# I No Gravity?

1. Jul 1, 2017

### Zutswang

If, according to general relativity, gravity doesn't exist and falling bodies simply follow curved space, what starts them falling to begin with? If a car parked on a hill slips its brakes what starts it rolling downhill, and what force accelerates it?

2. Jul 1, 2017

### Brian blake science

See gravity indeed exists !! The curved space which you talked about is the way gravity works . To the point of your example the car slipped cause of failing of the brakes which could be due to immediate loss in the friction ; Now the car slips downward it is accelerated by gravity itself because it has the gut of standing on a slippery slope . Now you would ask hey Brian but this could also be applied to planets but my friend , plants were already revolving and the Damn car is at rest !
Hope it helped !

3. Jul 1, 2017

### David Lewis

"Gravity" in this context refers to Newton's conception -- Newton doesn't explain what is going on -- which is perfectly valid and useful. In General Relativity, however, the definition of "gravity" is slightly different.

4. Jul 1, 2017

### Zutswang

What is the GR definition?

5. Jul 1, 2017

### Brian blake science

It's an attraction pull exerted by objects (having mass)

6. Jul 1, 2017

### Brian blake science

The simplest definition I think of

7. Jul 1, 2017

### David Lewis

In GR, gravity is curved space-time.
In Newtonian physics, gravity is a force.

8. Jul 1, 2017

### Brian blake science

I think Einstein was the Right guy . Different people, different views !!

9. Jul 1, 2017

### David Lewis

It is assumed that objects at rest with respect to the Earth are moving along the time dimension.

10. Jul 1, 2017

### Zutswang

I think my original question still stands. Einstein said once that he had a Eureka moment when he realized that a falling man has no sense of his own weight, and he said that day was the happiest of his life. The falling man feels no force, acceleration yes, but no force. This was the idea that started him on GR, culminating in the postulate that gravity is not a force. So the question is: If gravity doesn't start the car accelerating downhill, what does?

11. Jul 1, 2017

### Staff: Mentor

Gravity certainly exists in GR, it just isn't a force.

To understand how GR treats gravity, it is important to understand some basic concepts. In GR an object with an attached accelerometer which reads 0 is called inertial and has no external forces acting in it. If there is no gravity then such objects trace out a straight line in spacetime. If there is gravity then the spacetime is curved and the line traced out is a geodesic, which is a "straight line" in a curved spacetime.

It is very important to understand that it is spacetime which is curved, not just space. So an object at rest is still "moving" through time.

12. Jul 1, 2017

### Brian blake science

Ok now I go deep into it !
Gravity according to Wikipedia
"Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward (or gravitate toward) one another, including planets, stars and galaxies"

According to the phenomenon which was observed by Einstein during his Elevator thought experiment a man experiencing a free fall wouldn't recognize his own weight and the opposite would happen if he was moving upwards: He would be glued to it . Now the elevator was being pulled down by the Earth's gravity which was even observed by Newton but the point where he missed was that how gravity works ? Which was explained by Einstein through the Space time curvature. I think you need a visual explanation . You can refer here to clear what you are saying and what the readers are thinking if the question you are asking -https://youtu.be/MTY1Kje0yLg

13. Jul 1, 2017

### Staff: Mentor

When the parking brake is on, an accelerometer placed on the car will show that the car is accelerating at 1 g directly upwards. The contact force between the wheels and the ground explains this acceleration. When the brakes are turned off, then that contact force is reduced and the accelerometer shows that the car is accelerating at less than 1 g. Since the car is not accelerating as much as the ground, the hill starts moving relative to the car.

14. Jul 1, 2017

### Bandersnatch

Dale's comments may be better understood with some visual aids. One of our members (@A.T.) made these helpful videos illustrating the difference between Newtonian and relativistic concepts of gravity:

Brian blake science, you are clearly enthusiastic, but please refrain from obfuscating the issue. The video you linked is not in fact a good representation of gravity in relativity - just ask yourself, which part of the trampoline is supposed to represent the time dimension?

15. Jul 1, 2017

### Brian blake science

The trampoline is the space time curvature and as depicted in the video the still objects just met the massive objects at the center but when they were suspended to roll along the sides the just orbited like moons and planets

Isn't the model in which Einstein viewed the gravity. Ask yourself and if you were asking something different , sorry cause I didn't get what you're saying
. Apologies!

16. Jul 1, 2017

### Brian blake science

The space and time are wrapped together so the trampoline is space time . A combination of the two - THE SPACE TIME BLANKET . Einstein said it himself and this was indeed his picture of gravity

17. Jul 1, 2017

### Ibix

The problem with the rubber sheet model of gravity is that the only reason a stationary ball starts moving is the real force of gravity. So you can't model gravity without using gravity.

In GR the answer is the curvature of spacetime, bot of space. A.T.'s video, which Bandersnatch posted, illustrates this correctly.ï

18. Jul 1, 2017

### David Lewis

Analogous to the water pipe model of electric circuits, some people love it; others can't stand it.
The simplifications, limitations and weaknesses of a model must be kept in mind lest you stretch it to cover something of which it's not capable.

19. Jul 1, 2017

### Staff: Mentor

The point others are trying to make is that a trampoline is a 2D surface, and both directions are spatial. In the videos by @A.T. one direction is space and the other is time. They are clearly labeled. The trampoline has no clearly identified time direction.

20. Jul 1, 2017

### Ibix

...and if it did, some of the balls would be moving in opposite directions in time.

21. Jul 1, 2017

### Brian blake science

You said that we can't figure out a model of gravity without gravity? I think you should reconsider your statement cause the balls are attracted to the object placed at the middle due to the massive object makes a greater curvature in space time blanket than the balls . Thus it is not the gravity it is the curvature !

22. Jul 1, 2017

### Ibix

No. I said the rubber sheet model cannot explain certain things without invoking gravity. The rubber sheet model is not general relativity. (On a re-read, I see that wasn't entirely clearly written in my previous post).
If you believe this, place a ball on the sheet, stationary with respect to the mass in the centre. Now explain to me why it starts moving without reference to the actual force of gravity. Note also that if gravity plays no role, this experiment ought to work on the ISS. Do you think it will?

I repeat that it is, of course, possible to describe gravity without using gravity. But the rubber sheet model does not do it.

Last edited: Jul 1, 2017
23. Jul 1, 2017

### A.T.

As others stated, the trampoline has nothing to with how curved space-time results in gravity according to General Relativity. It lacks the crucial time dimension as an axis within the sheet, and the balls aren't following geodesics on the sheet.

24. Jul 1, 2017

### A.T.

Space-time, not space.

Things always advance in time, even when at rest in space. Distortion of space-time makes them deviate from purely temporal advance to wards spatial advance.

25. Jul 1, 2017

### Staff: Mentor

Another point about all this is that, although an object in free fall within a gravitational field appears to be accelerating(according to conventional 3D measures of acceleration), as reckoned based on 4d space time, its acceleration is zero, as verified by a zero reading on an accelerometer. So there is no force acting on it and it's 4d acceleration is zero.