Jump to content


Senior Members
  • Posts

  • Joined

  • Last visited

Posts posted by Toffo

  1. On 6/11/2018 at 3:11 PM, Moreno said:

    How can it be explained that Platinum has the smallest Seebeck coefficient among pure metals which is conditionally taken for zero?




    When two blocks of platinum are brought together electrons do not tend to move from block to another block, because the blocks are identical.  Zero motion means zero Seebeck effect.

    When a block of platinum and a block of iron are brought together electrons do tend to move from one block to another block. Then some works must be done to separate the blocks. The blocks form a charged capacitor. 

    If there was a way to connect wires to that capacitor so that there is no Seebeck effect between the blocks and the wires, then we would have a perpetual motion machine. 

    Actually there is a way to adjust the contact voltage between the wires and the blocks: Temperature. 

    Heat in the contact decreases the contact voltage, I guess. The effect is called contact voltage if I remember correctly. I mean the effect of contact becoming a charged capacitor.


    What happens when a cool platinum block and a hot platinum block are brought together?

    Interesting question there ... some electrons move from the hot block to the cool block. Electrons have less potential energy in the cool block, so electrons tend to fall from hot block to the cool block.  This effect is probably same in all metals, so this effect's effect on the Seebeck effect is zero, probably.
















  2. 3 hours ago, jajrussel said:


    Is it correct to say that a medium effects the photons velocity?

    There are many kinds of mediums. Below there is a road and three cars that are driving to the right, always at constant speed, we are looking from above. The road is the medium to the cars. When the cars enter the curvy part, they lose some rightwards pointing velocity and momentum. The road absorbs the momentum. When the cars leave the curvy part, the rightwards pointing momentum goes back from he road to the cars.




    Now let's say the  ''road'  is an optical fibre, and the 'cars' are photons.  The story is the same as above.


    That was trivial. But what if the picture is a road profile where the bumps are hills?



  3. On 11/15/2017 at 4:02 PM, jajrussel said:

    If i were to jump off a very high cliff i would assume a steady increase in kinetic energy. If instead of jumping I pointed a flash light down and turned it on, being clueless I would assume the gravatational acceleration would be too small to effect a photons kinetic energy to any measurable degree. But, what if I pointed the flash light at a black hole. what then? If gravity causes a change in a photons direction of travel, implying that gravity does effect the photon what happens to the photons energy? C is th speed of light in a vacuum. What is a black hole to the photon? If all direction is a oneway street, is that the same as a vacuum? Do I need to put the word true, or perfect in front of vacuum?


    If you were to jump off a very high cliff your momentum would be increasing steadily, and your velocity would be increasing steadily. Your kinetic energy would be those two things multiplied.

    Now let's try to replace 'you' with  'photon' in that sentence above:

    If a photon were to jump off a very high cliff its momentum would be increasing steadily, and its coordinate velocity would be decreasing. Its kinetic energy would be those two things multiplied.

    The above sentence may be incorrect, as the kinetic energy would be decreasing, which does not sound quite right.


    Let me try again:

    The momentum of the downwards falling photon is increasing at the same rate as its potential energy is decreasing, and the coordinate velocity of the photon is decreasing at the same rate as its potential energy is decreasing. The product of the momentum and the coordinate velocity is unchanging.

    That's much better.


  4. What is this proper velocity thing? If a driver measures his speed relative to road to be 0.5 c, then a cop standing on the road agrees that the car moves at speed 0.5 relative to the road.

  5. What is the magnitude of the proper acceleration of an object moving in a circle at a given (unchanging) relativistic speed?



    Let's consider forces affecting an object swung around using a rope.


    If the Newtonian centripetal force measured on the rope is F, then the relativistic centripetal force is gamma * F, because the transverse mass of the object is gamma*rest mass. This is the force at the other end of the rope. I mean at the end that is not moving relativistically.


    At the end that is moving relativistically forces are measured to be gamma times bigger. I like to think that's because of time dilation.


    The proper acceleration is measured by an accelerometer, which is a force meter.


    So a force meter attached to the object measures this force: Newtonian centripetal force times gamma squared.


    So when used as an accelerometer the force meter measures this acceleration: Newtonian centripetal acceleration times gamma squared.

    What is the magnitude of the proper acceleration of an object moving in a circle at a given (unchanging) relativistic speed?


    For an object moving in a straight line proper acceleration = acceleration*gamma^3


    My question has to do with the relationship between transverse mass and longitudinal Mass


    Longitudinal acceleration and transverse acceleration are quite different:


    Longitudinal acceleration changes the longitudinal mass and the transverse mass.


    While transverse acceleration does not change the longitudinal mass or the transverse mass.


    So it makes sense that longitudinal mass and transverse mass are quite different.

  6. Let us consider million rocks arranged in a sphere formation. Every rock is simultaneuosly and abruptly pulled outwards by some force. We know that extra energy is used in this pulling procees, because a rock thinks the distance to the other rocks is shorter than the real distance.


    But we also know that no gravity waves are generated in this kind of situation, because experts have said so. So we conclude that the extra energy will absorbed by the rocks at some later time.


    Now let us remove one rock from the formation. Quite obviously the hole will emit some gravity waves, spherical gravity waves.


    If we remove more rocks every hole will be a source of spherical gravity waves.


    If we remove all rocks except two at opposite sides of the sphere, there will be 999998 holes that emit spherical gravity waves that interfere with each other. Distant gravity wave detectors will detect a maximum of gravity waves at some place and minimum of gavity waves at some other place.

  7. Interesting problem here. Let's see ...


    Bob revolves around Alice observing Alice.


    When Bob observes Alice, there is a constant Dobbler blue shift, but despite of that all the frequencies Bob sees are the real unchanged frequencies. They must be the real ones, otherwise there is a problem, as has been pointed out by the OP.





    Let's consider some optical fibres on a spinning carousell, fibres are installed radially. At the middle of the carousell there is some photon gas in some container, that gas is steered into the fibres. At the other end of the fibres there are containers into which the photon gas goes.


    We are interested about the energy of photon gas in a container in the container frame. The gas was given kinetic energy by the carousell, but in the container frame that energy does not exist. So we conclude that no increase of energy caused by the rotation of the carousell will be observed inside a container. Right?

  8. Was just thinking that transferring the spin from a disk from the bottom of a container to spin a disk on the top of the container using AC current, how would the system know which way to move to keep the center of mass at rest in an inertial frame.


    AC current, well there are some propagating electromagnetic fields carrying energy and momentum, not really different from a laser.


    DC current? Now we have some kind of ion thruster.


    Well how about a drive shaft then? Probably when you press a brake pad on a spinning drive shaft, the shaft will try to move to some direction, the direction opposite to the energy flow.


    Do you violate conservation of momentum with this?

    No. I guess. As Strange said, amusing energy juggling gadgets are almost like ordinary matter bouncing apparatuses.

    If you have a closed system with 2 disks and spin the disks in opposite directions, would the system stay at rest in an inertial frame?

    Yes. Very very very likely the center of mass would stay at rest.

  10. Two spiral cords are connected to the terminals ot the moving battery. Now the moving battery can be easily drained to a static battery.


    So what will happen:


    When the batteries move away from each other: We try to remove energy from the battery but what happens is that some electro-chemical energy turns into kinetic energy. A reduced amount of energy arrives to the static battery.


    When the batteries move towards each other: We want to remove the electro-hemical energy from the battery but some kinetic energy turns into electro-chemical energy. an increased amount of energy arrives to the static battery.


    In both cases the cord tries to move away from the moving battery ... like a toy that consists of a whistle and a rolled paper tube ... what is that called?


    (You see, the battery is blowing energy into a curly hose, and the energy is redshifted or blueshifted)


    It is exactly the same as yours: you add energy to an object, move it from one place to another, remove the energy, then move it back again. Again, no net change in momentum.


    A battery is loaded -> it becomes more massive.

    Then battery is pushed so that it slides along a surface.

    On the surface there are metal strips that short circuit the battery.

    Then the drained battery, which is less massive than the battery that was pushed, is stopped.

  12. I was thinking that this could be used to accelerate spaceships.

    Here is the hypothisis:


    If i have a closed system with a disk that moved up and down, and the disk was made to spin by a motor at the top of the system and made to stop spinning at the bottom of the system, the whole system would feel a force that would accelerate it upwards.


    The system could have a similar disk that moved up and down but spun the opposite direction than the first to stop the whole system from spinning.


    I have thought about how It mightn't work, such as if you spun up the disk, would it lose translational inertia?

    I think no, since according to relativity, there is no preferred frame to slow down against.


    I understand that there would not be much translational force produced for 'non relativistic' speeds, but sensors are quite good, so this might even be experimentally testable on earth.


    Here's an alternative version:

    A battery is loaded, then the loaded battery is thrown, then the flying battery is drained, then the empty battery is stopped.

  13. Let's do some math. smile.png


    First an accelerating rocket at non-relativistic velocity. We are interested about the shrinking velocity of the rocket according to an outside observer.


    At non-relativistic velocities: coordinate acceleration = proper acceleration.


    Let's say the acceleration is constant.


    shrinking velocity at some time from the beginning of the acceleration = time * (rear's proper acceleration - front's proper acceleration)


    The term (rear's proper acceleration - front's proper acceleration) can be written as: length*acceleration*some constant.


    "Some constant" might be the gravitational constant, definitely the gravitational constant must be there.



    Hey I think I can do the relativistic case too:


    shrinking velocity at time t from the beginning of the acceleration = integral from 0 to t ( (rear's proper acceleration - front's proper acceleration) / gamma^2)



    (gamma squared because the acceleration drops as gamma^2, a velocity change is smaller AND it takes a longer time in the outside observers frame)




    ADDITION: Gravitational time dilation can be thought as causing the differences between front and rear, and I think there's should be some very simple equation regarding that.

  14. Unless their timing and acceleration is set up to exactly counteract length contraction from their relative velocity, which is the condition of "Born rigidity."


    Yeah. For example: Constant proper acceleration for a leading space ship, a little bit larger constant proper acceleration for the space ship following the first one.

    Unless their timing and acceleration is set up to exactly counteract length contraction from their relative velocity, which is the condition of "Born rigidity."


    Actually there's no room for any unlesses here.


    Same frame = no relative motion = rigidity

  15. That was my point, though I don't see it as a contradiction.


    Any nonzero distance (as measured in every current rest frame) between two objects cannot be maintained while they are accelerating (in the direction of the distance between them) while they stay in shared rest frames.


    A distance (a maximum of which is limited by their separation and rate of acceleration...ie. Frank can't break the laws of physics) can be maintained with respect to the rest frame of one or the other, but not both.

    I don't know if this has been said but:


    When they think that they are staying in a shared rest frame, then they think the distance between them stays the same.


    When they think that they are not staying in a shared rest frame, then they think the distance between them changes.



    shared rest frame = no relative velocity = no change of distance

    Born rigidity seems applicable. The original question might be expressed as "what happens if a Born rigid system stops accelerating and comes to relative rest?"


    In that case I think the timing could be set up so that one observer sees the length remaining constant, but both observers couldn't. I think this agrees with what J.C.MacSwell wrote. The two observers couldn't remain in sync, as described in the link: "although the proper distances with respect to the instantaneously co-moving reference frames remain constant, the proper times of the different parts of the object do not remain coherent. In other words, if we contrive to hold the spatial relations fixed during an acceleration, a phase shift is introduced between different parts of the object, just as, if the phase is held constant, there is spatial stretching."



    Edit: But then, according to the main observer, the other suddenly goes from having a relative velocity to being at relative rest. I don't know how the change in length contraction and relative simultaneity would appear or if it could be compensated for. There must be a simpler and more interesting way to look at this question.



    Two spacecrafts accelerate so that they think the distance stays the same.



    => =>




    Outside observer says the distance shrinks.


    Then an engine break happens to both crafts at the same time in the outside observer's frame.


    The last craft collides to the rear of the first craf. The collision speed is the contraction speed at the time of the engine break.


    Assume an elevator that is 1LY wide, at rest in an inertial frame. Light is flashed into the elevator horizontally at time t=0. Simultaneously, the elevator begins to accelerate upward at 1g, which I believe is 1.0326 LY/Y/Y. In the inertial frame, the flash takes 1Y to strike the opposite wall. In 1Y, an object accelerating at constant proper velocity 1.0326 LY/Y/Y travels 0.4236 LY in the inertial frame. So an observer in free float in the elevator would observe that the light was bent, with a total 0.4236 movement downward for 1.0 movement across.




    gamma * 0.4236 is the distance in the elevator frame, if 0.4236 is the distance in the inertial frame. Seems very simple ... but there might be something wrong with even that.


  17. Toffo, let's push the question a bit further. Are you using homogeneous gravity or actual gravity? Also, say the observer carries his own meter sticks. Does the light bend the same amount in his meter sticks in both the gravitational field and the accelerating elevator? If the light hits the same place on the wall in both cases, it might be because of the offsetting effects you describe, which means that the observer could use his own meter sticks to determine that light bent a different amount in the two cases, so the two frames are not equivalent.


    Of course there is a homogeneous pseudo gravity field inside an accelerating rocket. And of course the measuring devices are owned by the person that is measuring with the devicessmile.png


    Does the light bend the same amount according to an observer's meter sticks in both the gravitational field and the accelerating elevator? That's the question. Maybe we should calculate the effect of my offsetting effect.


    My offsetting effect can be expressed like this: A slow bullet spends its flying time in a gravity field that is smaller on the average than the gravity field that is effecting a faster bullet, because the gravity field decreases with time.


    It was an outside observer that observed the bullets and the rocket, and the observer imagined a gravity field effecting the bullets, that observer is the observer that observes a weakening imaginary gravity field.


    (I should mention that the rocket accelerates with constant proper acceleration.)



    It takes time t for the rocket's speedometer needle to go from speed x to speed y, according to a clock inside the rocket.


    Outside observer says the time is t*gamma , according to his clock. (gamma is the relativistic factor of change)


    So we see: acceleration = initial acceleration/gamma


    ... And here end my mathematiclal skills.frown.gif

  18. If instead you accelerate the elevator's parts in a Born rigid manner, the bottom will accelerate at a greater rate in the observer's frame than the top, so the elevator should length contract at a greater rate than the elevator at rest described above (which the "falling" observer could presumably measure). And, if the elevator is really tall (such that its proper length is greater than c^2 divided by the proper acceleration of the top end), then even Born rigid acceleration will not prevent the elevator's proper height from expanding, and the elevator will tear apart toward its bottom. The "falling" observer will be able to observe this. Does any of this occur, such that it could distinguish the two reference frames?




    It's impossible for a very long rocket to accelerate rapidly.


    And it's impossible for a very tall rocket to stand on the ground in a strong gravity field, because the ground can not exist, because matter is pulled into a singularity in the place where the ground is supposed to be.


    1. It has been suggested that the degree to which light bends in the accelerating frame is less than that which light bends in equivalent gravity. See post here: http://www.scienceforums.net/topic/67141-equivalency-principle/?p=684962. The "falling" observer could theoretically determine this by measuring the light bending and the rate at which his speed relative to the walls grows. Then S1 and S2 would be distinguishable. Does this occur, so would it distinguish the two reference frames?


    Let me think ... The length contraction motion that is slowing down! That must be the solution.


    A bullet shot across an accelerating rocket: One might think that length contraction would cause the bullet to hit "wrong" place.


    But the bullet has some initial length contraction motion velocity ... and the target will lose some length contraction motion velocity during the flight of the bullet.


    Bullet lands where it's supposed to. Same thing with laser beam.

  20. With this kind of photon container we can produce photon wave bars of any desired length:iiiv.png

    With this device any photon wave bars produced by the previous device can be converted to any desired length:



    With this photon container we can do interference experiment with photon waves of any length:


  21. I don't know what you mean with long or short photon. If you mean the wave length, that does not matter in this set-up. The detector can be regarded as a new photon source. A triggered photon source (if possible) could also be used. All input photons of the 50% mirror have the same wave length, short or long.


    I think this has been covered already. Photon sorce launches photons, remembers the launch times --> photon waves will be short ones.


    if we want to produce long photon waves, a photon container with a small hole is good for this purpose: no triggering --> no definite launch time --> long photons, I mean photon waves.

  22. Now a snack in between. The left setup F gives an interference pattern. So the right one too, isn't it?




    But the path lengths are quite different. So if you measure the travel time between incoming detector and main detector, you know which path the particle went. Where is the error in this argumenting?



    Again there seems to be some advanced detector: The incoming detector.


    The incoming detector may detect a quite exact time, or quite uncertain time. I quess we have here such detector that it detects a quite exact time.


    So then, that makes the photon short. I mean the photon that the detecrtor emits. Or the photon that the detector observes passing by, I don't know what this detector exactly does.


    If incoming detector shortens long photons, it causes a loss of interference.

    If incoming detector elongates short photons, it causes an interference to pop up when otherwise there might not have been interference.

  23. But the standard set-up is detectors at both slits, which gives a signal in the main detector without interference pattern. Those detectors must detect+pass each photon, otherwise there would be no signal in the main detector.


    Well, it sounds to me those detectors are some kind of advanced and modern detectors.


    So, where can I read about that standard set-up? A link to a simple description of the set-up would be nice.

  24. This is an example of an ill posed question resulting into a lot of confusing/meaningless answers. The issue is that indeed, "relativistic" mass increases with speed by [math]\gamma[/math] while volume decreases by [math]1/\gamma[/math], so it would appear that COORDINATE-DEPENDENT density would increase by [math]\gamma^2[/math]. But, this is a meaningless exercise, since COORDINATE-DEPENDENT density is not a meaningful physics quantity. Case and point, charge density is frame invariant.


    I checked wikipedia, it says: "charge density is a relative concept"




    Wikipedia also says that Anthony French has described how magnetism is a result of relativity of charge density.


    So therefore I say that gravitomagnetism is a result of the relativity of mass density.

    When you have an object moving near the speed of light, would the density or the volume of an object change from mass increase?

    You either need to have the density or the volume to change in order for the mass to change


    Oh yes. And also from volume decrease.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.