Speed of light and action at a distance question

[wallflash]

OK, I'm new here, and not a college educated guy or one with any professional connection to science or teaching, and don't have the math background to grasp the equations of physics and astrophysics. Just someone interested in astronomy and astrophysics, as well as some others like evolution.

 

But I have a few questions I would like to run past posters. I've seen some things said in books like Brian Greene's The Elegant Universe that seem more guesswork than solid proof, and the following comes from this sort of "it just has to be" mentality that I see in some of Greene's explanations.

 

The following has to do with the relay of information faster than the SoL, or the premise that none can be.

 

Imagine that in our future scientists have tethered a small space station to the Earth at a distance of 558,000 miles from Earth . This can be done by a physical cable or some future gravity beam. So after a few orbits the space station assumes constant velocity and maintains a position straight up from the tether point. They also place a second space station at a distance from Earth of 1,860,000 miles in a stationary position relative to the Earth, so that the view of the Earth is always the exact same spot in the monitoring screen, and the distance from Earth to the second station and the first station to the second station are identical. So the first station is 3 light seconds away from Earth, and the second one is 10 light seconds. The scientists set up a laser signal that fires at precisely the time each day when the first station is at a 90 degree angle from a second station to earth axis, so that both Earth and the first station are visible and the same distance from the second station, and another beam from the first station to the second at this same point. Happens twice a day obviously. Tests are done to calibrate the beams to fire to arrive at the second station simultaneously.

 

Once everything is calibrated, the experiment becomes this. The calibrating laser on the first station is turned off and replaced with a laser that fires the instant the station detects outward motion away from Earth as it is released from its tether to Earth. At the precise time the Earth and the first station are the same distance from the second, the tether holding the first station to the Earth is released at the Earth level, and simultaneously a laser is fired from Earth to the second station signalling this release. When the first station detects the outward motion of being cut loose from Earth, it fires its laser to the second station. The SoL physics (IF I understand them right ) say that the second laser will arrive 3 seconds after the first laser, because it will take 3 seconds for the first station to receive the info that it has been cut loose and fire it's laser to the second.

 

This seems nonsensical. Am I simply misunderstanding something basic? If this is true, then what holds the first station in place for that 3 seconds before it detects it has been cut loose? It has instantly lost what holds it to Earth. What holds it stationary for a full 3 seconds after the release? You could stretch this to a minute with enough distance from Earth to station. It seems to me it should move the instant it is released at the Earth point , but if it does, isn't releasing it at the Earth level and having it react simultaneously 558,000 miles away a transfer of info faster than the SoL? If it doesn't move instantly, what keeps it in place for the 3 seconds after it has been cut loose?

Edited March 4, 2017 by wallflash

[Strange]

A couple of immediate things occur to me. The main one is that nothing is transmitted instantly. So, for example, when the tether is cut (if I have understood the scenario correctly) it will take time for that information to travel up the tether. This can not travel faster than the speed of sound in the material of the tether (because the speed of sound is defined the speed at which mechanical forces "ripple" through the material).

 

So, in principle, the observers on the outer satellite could see that the tether has been cut long before they feel them selves set free.

 

So, what keeps it in place is the inertia of the material that the tether is made of.

 

The other subtlety (that could become relevant) is your mention of "simultaneously". Two events that are seen as simultaneous by one observer may not be simultaneous for another (and may be seen to occur in the reverse order for a third).

[Delta1212]

You don't even need to wait for the future or operate at the speed of light to see how this would work:

 

 

 

If you hang a slinky by one end and release, the bottom of the slinky won't begin to fall until the release of tension has been transmitted from the top to the bottom in the form of a pressure wave.

[imatfaal]

Great find - sometimes youtube really is good

[wallflash]

You don't even need to wait for the future or operate at the speed of light to see how this would work:

 

 

 

If you hang a slinky by one end and release, the bottom of the slinky won't begin to fall until the release of tension has been transmitted from the top to the bottom in the form of a pressure wave.

 

 

OK the Slinky thing is cool. My question on that would be, does the fact that its coiled and not stretched taut and essentially out to its limits have any impact on that? Obviously if you did the same thing with a cable and a 100 lb weight, the weight at the bottom will appear to move at the same time as the cable is released, and not wait for the top end of the cable to reach it before moving downward . Do the coils cause the delay entirely, or merely accentuate the delays for human eyes to notice?

 

Lets replace the space station in my scenario with a powered spaceship pulling against a tether, so that we are not dealing just with inertia. What keeps the spaceship from instantly moving even a quarter inch to trigger the laser when the tether is released at Earth level? We are not talking about a transference of energy up the cable, as we would be if we were moving the cable to create a wave action along the cable. We are simply removing an action at one end . Its hard to conceive that the ship at the other end pushing to move away would remain motionless even though there is no longer any restraint upon its movement, especially that we are now talking about it remaining motionless for around a minute with a powerful engine trying to move it with (now) no actual restraint, instead of 3 seconds.

 

Or to go back to the gravity beam, so that we are not dealing with pressure waves . If this beam is simply ended at Earth level, what keeps this ship motionless for 3 seconds with the actual restraining force removed?

Edited March 5, 2017 by wallflash

[Janus]

OK the Slinky thing is cool. My question on that would be, does the fact that its coiled and not stretched taut and essentially out to its limits have any impact on that? Obviously if you did the same thing with a cable and a 100 lb weight, the weight at the bottom will appear to move at the same time as the cable is released, and not wait for the top end of the cable to reach it before moving downward . Do the coils cause the delay entirely, or merely accentuate the delays for human eyes to notice?

It just accentuates it

Lets replace the space station in my scenario with a powered spaceship pulling against a tether, so that we are not dealing just with inertia. What keeps the spaceship from instantly moving even a quarter inch to trigger the laser when the tether is released at Earth level? We are not talking about a transference of energy up the cable, as we would be if we were moving the cable to create a wave action along the cable. We are simply removing an action at one end . Its hard to conceive that the ship at the other end pushing to move away would remain motionless even though there is no longer any restraint upon its movement, especially that we are now talking about it remaining motionless for around a minute with a powerful engine trying to move it with (now) no actual restraint, instead of 3 seconds.

If the cable is under tension it will be stretched. Upon release at one end it will start to return to its non-stretched length and the effect of this will take time to reach the other end.

Or to go back to the gravity beam, so that we are not dealing with pressure waves . If this beam is simply ended at Earth level, what keeps this ship motionless for 3 seconds with the actual restraining force removed?

Changes in gravity can only propagate at the speed of light. So even if you could build a gravity field generator, and then shut it off, the effect of doing so would need to propagate at c to any remote point before anything at that point could feel that effect.

[studiot]

I couldn't agree more that this is a brilliant video, Delta1212. +1

 

 

 

 

 

OK the Slinky thing is cool. My question on that would be, does the fact that its coiled and not stretched taut and essentially out to its limits have any impact on that? Obviously if you did the same thing with a cable and a 100 lb weight, the weight at the bottom will appear to move at the same time as the cable is released, and not wait for the top end of the cable to reach it before moving downward . Do the coils cause the delay entirely, or merely accentuate the delays for human eyes to notice?

 

Lets replace the space station in my scenario with a powered spaceship pulling against a tether, so that we are not dealing just with inertia. What keeps the spaceship from instantly moving even a quarter inch to trigger the laser when the tether is released at Earth level? We are not talking about a transference of energy up the cable, as we would be if we were moving the cable to create a wave action along the cable. We are simply removing an action at one end . Its hard to conceive that the ship at the other end pushing to move away would remain motionless even though there is no longer any restraint upon its movement, especially that we are now talking about it remaining motionless for around a minute with a powerful engine trying to move it with (now) no actual restraint, instead of 3 seconds.

 

Or to go back to the gravity beam, so that we are not dealing with pressure waves . If this beam is simply ended at Earth level, what keeps this ship motionless for 3 seconds with the actual restraining force removed?

 

 

In particular, wallflash, you should study it more carefully.

 

The presenting Professor of Physics, who also deserves full credit, actually describes why your post#5 is wrong between 2:40 and 2:57 on the video.

[Delta1212]

 

 

OK the Slinky thing is cool. My question on that would be, does the fact that its coiled and not stretched taut and essentially out to its limits have any impact on that? Obviously if you did the same thing with a cable and a 100 lb weight, the weight at the bottom will appear to move at the same time as the cable is released, and not wait for the top end of the cable to reach it before moving downward . Do the coils cause the delay entirely, or merely accentuate the delays for human eyes to notice?

 

Lets replace the space station in my scenario with a powered spaceship pulling against a tether, so that we are not dealing just with inertia. What keeps the spaceship from instantly moving even a quarter inch to trigger the laser when the tether is released at Earth level? We are not talking about a transference of energy up the cable, as we would be if we were moving the cable to create a wave action along the cable. We are simply removing an action at one end . Its hard to conceive that the ship at the other end pushing to move away would remain motionless even though there is no longer any restraint upon its movement, especially that we are now talking about it remaining motionless for around a minute with a powerful engine trying to move it with (now) no actual restraint, instead of 3 seconds.

 

Or to go back to the gravity beam, so that we are not dealing with pressure waves . If this beam is simply ended at Earth level, what keeps this ship motionless for 3 seconds with the actual restraining force removed?

Re: the gravity beam

 

Imagine you have a faucet running in your sink. When you turn it off, the water doesn't just vanish. Whatever was already in the column of water between the faucet and the basin of the sink continues to fall, so there is a delay between when the water is shut off and when water stops hitting the basin as that "shut off information" propagates downward. The same is true for light rays or, if you could create a directional source of gravity in the form of a beam, gravity.

 

Changes in gravity propagate out from their source at the speed of light. This is quite well modeled by General Relativity and we just last year got additional confirmation that gravity does indeed behave as it is modeled in this way by the first detection of predicted gravity waves.

 

It does intuitively very much seem like there should be some way to arrange things so that the effect of some action happens instantly over great distances. I had many of the same or similar thoughts when I was first learning about these concepts and trying to come up with ways around them myself. The reason for that intuition we have is that for a lot of similar actions we encounter in our daily lives, it seems like the effect instantaneously follows from the cause. But it seems that way because the effect propagates out from the cause very, very quickly and we are dealing with extremely short distances, so for all practical purposes of a typical person these effects might as well be propagating instantaneously.

 

When you are multiplying the very tiny difference between "might as well be instantaneous" and "actually instantaneous" over the unimaginably vast distances found in space, however, it starts being very noticeable indeed.

[wallflash]

I couldn't agree more that this is a brilliant video, Delta1212. +1

 

 

 

 

 

In particular, wallflash, you should study it more carefully.

 

The presenting Professor of Physics, who also deserves full credit, actually describes why your post#5 is wrong between 2:40 and 2:57 on the video.

 

 

 

 

 

 

Naw, I thought I would ask a question just so I could ignore the answers

Edited March 5, 2017 by wallflash