Time and Acceleration questions

[Mot]

I think these questions are linked, but I don’t really know how to find the answers.

 

1) Does time exist outside of a gravity well? I was listening to Science Friday and remembered these questions. I believe that time only exists if there is a mass acting on the observer even if that mass is the observer.

 

2 If you are in a space station that is spinning to give artificial gravity, then will spinning the space station always cause artificial gravity? My guess is that in order to have the acceleration, the space craft has to be in a gravity well. The corollary question is, does a change in the gravity well affect the perceived gravity inside the space station?

 

Thanks

Mot

[swansont]

I think these questions are linked' date=' but I don’t really know how to find the answers.

 

1) Does time exist outside of a gravity well? I was listening to Science Friday and remembered these questions. I believe that time only exists if there is a mass acting on the observer even if that mass is the observer.

 

2 If you are in a space station that is spinning to give artificial gravity, then will spinning the space station always cause artificial gravity? My guess is that in order to have the acceleration, the space craft has to be in a gravity well. The corollary question is, does a change in the gravity well affect the perceived gravity inside the space station?

 

Thanks

Mot

 

1. Yes. It actually flows fastest there.

 

2. Yes and yes. Classically speaking, gravity is another force and they are vectors.

[Mot]

I don't understand why you say yes to 2a. Do you know a reference or two that would explain in more detail for both question 1 and question 2?

 

thanks

Mot

[J.C.MacSwell]

 

2 If you are in a space station that is spinning to give artificial gravity' date=' then will spinning the space station always cause artificial gravity? My guess is that in order to have the acceleration, the space craft has to be in a gravity well. The corollary question is, does a change in the gravity well affect the perceived gravity inside the space station?

 

Thanks

Mot[/quote']

 

2. No. The spinning will have virtually no effect on a mass that is not spinning with the space station.

[Mot]

2. No. The spinning will have virtually no effect on a mass that is not spinning with the space station.

The question I am trying to ask is different. Let's assume all the mass in the space craft is spinning too. Now take away all mass outside the space station. If you are outside any gravity or reference how can you even say that the space craft is spinning and how can there be any artificial gravity acting on the mass inside the station? Now let us put an identical spacecraft next to ours and on the same axis. Have it not spinning? Which one has artificial gravity? How can you tell?

[Xyph]

The one that is spinning has artificial gravity on the inside surface. You can tell it is spinning because it has artificial gravity, and the other one doesn't.

[swansont]

The question I am trying to ask is different. Let's assume all the mass in the space craft is spinning too. Now take away all mass outside the space station. If you are outside any gravity or reference how can you even say that the space craft is spinning and how can there be any artificial gravity acting on the mass inside the station? Now let us put an identical spacecraft next to ours and on the same axis. Have it not spinning? Which one has artificial gravity? How can you tell?

 

A spinning reference frame is not inertial, and you will observe that Newton's laws do not hold. From that you can tell that you are spinning.

[Bill Wolfe]

Mot. . .

To answer your questions:

 

"Does time exist outside of a gravity well?" Yes, time exists everywhere, even in the meanest nastiest black hole. It just moves more slowly FROM THE PERSPECTIVE OF ANY OBSERVER OUTSIDE THE BLACK HOLE. The view from the inside would be the opposite, the rest of the universe would appear to speed up.

 

"If you are in a space station that is spinning to give artificial gravity, then will spinning the space station always cause artificial gravity? " The centripetal (or centrifugal, depending on where you are in the system) force mimics gravity but it is not. It's just Newtonian necromancy. And yes, no matter where you are in the universe. . .even in an area of space so empty that galaxies are not visible. . .it will work exactly the same as in free fall or at a la Grange point.

 

"My guess is that in order to have the acceleration, the space craft has to be in a gravity well. " This statement makes no sense to me. If you mean the radial acceleration of the spinning body. . .no, it has nothing to do with a gravity well. Don't forget that if you entered your hypothetical spinning station at the axis and never touched the sides, you would be in free fall until you grabbed something moving and let it pull you around and around.

 

If you did it just right using suit thrusters, you would hover just above the moving 'floor' of the space station while people standing on the 'floor' would zip past you. This scenario assumes no air in the station, by the way. The moving air inside would eventually cause you to 'catch up' with the spin of the station and you would 'fall' to the 'floor.'

 

"The corollary question is, does a change in the gravity well affect the perceived gravity inside the space station?" The answer to this is probably 'no.' If you are in a gravity well and the spinning space station is in free fall--either because it's in a stable orbit or because it is freely falling toward whatever massive body is the source of the gravity. . .then there would be no effect at all. But if thrust is being applied to the spinning station to keep it from falling into the gravity well, that thrust would tend to tilt the forces inside the station in a direction opposite that of the thrust.

 

Does any of this make sense?

[Bill Wolfe]

1. Yes. It actually flows fastest there.

 

2. Yes and yes. Classically speaking' date=' gravity is another force and they are vectors.[/quote']

 

Uh, no swantsont, it doesn't. Time 'flows' more slowly in the stronger gravity well.

 

As for the yes and yes answers. . .if you're interested, see my other post to the original question.

[J.C.MacSwell]

Uh' date=' no swantsont, it doesn't. [b']Time 'flows' more slowly in the stronger gravity well. [/b]

As for the yes and yes answers. . .if you're interested, see my other post to the original question.

 

Then you should agree that the answer is "yes"?

[swansont]

Uh, no swantsont, it doesn't. Time 'flows' more slowly in the stronger gravity well.

 

 

...so it flows fastest where there is no gravity, i.e "ouside of a gravity well," which is what I said.

 

I think one of us is missing a minus sign, as it were.

[Mot]

Ok let me try again. Take away all matter from the universe. Put in one small jumbo jet sized space station. Let's apply some force at right angles to the axis of rotation. I agree that during the application of force to get the space station spinning that there will be artificial gravity associated with the force applied. After the jets are cut off, what reference can say that the space station is spinning? I don't buy that you will just feel artificial gravity from the spin. There has to be something acting on the body to make the observer in the body feel the 'gravity'. I am assuming it is an aspect of gravity. Even in free fall, a space ship is still being acted on by gravity. I only have four forces to work with and it seems the most likely, so take away any gravity well then it shouldn't matter how fast the space ship spins, there will be no spin felt once the jets quit. I think I am outside the realm of counting infinity and in the realm of Aleph One or Two to make a math comparison. If you assert that there is an artificial gravity if there is a spin, then explain where your reference point is. Did you set a small camera in space before you started this thought experiment to watch the acceleration of the space craft? Anyone know where a good source of information for this type of inquiry is? I like Brian Greene, but it is not in his books, and I am limited to internet at the moment.

 

On the time question. The reason I asked it is because I think time is meaningless in an area with no gravity and also in an area with infinite gravity. In between time flows slower in high gravity such as a black hole has and faster as the gravity decreases, but if the observer is in the black hole or in a place with little or no gravity acting on them, then they would not notice a change in time. You could say that time and weight are in some sense inverse to each other. I think this question is easier to tackle than the spinning space craft though they are linked. I just don't have an understanding of how they are linked.

[swansont]

Ok let me try again. Take away all matter from the universe. Put in one small jumbo jet sized space station. Let's apply some force at right angles to the axis of rotation. I agree that during the application of force to get the space station spinning that there will be artificial gravity associated with the force applied. After the jets are cut off, what reference can say that the space station is spinning? I don't buy that you will just feel artificial gravity from the spin. There has to be something acting on the body to make the observer in the body feel the 'gravity'.

 

Which would be the circumference of the station following a curved, rather than straight, path. Someone standing on that surface would have to feel a force toward the center of the station.

 

Spin by itself doesn't create the force. You are only going to feel the force if you are touching the floor of the station. (the floor here being the surface at some value of R)

[J.C.MacSwell]

Ok let me try again. Take away all matter from the universe. Put in one small jumbo jet sized space station. Let's apply some force at right angles to the axis of rotation. I agree that during the application of force to get the space station spinning that there will be artificial gravity associated with the force applied. After the jets are cut off' date=' what reference can say that the space station is spinning? I don't buy that you will just feel artificial gravity from the spin. There has to be something acting on the body to make the observer in the body feel the 'gravity'. I am assuming it is an aspect of gravity. Even in free fall, a space ship is still being acted on by gravity. I only have four forces to work with and it seems the most likely, so take away any gravity well then it shouldn't matter how fast the space ship spins, there will be no spin felt once the jets quit. I think I am outside the realm of counting infinity and in the realm of Aleph One or Two to make a math comparison. If you assert that there is an artificial gravity if there is a spin, then explain where your reference point is. Did you set a small camera in space before you started this thought experiment to watch the acceleration of the space craft? Anyone know where a good source of information for this type of inquiry is? I like Brian Greene, but it is not in his books, and I am limited to internet at the moment.

 

On the time question. The reason I asked it is because I think time is meaningless in an area with no gravity and also in an area with infinite gravity. In between time flows slower in high gravity such as a black hole has and faster as the gravity decreases, but if the observer is in the black hole or in a place with little or no gravity acting on them, then they would not notice a change in time. You could say that time and weight are in some sense inverse to each other. I think this question is easier to tackle than the spinning space craft though they are linked. I just don't have an understanding of how they are linked.[/quote']

 

I think you are underlining an old idea of mach's (or his followers). If you doubled the amount of mass in the universe your (you at the same mass)inertia would double, if you halfed it your inertia would decrease by half.

 

Obviously no experimental results on that one!

[Mot]

I think you are underlining an old idea of mach's (or his followers). If you doubled the amount of mass in the universe your (you at the same mass)inertia would double' date=' if you halfed it your inertia would decrease by half.

 

Obviously no experimental results on that one![/quote']But how does my mass know the other mass is out there? Is it a constant in the universe or does it depend on how close I am to other mass? If it is proportional to distance then there must be a critical mass (or event horizon). I don't know how to design an experiment to test this unless somebody has a handy anti-gravity chamber they could lend. I am a math guy, so the only lab I have ever used consisted of a pad of paper and a pencil (and some computer simulation these days).

 

Thanks for pointing me to Mach. That will probably keep me busy for a bit while I figure this out some more.

Mot

 

PS This is par for the course for not being on the cutting edge of science, but at least I was expecting this to be old hat to somebody this time. I came up with a bunch of new math in the area of infinite series to work out some 2D shallow water equations in a discrete environment back in the 90's only to discover after a bit of research that it was all done and published in the early 1900's. Talk about a bummer. I was excited for an hour or two. I still found a leaf or two of knowledge but not a whole branch like I first thought.

[Severian]

But how does my mass know the other mass is out there?

 

This is actually a very good question. It was think sort of question that led Einstein to think up GR. You have demonstrated that motion in a gravitational force looks exactly like motion in a non-inertial frame. Einstein went on to postulate that they are the same thing - given the information that you have in the closed system you can't tell whether or not it is gravity or motion. (Einstein used a lift as his non-inertial frame, but it is the same thing.)