# Tag Info

98

A "perfectly efficient" computer can mean many things, but, for the purposes of this answer, let's take it to mean a reversible computer (explained further as we go). The theoretical lower limit to energy needs in computing is the Landauer Limit, which states that the forgetting of one bit of information requires the input of work amounting $k\,T\,\log 2$ ...

66

Since cables carry electricity moving at the speed of light, why aren't computer networks much faster? Perhaps I can address your confusion with a rhetorical question: Since air carries sound moving at the speed of sound, why can't I talk to you much faster? The speed of sound is much slower than light, but at 340 m/s in air, it's still pretty damn fast. ...

33

Landauer's principle (original paper pdf | doi) expresses a non-zero lower bound on the amount of heat that must be generated by computers. However, this entropy-necessitated heat is dwarfed by the heat generated through ordinary electrical resistance of the circuitry (the same reason light bulbs give off heat).

25

As you've probably guessed the speed of light isn't the limitation. Photons in a vacuum travel at the speed of light ($c_o$). Photons in anything else travel slower, like in your cable ($0.64c_o$). The amount the speed is reduced by depends on the material by the permittivity. Information itself is slower still. One photon doesn't carry much information....

11

A transmission line is made of a pair of conductors which have some resistance, inductance, capacitance, and leakage conductance. We can take all of these per unit length: The wave equation for signals in this line, in the limit of a lossless cable with $R=0$, $G=0$, is $$\frac{\partial^2 V(x)}{\partial x^2} + \omega^2 LC \cdot V(x) = 0$$ You have to be ...

10

this is a broad, complex, somewhat tricky question with many angles that an entire survey or book could be written on but unfortunately it seems one hasnt yet. heres a "grab bag" of some deep parallels noticed over the years that such a book might cover & "research leads" for further inquiry. Modelling and simulation. as computing capability has ...

8

There exists substantial evidence that integers that appear in physics are emergent in character, see e.g. this essay by Prof David Tong http://fqxi.org/data/essay-contest-files/Tong_integers.pdf which won a silver prize in a related contest http://fqxi.org/community/essay/winners/2011.1 Because the integers are emergent, they're not fundamental ...

6

Computers manipulate internal stored values "0" and "1" represented as different voltages. Every change 0-to-1 and 1-to-0 involves an electric current I passing through a circuit resistance R, which gives rise to ohmic or "Joule" heating.

5

"Surely this is a bottleneck" - No, it's really not. Any real-life network connection is not speed-limited by the propagation speed of the signal in the cable, but by the processing delays in the various routers, switches, and network interface processing at each end.

5

The computer program can be very simple and they have existed in one form or another for many years. At least for the purpose of defining variable star pulsation timings. The problem lies in both in signal to noise ratios and total timings. The variation in light can be lower than milli-magnitudes for planetary passes and as such this is very tricky to ...

5

Re question 1: A processor is obviously a very complex object, but it's made of from basic structures called logic gates. A logic gate consumes power mainly when it's changing state, and the frequency with which it changes state will probably be, on average, proportional to the clock frequency. To work out the work done whenever the gate changes state you ...

5

Hard disks use magnetic storage, http://en.wikipedia.org/wiki/Magnetic_storage i.e. the information is stored in the South/North orientation of small pieces of the magnetic medium (also the case for tapes in the old tape recorders; and credit cards; these applications differ by the "geometry" how the bits are arranged but not by the essence how a bit ...

5

What a quantum computer can do it take lots of results and options and do them in parallel, and then if you can figure out how to get bad or wrong results to cancel each other, then you can efficiently combine them all together and get the right answer. It's being able to efficiently combine all the things done in parallel that is useful and special. For ...

5

You can do all kinds of things like that, but in the end you will find yourself returning to electrons. Using light in particular is not a very good idea because it is very hard to do non-linear operations with it. Almost all non-linear effects require a very large number of photons (I would guesstimate on the order of 1e8 or more), whereas one can ...

4

There are numerous examples of people using genetic algorithms, for example, to optimize some output where an actual solution of the equation would be otherwise impossible. Information entropy, which is a generic computing concept, has some hold on statistical physics. But I cannot think of a case I have seen where a concept from cutting edge computer ...

4

According to Seth Lloyd's Ultimate physical limits to computation, the time taken for an operation is limited by the uncertainty in the energy available $\Delta t=\pi\hbar/2E$

3

Two reasons: 1) The speed of light in a "medium" is (almost*) always slower than the speed of light in a vacuum. 2) Electricity propagating in a wire is subject to inductive and capacitive effects which slow it's progress. And even if wires were infinitely fast, integrated circuits are not. Again, inductive (a little) and capacitive (a lot) effects limit ...

3

Why only 64% What does propagation speed mean? I know there are other variables effecting the latency and perceived speed of computer network connections, but surely this is a bottle neck. Speed of signal propagation is distance the signal (packet) travels in one second. It is usually lower than $c$ because EM waves that carry the information travel in ...

3

How sure are you that electricity travels at the speed of light? Although electricity propagation moves at the speed of an E/M wave, and not electrons, its speed depends on the dielectric constant of the material. Only in a vacuum, I think, would it travel at the speed of light.

3

I'm the developer of a project called the Physics Derivation Graph. http://allofphysicsgraph.github.io/proofofconcept/ github.com/allofphysicsgraph/proofofconcept My intention is to develop a set of derivations into a graph which would capture the current state of knowledge in Physics. Although I consider automated reasoning outside the scope of my project, ...

3

Simulation implies an author, so this is another attempt at trying to find a creator for the universe, imo. Thus it is metaphysics and not relevant to the subject of physics. Physics as a discipline starts from observations and fits them with mathematical models that have predictive power, having accepted axioms and postulates. The mathematical forms are ...

3

I am on record of having the opinion that there is no real argument against us being a simulation in a general sense, however we frequently find people jumping to quick into the simulation pool and stating there new what-ever-it-is proves the universe is a simulation. The example given above sounds like one of them. First off, Quantum Error Correcting Code ...

3

The link between Computer Science and Physics can be very subtle sometimes. For example, consider this article: http://arxiv.org/abs/1010.0128 The point is the following. Consider a quantum algorithm in order to solve an NP-complete problem (i.e.: a hard problem, which is conjectured not to be solvable in polynomial time). Now, consider a classical ...

3

Optical Drives are used for the storage of data (Using Photochromism) in several disk formats with the help of laser diodes. Popular optical drive formats include CD-ROM, CD-R, CD-RW, DVD, DVD-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-R DL, DVD+R DL, BD-R & BD-RE. As we all know, computer understands binary data (either 0 or 1). Here, 0 (false value) says ...

3

A flip-flop (bistable multivibrator) is, in simple terms, two transistors wired together in such a way that there are two stable conditions: (1) one transistor is full "on", while the other if full "off" (2) vice versa If the circuit happens to be in a state "in-between" these two states, it will, due to positive feedback, very rapidly move towards one of ...

3

I am the author of the astronomy and physics ontology mentioned in the original question. The original purpose of that ontology was to improve search for data and articles in astronomy. The idea was to have data sets tables and individual columns in tables marked up with relevant keywords. The rows of data in astronomy are usually different astronomical ...

3

No. Power and temperature are related, but not directly. Thermal environment and surface area also matter a great deal. A tiny chip has less area to dissipate heat than a larger one. So for the same power draw, it may easily produce a higher temperature than a larger chip. Imagine a 40W incandescent light bulb. Both the filament and the bulb as a ...

2

That is a very good question. I think the answer is that it is a quest for the unexpected. Programs have been developed to detect the transits, but perhaps there are new kinds of discoveries waiting to be made by having thousands of people looking at the data. From Humans vs. Machines on the Planet Hunder's site: ... developing computer algorithms to ...

2

It's because present-day computers are syntactic devices, not semantic devices. Humans use both levels (and possibly others) to figure out our way in this world, whereas computers only have access to the first level. More simply put, computers lack the big-picture perspective, and the hugely interconnected network of representations that our brains are so ...

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