Einstein's theory of relativity establishes that time dilation occurs as an object approaches the speed of light, affecting the perception of time for both the observer and the moving object. Light itself does not experience time, making discussions about its aging irrelevant. The muon decay experiment serves as a practical demonstration of time dilation, showing that while muons decay slower when moving at high speeds, they appear to live longer from the perspective of a stationary observer. The concept of reference frames is crucial, as all observers measure the speed of light as constant, regardless of their own motion.
PREREQUISITESStudents of physics, educators in relativity, and anyone interested in the fundamental principles of time and motion in the universe.
Actually I am not trying to debunk anything. Just asking questions and trying to grasp concepts which at first sight are riddled with inconsistencies.PeroK said:I'm glad to see you've given up trying to debunk relativity, but I suspect you'll have no more luck debunking the big bang theory!
Try this for starters:
https://www.physicsforums.com/insights/big-bang-happen/
Simon Bridge said:When attempting a critique it is important to understand what is actually being said, otherwise we end up inadvertently attacking a straw man.
The important rule is that the speed of light is the same for all observers - not that the velocity is the same.
Remember the speed is the magnitude of the velocity vector.
There is no rule to say that relative motion cannot change anything about light. For instance you can change the kinetic energy and momentum carried by the light by changing the motion of the source, and you can also change the direction that the light travels in... put more precisely: different observers may determine different energy, momentum, and direction, for light; even though they always measure the same speed.
This thing about the speed has fun consequences when you compare observations made in different reference frames.
For instance - a directed pulse of light is fired to a detector in the ceiling of a rail car ... that is moving along tracks.
At the instant the pulse is fired, a flash bulb goes off so everyone can see when that happened. Everyone will agree call that this happens at t=0.
When the pulse hits the detector, another flash bulb goes off. Everyone agrees to take note of when and where the flash goes off, in terms of their personal coordinates and clocks. For the sake of simplicity, everyone agrees to account for the speed of light travel times when working out the times that things happen.
Bear this in mind when considering the following discussion. I am going to attempt to illustrate the stuff about what is apparent vs what happens in reality... I'll start by talking about what is apparent to one observer without making claims about what is real or "actually" happening.
According to Alice, standing on the tracks, the pulse was emitted at the floor of the car when it was right next to her, but was detected at the ceiling of the car some distance down the tracks. Therefore the trajectory of the pulse looks to have been at a diagonal to the tracks. But was that actually the case, or is this just what it looks like and the reality is different?
She may reason that the pulse of light should have just shot straight upwards from her position ... after all, if she held the emitter and fired a pulse, that is what would happen. Maybe, she speculates, there is a spherical pulse of light and the part the detector intercepted is not the part that kept going upwards; maybe it only appears that the the pulse was diagonal?
She can prove it by having a second detector directly above the first at t=0, but which does not move with the car. This detector does not go off, indicating that no part of the pulse lagged behind the train. It is also possible to repeat this experiment at speeds arbitrarily close to the speed of light, or using rail-cars that are arbitrarily tall ... this makes the observed (by Alice) horizontal displacement quite large so what is "actually going on" is going to become more and more apparent. Lastly, the experiment was rigged with a highly directional pulse, the physics of which Alice can examine and see that it behaves as reported.
She is forced to conclude that the pulse of light traveled diagonally with no spreading, no lagging, nothing.
How can this happen? Easy - the emitter was cleverly devised so that the pulse was actually aimed ahead of the detector-position. This is, after all, how a marksman hits a moving target.
After the experiment is over she talks to her good friend Bob who rode along on the train. They compare notes over coffee.
Bob is like, "No, I aimed the pulse straight up like you said to."
Moreover, the time between emitting and detecting the pulse was less by Bob's clock.
Which report is the reality and which the seeming?
It it possible to devise an experiment to demonstrate which is the real reality?
Notes:
1. it may be that there is something else in mind, in which case please state it clearly - there are lots of fun situations that can be, and have been, devised;
2. the above example is not as careful as it should be - this is why the standard thought experiment involves a return trip. The whole thing can be reformulated with the emitter also being a detector and the ceiling detector replaced by a mirror for anyone who wants to be more rigorous.
Francis Ward said:Now, if we replace the aircraft with a rotating podium on which an observer, A, stands, and that podium is rotating at 6000 rpm. Then place another observer, B, a distance of 478 km from the podium. If my maths is right to A they will simply see B rotating at 100 hz, but to B they will see A flash past them at marginally higher than c, 100 times per second.
Francis Ward said:Now replace B with a light source emitting a straight source of light, which does not diverge. Build a circular wall at 478 km radius from B, on which the spot of light can project. To all observers in the frame of reference of the wall, that spot of light would travel at marginally higher than c (again assuming my maths is correct).
But even if time existed before the Big Bang, there is still another reason not to imagine the Big Bang as happening at one point in a preexisting empty space. Observations of the universe show a nearly complete lack of structure on very large scales, and the cosmic microwave background is also extremely uniform (with fractional temperature differences on the order of 10-5). For this reason, realistic cosmological models must be almost exactly homogeneous, meaning that no point in space has properties that differ very much from those of any other point. Therefore the best evidence is that the Big Bang happened uniformly, everywhere at once.PeroK said:I'm glad to see you've given up trying to debunk relativity, but I suspect you'll have no more luck debunking the big bang theory!
Try this for starters:
https://www.physicsforums.com/insights/big-bang-happen/
No. On the contrary, it is probably the most magnificent concept I have heard. The imagery is awesome. Do you have thoughts on how such an event would be initiated?weirdoguy said:Is that a problem for you?
Francis Ward said:No. On the contrary, it is probably the most magnificent concept I have heard. The imagery is awesome. Do you have thoughts on how such an event would be initiated?
... that would be common sense logic rather than logic based on very careful observation.Francis Ward said:Hi Simon,
That is precisely what I am trying to understand, and thank you for taking the time to depict it so well. I come from an engineering background and as I said I am new to pondering the whole relativity stuff. So my views are initially constrained by a very logical approach eg if you add speeds in a single direction there should be no limit how fast you can go.
... as an engineer you would know that those speeds are estimates. They should be quoted with their uncertainty.So, pondering the many comments my initial post had generated, leaves many questions. For instance, that raised by the question of relative velocities, such as, to a person on the ground, observing an aircraft passing over head, they see it moving at, say, 600 km/h. However to the person on the aircraft, observing the person on the ground they see precisely the same of the person, that he is moving at 600 km/h. Simple and obvious stuff
You no longer have the kind of equivalence you had with the aircraft example since A is in a non-inertial frame.Now, if we replace the aircraft with a rotating podium on which an observer, A, stands, and that podium is rotating at 6000 rpm. Then place another observer, B, a distance of 478 km from the podium. If my maths is right to A they will simply see B rotating at 100 hz, but to B they will see A flash past them at marginally higher than c, 100 times per second.
... there is nothing in relativity to contradict this, and the effect has been observed in Nature.Now replace B with a light source emitting a straight source of light, which does not diverge. Build a circular wall at 478 km radius from B, on which the spot of light can project. To all observers in the frame of reference of the wall, that spot of light would travel at marginally higher than c (again assuming my maths is correct).
Replace B with a machine gun. Does any individual bullet move along the wall at greater than c? Or does each bullet move from the gun to the wall at the usual muzzle velocity, well below the speed of light?Francis Ward said:Now replace B with a light source emitting a straight source of light, which does not diverge. Build a circular wall at 478 km radius from B, on which the spot of light can project. To all observers in the frame of reference of the wall, that spot of light would travel at marginally higher than c (again assuming my maths is correct).
HI George,GeorgeDishman said:Replace B with a machine gun. Does any individual bullet move along the wall at greater than c? Or does each bullet move from the gun to the wall at the usual muzzle velocity, well below the speed of light?
You say you're an engineer, me too, so make your light source an LED connected to a pulse generator. What happens to each flash of light emitted?
The situation you are describing is a variant of a paradox called the "Superluminal Scissors".
Hi Simon,Simon Bridge said:... that would be common sense logic rather than logic based on very careful observation.
Common sense is what tells you the Earth is flat ... though, to be fair, common sense is pretty good for day-to-day experience.
... as an engineer you would know that those speeds are estimates. They should be quoted with their uncertainty.
So ... off those figures, the speed would be ##600\pm0.5##kmph. This is important to notice since, for such slow speeds, the difference between common-sense and relativity is usually much smaller than the uncertainty.
The next thing I want you to notice is that you have described two inertial reference frames (apart from gravity being in both of them) ... so they are equivalent from the point of view of the physics you can do in them. The only way to distinguish them is to notice that the flying aircraft is making a lot of noise and is using up fuel ... but I'm sure you can think of a way to get rid of such clues.You no longer have the kind of equivalence you had with the aircraft example since A is in a non-inertial frame.
Rotating frames are accelerating.
The rotating observer thing is handled in general relativity.
http://abacus.bates.edu/~msemon/SemonMalinWortel.pdf
(Slideshow discussion... http://luth.obspm.fr/~luthier/gourgoulhon/fr/present_rec/imcce_syrte10.pdf )
... however, it is probably best to get used to special relativity first, otherwise you are trying to make links to things that are harder to understand from a fuzzy understanding of something else. The tldr answer is that the common-sense, euclidean, geometry you are used to becomes non-euclidean for rotating observers.
... there is nothing in relativity to contradict this, and the effect has been observed in Nature.
ie. http://www.mtu.edu/news/stories/201...may-help-illuminate-astronomical-secrets.html
Another example would be two spacecraft flying in opposite directions away from a space station, each at 0.6c wrt the station. Clearly an observer in the station will see the distance between the spacecraft getting bigger at the rate 1.2c.
Distant galaxies can also exceed c - due to cosmological expansion.
All observers measure the same speed for light in a vacuum ... this does not mean that nothing, no effect of any kind at all, can travel FTL, just that no message can be sent FTL.
Francis Ward said:Hi Simon,
Once again thank you for the time and effort you put into your responses. It is highly appreciated.
Can you explain the inertial/non-inertial frame?
Francis Ward said:So, going back to the light emitter traveling at 90 degrees to the direction of the light. With the 'physical' things, such as the bullets or the ball being fired/thrown, I can see clearly that the velocity of the train 'adds' to the velocity of the ball/bullet and gives a resultant. Can we apply the same principle to the pulse of light? As Brian Cox put in his book, does the light get a 'helping hand' from the motion of the train? My understanding, based on the fact that if you switch the direction of the light emitter to one that is parallel to the direction of the light, you cannot 'add' to the speed of the light, is that is should not. This was refuted by an earlier responder, saying that is was 'a demonstrably false assertion', and Einsteins light clock thought experiment seems to support this, inasmuch as the light travels from mirror centre to mirror centre, exactly as it would if the train were stationary. (Which of course, to the observer on the train, it might well be)
Yes, you can apply the same principle but you have to use the relativistic maths, not conventional version that simply adds the velocities which only works for low speeds. For light parallel to the direction of motion, the combination of c and v gives c and still in the same direction. For light at any other angle, the angle changes but the speed remains c. That change of angle essentially throws the light from a moving source forward, it is sometimes called the "relativistic headlight effect" or "relativistic beaming" or aberration. A real world example occurs on the jets from some super-massive black holes, see the comparison of M87 and 3C31 in the following article. The jet moving away from us is so dim we cannot see it for this reason.Francis Ward said:Can we apply the same principle to the pulse of light? As Brian Cox put in his book, does the light get a 'helping hand' from the motion of the train? My understanding, based on the fact that if you switch the direction of the light emitter to one that is parallel to the direction of the light, you cannot 'add' to the speed of the light, is that is should not.
Francis Ward said:Hi Simon,
So my views are initially constrained by a very logical approach eg if you add speeds in a single direction there should be no limit how fast you can go.
Francis Ward said:HI George,
My understanding, based on the fact that if you switch the direction of the light emitter to one that is parallel to the direction of the light, you cannot 'add' to the speed of the light, is that is should not. This was refuted by an earlier responder, saying that is was 'a demonstrably false assertion', and Einsteins light clock thought experiment seems to support this, inasmuch as the light travels from mirror centre to mirror centre, exactly as it would if the train were stationary. (Which of course, to the observer on the train, it might well be)
Thanks for that. This is now my favorite analogy.Mister T said:You can add speeds in a single direction, and there is no limit on the sum that you get when you do that.
But there are other ways besides addition that one can combine numbers. The logical question to ask is which is the right way to combine them when trying to figure out how fast something is moving. If B moves with respect to A at speed ##v_{AB}## and C moves with respect to B with ##v_{BC}##, then what is the speed ##v_{AC}## that C moves with respect to A?
One possible way to figure that out is by adding ##v_{AB}## to ##v_{BC}##. When you do that you of course get a speed, but it's not equal to ##v_{AC}##.
Note that common sense is just something that appears to be true because it has been demonstrated to be true, and therefore appears to be obvious.
But in fact this way of combining speeds to get ##v_{AC}## works no better than it does when combining slopes. If you're on a roof that has a slope of 5/12, meaning it rises 5 inches for every 12 inches of run, and you place a rod on that roof so that it is tilted at a slope of 5/12 relative to the roof surface, then the rod will not have a slope of 10/12 relative to the horizontal. Instead, you would add the angles, which are the inverse tangents of the slopes. (Note though, that all you ever deal with is very small slopes, you find that adding the slopes works just as well as adding the angles. You don't notice that one way works better than the other until you have some experience with larger slopes. The same is true of slow speeds!)
Likewise, you would add the inverse hyperbolic tangents of the speeds. That's the right way to combine them.
Battlemage! said:This is now my favorite analogy.
Francis Ward said:the static observer.
Define a tick to be a round trip instead of a single leg and this problem goes away. It's surprisingly difficult to assign any sensible and observer-independent meaning to the one-way (as opposed to round-trip) speed of light - search this forum for threads about the one-way speed of light to understand the problem here.Francis Ward said:as it appears the light travels further when heading in the direction of train travel and less when returning. I must be wrong with this!
Replace the single observer on the platform with multiple observers all lined up along the platform. Each one observes only what happens right under his nose, records that and the time his wristwatch reads when it happens, and then after the fact we can collect all of their notes to construct the history of the light beam as seen from the platform.Secondly, how is the time that it takes for the light from the train to reach the observer on the platform accounted for? For the 'image' of the light clock to reach the observer on the platform, there must be a delay, albeit very small, and that delay is not the same at the start of the journey as it is at the end of it because the train is now further down the platform by distance vt) and therefore there would be an addition time for that light to travel vt, which would be vt/c. The only way this could be eliminated in the thought experiment is if the journey was circular, returning to the same spot it started, immediately adjacent to the platform observer. That then would make the journey subject to acceleration, which messes up the simple maths. (Sorry I had too much time to think on the flight)
Francis
I'm please you spotted this yourself - well done - I was aware of the problem of the one-way trip for light when I wrote post #30 and, if you had not spotted it, I would be forced to point it out or risk leaving a big pothole in your path to understanding....it appears the light travels further when heading in the direction of train travel and less when returning. I must be wrong with this!
An additional complication for the parallel light clock is the distance between the mirrors. In SR we have length contraction in the direction of motion. If you don't take it into account the derivation of the time dilation will be a bit off, but you also can't assume it upfront. You have one equation with two unknowns.Francis Ward said:If any of you have the patience - could you explain how the thought experiment works if the light clock runs parallel with the direction of the train?
Replace the single observer on the platform with multiple observers all lined up along the platform. Each one observes only what happens right under his nose, records that and the time his wristwatch reads when it happens, and then after the fact we can collect all of their notes to construct the history of the light beam as seen from the platform.
Francis Ward said:I see what you are saying, but isn't that in effect the same as the platform observer traveling with the train?
You don't have the same t in both directions.Francis Ward said:From the platform I think it is d+v*t for the outward journey (t = time for light to travel one way from mirror to mirror, v = speed of the train); then for the return journey it is d-v*t. Hence for the whole journey it is the sum of these 2, which is simply 2*d. Where is my mistake?