If Gravity affects Time, does Time effect Gravity?

There's some nice little videos on YouTube you might find helpful. Even if they're not helpful, they're interesting for someone with an interest in science.

It depends on the method of time travel.

If one were to travel to the future, for example, by the (more or less) irreversible method of traveling in a spaceship near the speed of light, so that time outside your spaceship (say, on earth) were going by much faster relative to time inside your spaceship, and when you slowed down and returned to earth, you are effectively in the future, despite having aged very little yourself, then yes, this method would necessarily have an affect on gravity. The reason doesn't have to do with the time travel itself, but rather the method of time travel. A spaceship moving that fast will necessarily gain "mass" via its kinetic energy (mv^2/2) through Einstein's mass-energy equivalence (E=mc^2), and the energy-mass density of your high-speed spaceship will indeed bend the space around it due to General Relativity, giving it gravity beyond its natural, rest-frame mass.

Time, space, and gravity are very intertwined, yes, but as far as I know, I don't think time travel must necessarily affect gravity, but the semi-realistic methods I can think through which time travel will work, do invariably affect gravity in some way. To use another example, if you were to, say, use a wormhole to time travel, then space will bend around the entrance to the wormhole, much like it does around the entrance to a black hole.

Whether "anti-gravity" and "anti-inertia" dampers affect time depends largely on how such dampers work in your fictional universe.

small axe said:

I know Science Fiction (unless very "hard") often plays fast and loose with Physics ... but would I be wrong if I demanded that TIME TRAVEL creates some unexpected but unavoidable massive Gravity disruption, simply because the forces are interwoven?

Insights and explanations would be appreciated!

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Time isn't a force, it's just a coordinate.

But anyway, to answer your question more directly, it depends on how your method of time travel works. I think you can definitely get away with it, especially crediting it to the massive amounts of energy necessary for time travel.

The problem is, I can't see a situation that would create a massive gravity disruption. Gravity is a very weak force, the weakest of all the fundamental forces. It takes a massive amount of mass and an even more massive amount of energy to create a noticeable gravitational field. Your method of time travel would probably have to use energy equivalent to several suns to create a noticeable gravitational effect.

Of course, needing such a massive amount of energy to create something like a stable wormhole isn't completely unrealistic, so that could work. Where your characters/civilization gets so much energy is beyond me.

small axe said:

The "check my physics" thread got me thinking ...

If acceleration affects the rate of Time passage, and the amount of Gravity distorts Space/Time so that Time passage is influenced too ...

Would TIME TRAVEL necessarily have to affect GRAVITY?

Would a starship using "anti-gravity" or "anti-inertia" dampers ... necessarily have repurcussions with TIME?

Are they (Time and Gravity and Space/Time) as interwoven and co-dependent as Time/Space by their very natures?

I know Science Fiction (unless very "hard") often plays fast and loose with Physics ... but would I be wrong if I demanded that TIME TRAVEL creates some unexpected but unavoidable massive Gravity disruption, simply because the forces are interwoven?

Insights and explanations would be appreciated!

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The simple version is "time doesn't exist so it can't affect gravity".
The slightly more complex version is that time does not have an independent existence but is a phenomenon that comes about from other natural forces. So you cannot alter time as such and see what that influences. Whether "altering" time will have gravitational effects thus depends on the method you use to influence time.
That being said, pretty much everything in physics is totally interconnected and codependent. You really cannot change or circumvent any natural law without completely breaking the way the world works. Take just for example the proverb "Causality, relativity, faster than light. Pick any two."
So, the only you thing you can really do is break or circumvent as few physical laws as necessary for your story and try to conceal the unwanted results with some sleight-of-handwavium. I mean, who wants to deal with the fact that antigravity or inertialess spacedrives break conservation of energy, collisions at faster than light have an infinite energy or time travel always results in worldbreaking paradoxes?
So, no you are not wrong in demanding gravitational anomalies resulting from time travel, you have a right to deman anything you want. The author of course has the right to totally ignore your demands. It's certainly not something that has to result directly and always from time travel mechanisms (the way that the implosion of the law of conservation of energy is the direct and inevitable result of a reactionless drive).
But time travel is so extremely problematic, scientifically and even logically, that you should probably just treat it as magic. Maybe magic with some technobabble.

These are some interesting questions, and it made me wonder, if inertial mass and gravitational mass are equivalent, both increasing without bound as a ship approaches c, then wouldn't the ship--not to mention the people on board--be crushed by self-gravitation?

Julie Worth said:

wouldn't the ship--not to mention the people on board--be crushed by self-gravitation?

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No. At least, it's very unlikely.

To begin with, are we talking about an accelerating ship, or a ship that's moving at a constant velocity .99999999999c?

Let's start with constant velocity. This part is easy. Just think about relativity. If you're at rest outside the spaceship, you'll observe the spaceship as moving very, very fast, and thus you can measure the gravitational effects from the ship's mass-energy density bending spacetime. Inside, the ship, however, you are at rest in the ship's own frame. Thus, the ship's mass is equivalent to its rest mass, and this is equivalent to you just standing on a ship that's at rest on Earth.

But how about an accelerating ship? According to Einstein's equivalence principle, you can't tell the difference between a constant gravitational field and a constantly accelerating frame. For example, if you were locked inside an elevator with no way of seeing outside, you wouldn't be able to tell the difference between an elevator accelerating upwards in space at 9.8 m/s^s and an elevator sitting stationary on the surface of Earth. Thus, those in an accelerating ship will feel the equivalence to a "gravitational field" in the direction opposite the acceleration. Of course, this force is completely different than the gravity a fast-moving spaceship might make due to its mass.

But to be realistic, gravity is so weak a force, that in the first example, there's almost no way you'd really be able to notice the gravitational force outside the spaceship. In the second example, you'd just have to accelerate really, really fast, which is much more plausible.

Let's say we somehow found a way of scooping up vast amounts of interstellar gas and accelerating to a hair short of c. We do it at 1 g to keep things comfortable. There shouldn't be any difference inside so far as we can tell, that's what I would have said an hour ago. But now this conundrum--all that kinetic energy, which is equivalent to mass. How can we appear extremely massive to outsiders (and therefore possessing an enormous gravitational field) but not to ourselves?

Well, you will notice a force between attracting you to the outsiders at rest. From your frame of reference, though, it will be because they are moving at an incredible speed, thus they are the ones with such high kinetic energy.

So you will notice the same gravitational field (it likely won't be "enormous" though--remember, gravity is extremely weak), it will just seem to originate from a different source, to you.

However, since you and the spaceship share the same reference frame, and you're at rest with respect to each other, there won't be any "crushing" self-gravitational force.

kuwisdelu said:

Well, you will notice a force between attracting you to the outsiders at rest. From your frame of reference, though, it will be because they are moving at an incredible speed, thus they are the ones with such high kinetic energy.

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So, if two star systems pass each other at nearly the speed of light, each will sense an enormous gravitational field from the other, and yet each are deflected by exactly the same amount. Seems paradoxical.

How about coming and going. Is there a gravitational Doppler shift?

Relativity is around when I stopped being a physics geek so fast I changed gravitational mass.

Julie Worth said:

So, if two star systems pass each other at nearly the speed of light, each will sense an enormous gravitational field from the other, and yet each are deflected by exactly the same amount. Seems paradoxical.

How about coming and going. Is there a gravitational Doppler shift?

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You're passing a bit beyond my knowledge. I haven't really done much work with general relativity yet. I'll try to read up a bit more, and in the meantime hopefully someone with more experience can give you a better answer.

small axe said:

Personally, I don't understand gravity: if it's a distortion of space/time ... how can they be looking for a carrier particle? How can it be a cosmic force, acting upon something as unreal as "time" actually "bending" space?

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*shrug*

The same way light is a wave and acts like a wave, except when it acts like a particle.

All we really know is we have these two theories--general relativity and quantum mechanics--and they both seem to describe reality pretty damn well. They describe pretty different ways of looking at a fundamental force, but they've been right so far.... So we'll see.

small axe said:

Personally, I don't understand gravity: if it's a distortion of space/time ... how can they be looking for a carrier particle? How can it be a cosmic force, acting upon something as unreal as "time" actually "bending" space?

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In relativity, space and time are defined in terms of the things that measure them.* Time is thus that which is measured by clocks (or any clock-like processes), while space is that which is measured by meter-sticks (or any meter-stick-like objects or the processes that can substitute for them). To say that gravity (or high relative velocity) "slows time" means only that it slows all possible clock-like processes (which includes everything from the mechanisms of actual clocks, to the vibrations of microscopic particles, to the movements of photons, to the firing of synapses in the brain); and to say that it bends (or compresses) space means only that it compresses all meter-stick-like objects.

To illustrate, if every single particle (photons, electrons, quarks, etc) in a room were suddenly to be held in a magical field that froze them perfectly still, then time as defined in relativity would have come to a halt in that room. Time, in a more metaphysical, ephemeral sense, might still be 'flowing' (if time can be said to 'flow' at all), but from the perspective of anyone in that room, time has stopped. A hundred years (of metaphysical time) could pass by before the field collapsed, yet all those years would pass by in a single instant for anyone or any kind of clock inside the room. The more metaphysical notions of time and space are unpopular with scientists -- they prefer notions that are physically meaningful (i.e which lend themselves to measurement).

* See e.g. Einstein's paper, On the Electrodynamics of Moving Bodies, section 1 (where he defines simultaneity).

small axe said:

Indeed. But one would think that the thing measured has a real existence independent of the method by which they are measured (however they are defined) ...

I wonder what Time "really" is.

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By defining time and space instrumentally, they are basically saying that time and space (for the purposes of relativity) are nothing more or less than the properties of the measuring instruments. Which is to say, if the clock-like processes over there are running slower than those over here, then "time" over there is running slower than over here. And if the meter-stick-like objects over there are shorter than those over here then "space" over there is compressed relative to here.

I mention this because relativity only describes gravity as the warping of spacetime as instrumentally defined. Assuming it is meaningful to ask what 'real' time and space are (in contrast to instrumental spacetime), one shouldn't assume that 'real' spacetime is warped by gravity just because the theory of relativity says so (because the theory isn't really talking about 'real' space and time).

Gravity doesn't affect time. Gravity effects relative time. That is a HUGE difference and the reason it is called 'Relativity'. The person or object undergoing the acceleration is completely unaware of any difference in perceived or measured time. Only when time measuring devices are compared does the difference reveal itself.

As far as time travel is concerned, the question is moot. Time travel is, as far as we know, impossible so can not be tested. However, what you propose is a very interesting side effect that would play extremely well in a science fiction novel.

Makes one wonder what the universe is like in a high gravity environment such as near a black hole. Within the event horizon the passage of time would be extremely slow compared to what we are experiencing, which holds for chemical and nuclear processes, so free neutrons for instance would have extremely long, possibly close to infinite half lives. Also, the ramifications for objects measured as traveling at extremely high speeds relative to Earth, such as quasars, is quite intriguing. Measured from the quasar it is we that would be observed to have the large red shift. So which body is experiencing the time dilation? It, or us?

MelancholyMan said:

Also, the ramifications for objects measured as traveling at extremely high speeds relative to Earth, such as quasars, is quite intriguing. Measured from the quasar it is we that would be observed to have the large red shift. So which body is experiencing the time dilation? It, or us?

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Neither, since the high relative velocity of distant objects is due to the expansion of space itself.

Possible books: The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics by Leonard Susskind.

Introduction to Black Holes, Information and the String Theory Revolution : The Holographic Universe by Leonard Susskind and James Lindesay

small axe said:

Someone was mentioning that on the Book Channel the other night, and I thought he was saying that in movies we see the scene where the spaceship and crew fall into a black hole, and they die screaming as it stretches them down a horrible "whirlpool" that rips them and their spaceship apart atom by atom ... but actually theu might never know anything was wrong, because Time might be slowing down so much TO THEM that they just live out their normal lifetimes ... as the outside observer watches them destroyed? That's where i switched in, and there was some channel jumping that broke up his comments, but I think it was something like that. The crew doesn't know they're dying.

He later tied that in to some comment that ALL the "information" that falls into a black hole is "frozen" timeless and eternal in a shell at the event horizon,

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I can see how that happens. With higher gravity, time slows down (or it appears to slow down to us looking at it from our position of being in lower gravity - it's all "relative," you know).

Any "small" or "normal" sized black hole (one with the mass of the Earth has a event horizon about 1/3rd of an inch in diameter, and one with a star's mass will have one a mile or two in diameter) is going to have a substantial gravitational gradient, also known as tidal forces (read Larry Niven's title story "Neutron Star" in the book of short stories with that name) near it, and I suspect anyone indeed will be torn apart before they get close enough to notice the time dilation.

If you're standing up relative to the black hole so that your feet are about five feet closer to the black hole than your head, and the gradient is one gee per foot, you'll certainly feel yourself being pulled. At 50 gees per foot, you're definitely being pulled apart. But don't worry, you've already passed out from your blood pouring out of your midsection and collecting in your head and feet.

But a black hole at the center of a galaxy has the mass of millions of stars, and you may get in close enough to where time dilation becomes significant before the tidal force hurts you.

small axe said:

so that the universe becomes a holographic image (?) ... that OUR entire universe and reality is a type of holographic image formed by information located on some inner surface?

It was rather mindbending, I wished the show had repeated. He was the guy who had that bet with Stephen Hawking about the nature of black holes.Can anyone help me out and tell me his name, I keep meaning to look him up! But the holographic universe was one of his main things ...

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I remember that, poor Dr. Hawking lost the bet. But no doubt he'd agree that's a small price to pay for the advancement of science.

Would that be John Preskill as mentioned in this story?
http://www.newscientist.com/article/dn6193-hawking-concedes-black-hole-bet.html
The article also mentions Kip Thorne, a prominent scientist and author who was on Hawking's side of the bet, perhaps that's who you're thinking of.

small axe said:

.but actually theu might never know anything was wrong, because Time might be slowing down so much TO THEM that they just live out their normal lifetimes ...

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That's exactly backwards. If they have ten minutes to live before they're torn apart by tidal forces, and time has slowed for them by a factor of ten, then they only experience one minute. Only if time were vastly speeded up for them could they live out their normal lifetimes.