# Tag Info

5

If "spacetime" is assumed to have curvatures obeying general relativity then gravitational fields can do work, hence carry energy, hence have mass. So physically speaking this is not a universe "consisting of only two point masses" because spacetime is not empty. And yes, they will interact with the gravitational field, which will also interact with itself, ...

3

Yes to the first question, no to the second, but there will be motion, as they are drawn together, if I understand you correctly. The two point masses (let's assume they are Earth mass, but in theory any amount of mass will do), are drawn towards each other by Newton's law of gravity, that is the force pulling them together is proportional to the product of ...

0

Standard cosmology assumes homogeneity (as well as isotropy) of the universe. Though we know that this is only an approximation, we also know that for cosmic scales it is a good one. This can be seen in the cosmic microwave background radiation (CMB). However, the initially small inhomogeneities are responsible for galaxy and star formation through ...

-1

As Sebastian Riese points out, quantum mechanics is computable. Interestingly, classical mechanics is known to be non-computable. If classical mechanics were valid on all length and time scales, then you could construct a so-called "rapidly accelerating computer", which is a computer that accelerates such that the next clock cycle takes half the time to ...

1

Reading about Computable numbers I wondered if there is any physical experiment that returns non-computable numbers or if there is any physical theory that needs non-computable numbers. Because if that would be the case, we would have prove that the universe is not "simply" a simulation inside a Turing machine. Measurements and experiments result in ...

1

If two black holes hit head on at 99% the speed of light, the result would be one black hole sitting stationary in the center of mass frame with roughly twice the mass and some fraction of the energy expended as gravitational radiation.

0

Actually its perfectly possible within standard GR to do so! The question then becomes at what point does GR break down, and if it formed a mass at that point would it have time to decay via hawking radiation down to a Planck remnant? Many types of matter exhibit what is called type II critical collapse where if you carefully tune initial data, you can ...

0

There are many different forces which can affect various types of particles: electromagnetism, gravitation, weak force and strong force. These forces act in different ways to change the state of other particles which are sensitive to that particular force and transfer information from one part of the system to another via these interactions. ...

1

Gravitational waves are not dark energy. Dark energy is closer to a fluid that is created when space expands and is destroyed when space is destroyed. That a fluid can do that requires a particular balance between energy and pressure one that is not normally achievable but if there were a fluid like that it would just keep filling everything. Classical ...

3

If I understand correctly, you are just asking about the relation between energy and distances in both radiation and matter (and cosmological constant) dominated eras of the expansion of the universe. Consider the Einstein equation $$G_{\mu\nu} = 8\pi G T_{\mu\nu} \ ,$$ where $G$ is Newton's constant. In a FLRW unverse $G_{\mu\nu}$ is diagonal and using ...

2

Simply put, it is: how many aliens could we meet? More specifically: The Drake Equation is a way of predicting how many intelligent species there might be in the universe and the likelihood of them contacting us. There are a lot of things that can change the number of aliens we can expect to find. So we use what we know about the universe so far and ...

2

I don't think I can explain all the technical stuff, but first things first. Primordial means "at the creation", so if it was created today it wouldn't be primordial. Now "Primordial size" black hole, is probably what you mean and even that is a bit vague as estimates vary on the possible sizes of Primordial black holes, (and there's some uncertainty as ...

-1

If people brought all the trash in the galaxy to a single place and dumped it, a black hole could eventually form from the ever-increasing density without ever forming a star. Hypothetically, a bunch of heavier elements (incapable of starting a fusion/fission process to hold the gravitational collapse at bay) could happen to end up in the same region of ...

1

If a particle either "is" or "isn't", then its count is either 1 or 0. Even in quantum mechanics it's not possible that half a particle exists. It is possible to detect it with 50% probability, but if you set about counting all the particles one at a time, you necessarily end up with an integer answer.

1

Imagine a 1d space and a little vector in it that can change its location but not its length. It can't continuously change in a way that turns it around. If that vector was the momentum this says in 1d you can't continuously change you direction without your momentum being zero. A similar thing happens in relativity. A massive object has a location in 4d ...

-4

Could we be on the inside of a concave hollow universe? Nope. Nor are we on the inside of a hollow Earth. Recently I was discussing this theory again (a little drunk, I admit) and then tried to find answers, but couldn't find anything satisfying. That's because it's bunk. There is a theory (or several theories) that we could be living on the ...

7

For any transformed shape of the Universe, one may always easily define the theory in such a way that its results will be absolutely indistinguishable from the original theory. For example, one may describe the Earth and its vicinity by the polar coordinates $$R, \theta, \phi$$ so that $R\gt R_E$, the Earth's radius, corresponds to the space outside the ...

0

The simple answer to both questions is, yes, you exert a force on Pluto and on the universe. By simple, I mean that the Pluto question is treated as a "two body" problem (and nothing else). This answer is obtained using the formula given by dotancohen. For the universe question, it should be easy to see that the same process can be applied to you and any ...

3

Antonio -- sure: your mass affects Pluto. Here's a line of thought that might get your head around this amazing concept: You probably agree that the Earth as a whole affects Pluto .. right? Simply, consider the earth chopped up in to little pieces each about the size and mass of yourself .. say 100kg. (Funnily enough .. there's actually NOT THAT MANY of ...

6

The typical extinction for a line of sight out of our Galaxy (but avoiding the Galactic plane) is of order a few tenths of a magnitude at visible wavelengths (it is a factor of 10 less in the infrared and factors of a few more in the UV). This means that the typical attenutation of a signal arriving at the HST from outside the Galaxy is around say ...

2

You are approaching the question from the wrong end. The expansion of the universe is described by a particular solution to Einstein's equation called the FLRW metric. To derive this metric we have to make some assumptions, and the key assumptions are that the universe is isotropic and homogeneous i.e. that it is the same everywhere. So the universe being ...

0

Expansion of space is allowed by General Relativity. Which has made quite specific predictions that passed. So it seems valid to have solutions with an expanding space on the table (and leave it to observation to exclude them or to favor them). And there are two different situations where it comes up. A cosmological context where the expansion is the ...

2

Yes. You can make a model where you have coordinates $t, x,y,z$ where for any $x,y,z$ the universe looks the same. The metric ends up looking e.g. like $$ds^2=dt^2-(a(t))^2(dx^2+dy^2+dz^2)$$ and you can move your $x,y,z$ to have any value and everything looks the same (those things do loom different for different cues of $t$). You end up with the densities ...

1

You are not travelling faster than light in the sense that if you send some light to your destination it gets there before you do. It can be faster than light in the sense that if space is isotropically and homogeneously distributed with energy and such then there is an obvious global frame and distance in the global frame between two points can decrease ...

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Let's make this question a bit more operationally meaningful by asking if you can change the state of Pluto by choosing to do something here. As mentioned in the other answers, Pluto would feel the same force due to your mass even if you didn't exist, because the matter you consist of would be present on Earth anyway. However, you can still chose to move in ...

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The mass that makes up your body and everything inside your body is attracting Pluto. If you did not exist, then that mass would take other forms, and it would still attract Pluto by the same amount. I don't want to offend, but you know where all your mass came from, and where it's going. It would likely be in one of those forms had you not been conceived, ...

7

You -- and more to the point the matter you are made of -- are part of the Earth. The 65% water content of your body was part of the oceans before you were born and will be again. Likewise the other 35% of minerals and other substances come from Mother Earth. Thus whatever gravitic influence you have is both imperceptible and inconsequential in the context ...

102

While the atoms that make up your body are exerting gravitational force on very distant objects you as an entity are only exerting gravitational force on objects out to about 14 light years distance (assuming the age shown in your profile is correct). Because the "speed of gravity" is the speed of light. And toward the outer edge of that sphere the forces ...

5

I think you ask some good questions here, though many have been asked before. I tend to deny any claim that X is infinite, where X is anything from our observable universe. For instance: This is correct. Nothing in our observable universe is infinite. The observable universe is very large, but not infinite. The number of grains of sand on ...

4

Is the universe infinite? We don't know, we can't know. And we never will know. But there are some solid reason to think it is, and some solid counter arguments. One argument is based on the fact that we look out at things far away and notice they are all moving away from us. We argue that it is unlikely we happen to be at a unique place in the universe ...

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