we shall further account for energies and the kitchen sink... $T^{ab}$ is the stress energy tensor, which is to say how much stress and energy of all types identifiable in our theory, are present here and there. Go one step at a time.

The stress energy tensor, $T^{ab}$, is the gravatational attraction of matter considering MOMENTUM and ENERGY, and also circulations... and the kitchen sink.

Am I making progress?

### General Relativity stress-energy tensor

I think I can get you there. This is the representation of mass-energy sources in GR, the right-hand side (RHS) of the equations of non-empty space. Start with the accounting of masses, and since I don't recall at the moment how to write a matrix, look at the 4-vector of velocity distributions defined as: $v^a=<c, v^1,v^2,v^3>$. Simply, the tensor is defined, with mass density field $\rho$, by saying: $T^{ab}=\frac \rho c^2 v^a v^b$. There's more but the sun is out. Later!

Lets make sure I understand each character:

c=speed of light

v=volume and not velocity

$\rho$ = mass density (Different then just mass in KG? how is this determined?)

$T^{ab}$ What does this represent?

### General Relativity stress-energy tensor

Sorry about not wanting "complicated" math, but a tensor is a mathematical object. I don't think there's a way around it.

I do not mind some math (like Norman used) but what I usually get is an equation with a bunch of characters that are not defined plus the goal is also not defined.

Here is what I was looking for to get started but corrections will be needed of course as I do not know the facts or I would not have asked the question.

GR has to do with gravity, but unlike Newton who used only mass, GR will consider things like density and volume to...P=m/v with p=density, m=mass and v=volume....and then we...

See what I am looking for?

The first question is:

What is the stress-energy tensor going to tell us when we do the math? The gravitational attraction of certain matter?

### General Relativity stress-energy tensor

I have been trying to grasp the basics of "stress-energy tensor" and most information I have found starts above my head. I hope the basic concept can be put in English words without a ton of complicated math. It seems like more then just mass is being considered in GR when calculating the curvature of spacetime. Is that what it is all about? Links and explanations welcome.

### How many stars do dwarf galaxies have?

I learn something new every day......I watched the science channel last nite and always thought that the universe was moving away and spreading out, but learned that the our galaxy the milkyway is moving toward andramida galaxy. Interesting stuff:)

It seems that local gravitational forces causes many galaxies, and even galaxy clusters, to collide. However, after a certain distance away, the expansion of space makes sure all galaxies are moving away form us.

### Frozen earth

Wouldn't that be like freezing a beam of light? It may take absolute 0 -1

### what is the universe expanding into?

NowThat,

aaaarrrrgh! Please don't use the word singularity as if it was something real that actually existed!

...So it misleads people when one refers to a singularity as a physical natural something that could do something, like explode...

Hey, I started with "Even if". Isn't that worth something? Some of your post sound like NowThatWeKnow and that makes me go aaaarrrrgh!

### what is the universe expanding into?

Good suggestion! It reminds me that Ned Wright has an alternative version of the calculator here:

http://www.astro.ucla.edu/~wright/DlttCalc.html

where instead of putting in the redshift z, you put in the travel time (in billions of years, not billions of lightyears ) that the light took to get here...

...Anyway any sort of physical explosion is a misleading image and gives people the wrong idea. In an explosion, crud moves thru space and that simply doesn't fit the reality...

I like that calculator better. At first I thought they left out "z" but then I saw it at the top.

Even if the Big Bang was an explosion of singularity or a region, it seems it is not driving what we observe today. Maybe the BB ran out of energy and something else took over the expansion.

### what is the universe expanding into?

...then we are already admitting that the outer edges of our visible universe had only a little over 13 Billion years to travel 24 Billion LY in opposite directions, for a combined separation speed of about 48 Billion LY in only a little over 13 Billion years, then the light speed limit is broken anyhow. How do you explain that?

Airbrush,

Martin pointed me to a really neat calculator that helps put things in perspective.

http://www.astro.ucla.edu/~wright/CosmoCalc.html

The light from a galaxy that took 12 billion light years to get here started towards us when it was only 4.8 billion light years away. That galaxy is currently 23.58 billion ly away. The space the light traveled through had an expansion rate of 4.9. If you are interested, a few simple rules and definitions are needed to understand the calculator. If you put 3.9 for a value of "z" and click "flat" you may see where the #'s come from.

The light speed episode of "the Universe" program said "the only thing faster then light is the expansion of space. This may be the answer to the question. Also, a third party seeing two objects separating at faster then the speed of light does not mean those two objects see themselves separating faster then light. That crazy space math.

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html

and

http://www.cthreepo.com/cp_html/math1.htm

### i still dont understand how someone can "escape"time by traveling faster than light.

Do a google search for "faster then light speed" and you get all sorts of stuff.

### Toy model space that is not expanding into

NowThatWeKnow , start thinking 4-d and not 3-d with an "external" parameter = time.

In doing so you see that the expansion is really an "illusion" of the 4-d nature of space-time. Thus, no need to expand into anything.

By geometry being dynamical one simply means the metric depends on both where and when you are...

I try to think of space/time as a single construct. As time passes it stretches the metric of space with it. It sounds like you are implying that maybe space is not expanding. Would that require time to continually speed up for us to observe the expansion we see? It is hard enough to understand with out illusions making it even harder.

It is probably just as hard to imagine a finite universe with a boundary we can not cross as a infinite universe without a boundary to cross.

### Toy model space that is not expanding into

NowThat,

why should it be expanding into anything?

I was hoping to get ajb to elaborate on "Then we see that space gets larger as time does, yet it is expanding into nothing."

What could be closer to nothing then a vacuum that is void?

A thing does not need any surroundings in order to exist. Or?

Do you think that because the universe exists it must have some surroundings?

Not having a surrounding woud be easier to visualize if we observed a static universe but that is not the case. Our observations show that the distance between matter is expanding. Maybe it is not expanding into anything but is expanding through nothing.

None of us know for sure what is happening and it would be foolish to rule out any of the many possibilities. We all just put a different likelihood on each scenario. your knowledge allows you to grasp what you say much better then my knowledge allows me to grasp what you say. However, my respect for your knowledge tells me to give your views much consideration and I listen when you speak.

Now, if someone would get off their double sided end and figure out the real answer.

### Toy model space that is not expanding into

Geometrically, I liken the expansion of the universe to a cone. The cone is topologically (lets cut out the apex) $R^{1}\times S^{1}$. Now if we take time to be $R^{1}$ and space to be $S^{1}$. Then we see that space gets larger as time does, yet it is expanding into nothing.

Do you think it is possible that time is responsible for the expansion of space? Maybe time could be a type of ether. It seems the universe is a vacuum full of matter and electromagnetic radiation. Is it expanding into a vacuum that is void?

### i still dont understand how someone can "escape"time by traveling faster than light.

Actually, only electromagnetic radiation (like light and radio waves) can move at the speed of light because the photon has 0 mass. All matter is restricted to speeds below this. This short power point presentation may help you understand time dilation.

Also my thread in "relativity" "1G relativistic rocket ride Wheee" gets into it a lot.

### what is the universe expanding into?

you guys are right, im sorry. its not lazyness at all, i was just kind of pissed my question wasnt getting agreed with. no, i know when to stop and say im wrong, and im wrong. i agree, and finally understand{ i dont know why i wasnt getting it, just something didnt click}. i don like to consider myself a simpleton, but a 15 year old still has a long way to go i guess.

The thing is, it is possible that you are not wrong. Have you heard any one here that had the answer? You are doing fine for 15.

### 1G Relativistic Rocket Ride, Wheeee

According to the light speed episode of "The Universe" program on the History channel, .99c woud give you a very distorted view directly in front of you and you lose the distance advantage to the side. That program and these forums in the last 30 days is my total experience with relativity. But I do like to give my opinion. And I did stay at a Holiday Inn Express the other night.

### 1G Relativistic Rocket Ride, Wheeee

Still, the fizziks IS. Now help me see the far-field question. I guess that this does not magically make the "locally speeding" observer see farther than we can in terms of the early universe. They should perceive different distances, and times???

After you looking at my questions I am sure you do not want me answering your questions. My question is have we actually observed anything moving close to c that had transverse movement?

Edit - You bring up a very good point. The edge of our known universe would be very close at .99c.

### 1G Relativistic Rocket Ride, Wheeee

...Dick has an extended system corrected for the delays they perceive between themselves, all moving to our observations. ....Anyway, this is gonna be a mess because y'all at your different points of view do not agree on simultaneity at a distance.

I have "simultaneity at a distance" when I run these scenarios through my head.

### 1G Relativistic Rocket Ride, Wheeee

Oh, sorry, I didn't figure the relation.

Twins

### what is the universe expanding into?

so its like space is stretching, i understand, and i like it. but when you inflate a balloon it gets bigger, hence expanding into something, like the space around it.

and the idea that we just dont have the "equipment" to visualize space, although legit, seems to me like sheer laziness.

When my parents were young the Milky way was the known universe. Today that 100,000 ly has turned into 13 billion ly. The expansion of space and the speed of light keeps us from seeing any further (at this time). I do not see it as laziness.

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