# NowThatWeKnow

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391

## Posts posted by NowThatWeKnow

### 1G Relativistic Rocket Ride, Wheeee

They would have been better off spending their time together, no?

I wouldn't want to spend that much time with my brother or sister but maybe twins would.

### 1G Relativistic Rocket Ride, Wheeee

Also, it is so that the younger of the twins experienced acceleration, right? There has to be such a logical way to distinguish them, at least if they start in the same frame and end up similarly.

I think the younger twin is the one that accelerated but am confused by "The one who doesn't accelerate. Being at rest in a gravity field is an accelerated frame, which is one of the quirky concepts of GR."

I posted these four twin scenarios in another thread and there were no replies. I know that 1 and 2 are true so 4 should also be true. The calculators say 3 is not true unless Jane stops separating from the Earth and waits for Dick to make a similar trip speed and acceleration wise.

Since I am wrong much of the time would someone confirm that.

1. Dick takes a space trip at relativistic speeds and when he comes back to Earth he is younger the Jane.

2. Jane doesn't want to be old so she takes a similar space trip and comes back the same age as Dick.

3. Jane wants to be younger then Dick so she starts off an a space trip. Some time later Dick decides he doesn't want to be older so rather then wait for Jane to turn around and come back, he jumps in his space ship and goes to her. They are the same age?

4. They both want to be younger and take off an similar space trips but in the opposite direction. When they get back they will be the same age even though they were in different inertial frames the entire trip. Is that right? The separation and closure during the trip was equal.

### 1G Relativistic Rocket Ride, Wheeee

As you can see, it would take more work to lift the object agains 1g the whole way than it would to lift it against Earth's decreasing energy.

And even though thare is no difference in g-force in the uniform field between the surface and the distance of the Moon, there is a large differnce in potential, which is what effects gravitational time dilation.

With the spaceship accelerating at 1g, you have to assume a uniform gravitational field that extends for inifinity, and that the Milky Way is falling through that field if you want to use the GR approach to the problem.

Excellent! Another piece of the puzzle starts to come into focus. Could you explain the difference between 1G from the rocket engines or 1G from (the apparent) centrifugal force. Inertia could keep a rocket spinning for 1G and not use fuel. Does the 1G have to be consistent with the direction of travel?

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Location, location, location. The potential is the depth of the well. The acceleration is the slope of the sides. e.g. if g is large, it says that a small change in position gives a large change in potential, but it doesn't tell you what the potential is.

The one who doesn't accelerate. Being at rest in a gravity field is an accelerated frame, which is one of the quirky concepts of GR.

Between you and Janus I am making progress. It does seem that when things seem clear, I can view things from a slightly different angle and take them out of focus again. The first paragraph above is an example of what sinks in and should stick. The second is what makes my head hurt. I am hoping that others might benefit from my questions and your efforts.

### what is the universe expanding into?

NTWK, I'd warn against taking History channel literally! All the comment I heard about their "The Universe" program was that it was embarrassingly inaccurate. Didn't watch it myself...

From a layman's point of view there are good and bad episodes. Some is pure speculation but they do not misrepresent what they say. Some seems factual but I do not have your eye for it. They did peek my interest and that is why you are having to put up with me.

### 1G Relativistic Rocket Ride, Wheeee

And they will observe the maps to be wrong when they are moving. You're taking measurements from two different frames, and that's not how you measure the speed of light.

The acceleration is not the right thing to look at, it's the potential...

...gravitational potential from accelerating at g is g*d/c^2. If you recast that in terms of speed, it's v^2/2c^2, which is what you get if you assume it's all from the speed....

I was mixing frames trying to better visualize what is happening in both frames. In the end they traveled over 100,000 ly and and aged 12 years while the galaxy aged over 100,000 years but never exceeded the speed of light.

I understand the definition of gravitational potential but am not getting a clear picture on how it fits in to this picture.

Is the "g" in the above formula gravitational potential that should be "G" so google can work with it? Or is it # of g's acceleration.

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Length contraction is not a "visual distortion", according to the crew, the Milky way really shrinks in the direction of travel. You can't mix measurements from different frames (distance from the frame of the Galaxy and Time from the frame of the ship) and get a result that has a physical meaning.

It was more like keeping up with both frames while being temporarily in one. The space travelers real world is at rest in the Milky way.

How did you arrive at this answer? Did you use the standard gravitational time dilation formula? If so, it is invalid for this situation. It is for use for gravitational fields that fall off by the square of the distance. Remember Gravitational time dilation is dependent on difference in gravitational potential, not differences in local gravitational strength or local acceleration due to gravity.

(G * mass of Earth) / (radius of Earth * (c^2)) = 6.95453588 × 10^-10

But as you pointed out it does not apply here.

If Gravitational time dilation is dependent on difference in gravitational potential, what influences the change in gravitational potential? Not at all clear here.

In this situation you can either calculate strictly using speed and SR or using GR (ship standing still in 1g uniform gravity field while the galaxy falls past you), but you do not combine them.

But who gets old is determined by who accelerates to a different frame?

This has been demonstrated by using high speed centrifuges (up to 10^18g). It has been shown that the time dilation only depends on the speed at which the object is moving and not the g's it experiences. With different radii of centrifuges you can have the same speed but experience different g forces, but the sample on the centrifuge will show the same time dilation.

The online dictionary says:

"an accelerating dragster or space shuttle. Spinning objects such as merry-go-rounds and ferris wheels are subjected to gravitational time dilation as an effect of their angular spin."

### 1G Relativistic Rocket Ride, Wheeee

I have a couple of questions later for Janus and swansont but it is Saturday night and time to chase skirt.

### what is the universe expanding into?

i agree, and where is this thread at? i havent seen it yet, id like to read it.

Where Does Space End? It Must End Somewhere!

Martin warned you and I agree with him. However, you may learn something if you can wade through the trash.

I should have said if the universe is finite and flat, there may be an edge. "The universe" program on the History channel says so far the laser measurements say it is flat. Even if it is a 2d spherical surface it could be getting larger like the Hubble balloon. It is expanding into...

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I think you still might have to explain your point a bit more...

I am glad that I am not the only one.

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map=space=metric

territory=reality

reality=????

### Hot Jupiters.

Yes, notice how it says CHARGED PARTICLES, not ALL particles. Gamma rays and regular light can penetrate the magnetic field with ease. It supports my position completely.

So you are saying that charged particles from the solar wind would have no influance on the atmosphere of a planet if they got through the magnetic field? Just a question, I do not know the answer.

### what is the universe expanding into?

if the universe is expanding, then it has to expand into something. just because we dont know why its doing this doesnt mean it doesnt make sense.

Maybe the question should be is the universe infinite or finite and if it is finite, what is past the edge. Seems like there is a long thread about that.

### Hot Jupiters.

No it would not. Magnetism only works on other materials that are magnetized, and ...

http://en.wikipedia.org/wiki/Earth's_magnetic_field

"The magnetosphere shields the surface of the Earth from the charged particles of the solar wind and is generated by electric currents located in many different parts of the Earth. It is compressed on the day (Sun) side due to the force of the arriving particles, and extended on the night side."

### 1G Relativistic Rocket Ride, Wheeee

1. False. The ship will observe length contraction of the path of the trip.

2. False. Two sides of the same coin. You can approach the problem using either solution, and if you do it right you'll get the same answer.

Let me rephrase #1:

Speed as observed by the crew and shown by the ships clocks well exceeds the speed of light relative to the Milky Way when star map time and distance measurements are taken. This would be an observation from math measurements and not a visual view distorted by length contraction. We know that star A and star B on our map are x ly apart when at rest in the Milky Way frame.

2. I was afraid you were going to say that. Help me out here. I view the 1G force as a byproduct of acceleration. My calculations show 1 G vs way out in space as .22 seconds per year. Nothing to get excited about. Using the calculators on

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

shows a non accelerating frame speed of .97c having a time dilation of 4.11

Consider my chart.

After 2 years accelerating you reach a speed of 0.97c with a time dilation of 3.99

Is the small difference because of 1G gravity? By looking at this it seems that speed, and not gravity, is responsible for most of the time dilation. Also as the speed increases and gravity remains constant, the time dilation and length contraction increases.

### 1G Relativistic Rocket Ride, Wheeee

i'll go!

interesting point they make about C.B.R. getting blue-shifted enough to melt the ship.

We leave tomorrow if the two questions are answered. You don't have any thought on that?

### 1G Relativistic Rocket Ride, Wheeee

We are going to travel 113,242 light years across the Milky Way starting at rest in our galaxy's inertial frame near an outer edge. We will use a continuous 1G of acceleration the entire distance. The table below is from

http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

T= time in rockets inertial frame t= time in earths inertial frame

d= distance in light years v= light speed Y= time dilation

T --------- t ----------- d ----------- v -------------- γ

1 year --- 1.19 yrs ----- 0.56 lyrs --- 0.77c ----------- 1.58

2 ------- 3.75 -------- 2.90 -------- 0.97 ----------- 3.99

5 ------- 83.7 -------- 82.7 -------- 0.99993 --------- 86.2

8 ------- 1,840 ------ 1,839 -------- 0.9999998 ------ 1,895

12 ------ 113,243 ---- 113,242 ------ 0.99999999996 -- 116,641

Relative to the clocks in the space ship and as observed by the crew, they will travel well over 100,000 light years in just 12 years. Their speed increases during the trip, as the distance covered according to their galaxy map shows.

True or False:

1. Due to time dilation, the ship's inertia frame as observed by the crew and shown by the ships clocks well exceeds the speed of light relative to the Milky Way. Especially during the last half of the trip.

2. 99%+ of the the time dilation is caused by SR with only a small part of the time dilation caused by GR because of the 1G from acceleration.

### Stopping the clock

decimal points are slippery little suckers. fortunately, you can always catch mistakes later(but hopefully before anybody reads your post).

It seems I would have easily seen a decimal point out of place. Not sure what I did at this point.

### Stopping the clock

Mass is probably kg and length (radius) metres, these are the SI units.

That was my first thought but...

radius of Earth = 6,378.1 kilometers

(G * mass of Earth) / (radius of Earth * (c^2)) = 6.95453588 × 10^-10

Of course. If I use meters instead of kilometers.

(G * mass of Earth) / ((6 378.1 * (10^3)) * (c^2)) = 0.695453588 nanometers.

Can I blame it on being old?

Thanks

moth, Thanks for the link. I understand what is happening but like you, I have a little trouble with the math. Even with the simple math sometimes as you see above.

### Black Hole collapse

I don't know about matter and antimatter reacting. If there was any antimatter around it would have been annihilated immediately on contact with matter, before approach to the black hole.

I agree. If matter and antimatter can not coexist, where in the universe would you find antimatter and what produced it?

Welcome.

### Stopping the clock

It seems our local gravity does not have a large influence on our clocks

--------

(G * mass of Earth) / (radius of Earth * (c^2)) = 6.95453588 × 10^-10

Because of our Earth's mass and radius, clocks are slower by about 0.7 parts per billion. 60.5 microseconds per day. .022 seconds year compared to way out in space.

(G * mass of the Sun) / (radius of the Sun * (c^2)) = 2.12324397 × 10-6

Because of our Sun's mass and radius, Clocks on the surface would be slower by about 2.1 parts per million. .1728 seconds day. 63 seconds year compared to clocks way out in space.

(GPS satellite - Altitude 12,000 miles) Our Earth clocks are slower by 45 microseconds per day.

---

I was trying to figure out how to use these formulas to calculate the 45 microseconds difference in time between the GPS satellite and the Earth surface. I can not even tell what unit of measurements are being used for "radius of Earth" or "mass of Earth". I would also like to know the time difference between the Sun's surface and the Earths surface. Can anyone help?

### What is time?

Then the question would be, does time exist?

Yes! Time is the power to produce an outcome or achieve a result. When I look in the mirror I can see that time does exist. However, with out consciousness it may not exist.

### galaxies receeding at the speed of light

Martin has a sticky on "Cosmo Basics" with several good links in it. I liked this one and it answers many questions and is a fun read.

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Local gravity keeps the galaxies and even galaxy clusters from expanding but the space in between expands. As Baby Astronaut pointed out.

### What is time?

I'm confused. Do you think that there is a problem with that? After all every single word that has ever existed on this planet was invented by a human and defined by a human. What's the objection to that?

Not at all. You pointed to the answer. Why struggle with words when someone already did it for you? Good post. I'm on you side here.

### What is time?

Here is a good working definition of time

http://en.wikipedia.org/wiki/Time

There you go. A word invented by man and defined by man. I like Bookshelf's definition:

"A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future."

### What would proving there's life on Mars do for science?

In stead of worrying about us killing the life on Mars, many will worry about Mars' life killing us. I'm sure this would also be investigated...

Exactly! Not that they expected life on the moon but the astronauts were quarantined after the first moon trip. Who were we protecting? Us or them?

### Where Does Space End? It Must End Somewhere!

this is more theory than fact but you have to think what was here before time what was there before space what was there.com think of it like this im guessing yes the universe has boundaries but the boundaries have no boundaries if there was a place before time and space surely the universe can expand into that

Hi "12 year old boy with a 1200 IQ". Could you use some punctuation so us folks with a lower IQ don't have to work so hard?

### Stopping the clock

it would be interesting to find out if 1G in a rotating artificial gravity chamber caused the same amount of Lorentz contraction and dilation as 1G on earth, but when i try the math i get confused about that vx/c^2 term in the time part of Lorentz equation.

In another thread there were several that agreed that it was.

### Stopping the clock

The use of centrifugal force means you are in a rotating coordinate system, not an inertial one. You still have a gravitational potential.

...

Spinning a clock will slow it...

You say a lot with a few words. It is becoming more clear to me why "gravitational potential" is used often.

It seems the curvature of space is the key to clock speed and not just G forces that you feel. A merry-go-round warping space/time.

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Moreover, centrifugal force isn't real. It is best to forget that concept.

How about that apparent centrifugal force or maybe inertia force.

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