# A question about anti gravity

1. Apr 25, 2009

### uperkurk

Hello all. I am clueless when it comes to physics and I dont know where to place this thread so i'll just place it here. My question is: Is gravity just a force of nothing or can you actually take a sample of it. For example can you look through a microscope and see the atom or whatever it is, graviton?

If this is possible is there any chance at all that there is a way to make that particual or atom or whatever it is negative. So instead of it pulling down, it just floats around doing nothing. I thing I dont understand is in space, people and objects float around because theres no gravity., but there must be gravity because the earth and planets are held in position by gravity. If anti gravity was possible what would that mean in terms of space travel?

Sorry if its a dumb question but as I said im clueless. Thanks for reading.

2. Apr 25, 2009

### DaveC426913

Our best theories indicate that gravity is not a property or thing, but that it is the curvature of space.

Quantum mechanics proposes that may possibly be a graviton particle, yes. But no one's seen it.

You're on the right track, good critical thinking. There is gravity in space. There is no place in the universe that is completely free of gravity; it extends to infinity.

What you're seeing when you see people and objects floating around is space is one of three things:

1] Most man-made obejcts are in orbit. They're racing around the Earth at 25,000mph. When the space shuttle, the space station and the astronauts are all in the same orbit, they float motionless with respect to each other. But rest assured, they are all moving at 25,000mph and falling toward the Earth under gravity all the while (they just miss it!).

2] Sometimes, if things are floating far enough away from any gravity body, they really are in very low gravity.

3] You could very well be falling toward the Earth (or Moon) at terminal velocity, along with other astronauts and obejcts, but you would not know it unless there is something with which to measure your speed. Itr owuld appear that you are floating motionless.

Last edited: Apr 25, 2009
3. Apr 26, 2009

### uperkurk

wow thats really intresting, so if you was to just jump out of your spaceship into space would you fall through space and constantly build up speed? So you could end up moving at 100,000 miles per hour and it would just seem like your floating around? lol. Is this is the case how long would an object have to fall through space to reach 100's of thousands of miles per hour. Is there an equation involved?

4. Apr 26, 2009

### DaveC426913

Well, not quite. You would retain the velocity of your spaceship, so you'd merely float next to it. If your ship were in orbit, then so would you be.
Yes. Einstein's legacy of relativity is that motion is relative. There is no absolute motion, only motion relative to some reference point. If you are floating around in space, the only way you can claim you are moving at all is to measure your motion relative to an object of your choosing.

There is. But it's complex because in this case, the gravitational force would not be a constant. You'd start of hundreds of thousand of miles away where gravity is much more weak. It would take a loooong time. Weeks. Months.

5. Apr 26, 2009

### uperkurk

Thanks for the answer dave. Helped me understand alot more but one more thing I would like to add is if you for example was standing on the wing of your spaceship and fell off and started to float away would it be possible to do a swimming motion to get back or are you at the mercy of gravity or whatever :)

6. Apr 26, 2009

### DaveC426913

You couldn't swim back. That only works in an atmo or in water.

If you became separated from your ship, you would be in very grave danger. Unless you can jerry-rig some method for propulsion, you would drift away inexorably and eventually die. Space is very unforgiving.

But you're not dead yet. What you could do is detach any non-essential pieces of equipment from your suit and throw them in the opposite direction. The reactionary force will (hopefully) propel you back toward your craft. Even more desparate, you could find a way to vent some of your oxygen and use that as a propulsive force.

7. Apr 26, 2009

### uperkurk

But theres no resistence in space so how would throwing things behind you propel you forward? As I said im very curious so thats why im asking all these random questions :)

8. Apr 26, 2009

### DaveC426913

Asking questions is great. 'twould that more people did so.

Newton's First Law: Any action generates an equal and opposite reaction. It works in space quite nicely. It's how rockets propel themselves in space.

9. Apr 26, 2009

### uperkurk

ok one last question lol. Once inspace, would a massive square block of concrete be capable of falling through space at the same velocity as lets say a dart shaped rock? Would an aerodynamically shaped object pick up more speed faster then an object that isnt aerodynamic if where both cut lose at the same time?

10. Apr 26, 2009

### junglebeast

Dave is partially correct, but he is actually referring to Newton's third law of motion. However, this way of phrasing the law is somewhat antiquated, and cannot be interpreted literally. This often causes the law to be misinterpreted. What exactly constitutes an action, and what is the correct reaction?

For example, I have actually heard people that thought the law could be applied to the stock market: a stock goes down in price (an action), therefore there must be an equal and opposite reaction, therefore the stock must go back up in price an equal amount at a later time. Of course, that is completely incorrect! As you can see this way of stating the law is not actually useful.

A much more intuitive explanation for why you can propel yourself through space by throwing things behind you results from the law of conservation of linear momentum. This law means that in a closed system (ie, if there are no exterior influences), the total momentum of the system stays constant.

Momentum equation:
P = M*V

(P is momentum, M is mass, V is velocity)

Now, for the closed system we can consider the astronaut who is floating in space. Because space is [mostly] empty, the only particles we need to consider are the atoms that make up the astronaut and whatever gear he is wearing. The fact that the system is closed means that we will not be considering things like other particles flying in and hitting him from the depths of space, which would obviously screw up our analysis.

Ok, so every particle in the spaceman's body has some mass and velocity. Because the atoms are all held together by magnetism (ie, chemical bonds), we can roughly consider the entire astronaut as 1 big particle with 1 mass and 1 velocity.

If the astronaut takes off his glove and throws it behind him, it now has a different velocity, so we need to consider that as a separate particle. Let's represent the mass of the glove as Mg, and the mass of the astronaut without the glove as Ma. The original velocity of the astronaut is V1, the velocity of the astronaut after throwing the glove is V2, and the velocity of the glove after he throws it is V3.

Initially, we have

P = (Ma + Mg) * V1

The law of conservation of momentum tells us that the total momentum P (which means adding up M*V for every particle) is going to remain constant no matter what those particles in the system do. After throwing the glove, we now have

P = Ma*V2 + Mg*V3

Now we can just use elementary algebra to find out what the velocity of the astronaut has to be:

(Ma + Mg) * V1 = Ma*V2 + Mg*V3
(Ma + Mg) * V1 - Mg*V3 = Ma*V2
V2 = ( (Ma + Mg) * V1 - Mg*V3 ) / Ma
V2 = (Ma + Mg)/Ma * V1 - Mg/Ma * V3

In order to understand the result, we don't actually care how massive the glove or the astronaut is, so let's create some arbitrary positive constants (a mass cannot be negative):

V2 = C1* V1 - C2* V3

Ok, let's say his initial velocity was going in the positive direction. Then he throws the glove behind him, which means V3 is in the negative direction. Two negatives make a positive, so that means his final velocity V1 is still going forward, but it's faster than it was before.

One of the consequences of the law of conservation of linear momentum is that the center of mass of all the particles will always stay in the same place. Using that logic, you can also understand why he can propel himself by throwing something behind him...if part of the mass is moving away, then he has to move forward in order to keep the center of mass of everything in the same spot.

I hope that clears up your question

11. Apr 26, 2009

### junglebeast

Aerodynamics do not matter in space. Otherwise the Borg ship (which is shaped like a cube) would have trouble chasing after the Enterprise!

The reason aerodynamics matter on earth is because the air is a gas, meaning that it is composed of tons of little air particles bouncing around against each other. This is why air has pressure.

It's not hard to move through air at slow speeds, but the faster you are going, the more compressed all the particles in front of you become -- which makes them "more solid". If your ship is shaped with a point at the tip, then it can essentially squeeze through and push the air to either side of it relatively easily like an ice-breaker ship, but it would be a lot harder to push through it with a big fat square.

Space is empty. It's not a gas, so there are no particles bouncing around. There is nothing, so there's no need to worry about pushing against particles.

12. Apr 26, 2009

### DaveC426913

How embarrassing.

13. Apr 26, 2009

### uperkurk

ok thanks for that, I understand your answer about aerodynamics being important on earth ofcourse but I just wondered about in space. But your other answer about the linear motion means absolutley nothing to me

V2 = (Ma + Mg)/Ma * V1 - Mg/Ma * V3

About as meaningful as saying %%^&^&"%^"$£%£$^^%

lol :P

14. Apr 26, 2009

### junglebeast

This is not exactly true. The truth is, the question you have asked is one of the greatest mysteries in all of physics.

There are currently two contradictory theories which can be used to explain gravity. One of them is General relativity, which treats gravity is a curvature of space time, with no associated particle. This theory gives us a mathematical model which can be evaluated in simulations and appears to work.

However, the standard model predicts that gravity is caused by a particle caused by the graviton, and it would also be capable of explaining the behavior of gravity. Nobody has been able to actually detect a graviton, so we can't accept the theory yet. However, the standard model has shown us that ALL of the other forces in the universe (electicity/magnetism/weak force (or electromagnetic force / electroweak force) and strong force) are caused by particles. Therefore, it is a very attractive notion that perhaps gravity, which is the only force in the universe we currently cannot explain, would also be caused by the interaction with a boson (eg, graviton particle).

In short, the answer to this question is likely to yield the grand unified theory of the universe which Einstein spent his life trying to find.

The scientific community is divided on the issue of whether or not GR or the standard model is correct in terms of the graviton.

It is not surprising that nobody has been able to detect a graviton yet -- this fact cannot be used as evidence against the graviton's existence, because by its nature, it is an extremely difficult particle to detect.

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is simply the extremely low cross section for the interaction of gravitons with matter. For example, a detector the mass of Jupiter with 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the background of neutrinos, and it would be impossible to shield the neutrinos without the shielding material collapsing into a black hole.

Rothman, Tony; and Stephen Boughn (November 2006). "Can Gravitons be Detected?". Foundations of Physics 36 (12): 1801–1825.

However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are already underway (e.g. LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass.

Will, Clifford M. (February 1998). "Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries". Physical Review D 57 (4): 2061–2068.

15. Apr 26, 2009

### junglebeast

It's all good. The best way to learn is by teaching :)

16. Apr 26, 2009

### junglebeast

In that case, perhaps you will understand the alternative explanation I gave at the bottom of the same post.

17. Apr 26, 2009

### uperkurk

Yes I understand it sort of. I'm going to try to think logically here lol. Matter is everything correct? Everything that exsists in our universe is matter. Gravity exsists as we know so there must be some physical "thing" to show its presence. Gravity is space curvature or something but there can't be a force of any kind that "acts physically" on something. Maybe gravity is just so small its impossible to detect the presence of it? Something you can feel and not see at all. All very confusing for me but here is an example I have. Helium is a natural "opposite" to gravity, put it in a ballon and it repels against the force of gravity with no propulsion or anything, it just goes upwards. 100% natrual. so somewhere in the helium atom or partical or molocule or whatever its called must be a certain part that acts negatively towards gravity? Or am I just going the complete wrong way here?

18. Apr 26, 2009

### ZapperZ

Staff Emeritus
Hold a ball, probably something has heavy as a basketball.

Stand on a weighing scale and be as still as possible so that the weighing scale reads a constant value. Now toss the ball up into the air and watch the scale while you are in the act of tossing it. You will see that you momentarily 'weigh more' than when you were standing still. Why? While you are pushing on the ball, the ball pushes back on you. This is an illustration of Newton's Third Law, which results in the conservation of momentum.

So if you throw stuff behind you, those "stuff" also pushed on you when you shoot them out. The mathematics that you were shown are the direct mathematical derivation of what everyone has been describing in words. Those mathematics are necessary because every physics has an underlying mathematical description. The existence of a proper mathematical description in this case ensures that our explanation here is not merely some hand-waving argument.

Zz.

19. Apr 26, 2009

### DaveC426913

No, There is energy.

And the four fundamental forces:
weak nuclear
strong nuclear
electromagnetic
gravity

Last edited: Apr 26, 2009
20. Apr 26, 2009

### uperkurk

Thats a good way of explain it I understand now. The same as when your about to jump, you weigh more while your bending your legs and when you push down the scales shoot up. But your explaination is better.