Miscellaneous Science Questions

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

User avatar
doogly
Dr. The Juggernaut of Touching Himself
Posts: 5532
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Mon Jul 20, 2009 6:24 am UTC

Oh, it's not that unfortunate. I like fields much better than borin' ol' waves.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?

Klotz
Posts: 550
Joined: Thu Apr 17, 2008 11:27 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Klotz » Tue Jul 21, 2009 2:25 am UTC

Is the flag on the moon still oscillating? There's no real viscous damping to slow it down, although there might be some friction between the flag and the crossbar. So is it still going?

User avatar
scikidus
Posts: 792
Joined: Wed Nov 26, 2008 9:34 pm UTC
Location: New York, NY
Contact:

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby scikidus » Tue Jul 21, 2009 3:09 am UTC

Klotz wrote:Is the flag on the moon still oscillating? There's no real viscous damping to slow it down, although there might be some friction between the flag and the crossbar. So is it still going?

Probably not.
Happy hollandaise!

"The universe is a figment of its own imagination" -Douglas Adams

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Tue Jul 21, 2009 3:34 am UTC

Yeah, the flag would slow itself down. A finite sheet or string with one end free isn't a harmonic oscillator even under "perfect" (i.e. frictionless) conditions, I think. What you'd expect is movement on the end causing a wave to go to the fixed point/end, reflect back, and then more or less vanish when it reaches the end.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
thoughtfully
Posts: 2253
Joined: Thu Nov 01, 2007 12:25 am UTC
Location: Minneapolis, MN
Contact:

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby thoughtfully » Tue Jul 21, 2009 4:01 am UTC

I've never seen a picture of that flag in which it was flapping (flapping from what?). I'm pretty sure it is rigid, so it doesn't just hang draped around the pole, unrecognizable.
Image
Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.
-- Antoine de Saint-Exupery

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Tue Jul 21, 2009 4:28 am UTC

Well that's because pictures are still, silly face!

There is one bit of video where they're in the process of adjusting the flag (one of them has his hand right on it), in which there's a bit of wave motion. Which is only to be expected, since they're adjusting the flag and one of them has his hand right on it.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
scikidus
Posts: 792
Joined: Wed Nov 26, 2008 9:34 pm UTC
Location: New York, NY
Contact:

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby scikidus » Tue Jul 21, 2009 12:09 pm UTC

Also the flag has a supporting rod running across the top (how else would it stay up in a wind-less environment?), so that quickly dampened any oscillations.

EDIT: Also note how the flag slows even while they set it up:
http://www.youtube.com/watch?v=07kcPQyk7So
Happy hollandaise!

"The universe is a figment of its own imagination" -Douglas Adams

User avatar
oxoiron
Posts: 1365
Joined: Fri Jul 13, 2007 4:56 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby oxoiron » Tue Jul 21, 2009 3:54 pm UTC

scikidus wrote:EDIT: Also note how the flag slows even while they set it up:
http://www.youtube.com/watch?v=07kcPQyk7So
More evidence that the whole thing was faked. :roll:
"Whenever you find yourself on the side of the majority, it is time to reform (or pause and reflect)."-- Mark Twain
"There is not more dedicated criminal than a group of children."--addams

Mactabilis
Posts: 113
Joined: Fri Dec 19, 2008 6:06 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Mactabilis » Fri Jul 24, 2009 1:44 am UTC

i think i'm having trouble with what it means that the speed of light is constant regardless of the observers speed.

if i'm at point A and you have a light source right next to me. we both take off at the speed of light at the same time towards point B. To me , the light speeds past me at the speed of light , arriving at point b the speed of light faster than me, but to an observer at point b we both arrived at the same time?

i think i missed it, but i'm not sure what it means that weather your moving towards or away from a source of light it moves at the same speed, it still reaches you sooner because your closer to it, just not as soon as it should if you were moving towards it?

im lost.

User avatar
Sir_Elderberry
Posts: 4206
Joined: Tue Dec 04, 2007 6:50 pm UTC
Location: Sector ZZ9 Plural Z Alpha
Contact:

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Sir_Elderberry » Fri Jul 24, 2009 3:28 am UTC

Well, here's the thing. All motion is equally valid. Anyone can claim they're standing still. This isn't arbitrary, this is vital to science--every experiment ever done has been conducted on the surface of a rotating sphere. But that leaves us with pesky electromagnetism. Play around with maxwell's equations and you'll see that any possible EM wave has to travel at c. Since Maxwell's equations have to hold in all reference frames, it follows that everyone has to observe that as the speed of an EM wave. Because of this, the speed of light is the only universal benchmark--you measure distances, times, etc, relative to the speed of light. So yeah, everyone measures the speed of light the same--and because of that, time is slower, distances are shorter, etc. It's important to remember that the c thing is the cause of it all, not just an effect.
http://www.geekyhumanist.blogspot.com -- Science and the Concerned Voter
Belial wrote:You are the coolest guy that ever cooled.

I reiterate. Coolest. Guy.

Well. You heard him.

User avatar
PM 2Ring
Posts: 3713
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby PM 2Ring » Fri Jul 24, 2009 3:32 am UTC

Anyone can claim they're standing still.
Unless they're a photon. :)

Mactabilis
Posts: 113
Joined: Fri Dec 19, 2008 6:06 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Mactabilis » Fri Jul 24, 2009 3:12 pm UTC

So no matter what speed I'm traveling, the speed of light is always 299,7924,58 m/s faster than me? or its just always 299,7924,58 m/s? if its the latter that doesn't seem all that mind blowing. Is my first paragraph in the above post correct?

I understand that you can choose any frame of reference and be correct, there is no UP or DOWN, and you can just as easily say you are standing still and the universe is rotating around you, as the universe is still and earth is is the one spinning, i get all that. what i don't get is what that means to two different observers trying to tell when light from the same point A will reach point B when we both have different frames of reference. will someone not moving at all in empty space, get the same calculated time of arrival as someone standing still but on a rock that is spinning very fast and orbiting.

so if both observers find that light from point A one light year away from point B will reach point B in one year, why is that not what you would expect to find? why should my mind be blow?

gnunesjr
Posts: 5
Joined: Fri Jul 24, 2009 4:27 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gnunesjr » Fri Jul 24, 2009 4:47 pm UTC

but to an observer at point b we both arrived at the same time?


There is a difficulty using ordinary language to discuss relativity. But you have got at one of the fun things: a pair of events that appear to happen "at the same time" to one observer will appear to another observer--in a different frame--to happen at different times. But to really grasp the concepts (both the things that blow the mind, and the things that don't), you need to work through the math, which does not involve much more than the Pythagorean theorem. Blow $20 on the nice little paperback, "Introduction to Special Relativity" by Robert Resnick. Read. Understand. Enjoy.

User avatar
PM 2Ring
Posts: 3713
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby PM 2Ring » Fri Jul 24, 2009 4:54 pm UTC

Mactabilis wrote:if i'm at point A and you have a light source right next to me. we both take off at the speed of light at the same time towards point B. To me , the light speeds past me at the speed of light , arriving at point b the speed of light faster than me, but to an observer at point b we both arrived at the same time?

Correct. Almost. You can't take off at the speed of light, as you have non-zero rest mass. But you can get as close as you like to c, provided you have enough energy at your disposal.

Mactabilis wrote:So no matter what speed I'm traveling, the speed of light is always 299,7924,58 m/s faster than me?
Yes.

Mactabilis wrote:so if both observers find that light from point A one light year away from point B will reach point B in one year, why is that not what you would expect to find? why should my mind be blow?
If the observers are at rest with respect to each other, they will agree about the time of travel & the distance taken. Otherwise, they will disagree about the time & the distance. But they will agree that the light travelled at c. And if our observers who aren't at rest wrt to each other are looking at an object moving slower than c, then although they will disagree on times & distances, they will agree on the proper time experienced by that object.

Mactabilis
Posts: 113
Joined: Fri Dec 19, 2008 6:06 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Mactabilis » Fri Jul 24, 2009 7:37 pm UTC

Awesome looks like I understood the concept fairly well.


P.S. my mind is thoroughly blown

Zhatt
Posts: 35
Joined: Fri Apr 24, 2009 8:01 pm UTC
Location: Vancouver, BC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Zhatt » Fri Jul 31, 2009 6:16 pm UTC

PM 2Ring wrote:
Mactabilis wrote:so if both observers find that light from point A one light year away from point B will reach point B in one year, why is that not what you would expect to find? why should my mind be blow?
If the observers are at rest with respect to each other, they will agree about the time of travel & the distance taken. Otherwise, they will disagree about the time & the distance. But they will agree that the light travelled at c. And if our observers who aren't at rest wrt to each other are looking at an object moving slower than c, then although they will disagree on times & distances, they will agree on the proper time experienced by that object.


I get all that so far, but...

Lets say there are two planets, A and B, as well as three observers, Dan, Fred and Earl. Dan and Fred are at planet A while at planet B is Earl with a big knife (not a spoon).

Dan, being the evil man he is, sends a light-signal to his buddy Earl to kill Fred with his big knife as soon as he sees him at planet B. Fred is in his spaceship and takes off to planet B at the same time as the signal is sent. Fred is traveling arbitrarily close to the speed of light. Fred hopes he'll be able to disable Earl before Earl has a chance to read the message.

From Dan's planet of view, his message reaches Earl at nearly the same time as Fred reaches planet be. Earl doesn't have time to ready his big knife and is disabled by Fred. Fred stays alive- sorry, Dan.

From Fred's planet of view, the signal is going to get to Earl and his knife a lot faster then Fred will ever be able to go as the signal will always be traveling towards planet B at the speed of light away from him. By the time Fred reaches planet B, Earl has had much time (maybe even years) to sharpen his knife and hold it at the ready. As soon as Fred steps off the ship, he is struck. Poor Fred.

Question:
Is Fred dead?

More importantly for Dan, does Dan have to worry about Fred coming back and finishing him off?

Birk
Posts: 236
Joined: Tue May 19, 2009 5:08 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Birk » Fri Jul 31, 2009 6:30 pm UTC

Depending on how far apart A and B are and how close to c Fred is traveling, Dan might be dead from old age by the time fred gets there.

User avatar
TNorthover
Posts: 191
Joined: Wed May 06, 2009 7:11 am UTC
Location: Cambridge, UK

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby TNorthover » Fri Jul 31, 2009 6:33 pm UTC

Zhatt wrote:From Fred's planet of view, the signal is going to get to Earl and his knife a lot faster then Fred will ever be able to go as the signal will always be traveling towards planet B at the speed of light away from him. By the time Fred reaches planet B, Earl has had much time (maybe even years) to sharpen his knife and hold it at the ready. As soon as Fred steps off the ship, he is struck. Poor Fred.

As Fred accelerates length contraction kicks in, so by the time he's near c the distance to the other planet can be arbitrarily small. The photon covers this tiny distance at c, but Fred's also crossing it near c (well, the planet's coming towards him that fast) so he has time to intervene.

Zhatt
Posts: 35
Joined: Fri Apr 24, 2009 8:01 pm UTC
Location: Vancouver, BC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Zhatt » Fri Jul 31, 2009 7:17 pm UTC

TNorthover wrote:As Fred accelerates length contraction kicks in, ... so he has time to intervene.


Well that's that then. Thanks for clearing that up.

User avatar
QwertyKey
Posts: 49
Joined: Tue Aug 04, 2009 4:45 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby QwertyKey » Wed Aug 05, 2009 3:15 pm UTC

I have always wondered this, probably due to the fact the first thing I ever read was Fundamental Forces.(On wiki and everywhere else, excluding the useless textbooks :| )

Well, even though I have searched about it and asked about it, why does the Electron not 'slam' into the proton it is orbiting? What relationship does this have with Heisenburg Uncertainty Principle?

User avatar
Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Charlie! » Wed Aug 05, 2009 10:52 pm UTC

QwertyKey wrote:I have always wondered this, probably due to the fact the first thing I ever read was Fundamental Forces.(On wiki and everywhere else, excluding the useless textbooks :| )

Well, even though I have searched about it and asked about it, why does the Electron not 'slam' into the proton it is orbiting? What relationship does this have with Heisenburg Uncertainty Principle?

It has pretty much nothing to do with the uncertainty principle. It's even more fundamental than that.

The reason is, as far as we can tell, that the electron can only have certain amounts of angular momentum. It can have 1 unit, or 2 units, or 3 units, and as its angular momentum changes it'll move into different "orbits" (that don't look much like orbits) around the nucleus. But 0 is not an option. It's simply impossible. Odd, eh? It can pass through the nucleus, and in fact it does in order to do some nuclear processes, but it can't transfer any angular momentum to the nucleus, it can't stop orbiting, because it's already going as slow as it can go if it has 1 unit of momentum.
Some people tell me I laugh too much. To them I say, "ha ha ha!"

User avatar
QwertyKey
Posts: 49
Joined: Tue Aug 04, 2009 4:45 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby QwertyKey » Thu Aug 06, 2009 11:47 am UTC

Charlie! wrote:Stuff.


So what relationship do these units of angular momentum have with the intrinsic property of spin, if I'm seeing this correctly? Or it is about one of the Quantum numbers of the Electron?

And yeah, I guess the f,d,s,p orbitals look really weird :| .

User avatar
PM 2Ring
Posts: 3713
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby PM 2Ring » Thu Aug 06, 2009 1:01 pm UTC

Any electron has its intrinsic spin (which cannot vary in quantity), but a bound electron can also have orbital angular momentum (unless it's in an s orbital). Both are quantised, of course. Wiki has a good article, with nice diagrams. http://en.wikipedia.org/wiki/Atomic_orbital

Rhubarb
Posts: 98
Joined: Fri Dec 19, 2008 9:41 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Rhubarb » Thu Aug 06, 2009 7:29 pm UTC

I don't think angular momentum quantization quite explains it- the ground state of Hydrogen has zero angular momentum, which is why it's spherically symmetric. The uncertainty principle does come into it. It says [imath]\Delta x \Delta p \geq \hbar/2[/imath]. Let's assume for a minute the electron does crash into the proton. That means we can say exactly where it is, in other words [imath]\Delta x \sim 0[/imath]. But that means [imath]\Delta p[/imath] is infinite! If the uncertainty in p is that large, there is a large probability that the electron has enough momentum to escape the pull of the proton. Those two effects balance, so that the electron ends up being most likely found at some finite distance away from the proton.

User avatar
Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Charlie! » Thu Aug 06, 2009 8:08 pm UTC

Rhubarb wrote:I don't think angular momentum quantization quite explains it- the ground state of Hydrogen has zero angular momentum, which is why it's spherically symmetric. The uncertainty principle does come into it. It says [imath]\Delta x \Delta p \geq \hbar/2[/imath]. Let's assume for a minute the electron does crash into the proton. That means we can say exactly where it is, in other words [imath]\Delta x \sim 0[/imath]. But that means [imath]\Delta p[/imath] is infinite! If the uncertainty in p is that large, there is a large probability that the electron has enough momentum to escape the pull of the proton. Those two effects balance, so that the electron ends up being most likely found at some finite distance away from the proton.

Erm... the angular momentum quantum number is 0, but the angular momentum itself is never zero. Total angular momentum = sqrt(l*(l+1)) * hbar. (To people who don't know what that means: little l is a non-negative integer called a "quantum number," since it can only come in integer chunks. When you do that operation on it and multiply it by a very small fundamental constant, you get an always positive number that is the total angular momentum associated with that "level").

And if the proton was a point particle, sure, it would take infinite energy to put an electron in a proton. But since the proton has a very small radius, it takes only a finite energy to contained an electron in it. It's still a whole lot of energy, but it becomes a solvable fight between the 1/x dependence of the containment energy and the 1/x dependence of the electrical potential, and if there was not quantization of the angular momentum the electrical potential would win in the end, since the constants associated with that are much larger than hbar/(Rproton).
Some people tell me I laugh too much. To them I say, "ha ha ha!"

User avatar
PM 2Ring
Posts: 3713
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby PM 2Ring » Fri Aug 07, 2009 1:46 am UTC

Charlie wrote:Erm... the angular momentum quantum number is 0, but the angular momentum itself is never zero. Total angular momentum = sqrt(l*(l+1)) * hbar.
So for an s electron, the only angular momentum it has is that associated with its intrinsic spin; it's "orbital motion" is purely radial, it has no azimuthal component. Hence the spherical symmetry, as mentioned by Rhubarb.

User avatar
Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Charlie! » Fri Aug 07, 2009 5:42 am UTC

PM 2Ring wrote:
Charlie wrote:Erm... the angular momentum quantum number is 0, but the angular momentum itself is never zero. Total angular momentum = sqrt(l*(l+1)) * hbar.
So for an s electron, the only angular momentum it has is that associated with its intrinsic spin; it's "orbital motion" is purely radial, it has no azimuthal component. Hence the spherical symmetry, as mentioned by Rhubarb.

Hm, no, I'm pretty sure that that's the formula for total orbital angular momentum, which isn't 0 (but can be zero in any direction you measure it in). The conventions for spin and "spin + total orbital" angular momentums are distinct enough that I'd probably remember.

Pulling up hyperphysics to solve this... http://hyperphysics.phy-astr.gsu.edu/hb ... gm.html#c2
Some people tell me I laugh too much. To them I say, "ha ha ha!"

Rhubarb
Posts: 98
Joined: Fri Dec 19, 2008 9:41 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Rhubarb » Fri Aug 07, 2009 5:30 pm UTC

Well, what is 0*(0+1)?? (I'm sure I've made the same mistake in the past!) I think the angular momentum is never zero in the Bohr model, but it is in the full QM theory. The balance between the containment energy and electric potential explains why the Bohr radius is so small. You just get it from the energy equation, [imath]E = \frac{1}{2}mv^2-\frac{e^2}{4\pi\epsilon_0x} = \frac{p^2}{2m}-\frac{e^2}{4\pi\epsilon_0x}[/imath], then use the HUP: [imath]\Delta x\Delta p \geq \hbar/2[/imath], and say [imath]p^2 \sim \hbar/2x^2[/imath]. That gives [imath]E = \frac{\hbar^2}{8mx^2}-\frac{e^2}{4\pi\epsilon_0x}[/imath], which gives a ground state at the Bohr radius(ish).

User avatar
Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Charlie! » Fri Aug 07, 2009 7:23 pm UTC

Rhubarb wrote:Well, what is 0*(0+1)?? (I'm sure I've made the same mistake in the past!) I think the angular momentum is never zero in the Bohr model, but it is in the full QM theory. The balance between the containment energy and electric potential explains why the Bohr radius is so small. You just get it from the energy equation, [imath]E = \frac{1}{2}mv^2-\frac{e^2}{4\pi\epsilon_0x} = \frac{p^2}{2m}-\frac{e^2}{4\pi\epsilon_0x}[/imath], then use the HUP: [imath]\Delta x\Delta p \geq \hbar/2[/imath], and say [imath]p^2 \sim \hbar/2x^2[/imath]. That gives [imath]E = \frac{\hbar^2}{8mx^2}-\frac{e^2}{4\pi\epsilon_0x}[/imath], which gives a ground state at the Bohr radius(ish).

heh, oh, right... :oops:
Some people tell me I laugh too much. To them I say, "ha ha ha!"

User avatar
PM 2Ring
Posts: 3713
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby PM 2Ring » Sat Aug 08, 2009 3:12 pm UTC

Generally, an electron in an atomic orbital is in a well-defined momentum state. So its position state must be fairly delocalised. It's useful to work in spherical coordinates when looking at orbitals, and Hamiltonians can be written in any coordinate system we find convenient.

In particular, for an s orbital electron, the probability that the electron can be detected at a particular radius is dependant on the radius & is completely independant of the angles. This simplifies the analysis, as we can work with the Schrödinger wave of the radius parameter & ignore the angles. So when we Fourier transform the position state wave to get the momentum state, it must also be independant of the angles, ie the momentum must be a radial vector.

Sometimes, I like to think of the s orbital state as a quantum version of spherically symmetric bungee jumping. With an electromagnetic cord. :)

IIAOPSW
Posts: 131
Joined: Sat Dec 27, 2008 1:52 am UTC

space ships, observation at near light speeds

Postby IIAOPSW » Tue Aug 11, 2009 9:16 pm UTC

greetings glorious citizens of the peoples republik of the internet.

i recently found myself thinking about how a fast moving object would be observed from space. so lets say a ship starts out 20 light years away. it starts going towards you at .9c. you wouldn't observe the ship start to move until 20 years after it does. however, the ship would reach you 22.2 years after starting its trip. in other words, the ship would reach you only 2.2 years after you first observe it moving. thus from your perspective there would be the illusion that the ship traveled almost 10x the speed of light.

something about this seems wrong to me. i didn't think there were any relativistic factors because the observer is "stationary." I would like to think that a space ship 20ly away doesn't slow down time on earth.

am i wrong about something here or did i just rediscover some obscure principal?
-President of the peoples republik of the internet.

screw your coffee, i download my java!

User avatar
Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

Re: space ships, observation at near light speeds

Postby Charlie! » Tue Aug 11, 2009 9:39 pm UTC

This is, in fact, not relativity at all :D It's a thoroughly classical way to look at things, a lot like looking at the wrong place in the sky when you hear a fast airplane. The spaceship is jut moving relative to the light (from the planet's perspective).

It's possible to spice it up a little, though. Try reconciling the spaceship's perspective with the planet's so that everything meshes even though the speed of light is the same in both reference frames.
Some people tell me I laugh too much. To them I say, "ha ha ha!"

User avatar
Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Charlie! » Tue Aug 11, 2009 9:56 pm UTC

Okay, I've got a relativity question.

Let's say we have a spaceship airline. Flight 1 sets out 1/1/3001 for alpha centauri at 0.98 c, and flight 2 follows exactly a year later.

From the outside this looks fairly simple, they just get flattened. But what about from inside flight 2? Typically we imagine that the spaceship sees the distance to its target and home contract along the direction of travel. But it should be able to measure the correct distance with respect to flight 1. So it sees alpha centauri engulf flight 1? No, flight 1! Don't die!

I'm sure flight 1 doesn't get toasted, but I can't think of a sufficiently sensible reason why it doesn't appear to from the perspective of flight 2.
Some people tell me I laugh too much. To them I say, "ha ha ha!"

User avatar
Yakk
Poster with most posts but no title.
Posts: 11128
Joined: Sat Jan 27, 2007 7:27 pm UTC
Location: E pur si muove

Re: space ships, observation at near light speeds

Postby Yakk » Tue Aug 11, 2009 10:11 pm UTC

IIAOPSW wrote:am i wrong about something here or did i just rediscover some obscure principal?

In short -- yes, that is what the target world sees.

Well, not quite -- the ship leaving would probably not jump to .9 c instantly.

A fun way to think about it is to have laser pulses going off 100 times per year from the source star.

They see the ship leave when they see pulse #0.
They see the ship arrive just after pulse #2222. We know this, because we can work out what pulses the ship sees as it flies away from its homeworld.
We know that, because we can stand off in the distance, and watch the laser pulses bounce off the hull of the ship. As a stationary observer, far away and orthogonal to the travel vector of the ship, we see things relatively Newtonianly.

Now, a really neat effect you can get from the above is that if you engage in continuous acceleration, you actually create an event horizon behind you, beyond which you never see any new events occurring.
One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision - BR

Last edited by JHVH on Fri Oct 23, 4004 BCE 6:17 pm, edited 6 times in total.

User avatar
QwertyKey
Posts: 49
Joined: Tue Aug 04, 2009 4:45 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby QwertyKey » Thu Aug 13, 2009 11:04 am UTC

Sorry if I sound weird here, just really beginning to grasp relativity.

Why is there such a big fuss about science fiction and faster than light speeds?

A static clock would observe FTL speeds of a moving clock(assuming fast enough). Is this momentum too difficult to generate? Or am I really wrong in saying a "A static clock would observe FTL speeds of a moving clock(assuming fast enough)"

Or are people just too pedantic about *18 times the speed of light!*?

User avatar
Robert'); DROP TABLE *;
Posts: 730
Joined: Mon Sep 08, 2008 6:46 pm UTC
Location: in ur fieldz

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Robert'); DROP TABLE *; » Thu Aug 13, 2009 11:29 am UTC

The reason that FTL is such a big thing in sci-fi is because it's normally impossible. Relativity states that, as you go faster, your inertia (I think) increases, meaning you have to put more energy into your object to make it go faster still. This effect gets larger the closer you get to the speed of light, and eventually means that accelerating to c requires an infinite amount of energy.
...And that is how we know the Earth to be banana-shaped.

User avatar
QwertyKey
Posts: 49
Joined: Tue Aug 04, 2009 4:45 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby QwertyKey » Thu Aug 13, 2009 12:26 pm UTC

Robert'); DROP TABLE *; wrote:The reason that FTL is such a big thing in sci-fi is because it's normally impossible. Relativity states that, as you go faster, your inertia (I think) increases, meaning you have to put more energy into your object to make it go faster still. This effect gets larger the closer you get to the speed of light, and eventually means that accelerating to c requires an infinite amount of energy.


Assuming that we have a super-engine, we increase our Gamma(Not too physical, I know) linearly, and eventually your speed becomes high enough for you to 'go faster than' light. Because you travel such a great distance for a static(At rest) observer in less than 1 second, you are, I think essentially, faster than the speed of light.

I am not saying the clock goes at proper speed 1.1c which clearly violates stuff.

To show my point,
"A journey to the 4.3 light-years distant Alpha Centauri C, the closest star to our Sun, would take only 7.4 months in a space ship moving at 0.99c."

Now, to static observers, this 4.3 light years in 7.4 month is OBVIOUSLY FTL!
Figures according to google:
4.3 LY = 4.06802721 × 1016 m
7.4 months = 19 460 104.4 s
Avg. Speed = 2090444699.772525372474363498276(Calc Plus) m/s
=2.090444699772525372474363498276 x 109 m/s

Googling just 'c'
the speed of light = 299 792 458 m / s
2.99792458 x 108 m/s

From:
http://www.thebigview.com/spacetime/timedilation.html

Or my understanding of gamma and what not is still not too clear, but your post didn't help me :|

User avatar
Robert'); DROP TABLE *;
Posts: 730
Joined: Mon Sep 08, 2008 6:46 pm UTC
Location: in ur fieldz

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Robert'); DROP TABLE *; » Thu Aug 13, 2009 12:38 pm UTC

The reason you can travel to/from here to Alpha Centuari in 7.4 months at 0.99c is because traveling this fast as seen from Earth causes shipboard time to slow down. (That is, someone on Earth will see the ship's clock tick slower than it should. People on the ship will observe no difference.) Thus, although the travelers say it took them 7.4 months, someone on Earth will see them taking 4.04 years.

This effect can be used to (from the perspective of the traveller) travel an arbitrary long distance in an arbitrary short space of time, assuming you have the energy budget. However, observers on Earth will always see them as travelling at less than c.
...And that is how we know the Earth to be banana-shaped.

User avatar
QwertyKey
Posts: 49
Joined: Tue Aug 04, 2009 4:45 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby QwertyKey » Thu Aug 13, 2009 12:45 pm UTC

Which brings me to my point on people crying "Oh me yarm!" on FTL in sci-fi.

I agree your speed is not greater than c, but I don't agree how it is 'impossible' for you to have reached X place in X time for a relatively lazy observer Y. Obviously, assuming we have an interstellar engine.

Maybe this sounds more 'general' now since I am getting pretty subjective.

EDIT: Yes, not exactly science, but totally related to it.
Last edited by QwertyKey on Thu Aug 13, 2009 12:56 pm UTC, edited 1 time in total.

Birk
Posts: 236
Joined: Tue May 19, 2009 5:08 pm UTC

Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Birk » Thu Aug 13, 2009 12:51 pm UTC

Because actually being able to go 1000c would be incredible given how tragically confined we are to our part of the galaxy when traveling at sub-luminal speeds?


Return to “Science”

Who is online

Users browsing this forum: No registered users and 7 guests