# Sun disappears? consequences?

1. Nov 14, 2005

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 ^^

2. Nov 15, 2005

### SpaceTiger

Staff Emeritus
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

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.

3. Nov 15, 2005

### 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. I don't think I understand how gravity works well enough to get it.

4. Nov 15, 2005

### SpaceTiger

Staff Emeritus
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.

5. Nov 15, 2005

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?

6. Nov 15, 2005

### 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.

Last edited: Nov 15, 2005
7. Nov 15, 2005

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.

Last edited: Nov 15, 2005
8. Nov 15, 2005

### EP

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 [Broken]

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.

Last edited by a moderator: May 2, 2017
9. Nov 16, 2005

### SpaceTiger

Staff Emeritus
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.

No, this is incorrect. The changes cannot be propagated at faster than the speed of light. Otherwise, casuality would be violated.

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.

Last edited: Nov 16, 2005
10. Nov 16, 2005

and eistein's ideas DID make physical senses.

11. Nov 16, 2005

### Janus

Staff Emeritus
The Earth would have to been pretty much on a intersection course with Jupiter already for them to collide.

12. Nov 16, 2005

### EP

I'm not the one that said this idea was unphysical. You did. Or maybe I interpeted it wrong.

13. Nov 16, 2005

sorry, could you please explain that more? =)

14. Nov 17, 2005

### SpaceTiger

Staff Emeritus
Yes, I did say it was unphysical -- and it is. Then you responded with:

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).

15. Nov 17, 2005

### SpaceTiger

Staff Emeritus
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.

16. Nov 17, 2005

### 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.

17. Nov 17, 2005

### tony873004

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 Files:

• ###### noSun.GIF
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18. Nov 17, 2005

### SpaceTiger

Staff Emeritus
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:

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.

Last edited by a moderator: May 2, 2017
19. Nov 17, 2005

alright, i feel an argument coming up so i just like to clarify that i started this thread because i needed help on my physics project. my purpose is to derive and explain how the greek used geometry and algebra to calculate how far the sun is from earth. after that, i plan to explain further the consequences if the sun has suddenly disappeared. also along with more mathematical proofs on how 8.3 minutes (speed of light)was derived.
i was curious about this in the first place and that's why i chose this topic as my research project.

20. Nov 18, 2005

### SpaceTiger

Staff Emeritus
Okay, but I'm not seeing the relationship between these two topics. The Greeks used trigonometry to estimate the earth-sun distance when the moon was in an exact quarter phase (explained http://www.astro.washington.edu/labs/eratosthenes/rung3.html").

In the Newtonian limit, the consequences of the sun disappearing have already been explained and can be understood simply in the context of Newton's laws. If you're interested in the consequences of the sun disappearing in the relativistic limit, then you shouldn't dismiss what's said above as simply argumentative -- it addresses some of the fundamental reasons that Einstein thought general relativity was necessary in the first place!

The speed of light has been measured by many experiments, but again, this is only distantly related to the two things above. The speed of light measurement with the most relevance to astronomy was performed by Ole Romer in 1676 by timing the eclipses of Jupiter with its moons (more information http://www.wonderquest.com/calculate-speed-of-light.htm"). Once you have the speed of light and distance to the sun, deriving the "8.3 minutes" is just a matter of doing what I did in my first post.

Last edited by a moderator: Apr 21, 2017
21. Nov 18, 2005

well they DO relate in a way. you would need the distance to the sun and the speed of light to know how long it would take before the sunrays disappear from earth.

anyways, i don't want to argue, i just want to clear up a few questions i had. thanks who everyone to contriubuted. big help.

Last edited: Nov 18, 2005
22. Nov 18, 2005

### vincentm

Since the lgiht from the sun takes about 8 minutes to travel to Earth, wouldn't it be the same for them to disappear?

23. Nov 18, 2005

### pervect

Staff Emeritus
I thought there was a FAQ on the topic of "what happens if the sun suddenly disappears" but I can't find it. There is of course the FAQ about the "speed of gravity", which shows that attempting to find the speed of gravity by the direction of gravity is just as wrongheaded as finding the speed of light by the direction of the columb electrostatic force. This approach does not give the correct speed of light - nor does it give the correct speed of gravity. (You may read some popularizations that indicate otherwise. These are mostly the work of one person, Tom Flanderen. These popularizations are wrong).

Details on the "speed of gravity" can be found at

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

But back to the original question. There is no solution to Einstein's equations which correspond to the sun suddenly disappearing. The reason why there is no solution can be stated simply, though the deatails are complex: the differential form of the conservation of energy is "built into" Einstein's equations.

Thus you cannot find the solution for what happens when the sun suddenly disappears according to Einstein's equations, because these equations have built into them the assumption that the sun can't suddenly disappear.

Fortunately, there is no reason why it's actually _necessary_ to have the sun suddenly disappear to talk about the idea of the "speed of gravity". One can, in theory, "blow it up", instead of having it disappear. This gives a situation where the equations are soluble.

So the idea is this: one blows up the sun, then, sitting with a stopwatch, one looks for the amount of time it takes for the Earth's orbit to change from it's old predicted value.

There are a few additional wrinkles here. It turns out that in order to measure the speed of gravity, it is necessary to blow up the sun in a manner that is spherically assymetrical. If one blew the sun up symmetrically, the disturbance in gravity would propagate only as fast as the debris travelled - the Earth would not experience any effect in its orbit until the debris actually reached it. (How long this would be would depend on how fast the debris were moving. If they were moving at .1c, it would take about 80 minutes or so for the first of them to reach Earth).

With an assymetrical explosion, one can notice an effect on the orbit much earlier, one that travels at the speed of light. The magnitude of this effect will be small though - it would be determined by the amount of gravity waves emitted by the sun. It would be hard to blow up the sun vigorously enough to generate any significant gravity waves. It would probably be better to make the sun _implode_ rather than to explode it. The fact that the sun is rotating would generate the necessary assymetries, and the gravity wave production would be much higher.

People don't usually actually write out, much less solve, the equations that would be needed to describe these situations, with a few exceptions. The general characteristics of the extremely comlex differential equations that represent GR are already known, and one of the properties that the solutions to these equations have is the property that disturbances in them always propagate at 'c' or less.

Here is the exception - with the development of LIGO, people actually _have_ done the necessary modelling to try and understand what the expected gravitational radiation signals would be from a supernova, or from a binary neutron star inspiral. This is not exactly equivalent to "blowing up" the sun, and is much closer to the case of imploding the sun.

Last edited: Nov 18, 2005
24. Nov 18, 2005

### Danger

Pervect, I don't have time to search, but that other stuff about the sun disappearing was right here in one of the physics threads. It was one where someone was thinking that gravity was instantaneous. There might have been a FAQ about it as well, but this is where I saw it.

25. Nov 19, 2005

### Chronos

Humbug. The sun cannot just magically disappear. It's an unphysical and useless exercise in 'logic'. It is, however, a great excuse to look for ways to confirm a pet 'aha, Einstein was wrong' theory. Pardon my petulance.