Einstein already calculated the speed of gravity and found it to be exactly the same as the speed of light. There may be experiments, as you describe, but they can only be used to further verify Einstein’s theory of general relativity.
In answer to MC’s question: A black hole captures a photon when that photon gets closer than the event horizon of the black hole… actually, that pretty much sums up the definition of the event horizon. It’s not the velocity of a photon versus the velocity of a graviton, that’s at issue. It’s that the force of a gravitational field gets increasingly stronger as you approach it’s mass. When a photon gets close enough the force of gravity will prevent it’s escape.
Sorry, jab1. All that means is that the shadows that we see today are the result of 153 year old movements. If the planet were to disintegrate today, we would continue to witness the orbit of this planet every 3.523 days for the next 153 years…
You’ve changed a few of the parameters with this example. Now you’re panning the light source, not obstructing a steady source. But just for grins, let’s look at the implications of what your change means. Let’s say you’ve got a machine gun with lots of ammo. As you fire your machine gun at the wall and pan very slowly from left to right, you will cut a line in the wall. However if you pan the machine gun very fast, the line will disperse into a sequence of holes. The same thing will happen with your sweeping light source. The photons don’t get there any faster, however if the rate of change in distance from where one photon strikes the wall and the next exceeds the rate at which photons are arriving, there will be jumps between photon strikes, but these are not the shadow that was transmitted from your masked light source, but rather the darkness that was always there.
I see where you’re coming from and agree that the rate of the shadow across the wall will not be linear, however consider this. As each pellet approaches the wall, the perpendicular component of it’s velocity with respect to the wall will be less than it’s velocity perpendicular to it’s point of origin, except for the stream of pellets that are perpendicular to both the source and the wall. So in other words, the velocity of the shadow, if you will, will necessarily be less than or equal to the velocity of the pellets. It can’t be faster.
I previously wrote:
I now concede that this part was bunk; I was considering only shadows that are cast within a parallel stream of photons. All I can say is “Oops!” The first part is still correct:
Joey, I have the following problems with your post.
What does it mean to be perpendicular to a point? I’ll be generous and say you mean perpendicular to a line parallel to the wall going through the point. The component of the velocity in either parallel line, of course, will be identical, so maybe I shouldn’t be generous.
No one is saying that the pellets will move faster or slower. They will move in a straight line. But another pellet will come later and it’ll be X meters farther along the wall in less than X/c seconds.
But the velocity of the shadow and/or the point where the pellets hit the wall is independent of the velocity of the pellets!
How about an even simpler example? I shoot two pellet streams at a 1 ly square 1 ly away from me. They hit the wall simultaneously in 1 year.
Now 1 second after I start shooting I cover one of them for 1 second followed by the other one for 1 second.
Bam! 1 year and 1 second later there’s a shadow at one end of my imaginary square and 1 year and 2 seconds later the shadow has moved to the other side of the square.
Okay, JB. Let me see if I’m understanding you correctly. Let’s say, using my previous experiment (the one involving Nick’s head), that we add an additional element: We station another single-shot pellet gun at point A2 that shoots at the same speed of 1 meter/minute. Furthermore, we aim the gun at point B2.
Now, when the gap from Nick’s head starts moving away from A2, we fire the pellet gun. Are you saying that the fired pellet will keep up with the gap as it moves to B2? That although it only took a second to move Nick’s head from A1 to B1, it will still take 11,943,874 years for the gap to move from A2 to B2? (I finally got around to working the numbers.)
If that’s the case, read ravenous’ previous posts. I know that we all have had it pounded into our heads since 1st grade science class that nothing can exceed the speed of light. Keep in mind that “nothing” is exactly what a shadow is. Shadows are not real objects, and do not behave like real objects. They are a state of absense. As I said before, they have no mass and no energy.
In fact, the movement of a shadow is really more of an optical illusion. One microsecond, these photons are being blocked. The next microsecond a whole new set of photons are being blocked. Nothing has really moved (except for the photons and whatever is blocking them).
If you walk outside and look up at the stars facing east then turn your head and look west taking 2 seconds
( wear a mining helmet if you want to do the flashlight thing ) lets say your gaze started at point A a star 100 ly to the east of us then went to B a star 1000 ly to the west. It only took 2 seconds for your gaze to cover
the entire sky not just the 1100 light years between A & B. Because nothing actually moved between A & B
100 years later an alien looking thru his telecope after his sun (A) vanishes behind his planet for the evening and says Ha, look at that geak in a mining hat. Nine hundred years and 2 seconds (not 1100 or 1000) another alien makes the same observation but this time from around star B. Light from your geaky mining helmet headed toward A then 2 seconds later toward B. A & B are 1100 ly apart but what matters is the distance from the sorce. Alien 1 seen the light just 900 years before alien B. even though they’re 1100 light yrs apart. If we’d looked west first alien B would still not see it until 900 years after A. Light isn’t clearing 1100 ly in 900 yrs but thats the time in between event A (light from helmet hitting star A ) and event B (light from helmet hitting star B )
I don’t see why a shadow or light or
anything else couldn’t move across one point then another faster than light could move between the two.
They are independent events.
OK ravenous, I’ll concede that the shadow of Pikachu is everywhere and that we merely fail to notice it due to a lack of surrounding photons necessary to highlight it…
I meant perpendicular to the plane that is the wall.
I’m not sure what you’re trying to say here, but it doesn’t sound right. Pellets that are on the outer streams of our ‘spray’ will take longer to reach the wall than pellets in streams that are closer to the center of the ‘spray’. Therefore their velocity component that is perpendicular to the wall must be smaller than the pellets that are aimed straight at the wall. I don’t think I can state this any more plainly…
Ahh… Now I see your point. So using this same approach, let’s say I have a laser aimed at the left side of the wall and another laser aimed at the right side of the wall. Initially, the first laser is turned on and the second one is off. I then simultaneously turn off laser one and turn on laser two. By your theory I have just done two amazing things (1) I made a point of light move faster than light from the left side of the wall to the right and (2) I’ve made a shadow move faster than light from the right side to the left side.
Alright, you’ve convinced me… Einstein was a putz [sarcasm].
Actually, I see the difference between both your’s and MrKnowItAll’s viewpoints and my own. You guys seem to define a shadow as merely the absence of light, where I require that a shadow must be the absence of light where there once was light. At any point along the wall (and I mean point to be the diameter of a photon - whatever that is) where a shadow is approaching, first there will be a photon, then there will be the abscence of a photon. This abscence cannot arrive at the wall any faster than the photon that preceeded it. Likewise, the absence of a photon at the next point along the wall cannot arrive any faster than the photon that preceede it. The shadow is necessarily gated by the speed of the photons.
I think the real difference between your definitions of shadow is that MrKnowItAll and I are calling the image on the wall at the destination the shadow while you are calling the hole in the photon stream as it rushes from source to destination the shadow. Let’s call my definition of shadow the dougshadow, and yours the jbshadow. Of course, the photons are moving at the speed of light and of course the jbshadow is also moving at the speed of light.
However, the dougshadow which is the locus of points described by the intersection of the jbshadow and the surface of the wall can move arbitrarily fast.
In the one light-year example, it takes one year for any of the changes you make at the source to make it to the destination. But, once those changes reach the destination, the changes take the same amount of time as it took to make the changes at the source. Your two laser example above is a perfect example of this and says nothing about Einstein being a putz. The jbshadow caused by turning off one laser moves at c, and the newly turned on laser beam moves at c. The equivalent of the dougshadow which is a geometric construct and not anything physical, does in fact transit the face of the square instantaneously.
Change your experiment slightly so that you go back to the laser being swept from one side to the other in 1 second and you’ll see. It takes a year for the changes to get there (jbshadow) but once the changes get tot he wall, it (dougshadow) moves one light-year in one second.
Finally, the movement of a shadow is exactly equivalent to the movement of a lightbeam - you are just inverting photons to an absence of photons, but the answer is identical except in darkness instead of light.
Aha! I see now. JB is talking about the shadow en route to the target. Yes, I concede that that shadow cannot exceed the speed of light (or pellets or whatever).
That’s the reason I mentioned that planet eclipsing its sun. The shadow of that planet cannot arrive here any sooner than the light surrounding it.
Joey, I do know what it means if a star is 153 light-years away. It means the shadow of that planet and the light of the star both got here 153 years after they were made.
Technically speaking, though, there really is no such thing as a shadow. It’s an absence of light. How can an absence of something be something? Our eyes detect photons and our brain interprets the image. Our brain interprets the lack of light as a thing even though there is no thing there. (I deliberately wrote “no thing.”) It’s an illusion. Mechanical devices like cameras also detect photons, but, again, our brain interprets the dark areas of the subsequent images as a thing we’ve come to call a “shadow”.