# Search results

2. ### B Time traveling in the future and in the past

As far as i know, current special relativity allows for time travelling but only in the future. In sort what we have to do to travel to the future, according to special relativity, is to make a spaceship that can achieve speed comparable to the speed of light. Then while we are travelling with...
3. ### I Limit of (a^n)/n for a>1

We have the limit of the sequence ##\frac{a^n}{n}## where ##a>1##. I know it is ##+\infty## and i can prove it by switching to the function ##\frac{a^x}{x}## and using L'Hopital. But how do i prove it using more basic calculus, without the knowledge of functions and derivatives and L'Hopital...
4. ### I Do Maxwell's equations in integral form imply action at a distance?

Lets take for example Gauss's law in integral form. Suppose at time ##t## we have charge ##q(t)## (at the center of the gaussian sphere) enclosed by a gaussian sphere that has radius ##R>>c\Delta t##. At time ##t+\Delta t## the charge is ##q(t+\Delta t)## and if we apply gauss's law in integral...
5. ### I Would we ever be able to fully understand the laws of the universe?

So first of all ,we all agree that Universe exists and it is not totally chaotic , it seems to have some laws that govern its behavior. Would we ever be able to fully discover and understand the true laws of the universe. Or human intelligence is limited (the way the human brain is constructed...
6. ### Central Sleep Apnea

Hi all, I have been diagnosed with central sleep apnea. The medicine I ve been given doesnt seem to work on me. Is anyone here expert on this subject, knows what are the latest medicines for central sleep apnea and if they have been allowed in european union and more specifically in greece...
7. ### I Differentiation and Integration cannot always be swapped!

Well here is my (I hope successful this time ) attempt at 10. e) We consider the function ##f(x,y)=\begin{cases}\dfrac{e^{-x|y|}}{y}, y\neq 0 \\ 0, y=0\end{cases}##. For this function for ##y\neq 0## it is for any ##x##, ##\frac{\partial f}{\partial x}=-\frac{|y|}{y}e^{-x|y|}##. Also it...
8. ### Insights A Numerical Insight for the Fundamental Theorem of Calculus - Comments

Greg Bernhardt submitted a new blog post A Numerical Insight for the Fundamental Theorem of Calculus Continue reading the Original Blog Post.
9. ### B Water colour in Mars

I guess water in earth , in big lakes or seas, looks blue because the earth's sky is blue and the water's surface reflects the sky. Now if there would be some big lake of water at Mars, how the water surface colour would be? I guess the colour of Mar's sky which is red (right or wrong?) It is...
10. ### I Time component in physics

Are there any theories in physics that allow for a time component of the various vector quantities besides the x,y,z components? For example the velocity of a particle to have a time component ##v_t## besides the x,y,z components ##v_x,v_y,v_z##
11. ### Ohmic Resistance and Logic gates

Is ohmic resistance a "necessary evil" in order for transistors to be able to function as logic gates? I mean, I have seen some circuits for the NAND gate in RTL and TTL and they both seem to involve ohmic resistances. Can we make a super conducting NAND gate that will have total ohmic...
12. ### B Flow of time

I know that under some conditions time runs slower or faster depending on the speed we have relative to an inertial frame of reference. However if I understand correctly for an observer that is moving with say 90%c though time runs much faster, for him the time runs in a normal way (I mean he...
13. ### I Rational sequence converging to irrational

In the textbook I have (its a textbook for calculus from my undergrad studies, written by Greek authors) some times it uses the lemma that "for any irrational number there exists a sequence of rational numbers that converges to it", and it doesn't have a proof for it, just saying that it is a...
14. ### Caffeine molecule

I noticed that different brands of espresso coffee have different effect on my nervous system. Some are really good in the sense that they wake me up and make me feel fresh and dynamic, while others do nothing, and others make me really jumpy and nervous and stressed. Why is that? I am not good...
15. ### I Invariance of the laws of physics

The invariance of the laws of physics in space-time is a corner stone of physics and all science. A.Is this an axiom or can be derived from other more fundamental axioms? B. Are there any books that discuss how science could be if the laws of physics could be changed (for example if we could...
16. ### I Probability of an event in n tries

IF the probability for an event to happen in one try is m/n , what is the probability for the event to happen at least one time in n successive tries. In each try the probability of event is m/n. I care mainly for the formula that I suppose it involves m and n but if you kind enough to provide...
17. ### B The "true" vacuum

First of all I want to say that I am a *scrub* in relativity so when people say things like that the universe expands and/or inflates I still don't get the full grasp of it. Here is my question. When we refer to a vacuum I would expect it to be a region of space that has absolutely no...
18. ### I Rigid body with center of rotation other than CoM

Suppose we have a rigid body that rotates around a point P other than its center of mass, with point P being a point of the rigid body. This implies that there are external forces to the rigid body. If the external forces cease to exist(like we have an axis passing through P and we suddenly...
19. ### Medical Big problems with sleeping

My problem is as follows: During my sleep I wake up several times (as an example if I sleep say for 7-8 hour I ll wake up at least say 7-8 times, that is atleast once per hour). Usually I see a dream and wakeup. If I take some tranquilizer pill (tavor is called in my country Greece, I think its...
20. ### I Hall Voltage vs Laplace Force

Ok so when we have a current carrying conductor inside a magnetic field there would be Laplace force ##L\times Bi## which is the macroscopic form of the microscopic Lorentz force ##v\times Bq## in a large number of electrons ( or it is not ???) But also there will be hall voltage which will...
21. ### Medical Sleeping problems

Lately my sleeping problems have been gone acute. I cant sleep continuously for more than 3 hours (usually i wake up every 1-1.5 hour) i usually see too many dreams (usually i wake up because of the dream). I always had sleeping problems (they started about 20 years ago) but its the last 1 year...
22. ### CPU power consumed relates to information processed?

As far as i know CPUs consume power because of the ohmic resistance of the transistors that they are built of(i hope i am not wrong on this). But if we were able to build some sort of superconducting CPU with almost zero ohmic resistances, would it be able to operate and perform computations...
23. ### PF page messed up

Since about yesterday 16:00 GMT when i enter PF page via IE 11.0.9600.17420 the page seems messed up, like that the IE 11 cannot parse the html code correctly or i dont know what exactly. Maybe i changed some setting in IE and i didnt notice. I ve trouble writing this message as well. I hope you...
24. ### Maxwell's equations and conservation principles

Once we define energy and momentum carried by the field , is it possible to derive Maxwell's equations from conservation of momentum and conservation of energy (along perhaps with conservation of charge)?
25. ### Humidity definitions and measurement devices

I know there are three definitions of humidity. Absolute , relative and specific. Can you tell me if on a typical commercial device (such as the one i have in a clock i have) that measures humidity as percentage is it the relative humidity that it measures? Can the percentage become greater...
26. ### Raining effect on pressure in water pipes

Can we say that the pressure in the water pipes is increasing when it is raining heavy? Reasoning is that if the water company takes water from a nearby lake or river, rain causes higher water height in lake or river which causes higher hydrostatic pressure in the source which affects the...
27. ### Thread in black hole?

I dont know where to post this so here it goes I ve noticed that when i go to reply to some threads i get the error message that the thread has drop into a black hole? What is happening, is it some bug of the database that deletes threats or what?
28. ### Dark matter not really dark?

If we could observe a piece of dark matter with naked eye would it appear dark or invisible? Dark matter doesnt emit photons at all but it neither absorbs photons (it doesnt interact at all with photons right?) so it would appear invisible?
29. ### Wave property of particles

The wave property of particles (like electrons) is due to : 1) The wave function 2) The underlying fermionic field 3) Just because of the existence of de broglie waves? Or maybe someshow the above three cases are unified ?
30. ### Microwave beam width

I guess we all know laser beams. I wonder if we can make beams in the microwave frequency range and how the beam width relates to the frequency, e.g what is the minimum beam width we can achieve with a frequeny of 50Ghz.