A question for you physics guys

[daltoncalford]

Ok, being a very poor armchair physicist......

I have the following questions based upon certain things that I do not know if they are true.

1.) Although it takes light approximately 8 minutes to reach the earth from the sun, I have heard that if the sun suddenly dissappeared, we would be affected by it immediately - ie, the effects of gravity travel faster than light. Is this true?

2.) If you have three objects - A, B and C. And those objects each have their own light cone (based on the expansion of light from the source objects over time). The light cone of A crosses the light cone of B, while the light cone of C crosses the light cone of B. Neither A nor C's light cones overlap. A is travelling .6 the speed of light in regards to B while C is travelling .6 the speed of light in regards to B, while B can see that A and C are on inverse vectors. Now, A does not know that C exists or vice versa.
Does A's movement affect C's time reference - or better worded question, does the effects of relativity travel faster than light.

Just a question from a guy who is trying to understand the various effects of the cosmos.

best regards

Dalton

 

[Anthony]

1) As we have no examples of this we can't be entirely certain, but the answer is generally assumed to be no, the effects of gravity are not FTL.
2) No, though in your examples the light cones will actually eventually cross (I think from the perspective of A, C is moving at 0.96c).

 

[daltoncalford]

Thanks Anthony,

It is funny about 1 because I have heard it many times that the effects of gravity are instantanious.
It seems to be a common misconception.

What is your background? Are you an armchair physicist like myself or are you one of them thar educationified professor types?

As for the light cones in 2, yes, by definition their light cones would cross. My question is if this line of reasoning has even been addressed or if it was not considered as most math models as applied to relativity are applied to a closed referential system and not based on what if's out side of the immediate model.

best regards

Dalton

 

[far-trader]

As for question 1 I think there is still no absolute answer, and won't likely be for a long time as it's very difficult to measure. It's open to (often heated) debate even among those most knowledgable (and I'm not one of them ).

It comes down to the model of gravity/physics you choose, Newtonian or Relativistic. But even Sir Issac Newton had his doubts about instantaneous gravity. It was not long ago that it was accepted that gravity was instantaneous which is why you still see such references, especially in classic sci-fi and older physics text books. The current teaching is that it is limited to the speed of light.

In Newtonian physics the theory is basically that gravity is a force with an instantaneous "speed". It's not really speed because it's a force effect. It's a constant "now" effect by one body on another. Remove one and the result is the gravity effect is removed instantly. If you apply a light speed limit, and the resultant delay, to the effect of gravity on orbits you end up with unstable orbits that aren't shown to be the case in the solar system.

In General Relativity the theory is that gravity is not a force per se and orbital stability can be achieved by factoring position and velocity components into the interaction to balance out.

 

[Anthony]

Armchair physicist. What I meant is that A and C are actually visible to one another. Assuming that the light cones of objects actually do not cross (this is only possible due to expansion of space) they do not interact gravitationally.

 

[Scott Martin]

Thank You David Weber...

In case you're wondering where this misconception came from, it's a plot device in the "Honorverse" while trying to retain a (mostly) hard SF universe...

QUOTE]Originally posted by Dalton:
Thanks Anthony,

It is funny about 1 because I have heard it many times that the effects of gravity are instantanious.
It seems to be a common misconception.
[/QUOTE]


If you're going to bent the laws of physics in fiction, it's best to choose those that are really hard to prove or disprove given our current technology. If general relativity holds, then gravity propigates at C. That said, a number of folks have been trying to poke holes in general relativity for a while now, and in the early part of this century physics was pretty much wrapped up and mechanistic other than a few "loose ends" regarding electricity and light (which opened the pandora's box called quantum theory and relativity...)

Scott Martin

 

[aramis]

Several physicists have written that gravity doesn't propagate at all, but is merely a bend in the curvature.

In Traveller, we get a situation where we need to know: Jump drives. That's as close to instant as we can get for removing a mass.

The extrapolation of this is "How fast does the curve rebound." Asnwers vary, but orbital calcs do tend to show non-time-lagged effects, but the margin of error is high enough that the data is unreliable.

Real test? Send an asteroid at 0.1c by another one... Good luck

 

[Anthony]

Gravity behaves like a field, and doesn't obviously propagate per se. Changes in a gravitational field, like changes in an electrostatic field, are expected to propagate at the speed of light.

 

[daltoncalford]

So, what I am getting from this thread is that we do not have sensitive enough equipment/advanced enough equipment to test either conjecture but we do have some who argue both sides as to how gravity actually works.

As the math is simply a model constructed to match our observed understanding of things, we can not rely upon it to give an answer when the effects can not be observed. For all we know the observation may change the criteria of the mathmatical model.

For example, what causes the effects of relativity? We don't know, but we do know that with finite model mathmatics (a model that does not encompass the whole of the universe) it appears to properly predict some effects. We have never been able to test if the relitivistic effects occur outside the boundries imposed by the speed of light. I doubt that an experiment has even been thought of to test the concept.

I guess the biggest concern that I have been having over the current discussions (in this and other threads) is the absolutionist stance taken by some people. The mentality is anti-science and is, at best, a statement of faith.

Science has always been about questioning things. Of disbelieving the common perceptions and attacking problems from obscure viewpoints.

Engineers need absolute values, scientists need methods of attacking absolute values and testing all aspects of an idea.

Newtonian physics where the bible of the day, until relativity raised itself up to be the new truth. I am not saying that it is not true, but, I am also willing to believe that some nutbar who does not know the math, may be lucky enough to stumble upon something that causes some other nutbar to come up with the next briliant theory.

oh well, it is after midnight, I am tired and I am blathering on again.

g'nite all

Dalton

 

[Anthony]

Originally posted by Dalton:
So, what I am getting from this thread is that we do not have sensitive enough equipment/advanced enough equipment to test either conjecture but we do have some who argue both sides as to how gravity actually works.

Not exactly. There's not really any serious debate on the issue (but it's true that we can't directly detect gravity waves). FTL gravity would probably affect the macrostructure of the universe in a discernable way, but I'm not sure what that effect would be.

I guess the biggest concern that I have been having over the current discussions (in this and other threads) is the absolutionist stance taken by some people. The mentality is anti-science and is, at best, a statement of faith.

Huh? All I stated was 'generally assumed'. Gravity moving at lightspeed follows from our model of gravity, so unless that model is incorrect in significant ways, gravity moves at light speed.

Unless you're actually trying to test something, or have evidence otherwise, you should assume our current model is correct, because it's far more likely to be correct than any replacement you come up with based on no evidence.

 

[TheEngineer]

Hi !

GRT postulate gravitational waves, which propagate just at the speed of light, but those waves have not been detected right now.
Nevertheless there are a handfull of projects to catch gravitational disturbances and proove the postulation eg. VIRGO (http://www.ego-gw.it/virgodescription/pag_4.html) /LIGO (http://www.ligo.caltech.edu) experiments.

As for the relativity effect, the referential system has not to be closed. In fact one frame of reference is represented by the infinite set of objects and their coordinates (x,y,z,t).
SRT describes the neccessary transformation from one frame of reference to another, resulting in the effect that "realtity" for frame A has not to be the same as "reality" for frame.
The effect is neglegtable for minor differences in velocity, but in Your example relativistic speed cause, that the percepted realtity for A/B/C is just different. E.g. the observed velocity between A and C (from the standpoint of C respectively A) should be around 0,88c, according to the SRT velocity-addition formula.

SRT is a ggod example, that reality really is a crazy thing.

Dalton, if You are interested, I could give You a link or a PDF itself, which deals at least with SRT in a very practical way. The PDF would be perhaps easier, as I dont know exactly where I got it from...

regards,

TE

 

[daltoncalford]

Originally posted by TheEngineer:


Dalton, if You are interested, I could give You a link or a PDF itself, which deals at least with SRT in a very practical way. The PDF would be perhaps easier, as I dont know exactly where I got it from...

regards,

TE

I would love a copy! This is the sort of stuff that makes my wife shake her head and think that I am crazy.

best regards

Dalton

 

[Plankowner]

Yeah, I get that all the time from my kids too...

Once, we discussed Cyclotrons and their construction as dinner table conversation. (yes, I have a family of nerds).

Relativity will mess with your head and the best way to understand it is to do the math and believe the numbers. It is still a theory, but a theory that has been validated many times, but not completely; gravity waves are one example.

 

[Nick Nova]

Posted by Dalton:
1.) Although it takes light approximately 8 minutes to reach the earth from the sun, I have heard that if the sun suddenly dissappeared, we would be affected by it immediately - ie, the effects of gravity travel faster than light. Is this true?

There is a wonderful discussion of this very subject in "The Elegant Universe", with some excellent graphics to boot. If you haven't seen this series, there is a webcast of it here
In hour 1 "Einstein's Dream", watch chapter 2 and the first half of chapter 3 for this discussion. It is not very long.
Of course if you want to watch the whole series, that's ok too.

 

[aramis]

Dalton: I agree with your assertion of the attitude issue.

The problem I see is that, for the most part, Traveller Grogs tend to lean heavily towards the engineering mindset. Some of the worst offenders have been doing engineering in their jobs, but coming from a science background.

WRT Gravity, the rebound of 4d space is strongly theorized to be at light-speed, but no one is certain.

So, for gaming purposes, you can make any assumption that's reasonable (C, Near-C, instant, or just about any point in between).

Actually, I just thought of a possible experiment to see; Pluto's moons, over time, could be measured to see if the impulse is delayed. But that would require some VERY accurate measures over a long baseline. Just a thought.

Either that, or one successful jump!

 

[TheEngineer]

Hi !

Originally posted by Dalton:
I would love a copy! This is the sort of stuff that makes my wife shake her head and think that I am crazy.

best regards

Dalton

Well, again I mixed up languages. The documaent I had in mind is in german language, but recently I found another english resource for people interested in physics:

http://www.motionmountain.net/detailedcontents.html

The is one complete chapter on special relativity with many useful figures and explanations.

Best regards,

TE

 

[daltoncalford]

The reason I am asking the questions here vs on a dedicated physics site, is that I am trying to come up with a plausible reason for wormholes to exist between large masses in space.

In my universe, the method of travel is via wormholes that have to be found and opened via a keyhole drive.

The concept is that the effects of relativity are caused by the direct streaching/distortion of the underlying fabric of space time, and that relativistic effects do not "travel" per se, but are more like the pulling/bunching up of a table cloth. (closest analogy I have come up with so far)
If you put multiple heavy items on a table cloth, and some are pulled, pushed, or even having the whole table cloth shifted, the effects are fealt throughout the table top, even if the table cloth has only moved a tiny distance. Parts of the cloth would ripple or even fold.

With relativity and space time, the concept is that somewhere, someplace, there is some type of opject in the universe, travelling at relativistic speeds in relationship to any single body. This would cause local warping of spacetime, but since the effect modifies everything around the body, everything appears normal relative to everything else within it's local area of space time.

The large masses such as a sun, act as local anchors and normalize the effects in their local field. But as you get farther away from a single sun, you will find that time flows differently. As light travels, it takes on the local time, so the lightspeed barrier is caused by light traveling on the space/time fabric.

The folds in the space/time fabric allow for FTL travel to limited destinations from any one destination. Causality is comprimised, but, since you have to use the wormholes for all FTL travell and the time difference is minor unless you are travelling extreamly long distances (which means that the sub light time requirements makes time travel ineffective in "most" cercumstances).

So there you have the idea, FTL travel, caused by the "FACT" that relativistic effects are effective beyond the speed of light.......

The only hole I see in this idea is the re-emergence of the fabric of space time replaceing the idea of the old school thought of the "ether".

Any obvious flaws? (remember from a gaming viewpoint that may have a couple of physics nuts playing)

best regards

Dalton

 

[Anthony]

I recommend not going into the details of the physics; doing so just creates more holes for people to nitpick at. If you don't explain how something works, people can't complain that it doesn't make sense.

Relativistic effects don't travel anyway.

 

[Richard Saunders]

Hello.
Sorry but i dont understand.
If all the matter and energy in the universe was around at the big bang then all the gravity was there to and because the universe (depending on your beliefs) is expanding then all the gravity is everywhere already and dosn't propergate at all.
See where i dont understand????????.
Yes i have no university education and am only a Coms tech so shoot.
Await flames and possible enlightenment.

 

[Icosahedron]

Yes, gravity is everywhere at once, but the idea being examined here is that gravitating bodies move, and the question is about how closely their gravitational influence moves with them.

Consider this: a ship jumps into a new starsystem and a defence boat is trying to detect it using a long range gravitometer. Would the defence boat detect the ship as soon as it jumped in, or only after the gravity ripple had crawled across the intervening space at light speed?

This question is relevant regardless of the fact that all the atoms in the ship have been around since the year dot.

Hope that helps.