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View Full Version : Is there gravitational-type time dilation in a centifuge?

WaxRubiks
2017-Feb-09, 11:29 PM
If it goes very fast, would there be time dilation due to the pseudo-gravity?

swampyankee
2017-Feb-09, 11:36 PM
I like this question. My first thought is "no," because gravitational time dilation is due to mass.

Noclevername
2017-Feb-09, 11:45 PM
There's relativistic time dilation if it spins fast enough, but not gravitational.

WaxRubiks
2017-Feb-09, 11:48 PM
Yea, I suppose. Something fires up from the inner surface isn't in freefall back towards the inner surface....I think that's part of it.

grant hutchison
2017-Feb-10, 12:01 AM
There's also the whole "absence of gravity" thing.

Grant Hutchison

Noclevername
2017-Feb-10, 12:04 AM
Centrifugal force is not actually gravity. It's not even a force.

We may call it "pseudo-gravity" for its convenient macroscale effects, but physically it's just movement.

WaxRubiks
2017-Feb-10, 12:05 AM
It's acceleration.

Noclevername
2017-Feb-10, 12:08 AM
It's acceleration.

... Right. :doh:

ShinAce
2017-Feb-10, 01:13 PM
If it goes very fast, would there be time dilation due to the pseudo-gravity?

Yes. You can look up the Ehrenfest paradox to get a better idea of the physics and maths involved.

In the centrifuge frame, there's increased gravity. In the stationary frame, the observer in the centrifuge needs to be treated as a rotating observer.

Noclevername
2017-Feb-10, 01:44 PM

Noclevername
2017-Feb-10, 01:51 PM
(also :doh:)

grant hutchison
2017-Feb-10, 02:31 PM
Yes. You can look up the Ehrenfest paradox to get a better idea of the physics and maths involved.Ehrenfest is a special relativity "paradox", however - there's no gravitational time dilation involved.

Grant Hutchison

StupendousMan
2017-Feb-11, 01:11 AM
Physicists have used high-speed centrifuges to measure relativistic effects: look at the experiment (from 1963) detailed in the final section of this page:

http://spiff.rit.edu/classes/phys314/lectures/doppler/doppler.html

The original paper describing the experiment is Kundig, Physical Review 129, 2371 (1963).

http://journals.aps.org/pr/abstract/10.1103/PhysRev.129.2371

Buttercup
2017-Feb-11, 01:46 AM
Yes.

But I'm pressed for time.

VQkr
2017-Feb-11, 07:48 AM
Yes.

But I'm pressed for time.

Boo.

But if you spun a centrifuge such that the outside edge was very, very close to c, then wouldn't the mass (as measured by an observer outside the centrifuge) increase to the point in which a significant gravitational field formed around the centrifuge, dilating time?

Noclevername
2017-Feb-11, 08:15 AM
But if you spun a centrifuge such that the outside edge was very, very close to c, then wouldn't the mass (as measured by an observer outside the centrifuge) increase to the point in which a significant gravitational field formed around the centrifuge, dilating time?

Do you mean besides the time dilation of moving at near-C?

swampyankee
2017-Feb-11, 01:43 PM
Centrifugal force is not actually gravity. It's not even a force.

We may call it "pseudo-gravity" for its convenient macroscale effects, but physically it's just movement.

Could you tell it from any other acceleration in a strictly local experiment?

publiusr
2017-Feb-11, 08:32 PM
As an aside, how fast could a cosmological defect like a texture rotate--and could a neutron star spin fiast enough that things along the equator don't get crushed?

ShinAce
2017-Feb-11, 11:40 PM
Ehrenfest is a special relativity "paradox", however - there's no gravitational time dilation involved.

Grant Hutchison

Agreed. With some calculus and the Minkowski metric, you can resolve the paradox. However, it's a segue to general relativity.

swampyankee
2017-Feb-12, 03:47 PM
As an aside, how fast could a cosmological defect like a texture rotate--and could a neutron star spin fiast enough that things along the equator don't get crushed?

What kinds of things? People or steel blocks?

antoniseb
2017-Feb-12, 04:51 PM
What kinds of things? People or steel blocks?
Well. no in either case, at least not for more than a very short time, as these things spin down rapidly enough to prevent even steel blocks from having the strength to stand up without a lot of magnetic support.

VQkr
2017-Feb-12, 05:54 PM
Do you mean besides the time dilation of moving at near-C?

Correct. If the accelerating centrifuge's mass increases as measured by a non-accelerating observer, wouldn't this increase the gravitational field, eventually to the point that a nearby observer experienced relativistic effects due to the gravity alone (as measured by a more distant observer)?

Noclevername
2017-Feb-12, 05:57 PM
Correct. If the accelerating centrifuge's mass increases as measured by a non-accelerating observer, wouldn't this increase the gravitational field, eventually to the point that a nearby observer experienced relativistic effects due to the gravity alone (as measured by a more distant observer)?

AFAIK no, relative mass does not produce actual gravity.

ADDED: If it did it would make artificial gravity a snap.

1. Create a sturdy centrifuge
2. Spin matter at 99% C
3. Profit!

swampyankee
2017-Feb-12, 07:12 PM
Correct. If the accelerating centrifuge's mass increases as measured by a non-accelerating observer, wouldn't this increase the gravitational field, eventually to the point that a nearby observer experienced relativistic effects due to the gravity alone (as measured by a more distant observer)?

Many physicists do not like the term "relativistic mass increase," because it doesn't increase gravitational fields.

Grey
2017-Feb-13, 01:54 AM
If it goes very fast, would there be time dilation due to the pseudo-gravity?There should be. From the perspective of someone in the centrifuge, a clock closer to the center would be "higher" in a gravitational field, and so should run faster than a clock farther from the center. Similarly, in a linearly accelerating rocket, a clock closer to the front is "higher" in an apparent gravitational field, and so should run faster than a clock closer to the back. Remember the principle of equivalence. You shouldn't be able to do any local experiment that can distinguish between a gravitational field and a constant acceleration. For the centrifuge, there's also the sideways motion, and I suppose that might have some effects.

StupendousMan
2017-Feb-13, 03:06 AM
For the centrifuge, there's also the sideways motion, and I suppose that might have some effects.

Post #13 above refers to a paper published in 1963 which measures exactly the relativistic effects expected for the sideways motion. No need to suppose.

Noclevername
2017-Feb-13, 03:10 AM
So it's no on the gravity, right? All the time dilation is due to the relativistic spin.

Just checking to see if we're all on the same page.

VQkr
2017-Feb-13, 04:52 AM
Many physicists do not like the term "relativistic mass increase," because it doesn't increase gravitational fields.

Thanks for the clarification, to both you and Noclevername!

Noclevername
2017-Feb-13, 05:00 AM
Thanks for the clarification, to both you and Noclevername!

You're welcome.

Grey
2017-Feb-13, 02:18 PM
So it's no on the gravity, right? All the time dilation is due to the relativistic spin.

Just checking to see if we're all on the same page.You should be able to do the math from the perspective of the lab frame, where there's no extra gravity and all the relativistic effects come from the motion of the objects in the centrifuge. But you should equally well be able to give an answer from the frame of the accelerating centrifuge, treating that acceleration as a gravitational field instead, using the equivalence principle from general relativity. The net result should be the same (i.e., if you bring two clocks together in some manner, an observer in the lab frame and an observer in the centrifuge frame should agree on what the readings of those two clocks should be when they meet). I'd say that both frames of reference could be considered equally valid.

Grey
2017-Feb-13, 02:24 PM
Post #13 above refers to a paper published in 1963 which measures exactly the relativistic effects expected for the sideways motion. No need to suppose.Yes, but that's from the lab frame, using special relativity and treating the time dilation as entirely caused by the motion of the wall of centrifuge relative to the center. My point is that you should be able to treat this case using general relativity, considering the centripetal acceleration to be equivalent to a gravitational field. I know you can do this easily (well, "easily" in the sense that you still have to work with general relativity...) with a linearly accelerating rocket without having to worry about the velocity, but in the case of the centrifuge, I'm not sure if there would have to be an additional correction for the transverse velocity.

swampyankee
2017-Feb-13, 04:25 PM
Yes, but that's from the lab frame, using special relativity and treating the time dilation as entirely caused by the motion of the wall of centrifuge relative to the center. My point is that you should be able to treat this case using general relativity, considering the centripetal acceleration to be equivalent to a gravitational field. I know you can do this easily (well, "easily" in the sense that you still have to work with general relativity...) with a linearly accelerating rocket without having to worry about the velocity, but in the case of the centrifuge, I'm not sure if there would have to be an additional correction for the transverse velocity.

The important question for that is whether the acceleration can be distinguished from gravitational acceleration with a strictly local experiment.

WaxRubiks
2017-Feb-13, 04:38 PM
The important question for that is whether the acceleration can be distinguished from gravitational acceleration with a strictly local experiment.

My guess, that if a small ball is launched towards the centre, its trajectory would be different to a ball in a gravitational field.

grant hutchison
2017-Feb-13, 04:48 PM
My guess, that if a small ball is launched towards the centre, its trajectory would be different to a ball in a gravitational field.Sure - you'd have Coriolis effect. But swampyankee said "strictly local", which implies operating on a scale where these effects fall below measurement error. It's how the Principle of Equivalence is applied - you can tell a gravitational field from a uniformly accelerating environment if you operate on a scale large enough to detect tidal forces, for instance, but not if you tune your definition of locality to make the tidal effects unmeasurable.

There's an issue here of what you mean by "gravitational-type" in the thread title. It's possible to work the problem of an accelerating frame (like your centrifuge) using nothing but special relativity - in which case we can say there's no "gravitation-type" calculation being made. But you can also plug in the tools of general relativity, and write down a metric for the accelerating frame that looks a lot like gravity. So whether there is "gravitation-type" time dilation depends on which mathematical tools you use.

Grant Hutchison

WaxRubiks
2017-Feb-13, 05:40 PM
I just meant, other than time dilation bases upon speed.

I suppose a gyroscope could show that the thing was rotating.

swampyankee
2017-Feb-22, 06:16 PM
I just meant, other than time dilation bases upon speed.

I suppose a gyroscope could show that the thing was rotating.

Possibly, but then the question would be whether or not that's a strictly local measurement. Then, of course, you would run into the problem at some scale that the gyroscope would not be sufficiently sensitive to detect rotation.

Ken G
2017-Feb-27, 02:10 AM
Einstein actually had something interesting to say about this, quoted in that Ehrenfest paradox Wiki (the frame K' is a rotating frame which makes the cylinder seem static):
"But, according to the principle of equivalence, K' is also to be considered as a system at rest, with respect to which there is a gravitational field (field of centrifugal force, and force of Coriolis). We therefore arrive at the result: the gravitational field influences and even determines the metrical laws of the space-time continuum. If the laws of configuration of ideal rigid bodies are to be expressed geometrically, then in the presence of a gravitational field the geometry is not Euclidean."

Now, I have heard it argued that GR only deals with true gravity, so not only can all acceleration can be treated purely in special relativity, it just isn't GR at all. However, Einstein is clearly contradicting both those claims here. I don't really know if the thinking has evolved since Einstein said this, but I believe the "general" in GR was really intended by Einstein to mean that any observer frame, including accelerating or rotating, should count as a valid frame. So it's not enough to say that you can do with with SR by sticking to unaccelerating observers, it's GR if you can do it for any observer. What's more, Einstein seems to feel it should count as gravity any time the equivalence principle can be invoked, whereas many others have said it's only gravity if there are tidal effects and it comes from mass/energy, etc. So I think there is considerable latitude to use different language here, and personally I've always like Einstein's approach of viewing everything as geometric, so whether it is true gravity or just an accelerating frame isn't an important distinction.