# Multiple Curves in Space-Time: Masses on a Trampoline

• B
• BeedS
In summary: That's the problem. Spacetime is not a trampoline. It's not a sheet. It's not a fabric. It's not a grid. Those are all analogies that have limited use in explaining certain aspects of general relativity. They all fall apart when you push them too far, which is what you are trying to do here.The trampoline analogy is a useful tool for visualizing how objects with mass affect spacetime. However, it is not an accurate representation of how gravity actually works. In reality, gravity is the curvature of spacetime itself, not the result of objects sitting on a rubber sheet.So in summary, the trampoline analogy is not a reliable way to understand gravity and should

#### BeedS

When you put a mass on a trampoline it causes the trampoline to V. If you put another similar or same sized mass on the same trampoline at a great enough distance, will the curve on the trampoline/space-time look like a W or a U ?

If you can get the curves on a trampoline/space-time to look like W with the two masses spaced far enough apart. If you push mass_A towards mass_B will the curve peak between the masses (W) cause moving mass_A to repel/move mass_B?

If you add more mass gradually to mass_A side will it push mass_B away at first, until the curve peak between them comes down and then eventually they’ll start attracting or moving towards each other?

BeedS said:
When you put a mass on a trampoline it causes the trampoline to V. If you put another similar or same sized mass on the same trampoline at a great enough distance, will the curve on the trampoline/space-time look like a W or a U ?

If you can get the curves on a trampoline/space-time to look like W with the two masses spaced far enough apart. If you push mass_A towards mass_B will the curve peak between the masses (W) cause moving mass_A to repel/move mass_B?

If you add more mass gradually to mass_A side will it push mass_B away at first, until the curve peak between them comes down and then eventually they’ll start attracting or moving towards each other?

Unfortunately you cannot say any of these things, as the trampoline analogy is simply not realistic; there are many sources on the internet that will tell you this. Here is one.

You can't just add mass in general relativity, it has to come from somewhere. Thus there's no way for general relativity to tell us what would happen if a mass were to appear from nowhere - it's like asking a mathematician what would happen if we found an odd number that's an exact multiple of two.

@m4r35n357 has already pointed out that the trampoline is a bad analogy for the curvature of spacetime. If you search this forum for posts by member @A.T. you'll find a video he's made which shows a much better way of visualizing the way that curved spacetime leads to gravitational effects.

BeedS said:
When you put a mass on a trampoline it causes the trampoline to V. If you put another similar or same sized mass on the same trampoline at a great enough distance, will the curve on the trampoline/space-time look like a W or a U ?

If you can get the curves on a trampoline/space-time to look like W with the two masses spaced far enough apart. If you push mass_A towards mass_B will the curve peak between the masses (W) cause moving mass_A to repel/move mass_B?

If you add more mass gradually to mass_A side will it push mass_B away at first, until the curve peak between them comes down and then eventually they’ll start attracting or moving towards each other?

Leaving aside the issue of the rubber sheet, there is an important point here. Whatever your theory of gravity it must match the experimental evidence. A question like this is already answered if you think of the Solar System:

The planets orbit the Sun
Some planets have moons (or artificial satellites) that orbit them
A projectile fired on the Moon will travel in a parabola before returning to the surface of the Moon

That's the way gravity behaves, regardless of the theory that explains it. In particular, there is no experimental evidence for masses ever repelling each other. And, in fact, Newton's law of gravitation is sufficiently accurate to model the Solar system. So, whatever GR says, it must be very close to Newton's gravity on the scale of the Solar System.

BeedS said:
If you can get the curves on a trampoline/space-time to look like W with the two masses spaced far enough apart. If you push mass_A towards mass_B will the curve peak between the masses (W) cause moving mass_A to repel/move mass_B?

If you add more mass gradually to mass_A side will it push mass_B away at first, until the curve peak between them comes down and then eventually they’ll start attracting or moving towards each other?
In addition to what’s been said about how the rubber sheet is a poor analogy for gravity (or at least very, very, very limited in its application), your ideas about trampolines are incorrect. The peak in your “W” will shrink until the masses converge.

BeedS said:
If you can get the curves on a trampoline/space-time to look like W with the two masses spaced far enough apart. If you push mass_A towards mass_B will the curve peak between the masses (W) cause moving mass_A to repel/move mass_B?

Can you get this to happen on a real trampoline? If so, you have simply discovered something about the analogy that doesn't match what happens in the real world.

People have discovered things in a way that's comparable to what you're doing. For example, Dirac came up with a theory that explained the behavior of atomic electrons, but it allowed for the existence of an anti-electron. Something that may have seemed absurd until it was discovered.

Dirac, however, was working with a theory, not an analogy designed to explain the theory to people who are not familiar with it.

## 1. What are multiple curves in space-time?

Multiple curves in space-time refer to the concept of curved space-time as described by Einstein's theory of general relativity. It suggests that the presence of mass and energy can cause the fabric of space-time to curve, creating a gravitational field.

## 2. How is this concept related to masses on a trampoline?

The analogy of a trampoline is often used to explain the concept of space-time curvature. The trampoline represents the fabric of space-time, and the masses placed on it represent objects with mass. The heavier the object, the more it will curve the trampoline (space-time) and affect the movement of other objects on it.

## 3. Can you give an example of multiple curves in space-time?

One example is the orbit of the Earth around the Sun. The Sun's mass causes the space-time around it to curve, and this curvature affects the path of the Earth, causing it to orbit around the Sun.

## 4. How does this concept affect our understanding of gravity?

Einstein's theory of general relativity suggests that gravity is not a force between masses, but rather a result of the curvature of space-time caused by the presence of mass and energy. This has revolutionized our understanding of gravity and has allowed for more accurate predictions and explanations of gravitational phenomena.

## 5. Is there any evidence to support the concept of multiple curves in space-time?

Yes, there is a substantial amount of evidence supporting the concept of space-time curvature. This includes the observation of gravitational lensing, the bending of light around massive objects, and the accurate predictions of the orbits of planets and other celestial bodies. Additionally, experiments such as the Pound-Rebka experiment have also provided evidence for the curvature of space-time.