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Sun disappears? consequences?

by adjkgh
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adjkgh
#1
Nov14-05, 11:23 PM
P: 23
what are the consequences of earth if our sun had somehow disappeared?
can someone prove how long daylight on earth would evaporate if the sun disappeared?

I think it'd be equivalent to the speed of light, which is 8.3 minutes because that is how fast light travels from the sun to the earth but i'd like some derivations and explainations on how and why it is 8.3 minutes?

plus would there be any orbital changes amonst our solar system?

thank you ^^
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SpaceTiger
#2
Nov15-05, 12:16 AM
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Quote Quote by adjkgh
I think it'd be equivalent to the speed of light, which is 8.3 minutes because that is how fast light travels from the sun to the earth but i'd like some derivations and explainations on how and why it is 8.3 minutes?
Distance from the earth to the sun: 1.5e8 km

Speed of light: 3e5 km/s

Time for changes in the sun to be communicated to earth ->

Distance/Speed of light = (1.5e8 km)/(3e5 km/s) = 499 seconds = 8.3 minutes


plus would there be any orbital changes amonst our solar system?
Without the sun, the earth wouldn't orbit at all. It would continue moving in a straight line with whatever speed and direction it had prior to the sun's disappearance.
Chessguy
#3
Nov15-05, 04:27 PM
P: 8
Quote Quote by SpaceTiger
Without the sun, the earth wouldn't orbit at all. It would continue moving in a straight line with whatever speed and direction it had prior to the sun's disappearance.
SpaceTiger,
Would the earth break orbit the instant the sun disappears or would it continue to orbit normally for another 8 minutes before breaking off at a tangent?
I guess if gravity propagates at the speed of light then it would have to be the latter but it doesn't make intuitive sense to me. I don't think I understand how gravity works well enough to get it.

SpaceTiger
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Nov15-05, 05:50 PM
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Sun disappears? consequences?

Quote Quote by Chessguy
SpaceTiger,
Would the earth break orbit the instant the sun disappears or would it continue to orbit normally for another 8 minutes before breaking off at a tangent?

I guess if gravity propagates at the speed of light then it would have to be the latter but it doesn't make intuitive sense to me.
In the relativistic limit, we probably shouldn't take this question too far because it's really unphysical. The nearest physical situation I can think of would involve a detonation (like a supernova). In that case, it would take more than eight minutes for the matter and energy to leave the system anyhow.
adjkgh
#5
Nov15-05, 08:05 PM
P: 23
thank you for your responses =)

earth orbits the sun...and so does other planets as well, but is it true that other planets' gravity places earth at where it is? meaning that...if the sun disappears, it's gravitational pull is gone so would jupiter (or any other planets) alter our earth's tangent motion once the sun disappeared?
EP
#6
Nov15-05, 09:40 PM
P: 76
I'd imagine the other planets would affect Earths path, I dont believe anyone has ever calculated exactly how but I would think it would move out towards the larger planets. Also if the Sun instantly disappeared the Earth would move out tangentially immediatly not 8.3 minutes later. Atleast to my understanding of relativity. Basically the curvature of space will change instantly. Gravity is not a force pulling on something like a rope would, its an effect of curvature of space. Therefore it wouldnt take 8.3 minutes for the force to disappear.

In response to SpaceTiger these types of questions are the ones that Einstien asked him self in his famous thought experiments. They are very important to Theoretical Physicist.
adjkgh
#7
Nov15-05, 10:06 PM
P: 23
Quote Quote by EP
I'd imagine the other planets would affect Earths path, I dont believe anyone has ever calculated exactly how but I would think it would move out towards the larger planets. Also if the Sun instantly disappeared the Earth would move out tangentially immediatly not 8.3 minutes later. Atleast to my understanding of relativity. Basically the curvature of space will change instantly. Gravity is not a force pulling on something like a rope would, its an effect of curvature of space. Therefore it wouldnt take 8.3 minutes for the force to disappear.

In response to SpaceTiger these types of questions are the ones that Einstien asked him self in his famous thought experiments. They are very important to Theoretical Physicist.
but gravity has speed and i think i heard that it was almost equal to the speed of light which means it would take 8.3 minutes before it changes path?

side note: if the sun suddenly disappears, would earth crash into another planet then? consider jupiter's great mass and gravitational pull, wouldn't earth me traveling towards that way? because the sun isn't there to restrain it anymore.
EP
#8
Nov15-05, 10:13 PM
P: 76
I'm sorry your right the gravity does act at the speed of light. I got confused on the einstiens experiment. It was if Newtons law was correct the Earth would fly off immediatly. This sight briefly descibes the experiment. So It would take 8 min.
http://www.perimeterinstitute.ca/explore/einstein1.php


It might crash into Jupiter. That has many variables. How far away it is, the velocities, direction the earth is heading at time loss of gravity. You would have to calculate that.
SpaceTiger
#9
Nov16-05, 12:38 AM
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Quote Quote by EP
I'd imagine the other planets would affect Earths path, I dont believe anyone has ever calculated exactly how but I would think it would move out towards the larger planets.
These forces are extremely tiny. Yes, they have been calculated and yes, you would see slight deviations from a straight-line path, but you won't learn anything physical from it.


Also if the Sun instantly disappeared the Earth would move out tangentially immediatly not 8.3 minutes later. Atleast to my understanding of relativity. Basically the curvature of space will change instantly. Gravity is not a force pulling on something like a rope would, its an effect of curvature of space. Therefore it wouldnt take 8.3 minutes for the force to disappear.
No, this is incorrect. The changes cannot be propagated at faster than the speed of light. Otherwise, casuality would be violated.


In response to SpaceTiger these types of questions are the ones that Einstien asked him self in his famous thought experiments. They are very important to Theoretical Physicist.
Name one thought experiment that Einstein did that was unphysical. In fact, he did the opposite. He took the theories of the time and stretched them to their limits to see whether or not the results made physical sense.
adjkgh
#10
Nov16-05, 06:13 PM
P: 23
and eistein's ideas DID make physical senses.
Janus
#11
Nov16-05, 08:40 PM
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Quote Quote by adjkgh
side note: if the sun suddenly disappears, would earth crash into another planet then? consider jupiter's great mass and gravitational pull, wouldn't earth me traveling towards that way? because the sun isn't there to restrain it anymore.
The Earth would have to been pretty much on a intersection course with Jupiter already for them to collide.
EP
#12
Nov16-05, 08:47 PM
P: 76
Quote Quote by SpaceTiger
In the relativistic limit, we probably shouldn't take this question too far because it's really unphysical. The nearest physical situation I can think of would involve a detonation (like a supernova). In that case, it would take more than eight minutes for the matter and energy to leave the system anyhow.
I'm not the one that said this idea was unphysical. You did. Or maybe I interpeted it wrong.
adjkgh
#13
Nov16-05, 11:05 PM
P: 23
Quote Quote by Janus
The Earth would have to been pretty much on a intersection course with Jupiter already for them to collide.
sorry, could you please explain that more? =)
SpaceTiger
#14
Nov17-05, 02:49 AM
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Quote Quote by EP
I'm not the one that said this idea was unphysical. You did. Or maybe I interpeted it wrong.
Yes, I did say it was unphysical -- and it is. Then you responded with:

In response to SpaceTiger these types of questions are the ones that Einstien asked him self in his famous thought experiments.
I'm saying it's nothing like those thought experiments because Einstein wouldn't have started by thinking about a situation that none of the theories of the time predicted could occur (including his own).
SpaceTiger
#15
Nov17-05, 02:57 AM
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Quote Quote by adjkgh
sorry, could you please explain that more? =)
I'm pretty sure he means that the two could collide if the paths that they leave on intersect one another. This could happen, but it would be pretty improbable. The main point here is that the planets would not be bound to one another (i.e. in orbits about one another) after the sun disappears. Their relative velocities would be much too large.
EP
#16
Nov17-05, 10:35 AM
P: 76
What I interpeted as you saying was unphysical was having the Sun disappear. So there was no point in discussing the implications of this happening. Which is why I put that link up explaining the experiment that I was refering to. Einstein did imagine what would happen if the Sun just disappeared.
tony873004
#17
Nov17-05, 11:06 AM
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Even though it's unrealistic, It's still worth considering if it helps you understand something about the Sun's role in the solar system. It's unrealistic that humans could exist on an Earth with no atmosphere, but lots of beginning Physics problems tell you to ignore air resistance to help you better understand momentum.

Here's a pic of what would happen if the Sun just vanished. Watch how fast Mercury leaves!
Attached Thumbnails
noSun.GIF  
SpaceTiger
#18
Nov17-05, 11:51 AM
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Quote Quote by EP
What I interpeted as you saying was unphysical was having the Sun disappear. So there was no point in discussing the implications of this happening. Which is why I put that link up explaining the experiment that I was refering to. Einstein did imagine what would happen if the Sun just disappeared.
If anyone has a link that indicates this particular thought experiment really originated with Einstein, I'd be curious to see it. The article just uses it as an example, so it may be one of those crude popularizations (like the "warped space" analogy) that surfaced after the formulation of relativity. If Einstein had presented this as his reason for not believing Newtonian gravity, the naysayers could respond that it violated the conservation of energy. Matter simply cannot disappear. It would have been silly to use this thought experiment when there are countless other physically valid ones that can be formulated.

For example, if the sun were to suddenly explode and eject matter in an asymmetric fashion, its center of gravity would change before the news of the event could reach earth. Thus, Newtonian gravity would predict an instantaneous change in the earth's orbit. Why is this a problem? Well, it's not obvious from the article that you linked:

Quote Quote by http://www.perimeterinstitute.ca/explore/einstein1.php
So if the Sun were to suddenly disappear, the universal speed limit forbids us any means of knowing of this catastrophe until 8 minutes later. Light itself respects this required time delay: the Sun would continue to blaze in our sky during the whole 8 minutes it takes the last ray of light from the Sun to make its journey to the Earth. Only then would the Earth be plunged into darkness. Newton’s model of gravity does not respect this required time delay: it predicts that at the same instant the Sun disappears, the “gravitational force” it exerts on the Earth would also disappear, causing the Earth to immediately break out of its usual orbit. Observers on night side of the Earth could be immediately aware of this catastrophe by noticing a change in the apparent motion of the distant stars produced by the change in Earth’s motion. General relativity resolved this problem.
Why should it matter that observers can still see light from the sun after its gravitational influence disappears? Wouldn't the light just be delayed relative to the gravity?

The reason that it matters is that it violates causality, as I said. If the two events are simultaneous in one frame, you can boost to another frame in which the earth goes off course before the sun explodes. That is a real physical problem and it doesn't require invoking an impossible thought experiment.


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