On gravity, the speed of light, and a stick

[LoneWolf]

I'm no scientist but I love studying and reading on science. There are two questions regarding the speed of light and gravity that have had me thinking for a while. Maybe some of you can educate me.

1. I understand that supposedly nothing travels faster than the speed of light. But what about gravity? If the sun just miraculously vanished all of a sudden it would take about 7 minutes before we would SEE it vanish. But what about our orbit? Would we continue on our normal orbit for 7 minutes or would we instantaneously fly out of orbit?

2. Let's say we have an unbreakable stick about 1 light-year long. Let's say I am at one end of it and you are at the other end. I give the stick on my end a yank (I'm REALLY strong mind you). How long would it be before you saw your end of the stick move? Would it be a year or would it be instantaneous?

Any ideas?

[Brother Daniel]

1. Continue in our normal orbit for 7 or 8 minutes. Then it'd get weird.

2. More than a year. There'd be a big wave of tension propagating from your end of the stick to mine, very fast but considerably slower than the speed of light.

[dancer_rnb]

How elastic is the stick? :evil:

[Izmir Stinger]

1. I understand that supposedly nothing travels faster than the speed of light. But what about gravity? If the sun just miraculously vanished all of a sudden it would take about 7 minutes before we would SEE it vanish. But what about our orbit? Would we continue on our normal orbit for 7 minutes or would we instantaneously fly out of orbit?

Your question assumes a concept of simultaneity that doesn't exist. However, in a simplified model where things really can happen at the same time in different reference frames, yes, the Earth would continue in it's orbit for 7 minutes and then drift off under conserved momentum.

2. Let's say we have an unbreakable stick about 1 light-year long. Let's say I am at one end of it and you are at the other end. I give the stick on my end a yank (I'm REALLY strong mind you). How long would it be before you saw your end of the stick move? Would it be a year or would it be instantaneous?

It would take much longer than a year. What is the speed of sound through the medium of "unbreakable stick"? However long it takes to travel one light year at that speed is how long it would take before your effect on the stick would be observable by me. Thought the real answer is probably "I wouldn't see my end of the stick move" cause I'm not waiting that long.

[LoneWolf]

Thanks!

Now what the hell am I going to contemplate during my down times?

[ofro]

Thanks!

Now what the hell am I going to contemplate during my down times?

Considering your big stick, how softly to you speak?

[Loren Pechtel]

Considering your big stick, how softly to you speak?

Yeah--given that stick he must be mute.

[LoneWolf]

Considering your big stick, how softly to you speak?

Admins, can we move this post to this thread (http://www.secularcafe.org/showthread.php?t=442)?

;)

[Asha'man]

1. I understand that supposedly nothing travels faster than the speed of light. But what about gravity? If the sun just miraculously vanished all of a sudden it would take about 7 minutes before we would SEE it vanish. But what about our orbit? Would we continue on our normal orbit for 7 minutes or would we instantaneously fly out of orbit?
Gravity is weird. Gravity itself is instantaneous, so the Earth is being pulled to the place where the sun sits right now, not the place the sun was 8 minutes ago. However, and this is what get's strange: changes in gravity have to propagate at the speed of light. So we would indeed continue to orbit for 8 minutes after the sun vanished.

2. Let's say we have an unbreakable stick about 1 light-year long. Let's say I am at one end of it and you are at the other end. I give the stick on my end a yank (I'm REALLY strong mind you). How long would it be before you saw your end of the stick move? Would it be a year or would it be instantaneous?

The stick may appear solid to you, but it's really a bunch of individual molecules held together by electrostatic bonds. Forces can only propagate along the stick by traversing those bonds, and those forces therefore travel at the speed of light. So, your really strong tug would cause the stick to stretch, and the point it stretched at would move at the speed of light. In the end, it would be a year before the far end of the stick would move.

[Brother Daniel]

Gravity is weird. Gravity itself is instantaneous, so the Earth is being pulled to the place where the sun sits right now, not the place the sun was 8 minutes ago.
"The place the sun was 8 minutes ago" isn't even definable without reference to a specific reference frame. I think what you're saying is essentially right, but "gravity itself is instantaneous" is a somewhat misleading way of summarizing it, no?
The stick may appear solid to you, but it's really a bunch of individual molecules held together by electrostatic bonds. Forces can only propagate along the stick by traversing those bonds, and those forces therefore travel at the speed of light.
Slower. The wave we're talking about is really just a sound wave. See Izmir's response earlier in the thread.

[Redshirt]

I've heard that quantum connectivity would be faster than light. Instantaneous, in fact. If you take two identical atoms, where the act of observing them will cause the same spin rate or something, separate them by a billion light years, the act of observing one will instantly affect what we observe from the other. Perhaps some of the physicists here can elaborate. This would make a cool interstellar radio, should we take to the stars someday.

[LoneWolf]

I've heard that quantum connectivity would be faster than light. Instantaneous, in fact. If you take two identical atoms, where the act of observing them will cause the same spin rate or something, separate them by a billion light years, the act of observing one will instantly affect what we observe from the other. Perhaps some of the physicists here can elaborate. This would make a cool interstellar radio, should we take to the stars someday.

I have read that as well. I'm sure there is a catch. Nature always has a catch.

[Brother Daniel]

There's a serious catch.

Yes, the measurement of "this" atom will have an instantaneous effect on the state of "that" atom. However, you're getting a random result when you do the measurement, so you're not controlling the state of the other atom. If another measurement is done on the other atom (and then the two experimenters send messages to each other to discuss their results), you'll simply find that the two measurements are correlated. The two measurements could equally well be interpreted as having taken place in either temporal order: A then B, or B then A.

There's no way to use that effect for faster-than-light communication. :(

[Ray Moscow]

There's a serious catch.

Yes, the measurement of "this" atom will have an instantaneous effect on the state of "that" atom. However, you're getting a random result when you do the measurement, so you're not controlling the state of the other atom. If another measurement is done on the other atom (and then the two experimenters send messages to each other to discuss their results), you'll simply find that the two measurements are correlated. The two measurements could equally well be interpreted as having taken place in either temporal order: A then B, or B then A.

There's no way to use that effect for faster-than-light communication. :(

BD, in principle, could one do something to influence the state of one of the entangled particles -- which could then cause the opposite effect in its paired particle? If so, what prevents information from being transmitted this way?

[Brother Daniel]

You can measure something about the nearby particle, causing its wavefunction to collapse (as it "decides" which state to be in), thus in turn causing a collapse of the wavefunction of the far-away particle with which the nearby particle is entangled. That's the only thing you can do to have "instantaneous" effect on something far away.

And there's no way for your buddies in the other lab (on the far-away planet) to know whether you have or have not made such a measurement.

Let's take the simplest case: By prior arrangement, you and your friends have somehow managed to align the axes of your equipment so that you have a shared xyz coordinate system. Each lab has one particle of an entangled pair. The combined spin-state of the two particles is a 50-50 superposition of "A up, B down" and "A down, B up", with respect to the shared z-axis. You and your friends are both thinking of making a measurement of z-spin. (Maybe using a Stern-Gerlach apparatus or something similar.)

If they make their measurement when you haven't yet done yours, they have a probability of 1/2 of finding "up" and 1/2 of finding "down". Because the entangled particles are still in the original spin-state superposition.

If they make their measurement when you have done yours, they have a probability of 1/2 of finding "up" and 1/2 of finding "down". Because you had those same probabilities (a priori), so you may (probability 1/2) have found "up", forcing their particle into the "down" state, and you may (probability 1/2) have found "down", forcing their particle into the "up" state.

So their result is essentially unaffected, and they have no way of knowing whether you have messed with their particle or not -- because they have no way of knowing whether their measurement caused their particle's wavefunction to collapse, or whether their particle's wavefunction had already collapsed (and they were simply confirming your earlier measurement).

[Brother Daniel]

If you ever did make a faster-than-light communication device, you could make some pretty wicked time-paradoxes happen. Why? Hard to answer, unless and until I figure out how to attach diagrams to my posts....