# Faster than the speed of light

## Main Question or Discussion Point

I like to do whirlies with the kids down in my local park. I grab them by the feet and I spin them in a circle with thier whole body off the ground very fast, occasionally they throw up, or I do.

Sometime I do this at night.

when I spin them at night I look up at the stars and also at a start on the horison and the stars are spinning very fast.

Now one point of view is that I am stationary and the entire universe is spinning. Point of view number two is that I am spinning and the universe is stationary.

If the universe is spinning then the star I am looking at is travelling faster than the speed of light....oops

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when I spin them at night I look up at the stars and also at a start on the horison and the stars are spinning very fast.

when I spin them at night I look up at the stars and also at a start on the horison and the stars are spinning very fast.
SORRY THAT SHOULD BE

STAR ON THE HORISON

How Does This Fit With The Fact That There I No Preferred Frame ?

JesseM
Now one point of view is that I am stationary and the entire universe is spinning. Point of view number two is that I am spinning and the universe is stationary.

If the universe is spinning then the star I am looking at is travelling faster than the speed of light....oops
The rule that objects can't go faster than c is only meant to apply to inertial coordinate systems (as are all the other familiar equations of SR), it can certainly be violated in an accelerating coordinate system such as the rotating system you describe. Note that although objects will have a coordinate speed faster than c in this system, light in the same region can have an even higher coordinate speed...it is no longer accurate to use "faster than c" and "faster than light" interchangeably in non-inertial coordinate systems.

JesseM
How Does This Fit With The Fact That There I No Preferred Frame ?
Inertial frames are preferred over non-inertial ones (in GR as well as SR--see my post #8 here and post #14 here). It is only when you talk exclusively about inertial frames in SR (or locally inertial frames in GR) that the equations of physics, stated in non-tensor form, will be the same in every frame.

so my feet are spinning the entire universe thousands of times faster than C.....impressive

I thought the point of science was to reflect reality otherwise what is its use.....your reply implies that it is entirely possible that my feet are rotating the entire universe thousands of times faster than C ...... I wonder how much energy that would require... I wonder what the torque would be...I wonder what the impulse woulkd be when I stop...whats the friction..can the skin on the soles of my feet bear that :).... this is fanatsy, how come all the buildings around me arent falling down, trees lying flat. I hope the aliens living on the planets surrounding those distant stars had thier weeties

I may be a layman but there is just no way I am going to believe that. In fact those stars are doing millions possibly billions of times c. So we have matter doing billions of times C and not just a little bit of matter either, just the entire universe all powered by my quads and Ive got a dodgy knee

I was using my microscope the other night and I saw a virus spinning in a circle. To think a virus spinning the entire universe. An electron spins so it must be spinning the entire universe as well...not bad ..... sorry just a tad implasible

JesseM
I thought the point of science was to reflect reality otherwise what is its use.....your reply implies that it is entirely possible that my feet are rotating the entire universe thousands of times faster than C
No, this is just a feature of a particular coordinate system, not a physical possibility distinct from other physical possibilities. In some coordinate systems distant stars move faster than c, in others they don't. However, the laws of physics are adjusted to each coordinate system so that you'll make the exactly the same physical predictions no matter which one you use. Inertial coordinate systems are "preferred" in the sense that the laws of physics have a built-in symmetry which makes them work the same in every inertial frame.
rab99 said:
...... I wonder how much energy that would require... I wonder what the torque would be...I wonder what the impulse woulkd be when I stop...whats the friction..can the skin on the soles of my feet bear that :).... this is fanatsy, how come all the buildings around me arent falling down, trees lying flat.
Again, the laws of physics must be adjusted to every coordinate system--if you know the equations for the laws of physics in one system A (such as an inertial system where the laws take their usual form), and you know the coordinate transform between A and some other coordinate system B, then you just apply this transform to the equations of the laws of physics in A, and this gives you the correct form for the equations of the laws of physics in B. Using this procedure, it's guaranteed that A and B will make identical predictions about anything physically measureable, like the friction on your feet or the G-forces as measured by a physical accelerometer. So picking a weird coordinate system isn't giving you a new physical hypothesis about the universe, it's just a new way of labeling points in space and time with coordinates.

rbj
i dunno why JesseM (far more knowledgable about physics, particularly relativity) doesn't point this out, but there is a qualitative difference between translational motion (position moving in space) and rotational motion (spinning). even with "inertial" rotational motion (spinning at a constant rate), the matter particles in the spinning object experience acceleration and are not inertial. so, although there is no such thing as an absolute rest frame for translational movement to be measured against, there is an absolute difference between an object that is spinning and the same object that is not spinning.

so, no, there is no equivalence between you spinning (relative to the rest of the universe) and the universe spinning. you can have a spinning cooridinate system, but that is all that it is, a spinning concept. it is not spinning matter.

JesseM
I may be a layman but there is just no way I am going to believe that. In fact those stars are doing millions possibly billions of times c. So we have matter doing billions of times C and not just a little bit of matter either, just the entire universe all powered by my quads and Ive got a dodgy knee
You don't seem to understand the difference between actual statements about physics and statements about human-defined labels for the coordinates of events. If I want to label the event of me typing the first letter of this post with position x=0 meters, time=0 seconds, and to label the event of me typing the last letter with position x=20 trillion meters, time=1 second, then nothing is stopping me, and in this coordinate system my position has changed by 20 trillion meters in one second. But these are just labels, I'm not implying any hypotheses about the physical universe with these labels. Only if I ground my coordinate system in terms of readings on rulers and clocks with well-defined motion and synchronization schemes do statements about coordinates have physical meaning, and all coordinate systems will agree on facts about object's motion relative to these rulers and clocks, even if they disagree on the labels assigned to events like the object passing a particular marking on a ruler whose clock shows a particular reading.

JesseM
i dunno why JesseM (far more knowledgable about physics, particularly relativity) doesn't point this out, but there is a qualitative difference between translational motion (position moving in space) and rotational motion (spinning). even with "inertial" rotational motion (spinning at a constant rate), the matter particles in the spinning object experience acceleration and are not inertial. so, although there is no such thing as an absolute rest frame for translational movement to be measured against, there is an absolute difference between an object that is spinning and the same object that is not spinning.

so, no, there is no equivalence between you spinning (relative to the rest of the universe) and the universe spinning. you can have a spinning cooridinate system, but that is all that it is, a spinning concept. it is not spinning matter.
I did point out the basic difference between inertial and non-inertial coordinate systems in post #4, although you're right that I neglected to point out that all non-inertial motion can be detected in an absolute sense by measuring G-forces.

are you saying that you choose your coordinate system to ensure that the physics in one frame is the same as the physics in another different frame ..isnt that just a self fulfilling prophecy ? I mean how do we know the physics in one frame is the same as in another frame or is that an assumption?

In a lot of these posts ppl have have answered similar to
"you are travelling in a space ship and the other guy is in a car (just a hypothetical). The guy in the car doesnt know if he is stationary or moving and he also doesnt know if the guy in the space ship is staionary or moving, same for the guy in the space ship...they simply dont know. And apparently they cant use a reference point as one doesnt exist... therfore ipso facto there is no preferred frame of ref (FOR)

but if I am spinning a kid at night in my park and it appears to me that the entire universe is spinning then either:
im spinning and the universe is stationary;
the universe is spinning and I am stationary; or
both ...... which is it ?

its an easy enough question

I put my money on the universe is spinning :)

Ich
its an easy enough question
...and has been answered at least two times here.

Ok lets go back one step if i am spinning in my local park at night will the stars appear to be spinning yes/no?

Is the appearance of the stars spinning an arbitary fact open to interpretation. So if I got 1000 ppl who can see in my park spinning at night and asked them all are the stars spinning some would say no?

do we belive the ones who say no or do we discount them as being a few snaggers short of a barbie?

"is the universe spinning" is that a human defined label or a fact?

sorry I am a bit of a layman on these sorts of things and I just wanted to tell the kids which it was. So we have

1. im spinning and the universe is stationary;
2. the universe is spinning and I am stationary; or
3. both ...... which is it 1,2 or 3?

Im sorry for the slowness of my uptake but I am just a layman

I note im not interested in how fast it is spinning just that it is spinning yes/no answer only

JesseM
Only if I ground my coordinate system in terms of readings on rulers and clocks with well-defined motion and synchronization schemes do statements about coordinates have physical meaning, and all coordinate systems will agree on facts about object's motion relative to these rulers and clocks,

Ok well agree on a coordinate system and measurement system for me down at the park lets use the si system and cartesian coordinates

1. im spinning and the universe is stationary;
2. the universe is spinning and I am stationary; or
3. both ...... which is it 1,2 or 3?

Mentz114
Gold Member
No. Translational ( straight line) motion is relative and can only be defined wrt another inertial system. But rotation can only be defined for extended objects, and needs no external frame. It is not the same thing thing to say you are not spinning, and the universe is spinning around you.

Compare
"a state of rest cannot be distiguished from a state of uniform motion without reference to the outside world"

with

"you can always tell if you're spinning, without reference to the outside world".

If im walking in a straight line down at my park
1. is the entire universe moving and Im stationary? or
2. am I moving and the entire universe is stationary?

what is the significance of reference to the outside world? why is that such a no no? I mean everything we do is referenced to the oustside world. I would say the difficulty is not referencing the oustdie world after all thats were we live and that were physics happens?

No. Translational ( straight line) motion is relative and can only be defined wrt another inertial system. But rotation can only be defined for extended objects, and needs no external frame. It is not the same thing thing to say you are not spinning, and the universe is spinning around you.

Compare
"a state of rest cannot be distiguished from a state of uniform motion without reference to the outside world"

with

"you can always tell if you're spinning, without reference to the outside world".
If im walking in a straight line down at my park
1. is the entire universe moving and Im stationary? or
2. am I moving and the entire universe is stationary?

Mentz114
Gold Member
Neither, because you are not in uniform motion. Better to think about a spaceship. The crew fall asleep for a week and their ship wanders around at random. When they wake up, the engines are not running, so they are not being pushed. Nor are they spinning. Nor can they tell if they moving wrt nearby stars, until they do some measurements.

How Does This Fit With The Fact That There I No Preferred Frame ?
This is where your confusion is. There are preferred frames (i.e. spinning frames). The only concept of relativity (even classical relativity) is that there is no preferred inertial frame. Spinning frames are not inertial and therefore the laws of physics are different in them then in an inertial frame. Like you said, velocity is no longer limited by c in a spinning frame.

This does not mean you are spinning the universe. The reason you arent spinning the universe is because it is possible to measure if you are in a spinning frame or not. A person at rest in a spinning frame feels an inward acceleration, and can measure that the laws of physics are not the same as in an inertial frame. Because your kid feels this acceleration and can see stars moving faster than c, he is the one in the spinning frame, not the rest of the universe.

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If im walking in a straight line down at my park
1. is the entire universe moving and Im stationary? or
2. am I moving and the entire universe is stationary?
this is the whole concept of relativity (ignoring the external factors, assume ur moving at a constant velocity in space instead of walking). There is no absolute reference frame, and therefore it is relative which of you is moving depending on the frame. In your frame, your at rest and the universe is moving. In the "universe"'s frame, its at rest and your moving. There is no such thing as absolute position or velocity, only relative position and velocity.