Thought experiment Definition and 340 Threads

I am having problems with this presentation of the train thought experiment. I actually think it's incorrect, but I'm not sure. From Ray D'Inverno's book: -- Imagine a train travelling along a straight track with velocity v relative to an observer A on the bank of the track. In the train, B...

Hello everyone! New poster here. I would first of all like to thank you all, since in these threads i have found answers to many of my questions about astrophysics! I have a little thought experiment for you guys: Lets say that because of Hawking radiation, the radius of a black hole's...

Very simple thought experiment about time dilation: Let's say I have two clocks that are a substantial distance (light years) apart, at rest with respect to each other. The clocks are Einstein synchronized. Also, the clocks are specially designed to withstand any force or collision. However...

This has been on my mind for a while now; and I really need some outside input to either expand or put this idea to rest. So this idea relies on the Meissner effect; which during the transition of a material from a natural state to a superconducting state the magnetic permeability of that...

I've been wracking my brain for days trying to comprehend everything that deals with the twin paradox. I have a vague understanding of relativity and its effects, but am massively confused by most of the explanations given on this forum and other places. I think perhaps it extends from my lack...

I'm not sure where this thread should go, sorry if it's in the wrong place. In Newton's thought experiment about the rotating rocks going taut in deep space. With respect to what would the rocks be rotating?

Homework Statement cookie dough lies on a conveyor belt which moves at speed v. A circular stamp stamps out cookies as the dough rushes by beneath it. When you buy these cookies in a store, what shape are they? That is, re they squashed in the direction of the belt, stretched in that...

First, this thought experiment uses the conditions that the universe undergoes decelerating expansion and then contraction, as is the Big Crunch theory. **I know this is contrary to current evidence, supernovae surveys etc., but the thought experiment assumes a closed universe which will have a...

There is an empty container on some weighing scales. It is filled with electrons & positrons, the lid is closed and a reading of the weight is taken. Now they go boom and are turned into gamma rays... Lets say the box does not let the rays escape, instead are reflected back inside the...

Galileo is said to have reasoned that all masses must fall with the same acceleration despite their differing weights as follows. Imagine having a heavy object say weighing 200N and a lighter mass say weighing 100N, then we may suppose that the heavier object fall will an acceleration 2a, and...

Hey, I am 6 hours into a 12 hour audio course on Relativity. I love the course, but the bad thing is that you obviously can't ask questions. So here is a thought experiment I was hoping people could help me with. I am an observer standing between two rocket ships, R1 and R2. Each rocket...

I'm still really confused about the speed of light. I'm kinda new at this so bear with me. Does the speed of light depend on the frame of reference it is in? For example let's say you have a ball moving at 10m/s in a car moving at 20m/s. This would mean that from the Earth's frame of reference...

Assume that you placed a single atom (let's keep it simple and specify a hydrogen atom) in a theoretical container that could maintain a perfect vacuum. The perfect vacuum being defined as having no matter, electromagnetic radiation, neutrinos etc. What would be the ultimate fate of that...

Got this from extending the "faster than light" portion of the wiki page. Wiki notes that if a laser is swept quickly across a distant object, the point will move faster, but nothing else necessarily violates any rules so this experiment passes. Now what if you have an infinitely long rod...

Presume there are two massive bodies in open space, having zero relative velocity, and having the same space ambient temperature. That is, the incoming and out-flowing black body radiation is balanced and the temperature is unchanging. Presume these bodies become gravitationally attracted, and...

My 9 year old son made an interesting observation, and I couldn't explain it. Maybe you can help. He interlocked several gears on his Lego set, all in a straight line. He then put a handle on the first one and turned it. Simple enough. Then he said to me, "Dad, when I move the first gear...

H'mmm, my math and understanding isn't up to the discussion of relatavistic mass, so I am going to propose a reasonably simple thought experiment and if anybody can give me an answer, it would be much appreciated. A man is on a spaceship that accelerates to a point where time dilation...

I'm not sure if I'm posting in the right forum, or if you'll indulge a little mind experiment likely completely outside the possibilities of physics. Say something on Earth suddenly became incredibly massive, going in seconds from the density of a human body to a density of a planetary body...

It seems there is usually no mention of "redshift" when train thought experiment is used to teach relativity. Is there a reason for not bringing redshift (blueshift) into the experiment?

Supoose at the center of A huge cylinder of radious say 1km we place a high speed motor of say angular speed w > 3E5 . And in the motor we attach a laser directed towards the walls of the cylinder. When we rotate the motor with the laser lighted, won't the laser point move along the cylinder...

I was reading a chapter on "General Relativity" a few days ago. (I was reading Arthur Beiser: Modern Physics)I found it very hard to digest the relation between the "Equivalence Priciple" and "The General Theory of relativity".The equivalence relation states that "An observer in a uniformly...

Hi all, I was bored a little while ago, and somehow started thinking about the Big bang and all that fun stuff. I once heard it said that some of the galaxies near the end of the universe are at close to light speed. Now I'm not here to dispute that (would be nice if it were true though) but...

I have a theoretical question involving the properties of inductors, I hope someone is able to help! I know that the voltage across an inductor is proportional to the time rate of change of the current through the inductor, and that if the current is changed the inductor will generate a...

I've recently had this thought experiment, and wanted to share it. My HS physics teacher had talked about "perfect" strings that had no mass, does not stretch, and is completely flexible when describing physics experiments. Using such a hypothetical string means it does not interfere with the...

I have a thought experiment that may be of interest to some of you. If this has been discussed before, please direct me to the discussion. Thanks! The experiment involves a beam generator consisting of a laser, a parametric downconverter cut for operation at the 2nd harmonic, and mirrors to...

Hello everyone in PF land, I'm a long time reader, first time poster. My friend and I were conducting a thought experiment involving a spacecraft with a rotating section to produce artificial gravity. The rotating section would be accessed by a tube section that does not rotate. (I've attached...

Thought experiment: a sealed container with water in it, along with a balloon. You are able to control the balloon with a string from the outside somehow, and this string however is still sealing perfectly. No leaks in the system. The balloon is taken down to the very bottom of the container...

Hi Please excuse my poor electricity knowledge.. also if this is in the wrong place to post, let me know and I'll move Someone raised this simple thought experiment with me and I want to understand better: If you will, imagine a series of balls inside balls (ie multiple layers - for...

Thought experiment about pressure (intro physics) ..you have a U-shaped tube? that is open to the atmos on both the left and right side you fill it with water so the water level is the same on both sides. then you put a rubber stopper in one end (..the right end) (also the remaining air...

Here is a link to the text, chapter 9... page 30: http://books.google.com/books?id=gQ8LAAAAYAAJ&printsec=frontcover&dq=einstein&ei=tondSZLWOpnGywTa_pHyDg#PPA30,M1 So I have been analyzing this thought experiment from a causal perspective and have found it to be quite unsatisfactory. The...

I've wondered before about a little toy problem, and I think I've solved it, but just to be sure, I'll let you guys have it and see what you get. Alright, so you have a table and a (possible infinite) stack of circular disks of radius R and of neglible thickness (each disk is identical...

My friend just gave me a good thought experiment involving quantum entanglement violating special relativity that I can't seem to refute. There are two immortal people A and B. A and B synchronize their clocks far away from any gravitational influence (say spot Y). B departs for spot(at an...

All right, for arguments sake we shall assume that the neurons of the human central nervous system function in a simplified way, it shall soon become clear that this isn't relevant to the experiment, we shall assume they work as: 1: They receive an electrical charge via one to more inputs 2...

There is a hollow cylinder out in space (Cylinder A), shaped like a giant soda straw. Inside the cylinder are mirror-laser clocks keeping time by bouncing light back and forth across the Cylinder A's diameter. A similar cylinder of slightly smaller diameter (Cylinder B) is traveling at...

hi, i got a question Lets say we got 2 rockets X km appart, traveling parallel to each other in a straight line at the same constant speed. Now let's put a massive object like the Sun on one of sides on the plane the two spaceships make. Which one will get to the finish line first? Would the...

I just thought of a little thought experiment for those considering colleges for undergrad Suppose there is a HS senior student A (2100+ SAT ,3.8+GPA,research,science fairs) decides to go to a top 7 school not particularly known for grade inflation ie. Caltech,Cornell,MIT. Choose Caltech as...

AHHHHHHHHHHHHHHHH! It has happened to me again! This has to have been the 3rd or 4th post I lost just b4 finishing. This only happens in these forums. Does this happen to any1 else? I had just about finished explaining my thought experiment when, idk did i presss a wrong button? I lost...

1. This was just a quick question that I was thinking about. If you jump of a really tall building, let's say 100-story building and you fall most likely you won't live to tell the tale. But let's say you jumped with a cardboard (or any object) and a couple feet before you hit the ground...

Thought experiment: The 'perfect eye' (and a finite set of colours) Introduction: Human beings have trichromatic colour vision, our ability to distinguish different colours at different spectral frequencies, and wavelengths, of electromagnetic radiation, or combinations thereof, is...

I had this really random idea at lunch today - would it be possible to pull an electron apart? I know that it's a fundamental (point) particle and that most likely nothing is going to happen here, but do you think anything exotic could happen, if one were to suspend an electron between three...

I was thinking of this thought experiment the other day and it didnt quite seem to make sense to me. I hope some of you can explain it. I have drawn a quick example of the setup: and sorry for my english (c = 1 m/s) Imagine a 1 meter long spaceship with a velocity of 0,5 m/s (c/2). The...

Consider, a particle at position 0 at time = 0 Lets say it moves to 0.5 after 0.5 seconds It moves to 0.75 after 0.75 seconds and in general... It moves to 1 - 0.5^n after 1 - 0.5^n seconds So where will the particle be after 1 second? EDIT: n goes like 0,1,2,3...

This baffles me, because the device outlined below could create energy ... Theres two tanks A & B with a microscopic gate between them Air in A & B starts at say 25 degrees whenever a faster than average moving molecule approaches the gate in A a sensor opens the gate and let's in pass...

Ok please comment on this one. Let's suppose for our thought experiment you the reader were suddenly promoted to god-like status. You decided to take the our universe and remove all matter, but bearing in mind that all known laws of physics will still apply as they do now. You then create two...

A thought experiment: Reference frame: A surface in space with no atmosphere. A rocket ship is approaching just above the level of the surface. Its measured velocity is a constant 0.8c (0.8 times the speed of light) relative to the surface. Special relativity predicts that time in the...

confused about this thought. Object A and B are d distance apart, as measured by an object exactly in the middle of them, object C. A and B are moving towards each other at c/2, again measured by C. According to C's frame of reference, all three objects will collide at the exact same moment...

OK, so I understand mathematically why one can't square a circle, but when I do the following thought experiment I can't see how one could not square the circle: - I tie a string at both ends and lay it on a table so that it forms a perfect circle. - Now I place four pins, at equal distances...

I am not a physics expert but have been having a discussion about the effects of the Special Theory of Relativity with a friend. I came up with an example where time does not actually alter at high speeds, although it can be perceived to by a stationary observer because of the time it takes...

The explanation I've heard is that quantum superposition is an atomic-scale phenomenon, that it makes no sense to apply it to a macro-scale object such as a cat. But that's not what the experiment is doing. The radioactive isotope is subject to quantum effects, and it seems to follow that the...

If I lift an object, does its mass increase due to my investing it with potential energy? If I then drop it, does its mass increase further as it falls due to the acceleration? I don't see a contradiction here, but my intuition tells me that something is amiss.