h a l f b a k e r y
Flaky rehab

meta:

account: browse anonymously, or get an account and write.

 user: pass:
register,

Time dilation

See in to the past
 (+2, -2) [vote for, against]

According to the theory of relativity, time slows down as objects approach the speed of light. Theoretically, a person on a spaceship moving close to the speed of light would not age as fast as a person standing still. If the person on the spaceship could still travel fast and watch people walk around on Earth(maybe with his omnipotent powers?) then it would look like from his point of view that the people were moving around faster than normal. And to the people on Earth, the man in the spaceship would appear to be moving slowly. Now, say person x is standing inside a giant glass bell jar, and person w is standing outside the jar bouncing a ball. What would person x see if he was looking at person w if the bell jar was spinning close to the speed of light? Remember time is moving slower inside the glass, so would it appear to person x that time had slowed down outside the jar?
 — loneranger, Aug 05 2000

Cute little applet illustrating some aspects of the problem. [egnor, Aug 05 2000, last modified Oct 04 2004]

Now THAT'S an interesting question. I have no idea...
 — StarChaser, Aug 05 2000

 "If the person on the spaceship could still travel fast and watch people walk around on Earth(maybe with his omnipotent powers?) then it would look like from his point of view that the people were moving around faster than normal."

 This is not the case. From the reference frame of the person on the spaceship, the Earth and its inhabitants are the ones moving fast and would, therefore, appear to be experiencing time dilation. Their clocks and everything else would appear to be running slower, not faster, to the observer on the ship.

Assuming that person x was mashed, face down/out against the side of the bell jar as a result of the extreme rotation yet somehow managed to survive and see, he WOULD notice that objects passing before his eyes were time-dilated (and length-contracted). What's your point?
 — centauri, Aug 07 2000

 "This is not the case. From the reference frame of the person on the spaceship, the Earth and its inhabitants are the ones moving fast and would, therefore, appear to be experiencing time dilation."

That's exactly what loneranger said. As for the length contraction, it is my understanding that objects are "stretched out" in the direction of travel, so the bell jar should expand, if the bell jar was inverted (which I think is what loneranger meant) then why would the person be mashed against the jar?
 — da5id, Aug 07 2000

[dav1d]: [loneranger] said that the people on earth would appear, to the spaceship pilot, to be "moving around faster." This is not a possible result of time dilation due to "moving fast" in relation to another observer.
 — centauri, Aug 07 2000

 Yeah, sorry I meant to say the jar was upside down. I think this is interesing because there would be two different time frames existing at the same time. With a spaceship traveling near the speed of light in a straight line, it would be hard to study effects such as time dilation and length contraction.

Perhaps a better way of thinking of the person x analogy would be to imagine a blinking light bulb underneath the same inverted spinning jar When viewed from inside the jar, the light appears to blink normally. But from outside the jar might the light bulb appear to remain lit, or unlit? If so, might this be a way to "freeze frame" reality? This could be useful to scientist trying to observe experiments that happen rather rapidly.
 — loneranger, Aug 07 2000

centauri: As I understand this (and my understanding of special, let alone general relativity is far from complete) the situation is not symmetrical. That is, you cannot say that "From the reference frame of the person on the spaceship, the Earth and its inhabitants are the ones moving fast". There is a real difference between a person on Earth and a person on the spaceship: The person on the spaceship experiences accelaration away from the Earth and deccelaration as the spaceship turns around to head back towards the Earth.
Objects which have undergone a journey are different from those which haven't - they have experienced accelaration. This has been demonstrated by sychronising two atomic clocks, flying one around in an aeroplane for a bit and then comparing them. The one which has been on the aeroplane is a few billionths of a second slow. (Gravity is the same thing as accelaration for these purposes and so when this experiment is performed you hav to be careful to account for the weaker gravity up where the aeroplane is - clocks run slower under strong gravity and, in theory, stop completely where gravity is very strong, like on the 'event horizon' of a black hole.)
 — hippo, Aug 08 2000

 hippo, you're absolutely right about the effects of acceleration. Special relativity only applies to inertial (non-accelerating) reference frames. However, once the acceleration has ended, it is impossible to tell any difference between two reference frames.

Imagine, instead of the earth, you have two spaceships at rest with respect to one another whose crews are unconscious. Some time later, the crews awaken to find the ships moving apart at a constant high rate of speed. Both crews can adopt the convention that their ship is the stationary one and they have no way (other than checking fuel levels, I suppose) to learn which one originally accelerated until they are once again at rest with respect to one another.
 — centauri, Aug 08 2000

 Eek.

 The business about telling the difference between two travellers (both of whom ought to view the other as "time-dilated") is called the "twin paradox". I've added some references as links; they explain the situation better than I can. Suffice it to say that acceleration is mostly a red herring.

 The bell jar is mostly an interesting restatement of the twin paradox. It is by no means the only half-baked application of misunderstood relativity; crackpots love to grab onto the twin "paradox" and use it to construct all sorts of time machines and perpetual motion devices and whatnot.

As far as using time dilation to "freeze frame reality", it is of course impractical with current technology to accelerate anything larger than a few subatomic particles to speeds near c (the speed of light). However, in the realm of high-energy physics, extremely short-lived particles are frequently observed only because of relativistic time dilation.
 — egnor, Aug 08 2000

Crackpot? My "freeze frame" reality was mearly a suggestion. Of course it is impractical with today's technology, so is the other version of the twin paradox. I really just want to know what would happen.
 — loneranger, Aug 09 2000

An astronaut went out of sight,
At a speed that was faster than light.
He travelled all day,
In a relative way,
And came back the previous night.
[Recreated - based on the original in "Punch" I think.]
 — rayfo, Dec 05 2000

I would think that the two people need to be moving relative to each other, but with the bell jar, the person isn't moving towards or away from the other person, he's just spinning in place. Would that do anything (other than make him violently motion sick)?
 — PotatoStew, Dec 05 2000

So. Time dilation. If you're in a space ship travelling almost the speed of light, time will slow down? Now, you can't just count mississippis? (I apologize for my ignorance. This stuff is a little ahead of 9th grade science.)
 — ichinichi, Jun 12 2001

I think there's a misunderstanding about the spinning jar. The impression I had was that the JAR was spinning, not the person inside. That's why a 'bell jar', so he had somewhere to stand. If that should normally be transparent is moving at the speed of light between two people, what do they see?
 — StarChaser, Jun 12 2001

 Thanks, StarChaser, finally I understand the question. Seems to me if the person is looking through the glass perpendicular to its direction of travel everything is normal. If she moves her head so that it's close to the 99% lightspeed spinning glass and looks along it in the direction it's spinning, maybe images look reddish because the light is slowed (stretched) abnormally as it comes through the glass. Looking the other way I suppose the light would be blueshifted. (Alternatively way of looking at it: light propagating through the glass by means of interactions with electrons might(?) tend to gain energy where the glass moves in the directon of propagation and lose it where the glass moves agains the direction of propagation.)

 That's about my best guess. Nice thought experiment.

 I'm pretty sure, though, that if the experimenter puts her face right against the glass her head explodes.

 [later: But the index of refraction depends on the difference in the speed of light propagating through two substances. Now if a transparent substance is travelling at 99% lightspeed relative to a source of illumination, wouldn't time dilation effectively raise the index of refraction? So for a spinning cylinder, maybe 1.) the ior would vary smoothly from a very high value at the outer surface of the cylinder (99% c) to smaller values at decreasing radii--causing light to curve; and 2.) if high ior results in greater reflectivity (I'm not sure of that) then the spinning cylinder of glass would become completely reflective anyway.

But I don't really know anything about physics except gleanings from a couple of layman-level books on the subject. Somebody stop me before I pontificate again ;-)
 — Dog Ed, Jun 12 2001

 The same effect can be achieved with 4 ProPlus[tm] and a bottle of whisky. Your heart beats faster, yet everything around you slows down.

 You marvel as objects around the house begin to distort and fold in on themselves. When you wake up you and regain your focus you realise that you are staring at the U-bend on your toilet...

[perhaps this comment lacks sufficient gravity?]
 — riposte, Jun 12 2001

 I couldn't stand it, I had to email a physicist. Joeseph M. Izen, of the University of Dallas, replied (after chiding me for stating the problem in an ambiguous manner):

 "...you have overlooked the fact that to the molecules in the cylinder, the incoming photon looks like a gamma ray. Gamma rays passing through matter will compton scatter or even produce e+ e- pairs or an electromagnetic shower if the energy is high enough."

If I understand him correctly, someone inside the spinning cylinder can see nothing through it; no visible light passes through.
 — Dog Ed, Jun 24 2001

...and he dies of radiation poisoning. Neat! <grin>
 — StarChaser, Jun 24 2001

 [annotate]

back: main index