# Tag Info

## Hot answers tagged computation

82

What you're looking for is Landauer's principle. You should be able to find plenty of information about it now that you know its name, but briefly, there is a thermodynamic limit that says you have to use $k_BT \ln 2$ joules of energy (where $k_B$ is Boltzmann's constant and $T$ is the ambient temperature) every time you erase one bit of computer memory. ...

16

I think perhaps some of the other answers are taking computer science to be synonymous with computation. I guess that this is perhaps not what you mean, but rather theoretical computer science. There is obviously a huge overlap with quantum information processing of which I think you are already well aware, so I will ignore that. Much of physics (including ...

15

1) yes, it basically will find a non-optimal solution. At every point, the top of the ray looks for the bigger potential gradient, the charge in the surrounding volume grows, polarizing surrounding material (air, in this case) until a bigger gradient shows up and the ray continues over that direction. This is why the lightining path looks like a jigsaw; its ...

11

First off, physics tends to provide a very good background for people who move on to study problems in other areas, which is perhaps why there is a lot of cross-over to computer science. However, there are also a number of areas at the interface of computer science and physics which attract people from both sides: Computer hardware (which is generally ...

10

EDIT: This answer is specifically from the perspective of very computationally oriented fields like theoretical plasma physics. Most physicists can program, and in fact many are rather good programmers. It would be difficult to work in modern physics without being able to program. Unfortunately, many are also not terribly good programmers (I've read many a ...

9

Some info from ASIC world: For example, you processor have 300 mil. transistors, and most of these do some work. But, in order to make for example pure 32-bit add operation you need just about 1000 of them. Others are for caching and passing data back and forth - support functions which are impossible to estimate. So estimations from math side are very hard ...

8

Assuming a typical computer with CPU processing power ~1 GHz. It means that it can generate output byte sequence at ~$10^9$ byte/s, which is about ~$10^{-13}$ J/K in terms of von Neumann entropy. Also, the power consumption of a typical CPU is ~100 W, which gives entropy ~0.3 J/K at room temperature. So the (minimum ΔS) / (actual ΔS) ~ $10^{-14}$ This ...

6

This is, no doubt, one of the biggest challenges for realistic simulations: waves crashing, hair moving under wind and whatever other movement involving turbulence will be hard to solve. Though it is true that one can solve the equations of motion for each individual particle in a 'molecular dynamics' fashion, that is just infeasible for a system that goes ...

6

There's no flaw in your argument. A computer heats the room just as effectively as an electric heater of the same power and you could use the computer to do something useful (Bitcoin mining?) while it's heating your room. There are some practical considerations, though I think these have been sufficiently discussed in the comments. Computers would make for ...

5

Color forces are not like electromagnetic ones. There exist no unbound color carrying particles analogous to the electron, because the forces increase with the distance rather than decrease and collective effects appear only within nuclei through residuals of the colored forces which attract the nucleons and hold them in the nuclei. Collective effects ...

4

I think the main reason why this is so common is that many people who are of the tenured professor age now (50-60) were in graduate school before most colleges offered a Ph.D. in computer science. So back then, people who were interested in theoretical computer science got their doctorate in Mathematics, and people who were interested in applied computer ...

4

From my reasoning and knowledge of one CS professor who has a PhD in astronomy: Above all, the answer depends on your definition of what a "computer scientist" is. What do you mean by "computer scientist"? Someone who does research in a computer science department? Or does perhaps artificial intelligence, algorithm development, or grid computing for a ...

4

Can physical states be treated as information (strings over some alphabet)? There is a distinction between a state and a vector (see this mo question), but disregarding that, we can clearly approximate a vector to any desired precision using a finite-length string. I doubt that anyone can say whether the rounding errors involved grow uncontrollably or ...

3

The acceleration ${\bf a}(t)$ is simply computed from Newton law $F = m a$. It's a function of the forces on the particle, which is (assumedly) computable from the positions ${\bf r}(t)$ (of the entire system, i.e. all particles) at time $t$. This can be seen also in the figures, in the three schemes the acceleration is computed from (and only from) the ...

3

For all practical purposes today the answers above are very informative. However, as Marek has pointed out above, your fundamental theoretical model of the thermodynamics of computation, on which you are basing the question is, surprisingly, wrong, as we first began to discover 50 years ago (see refs. to Landauer Charlie Bennet, Friedkin, others). ...

3

Human power consumption can be guesstimated as 100W, similar to the power consumption of an ordinary computer, plus or minus a few orders of magnitude depending on one's idea of "ordinary". A computer can do billions of flops per second, and it would take me many seconds or minutes to perform one with pen and paper, and furthermore I will make many more ...

3

For modeling of physical (and chemical) systems on quantum computer even 25-30 qubits would be already quite nice, see Lanyon, et al, “Towards Quantum Chemistry on a Quantum Computer”, Nature Chemistry 2, 106 - 111 (2009) (see also http://arxiv.org/abs/0905.0887 ) Really, quant-ph section in arXiv.org is standard place for papers about quantum computers, ...

3

I think that Wolfram is arguing that the study of cellular automata and perhaps similar computational systems could serve as an organizational principle, providing a coherent framework to look at different problem (just like the more familiar frameworks provided by physics and chemistry). This explains the title of his new book, A new kind of Science (i.e. ...

2

The brain is massively parallel, so it tends to come out looking very good. The OP suggested using joules/flop as the measure of (in)efficiency. This leaves considerable ambiguity. I believe the way neurons typically work is that they form something like a weighted average of their binary inputs, and generate a binary output that is based on a threshold ...

2

The article is saying something different altogether. There is a difference between being able to run a program (Turing machine) and being able to decide whether that program evenutally finishes running. The latter is called a $\Pi_1$ decision problem. A classical computer can run any given program, subject to resource limitations, but there is no general ...

2

Although in one sense this question and answer is altogether beyond physics, (as discussed below) there would seem to be a very natural answer to this question, to wit: Beyond Quantum Computers are computers than can overcome the Church-Turing thesis (See the Wikipedia page with this name) and, more generally, can compute the truth value of propositions ...

1

You might be interested in this paper, NP-complete problems and Physical Reality, by Scott Aaronson. It doesn't discuss the next step "after quantum computers" per se, but compares the computational power of various physical theories. It surveys newtonian physics, nonrelativistic quantum mechanics, nonlinear corrections to QM, hidden variable theories, ...

1

This issue is considered, directly or indirectly, in other questions in physics.SE. The question Church-Turing hypothesis as a fundamental law of physics is nearly the converse of the question asked here and my answer raises the possibility that physics could allow other computational models, oracle machines being one of them. Then answering Final ...

1

even if (1) is true you cannot conclude (2). The reason being that differential equations are only an approximation to physical laws. Even if highly unlikely, the laws of physics could be non-computable, and only computable as an approximation. (see Wolfram's book "A new kind of Science"). In conlusion, there is no evidence that the evolution of the universe ...

1

The XOR gate is almost always called the NOT gate in quantum information and computation. To implement the XOR gate unitarily one must do it in a reversible way, since unitary gates must be invertible; conversely, any reversible logic gate defines a physical unitary operation. The XOR gate is itself not invertible, since e.g. both inputs "00" and "11" yield ...

1

Usually, a XOR quantum gate is implemented by the function : $XOR(Q) = a_1|00 \rangle + a_2|01 \rangle + a_4|10\rangle + a_3|11 \rangle$ The first bit is conserved, while the second bit is the result of an XOR operation between the first and second bit. For instance, if we have the combination $|11\rangle$, this means, after the transformation : $1$ ...

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