# Tag Info

1

I won't answer on the energy part of the question, but here a few remarks which in my opinion are worth adding: The laws of physics are not frozen, the evolve (more precisely, they get more and more accurate ; e.g. Newton, replaced with the general relativity, etc.). Maybe one day someone will come up with an even more general theory, a part of which is ...

-1

Time travel is not impossible because of conservation of energy, time travel is not impossible at all. Entropy does not allow you to go backwards in time, but forwards does not present any problems.

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Energy conservation stems from Noether's theorem applied to time (i.e., time-invariance leads to energy conservation, similarly to how spatial-invariance leads to momentum conservation). Since the universe is expanding (and accelerating at that), the state of the universe today is different than it was yesterday and will be tomorrow, hence energy ...

1

What we like to call the energy, i.e., the total matter/energy content of space-time, might not be conserved. However, there is a lot of reason to suspect that fundamentally the universe is some big quantum system, and that space-time and particles and fields are emergent from this underlying idea. In that case, we expect there to be a Hamiltonian $H$ and ...

1

Short answer: No. Momentum-energy conservation arises by dint of Noether's theorem, which says that if a system's Lagrangian is invariant with respect to a continuous transformation, there is one conserved quantity, called the "Noether Charge" for each such transformation (technically: for each linearly independent tangent vector in the Lie algebra of the ...

2

In as far as I know, the universe is actually gaining energy. And as far as we can see, the universe is expanding as a product of pressure, in the direction of the difference between opposing pressures, blah blah.... So would momentum be conserved under this rule? I suppose, if the exact same amount of energy were being "destroyed" as "created" within the ...

3

In this paper by Gott et al., on p. 466 they define the "future visibility limit", saying in the published version that "No matter how long we wait, we will not be able to see farther than this", and on p. 7 of the arxiv preprint they similarly say "If we wait until the infinite future we will eventually be able to see the Big Bang at the co-moving future ...

9

If our ideas about cosmic evolution are correct, galaxies that are visible today will in principle remain visible in the future. As time goes on, light from more and more distant galaxies will be able to reach us, and the number of observable galaxies will increase. However, there exists a cosmic event horizon$^1$, so this is an asymptotic growth: There's a ...

8

Consider that most elevators have a counterweight to store energy. The counterweight isn't perfect, but it reduces the overall energy needed to move the carriage. As the elevator moves up, the counterweight moves equally down. Likewise, a time machine would have to overcome the energy deficit/surplus caused when moving from one point to another, but it ...

9

No, conservation of energy is for the entire system. If you can travel from time A to time B then both time A and B are parts of the same system as far as conservation of energy is concerned. Even if you assumed that despite travel being possible the times were separated, time travel would simply require the transfer of equal energy from in the reverse ...

14

It is true that general time-travelling violates conservation of energy. If you transport yourself into yesterday, you appear twice in the universe for that day, which means twice your rest energy, which is a lot of energy. It may mean that time-travelling is inconsistent and therefore impossible. But not necessarily. In general relativity, it is very hard ...

0

Hmmm, a very interesting question. Your logic seems sound, but I'm not a physicist, so I can't really say. What I can say as a non-physicist is that time flows in the direction of increasing entropy, so moving back in time would mean that the Universe should go back to a state of lesser entropy, which is supposed to be prohibited. Interestingly, this in ...

7

If classical mechanics were valid at cosmological levels, the answer would be yes. But general relativity is what describes the dynamics at this larger scale, and it is not generically possible in GR (in an arbitrary spacetime) to define conservation of momentum or conservation of mass-energy.

0

If the Universe is infinite, then it had instantly grew from zero to infinity at the moment of Big Bang. Note, that this is a possible model from General Relativity (GR). But most probably GR is not applicable at the moment of Big Bang, because it contains singularity.

0

Brian Greene talks of many more types of multiverses...holographic, extra dimensional, simulated etc. It is also possible our universe is only a construct of our consciousness in which case consciousness would vary in a different universe..but that is in the realm of philosophy. Higgs field variation- a basic energy field value creating the constants and ...

0

Have a look at the Wikipedia article on Binary Black Holes. Essentially when two black holes come in close proximity they are believed to merge into a bigger black hole so you won't get one black hole sitting at the event horizon of another for infinity. Super massive black hole binaries are believed to form during galaxy mergers Regarding the answer ...

0

The universe was infinite at its creation, and continued to be so. It just got bigger during inflation. There may be a misconception that the universe begun as a "infinitesimal point", but singularity just means that it was non-defined. During inflation, the space between matter became wider, analogous in 2-dimensions, to the "spots on a balloon's surface" ...

0

So, we first have to understand what it actually means to be 'expanding.' The FLRW Metric describes our universe as homogenous (physics works the same at all points), isotropic (looks basically the same everywhere on a large scale), and expanding. In simplest terms, the further back in time we look, the closer things generally seem. Basically, objects that ...

0

It is a pseudoproblem of definition. Some people define universe as everything that could ever possible exist, to them the word multiverse is an oxymoron. But those who like to use the idea of a multiverse, use it encompassing different things depending on context. For instance, Max tegmark defines 4 different levels of universes/multiverses: Level I: ...

3

As far as I know, "a" Universe is caracterised by fundemental constants such as the speed of light, Newton's gravitationnal constant, the Planck constant and so on. You could distinguish between two different universe from the variation of these values I think.

1

Yes, you can simulate non-locality in celular automata, even if the rules are local. In such a case, contiguous cells do not represent adjacent points in our space. In these models space is an approximate, macroscopic emergent property (it is discussed in detail in NKS). This emergent space can have, at microspcopic scales, functional links that resemble the ...

-2

Black holes do explode after their life is over, thus vomiting out all the matter they ingested. Apologies for not providing credible explanation for my claim (in the form of mathematical equations). I read that (that black holes do explode) in Stephen Hawking's book titled Black Holes And Baby Universes. He stated that there is an inverse relation between ...

4

No, throwing matter out isn't contrary to a BH; there is often an accretion disc surrounding the black hole and that is what forms the jet. No, the ejected material cannot condense to form a galaxy. A galaxy requires the material to be gravitationally bound to some central point, a jet moving at $\sim c$ is moving too fast to be gravitationally bound to ...

3

There are scientists that do not accept the Big Bang (and more). That doesn't mean that there isn't evidence for the Big Bang (there is), nor does it imply it may be wrong. All we can say is that some scientists consider the Big Bang to be a fact because the evidence is overwhelming, some who think it is the best cosmological model because there is much ...

2

@GeorgeSmyridis: the big bang is a model that describes a bunch of cosmology. Physicists, when referring to it, rarely are talking about the event that happened at the very beginning of the model. In fact, the situation in the model goes outside of the applicable region of known physics before you get to the point in time when the explosion would have ...

0

People who are studying something something generally use a frame of reference which is reasonably close to the things of interest. While it might in theory be possible to measure the stature of a man by very accurately determining the distance from the center of the Earth to the bottoms of his feet, as well as the distance from the center of the earth to ...

1

The Universe is infinite. If the energy density of the universe is greater than zero, then the total energy must also be infinite.

0

Convert the masses into energy by multiplying them by c^2. Learn what percentage of each one there were, know the energy of the universe at that point... Find what percent of that is each particle... Remember, positrons and electrons are antiparticles and therefore have the same (rest)-mass. As for Photons at that time, determine their energy. Then sum it ...

0

According to quantum mechanics the time evolution of the universe is described by a path integral that will sum over all histories. If we consider a robot whose processor runs at a clock cycle of $\tau$ to simplify things, then all the possible time evolutions during that period of $\tau$ will contribute to explain the robot's observations, including the ...

1

Fortunately for experiments in physics we have better proxies than the accuracies of our five senses. We have detectors and computers and .... With these tools a theory of how the universe is made has been developed, from elementary particles with the theory of quantum mechanics building up the observables around us, to the astrophysical models that fit ...

2

Quite a philosophical approach. There is still the reliance on our four other senses in order to make sense of our physical world, however the same approach can be imply to those senses also with the delay in neurological impulses. One must also take into account, as you would call it, the in between frames of other people's perceptions, as well as those ...

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