It's often taught to us in physics that you can "feel a force". But what do statements about feeling force even mean, and are they accurate? The answer will surprise you. A follow-up video to "Why Solutions to the Twin Paradox are Wrong."
Now I can see why I never really "got" concepts like force, but rather memorized the treatment, and pretended to be comfortable with it, as presumably the teacher was also doing. The unsatisfactory feeling that the science was somehow settled and accepted, but I couldn't understand it, is maybe why I pursued mathematics, instead of physics. I'll make sure to teach my kids that it's ok (and desirable) to admit a lack of understanding, even on the most basic and common concepts.
I have not even started watching the video, but I am going to give you a like because of the mere fact that you chose to do a video on such a seemingly basic and simple concept. Topics like these are usually hand-waved away by teachers and professors alike as if they don't warrant a deeper explanation and have already been logically figured out by minds greater than our own. As someone who has always felt dumb for questioning the most basic of concepts, especially in physics, it is very refreshing to see that you have made explainers on these topics and taken them with enough sincerity in an attempt to properly formalise the most fundamental notions
Feel force, feel time, feel space? If you delve on that, you can doubt everything, not just forces. Stay simple. With clocks we measure time intervals. With rules we measure space distances. With springs we measure force differences. In F=ma, we can use clocks and rules to evaluate the acceleration, a spring to evaluate the force, and the thing we really need to assume to exist beyond our measurement instruments is mass.
But you didn’t understand what the video was talking about. Force, unlike the rest of the above, is introduced not as an independent physical quantity, but as the product of acceleration and mass. By the way with spring you can only measure spacial displacement of this spring
@@noavailablenamesatall My body is build using electromagnetic interactions. With these I am endowed with senses that can see, hear, smell, taste, and touch. When you shake my hand, my body feels the pressure. I can also watch accelerations. In F=ma the only thing that I cannot see, hear, smell, taste or touch is mass. That's what I need to assume.
Inside the closed room the person is standing and the accelerometer is hanging from the ceiling. That can mean atleast two things: 1. There is a force that is pointing downwards. 2. The person and the ball are arranged inside the room in such a way in free space free from any external force.( let us assume that the only matter left in the universe is the concrete room with the things inside it) In both the situation direction of acceleration can be observed. Situation 1. The person may fall if the acceleration is high enough. Situation 2. In case of an acceleration, the person will move towards a wall if the shoes he's wearing aren't fixed to the floor. If the shoes he is wearing are fixed to the floor , he will rotate and bang his head on a wall or floor.
We may miss the feeling of force when a train accelerates smoothly enough....because the acceleration is very small ...and we are often not paying too much attention to it. But that does not mean the feeling is not there. But great video....got me into thinking over the basics again.
Yeah but that feeling is caused by the seat’s matter hitting your body, and the effect it has on it is exactly the same it would have had if it was you who hit the seat. You still cannot discern whether it is you who is accelerating or the train.
If you were born free falling in a uniform gravitational field, and lived your life in free fall, and made free falling friends, you would never know that gravity exists and you would say you are inertial cause there are no forces acting on you. Your friends would also agree on the motion you describe for other objects, like kicking a ball (thus accelerating). This is what happens with our solar system. It is being accelerated uniformly by gravity coming from far away, and you BELIEVE it’s inertial because we all agree on accelerations outside the free fall. On the other hand if the gravitational field were not uniform, you would see other objects far from you accelerating and others would see those objects accelerating but you would disagree on the acceleration, because both observers are differently accelerated. A third person somewhere else would see you and the second person accelerating IN A DIFFERENT WAY that you and the second person see each other. Inertial frames may agree on accelerations, but accelerated people don’t. But no real inertial frame exists cause it’s impossible to tell if you are truly inertial. All the “accelerated” observers felt inertial in their frame of reference. The word “fictitious force” is what inertial observes named the forces they don’t perceive, implying that what they see is real, but an inertial frame can only be a local thing. For an inertial frame to be global you must have no interactions. There’s no such thing in the Universe. Summary: there is no global inertial frame only local. Those who agree on accelerations are just being accelerated in the same way, as if in a uniform gravity field. Those who don’t agree on accelerations are just being accelerated differently. In both cases, they are locally inertial, but the former insist they are globally inertial, just cause they have more people backing them up 😂
@@_Xeto We are indeed in a free fall under the Sun’s gravitational pull…. So we do not really feel Sun’s attractive force. It’s the hard crest of planet earth that does not let us freely sink to the centre of the Earth under earth’s gravitational pull … therefore we feel earth’s gravity.
Summary 3: To find out inertial vs non-inertial frames NO REFERNCE TO ANYTHING OUTSIDE THE FRAME IS NECESSARY. Experiments confined within the frame are good enough to tell us if we are in an inertial frame or a non-inertial frame.
Finally, at this moment, my spring have been awakened. It feels the force. Seriously "Force" does not have be felt. One of the easiest way to "observe" the force is the use of spring as a measure. In my humble opinion, spring does not "feel" anything. It does not have any knowledge about "inertial-ness". It also doesn't observe an acceleration, too. It's just a thing. It's different from me and my cat. But it perfectly & precisely react to the Force. How do you explain this? Quantum mechanics doesn't even bother the force!!??? man, quantum world is FULL of forces. It even tells how different kind of forces are transmitted through the particles. It doesn't covers Einstein's gravity yet but gravity is actually included in the theory as graviton (undiscovered yet, though). Okay, if you think, acceleration is caused by the "transformation of reference frame" you says, what about the "force" caused by the magnetic fields? how it's related to the "reference frame"? there is no acceleration but electron moves! I really liked your clam on "twin paradox" (at least half of it) but this is total nonsense. I agree that there are lots of interpretation of the laws of the physics as long as it consistent with the theory. But if you want to make your point, you should explain why your interpretation is better than the others. For example, Everett introduce his new interpretation for quantum mechanics to eliminate Von Neumann's projection. Because as long as we have it, we can't apply quantum theory to the universe itself where there is no "outside". He did not say, there are thousand of universe correspond to every quantum state...he said this is the reasonable assumption to improve the theory. So what is the point of your interpretation...no offence but seriously you need more research on the topic your interested in!!! Finally, this is the last claim. The reality of the variable in the physics law is actually questionable. In other words, the force may not be the "real thing". It only appears in the theory to clarify the relationship of the variable, that's all. But one thing for sure is, as I stated at the first of this comment, you can measure it actually. Therefore, I think it exists. But this is not a physics, it totally unrelated to the actual theory. It's more like philosophy than the physics.
An accelerometer such as a spring doesn't resolve the issue of how to measure force, because you are only still measuring something accelerating. As this video discussed, such a measurement has to be paired with additional information not intrinsic to the system in order for the conclusion to be made that a force is present. We discuss this topic further in our "Inertial Frames" video. We are not offering a "new interpretation" of any theory, only pointing out a flaw in the current interpretation. Many interpretations and theories have had their flaws pointed out without new theories being offered up in their place... critical inquiry and skepticism are an important part of the philosophical and scientific process.
@@dialectphilosophy do you not agree that for an object to accelerate, a force has to be applied? And from that we can deduce that a force has been acted upon the spring? Or do you not like the idea that deduction has to be used to come to the solution that a force has been applied? Also, all the examples you have given in this video are situations where objects have accelerated away from type of inertial frame. Force can be applied without the need to for objects to accelerate, as I can squish an egg in my hand with force without the egg changing it's position. Furthermore, the situation where a person is in a empty room and sees an accelerometer change position and thinking that maybe it's just being affected by a magnet and not that a force is being acted on the building seems flawed. As I can exclaim that the moon is made out of cheese but that doesn't change any facts what the moon is really made out of. I don't know, if you could come with counter arguments to my comments, maybe that would make me understand what you are trying to say.
@付和雷同 One of the fundamental forces, or interactions, such as strong, weak, and electromagnetic, is not the same as a specific quantitative force, the F in Newton's F = ma. As far as gravitons are concerned, even if they exist it may never be possible to discover them.
I think the point is that a spring is now essentially a 3rd observer. With only 2 objects its not possible to tell, which is why I think the more simple explanation of two balls going opposite is maybe more valid.
Force is mass * acceleration. So I would look like a dead pancake if a train hit me. But if I sit inside the train im not sure if this is a accurate formula.
3:06 Mass is not directly observable either. Mass can only defined with respect to an inertial frame. This is easy to fix in Newtonian physics because accelerations are invariant under Galelian transformations. The fix in relativity is more difficult, and leads directly to E=mc^2.
Fantastic. I hope you will tell us eventually more about the history and resolution of this discrepancy between your vision (Who else shares it? Does it have a name?) and that of the mainstream.
Pure classic philosophy. Thought-inspiring, but for other purposes it feels quite useless. And I guess the same reasoning can be applied to any physics concept. (What is mass/time/distance “REALLY”?).
Some broad thoughts im thinking right now: say, 1.) Force concerns how mass/matter reacts to motion. 2.) Matter/mass units can't (in essence) occupy the exact same space. So if you have multiple mass units, they are a distance apart from eachother. 3.) Max speed is C, so it will always take time to move mass across space. So you have a blob of mass spread across space, another mass comes in contact with the blob (there must be motion involved). They can't occupy the same space. 'internal pressure' of a mass or w/e will be in an equilibrium if outside conditions stay the same. Now after the collision, at the point of contact theres a bunch of back and forth of pressure-waves I guess.. It takes time for pressure to spread acros the blob of mass. This squeezing and stretching of the volume units of mass, because of localised pressure changes inside of the blob, are what they mean by 'feeling force'. If you happen to read this and think im an idiot its entirely possible. Got no real physics/math background. Would like to know why this would be wrong so I can see where i'm thinking wrong.
"Pressure" is defined as force per unit of surface. So, we do not really extend our intuition here by adding pressure to the equation. It's still about the nature of force, defined as the product of mass and its acceleration, i.e., its change of momentum, which we call its impulse. The problem is that we need to presume an inertial frame of reference to be able to define the background at which this acceleration can be observed. This also applies to light. We say that light in a vacuum travels at its maximum speed. But that is under the condition of a local frame, where space-time is considered to be undisturbed. Over larger distances, the speed of light does not necessarily equal 'c', the well-known value that we normally mention as the speed of light.
Summary 2: An inertial frame is one such that no mechanical and/or optical experiments conducted completely within the frame can ever tell us about the motion of the frame. Whereas, in a non-inertial frame we can easily detect motion without any reference to outside, just by conducting certain experiments confined within the reference frame.
As regards to "Can we 'feel' force?" No, we can't -- not because how you explained it, but because "feeling" is a biological phenomenon and in Physics we DO NOT FEEL ANYTHING. We measure things. And let me tell you, YES, we can measure real forces by doing experiments. Of course Einstein's General Relativity has been formulated in such a way that the gravitational force has been done away with by redefining the effect of mass & energy on spacetime and vice versa, thereby bringing accelerated frames due to gravity within the realm of inertial frames of reference. But acceleration of a reference frame due to any other of the 3 forces of nature (as discovered till now) are definitely detectable by simple experiments within the frame of reference.
Great video!! Continue doing them please. Just one question: if I am in a closed room and attach an accelerometer to the wall, that says 9.8m/S2, can I ensure that I am not in an inertial frame of reference?
That answer depends on your relative knowledge of the system; are you familiar with the accelerometer instrument, have you properly calibrated it prior in an inertial frame, can you guarantee that nothing else might be causing the accelerometer to move? etc.
@@dialectphilosophy ok, then assume - I am in our real universe - I calibrated it prior in an inertial frame( not necessary if i am using my laser beam method) - nothing else might be causing the accelerometer to move - the definition of inertial frame is just the same as in general relativity, where acceleration is still absolute.
I think you got your basic Physics terribly wrong. In this video and also in those in which you rudely refuted some of the explanations of the twin paradox, it's you who have completely misunderstood the concept of inertial vs non-inertial frames of reference. Here they are: An inertial frame is one such that no mechanical and/or optical experiments conducted completely within the frame can ever tell us about the motion of the frame. Whereas, in a non-inertial frame we can easily detect motion without any reference to outside, just by conducting certain experiments confined within the reference frame. The key point here is acceleration of a reference frame can be detected from within the frame without any reference to the outside world, whereas constant speed can't be detected from within the frame unless you refer to the outside world. This simply means REAL ACCELERATIONs (not pseudo accelerations) ARE NOT COORDINATIVE AND HENCE THEY ARE NOT RELATIVE. However, constant speeds (including v = 0) is very much relative. Quick illustration: Imagine you are in a completely sealed spacecraft. Sealed means no way you can "see", i.e., interact with the outside world. Imagine also that you are in an empty universe, which means even if you had windows in the craft, it's just blackness outside. Now, if you perform a myriad of experiments (mechanical and optical) confined within that spacecraft and yet there is no way you can figure out if you are moving or not, that means you are in an inertial reference frame. On the other hand, if any of your experiments (actually it would be ALL of them) shows clear signs of some kind of motion (again, without any reference to the outside world.... anyway it's blackness outside), then you are definitely in a non-inertial reference frame. The point is: To find out inertial vs non-inertial frames NO REFERNCE TO ANYTHING OUTSIDE THE FRAME IS NECESSARY. This is what most people (and of course you) have not at all understood. The abstraction regarding the empty universe is not necessary because a "sealed" aircraft is good enough to illustrate this simple physics of inertial vs non-inertial frames of reference. Sorry for my not-so-polite expressions.
Well, you could argue that when you can't detect any motion, it's simply because all particles in the ship are accelerating at the same rate, this is not something achievable with current technology, but in this hypothetical situation, if you have some inertial point of reference you would still be accelerating in relation to it even though you can't detect any movement
@@viktorbergman517 Not sure I got your point in its entirety, but there is no technology involved here. Acceleration due to the impact of any force is detectable by a simple pendulum (a heavy bob tied to a string). However, an accelerated frame can still be considered as a motionless frame if a suitable Gravity is brought into the picture. That's from Einstein's Genera Relativity. That way even an accelerated frame becomes an inertial frame. However, the two frames in the twin paradox are definitely different, and can't be considered equivalent by any argument as done in the video. So, twin paradox is not a paradox.
@@viktorbergman517 If "everything is accelerating at the same rate", that's equivalent to gravity and just means you're not accelerating in the absolute way at all. You are accelerating relatively, just like someone in free fall.
3:30 You talk about a train accelerating "smoothly enough so you don't notice it" but if I had an accelerometer (such as a 2 masses held together by a spring) that could measure to ARBITRARY PRECISION, then you'd always be able to tell whether you're the one accelerating or not, even if it's not noticable enough to feel with the limited human body. In other words, you can always tell whether your frame of reference is inertial or not. Acceleration is objective!
As discussed in "Newton vs. Mach" an accelerometer has to be calibrated beforehand in order to make any sense of its readings, and so does not measure absolute acceleration (which is an undefined and non-sensical concept) but rather acceleration relative to the frame of calibration.
Observing means interacting. You can't observe without receiving something (light,...). And if you receive something, you yourself must be an entity (object,...) within the system. In physics there's no such thing as a "neutral" observer.
Given the example of the twins in space with no reference. If one did accelerate away from the other, wouldn't they be able to deduce who is the one that accelerated? Let us assume that Alice accelerated away from Bob and both of them had a frictionless surface with a ball on it. Alice would see the ball move to the back of the plane whereas Bob would not. Given that there is no other information in the universe, then we can demonstrate that Alice is indeed accelerating and Bob is not. In the video, the explanation (5:00) would be that there is no way to tell if your refrence frame is accelerating because it can be due to some other mechanism that is moving it. Is it reasonable to state that there is no mechanism in this situation or would that count as measurement information?
Hey Mark, great questions! The issue with accelerometers here is that in order to associate a coordinate acceleration with a real acceleration, you first need to have an inertial frame. But once you take the twins out of the regular universe and put them in empty space, you lose any way to define an inertial frame. If you ascribe to Alice and Bob total knowledge of all the universe, and insert an accelerometer that distinguishes one of the twins' accelerations, then what you are asserting is that it is the addition of the accelerometer that is responsible for the definition of the inertial frame, and hence the breaking of the symmetry, which would in turn imply that your measurement tool somehow creates your reality.
@@dialectphilosophy I think it's resolved by the twins starting together, and one twin pushes something away from itself to accelerate. Whether they carry an accelerometer that is inaccurate or not doesn't matter, it's the twin that pushes something away that experienced more acceleration.
@@alexjohnward Exactly!! Yes, I've been trying to mention this on other videos of the twin paradox including one of Dialect's videos as well (I think anyway). This is a crucial piece they always leave out. The only way to accelerate is to exchange momentum with something else. This is literally in Newton's laws. You can compare your motion to the thing that you exchanged momentum with, whether that is something outside you interacted with (pushing an object) or ejecting a part of yourself (like a rocket).
In terms of observable linear momentum (assuming you're okay with that since mass is lumped in), can the video be summed up as the following statement? You can observe supplies but you can't know sources.
Inertia has nothing to do with "Feeling", it is simply the resistance to change in velocity. Or to put it in better terms it is the resistance to change from an inertial frame of reference. If two objects are accelerating away from each other there would have to be a force involved. This is why you can measure which train is actually accelerating. Since the force is only being applied initially to the train and not the objects inside it, the train will move relative to the objects until the objects come into contact with the train and the force is then applied to them via the wall or seat of the train pushing them forward. This is why if you had two trains in space with a ball inside each of them and only one accelerated away; the ball inside the accelerating train would seem to move relative to its train while the ball in the stationary train would appear to stay still relative to its train. This of course would stop once the acceleration stopped and both trains were moving at a constant velocity relative to each other again. This effect only occurs during the period of acceleration.
Hey thanks for the comment! If you define inertia locally via changes in velocity, then you are stuck at the fact that inertia will be relative, since all motion measurements are coordinative. That's why inertial frames are generally defined in reference to a global system, e.g. frames that aren't accelerating relative to the rest of the universe. In your two-train example, consider instead the frame where the accelerating train is stationary. Now it is the ball which is accelerating away from it, as well as the other train and the other ball. There is no method in the context of the thought-experiment for determining whose acceleration is "more real" than the others. You simply have two distinct but equally valid descriptions of the situation: one description where one train is accelerating away, and another description where two balls and the other train is accelerating. Neither acceleration can be "more correct" until you introduce some further feature distinguishing them.
@@dialectphilosophy So just to make sure I understand your stance; you are disagreeing with the Einstein himself as well as the entire physics community about the fact that acceleration is not relative? You believe that acceleration is relative in the same way as velocity? That given a discrete energy input, the change in relative velocity for one object is the same as all other objects being moved instead? Or do you simply not believe in energy and forces at all? Do you believe that the resistance to a change in relative velocity (inertia) doesnt exist?
@@WizardsOfTheGhost No we are not disagreeing with Einstein; Einstein actually agreed with our position, and it was part of his inspiration for developing GR. Our argument regards the locally un-definability of inertial frames. One could potentially argue a consequence of this un-definability is that acceleration must be made relative in some manner. However, the typical argument is that something else lends inertial frames their objective character (this was Einstein's stance). Acceleration, in-and-of-itself, cannot be responsible, since its definition is purely mathematical/coordinative.
@@dialectphilosophy Inertial frames are definable and can be tested and observed. For acceleration to continue, a force needs to be continuously applied. Which is why if you were accelerating in a direction and let go of an object you were holding it would not continue to accelerate unless the force were also applied to it separate from you. Without force objects do not change between inertial frames. Its the exact reason gravity works. Gravity is literally just acceleration caused by the warping of spacetime.
@@WizardsOfTheGhost Huh, no... Gravity lets you experience relative acceleration without the absolute acceleration that's the topic here. It's only when you fall against a planet, that you're "accelerated" absolutely by the surface of the planet. And then you've ceased to accelerate relative to your coordinate system.
You can always deduce if you are in a accelerating train with a ball, if it was at rest and stays at rest it is not an accelerating train but if it doesn't stay at rest it is an accelerating train.
Acceleration is simply a change in your worldline vector. The change requires energy. The force we feel is nerve endings in the mass of our bodies sensing is moving from our initial state, sending signals to the brain. We can also see water in a bucket, or changes on an accelerometer.
I think one can tell if it is on an inertial frame of reference or not by checking homogeneity and isotropy of space and homogeneity of time, rather than in a "feel force" argument. I belive the first method is really clear and unambiguous and the fact that we see objects accelerate is a consequence of interaction. From and accelerating object due to interaction one can check that objects which don't interact (this may be where the argument may be weaker) break this frame of reference spacetime symmetry properties.
Some people say the object "feels" a force (meaning that the force acts on the object) but they just say it as a figure of speech but it can be confusing because it sounds like the object is literally eperiencing a sensation, which isn't the case. If you're in the car and accelerating, you can feel an effect of the acceleration, which is that you get a bit squashed against the seat. That's not the force itself that you're sensing. It's a biproduct of the acceleration which is that you get a bit squashed.
Inertial means that a mass is giving a resistance against changing its movement away from what ever straight path thru spacetime it is taking at the moment. For example the accelerometer in our ears measures forces thru the liquid that wants to keep its initial movement and therefor is moving relative to our body if the body experiences a force. So you can actually feel a force working on your body.
AFAIK when you ‘feel’ a force you are feeling compression or decompression. This happens to all objects that are not a single dimensionless point, so therefore all objects. Even in a gravity field some atoms of an object will be closer to the center than others and experience it slightly differently. Of course that would be hard to measure, but that isn’t the point. Our mathematical approximations can’t distinguish which element is moving, but that is only because it is an approximation using dimensionless points that don’t exist. I could be wrong, so I’m curious how compression/decompression fits into this idea. Perhaps you can’t tell which atom is causing the compression, or which proton for that matter, but isn’t the whole body feeling a force if it is compressing?
Ok, maybe I'll look like an idiot here, but something is off in the thought experiment. When you're in a free fall, there is a spaghettification effect (your feet are slightly more attracted than your head) and thus, I can tell if there is a gravitational field around. I know that GR speculates that the "free falling object" is a size-less point. But then we cannot apply the reasoning of an apple (which has a size) and say it does not feel the acceleration. In the example of the passenger in a train, why does he feel the acceleration ? Because the train pushes the seat, then the seat pushes on the back of the passenger, etc. That's the equivalent of the spaghettification in acceleration. The reason why we don't feel like the passenger in a train when we are falling is because the gravity field is very smooth (almost continuously spread) and each part of the falling object is accelerating *almost* together at *almost* the same time. But that does not mean that there is not a theoretical/measurable difference.
When we measure systems to validate or develop theories of motion relevant to macroscopic objects do we ever involve anything other than electromagnetic forces now that we treat gravity as a curvature of space time? If not then is a relevant force just a deviation from some kind of electromagnetic equilibrium? Then is a frame shift actually a change in things like spin vector distribution but maybe other properties that add up to the appearance of special relativity?
These videos are great. My only question is when you say 'force', do you just mean gravitational force? As you reference General Relativity. Or do you mean ALL forces? For example, are you claiming that the Electro-Magnetic Force is a curvature in some Electro-Magnetic Field? Strong/Weak Nuclear forces, are curvatures in Strong/Weak Nuclear Fields? I'm just trying to wrap my head around the property of charge.
This video is regarding the classical definition of "force" as described by _Newtonian_ mechanics and the _Lorentz_ force for electrodynamics, so all contact and inertia (pseudo/fictional) forces on bodies with inertial and gravitational mass. The classical force was created with calculus and differential equation to determine causality, the action, change in momentum, and the path of point mass that deviates from a straight world line and being accelerated by a force. The video briefly mentions that in Quantum Field Theory, the concept of a "force" has been abandoned. The classical field theory of electromagnetism, _Special_ relativity, and _General_ relativity can be approximated using forces and field theory. The fundamental interactions and elementary particles (except gravity) are described as "excitations in quantum fields," rather than "curvature." Spacetime curvature versus flatness is something different.
None of this makes sense if we consider gravity a force. Gravity can't be a force, because for us someone freely falling due to gravity doesn't experience absolute acceleration. Only the relative kind. Absolute acceleration is what is responsible for time dilation.
omg I was always so confused when my teacher said f = ma was determined "experimentally" and was like "if it's measured in kg.m/s² because of the formula, how would he know before coming up with it how to measure it anyways?"
With any rotation isn't it plausible to know you're in an non inertial frame of reference, since distant objects appear to be moving faster than light? I agree with what you've said in the video, just confused with rotating reference frames.
Hey, thanks for watching! In GR, the speed of light is only constant locally; so in more global frames objects can move faster than the speed of light. This would likely indeed indicate a rotating or non-inertial reference frame. In this video here we suggest the possibility that force could be relative on a local scale, but on more global scales force is easily discerned through these sorts of fashions.
@@dialectphilosophy In which "global frames" can an object move faster than the speed of light? We are in a universe. in that universe, the speed limit is the speed of light (although it's actually the speed of causality ), PERIOD! BUT, we are in a expanding spacetime, so teorectically we would be moving at a speed higher than the speed of light. But it would be like an ant moving in a piece of elastic fabric. If the speed of the ant would be its speed plus the speed in which the fabric is expanding. You said "the speed of light is only constant locally". That is not true. Speed of light is the speed of causality and has the same value, unless it changes the medium, in this case, the photons interact with the new medium and it looks as if the speed is less than in vacuum, but this is well explained by Maxwell equations.
@@ranyeredt "Speed of light is the speed of causality" - Wrong. You can believe that philosophically, but the science does not show that. Science deals with observables only. It does not support claims about causation. Science is built upon reproducibility. It is built upon peer review. It is always up for adjustments when new theories are devised that provide better explanations than current ones. As such, it can not support claims about causation. "we are in a expanding spacetime" - in other words "I have bought into everything mainstream physics education tells me to think, regardless of how ridiculous it is". Inflationary cosmology is just a theory. It is still disputed somewhat. It is not absolute. Modern education is seriously lacking when it comes to teaching philosophy, logic, and skepticism.
This video is great and make a very compelling argument, but there is still one thing that confuse me. Let's say there is 2 boxes each with measuring instruments and one person inside in an otherwise empty universe, the boxes start next to one another with no speed relative to each other, then, the distance between the boxes start increasing faster and faster, let's say it increase like d(t) = t². In this situation as you point out, each of the boxes' passengers has an equal right to say that they are static while the other is accelerating or vice-versa, but! Let's now say that in one of the boxes, the passenger experience a constant push in the direction of the other box and the instruments on board read an "acceleration" a(t) = 2, meanwhile, in the other box, the passenger doesn't feel anything and the onboard instruments read 0. My question is, since acceleration is relative and each passenger has an equal right to claim that it is the other that is accelerating, where does the first box's passenger's experience and instrument's reading come from? How do we explain them? Does it have to do with space-time curvature?
We speak of "feeling" a force because we have the acceleration detection machinery built in, so it is something everyone is aware of, but obviously the phenomenon exists absent any knowledge of it. But in the imagined room with suspended ball, the person in the room would also experience the acceleration in other ways than observing the ball. The idea that it's impossible to know if your observer frame is inertial by local tests, as opposed to observation of other objects, is false. There are many reliable "blind" tests for acceleration.
2:46 I feel like this part needs to be elaborated more because it was confusing to me. To my understanding, you need force in order to measure inertial mass and passive gravitational mass (fictitious force for the latter perhaps). I can see that force is not needed for active gravitational mass, and if we can treat active gravitational mass and inertial mass as equivalents (because there's no evidence of deviation so far), then that part made sense. Though, I'm still unsure if that's what you were arguing.
In "Spacetime Physics", Taylor and Wheeler define an inertial reference frame as follows: "A reference frame is said to be inertial in a certain region of space and time when, throughout that region of spacetime, and within some specified accuracy, every test particle that is initially at rest remains at rest, and every test particle that is initially in motion continues that motion without change in speed or in direction." According to this definition, inertial frames are always local ones. An accelerating elevator in free fall can be an inertial reference frame.
Hey Dialect, I'm confused with this! Consider a man in rotation where he has nothing in the universe for relating with (you know!). Then according to him, he is still. But if his arms are lifted up, we could understand that the man is in rotation. Doesn't it mean that there is something that can be said relative to even in the nothingness (maybe the space itself!). What I mean is among all the possibilities, the man's arms are not lifted in only one scenario. And that is the case in which he is still (not rotating). So does the non-lifted arms mean that he is at rest relative to something eventhough we couldn't find any differences between any of the scenarios with our HUMAN PERCEPTIONS?
That's a great question -- it goes to the heart of Newton's Bucket Experiment / Mach's principle. When Einstein considered it, he felt that the lifting of the arms, if considered under Newtonian mechanics, could only be explained by adopting a "privileged space". But since such a space was not observable, he rejected the possibility of it and concluded that the cause for the lifting of the arms must lie outside the system. So to summarize, in the Einstein view, if the person's arms are lifting, then there MUST be something observable beyond them which they are rotating with respect to. A person in empty space could never rotate to begin with. As to the point you make about human perceptions, if you are asserting that there might be some hidden variable that is not measurable but which exists and which distinguishes local rotation from non-rotation, that is an interesting idea perhaps, but it might prove tricky building an empirical theory around.
@@dialectphilosophy Thank you very much, Dialect! Let me get some knowledge of what you've said above, as I have no idea about what is 'privileged space' or whatsoever. And a quick suggestion, if you add something behind your channel name, like 'Dialect - Exploring Science', that would be easy for your subscribers to search for your channel and that helps your channel to stand out among those creepy 'Language Dialect' channels, at least until you grow up(like VSauce!). And that's totally your wish, that's just a suggestion! Thank you very much!
If one imagines that to any single atom of your body is connected a infinitely thin and massles string and each string in turn is hooked, on the other edge, to an object that may accelerate, then your body will never experience the feeling of acceleration, because all atoms behave like a single object. That's in my opinion, how fields work.
"feeling" and "seeing" aside, surely by one pressing the throttle pedal you know you are mechanically applying "force" and causing accelerating and the other who is doing nothing is not given all else equal ... how would one translate this to absolute acceleration? ...
idk what about a *roller coaster?* like yeah i know that i am in it an can see stuff moving around me going faster, but I can feel being pushed into the back of my seat as it speeds up, or being pulled out of it from the centrifugal force when it spins.
Hi Dialect, I have seen all your videos and they are excellent... just what I need to think. I was upset with some other channels that misled me with the error of representing curvature of spacetime with the space time diagram tool. This video on Force is making me scratch my head however. If we are not 'feeling' a force when accelerating like in a centrifuge why are we feeling the spinning body pushing on our backs and compressing our bodies at high speeds? Another point is Centrifugal Force is considered a fake force, and there seems to be some kind of connection to Gravitational Force.. Is Gravitational Force fake too? To me there seems to be a similarity between Centrifugal Force and Gravity.
Indeed, Einstein deduced that all fictitious forces could be attributed to apparent gravitational fields; so yes, centrifugal and coriolis forces both are artifacts of accelerated observer motion, and thus are "gravitational" (though real gravity is what all observers will agree on, i.e. tidal forces). In this video we are examining more deeply the notion of what it means to truly be an accelerated observer. In the theories of relativity all observers will agree on who is truly accelerating (and it is such accelerated observers who will observe gravitational fields) but empirically speaking, absolute certainty of acceleration could never be possible for an observer, due to the relative nature of measurement-taking.
@@dialectphilosophy I look forward to more videos. Perhaps your take on Relativistic mass or maybe one on relativistic angular momentum would be interesting.
Profound stuff - sure to piss off the Star Wars fans! But seriously, you’re onto something here. Make some more videos, soon. All the best. A fellow astute thinker. 😊
The train example that you give is floored. It just shows the fallible human sensors are. If you asked a accelerometer what was happening it would tell you.
Yes, acceleration is one potential result of a force, but it can be resisted by a counteracting static force due to molecular forces causing friction and strain in the mass. So force is measurable. Force will change the velocity of mass if it is no restrained by an opposing force so again you can measure it my measuring the force required to prevent a change in velocity of the mass.
Great video. One of the few that tries to be rigorously logical. What I am confused about is compression and tidal forces. If someone if punched the area where there are punched becomes compressed. If I can tell where a force is applied doesn't that me a force was applied? In the same way one's back is compressed when a train accelerates forward..
To address your compression question: consider matter with two states, compressed and uncompressed, as measured relative to some standard. Now, the question becomes, is the compressed state the inertial rest state, or is the un-compressed state the inertial rest state? The only way to really know the difference is for there to have been measurements already made of the material's natural compression state in a frame known to be inertial. Thus you couldn't ultimately use something like compression to define what a non-inertial force is.
@@dialectphilosophy Thank you for addressing my question. You video is a real break through. I still do not think you answered my question. "The only way to really know the difference is for there to have been measurements already made of the material's natural compression state in a frame known to be inertial." Why can't I measure the service area of the object. Why can't I know if the surface area deformed? The point is. "The only way to really know the difference is for there to have been measurements already made of the material's natural compression state in a frame known to be inertial." We must have measurements of some kind or were couldn't identify the object or know that I exists. If you are talking about an unknown object or an unknowable object then your point becomes meaningless. They is something you aren't saying or some assumption that you have that I do not.
@@TomTom-rh5gk You’re probably overthinking it just a little. You certainly can make measurements of surface area, compression or whatever else you’d like. But those measurements would just be meaningless numbers if you don’t have an inertial reference system to compare them to.
@@dialectphilosophy No, I am under thinking it . There is something you haven't explained. You say, "But those measurements would just be meaningless numbers if you don’t have an inertial reference system to compare them to." 1. Inertia is the inertial reference system. Inertia is the most basic property of an object. Without inertia, you do not have an object. 2. Without inertia an object changes for no reason because It lacks identity. 3. Without inertia you have nothing. You can’t even say that something accelerated because there is no something. .. I think your explanation is missing something. Everything you said made so much sense that you must have a hidden assumption that you need to express in words.
I did not quite get why the accelerometer does not solve this? In another video you mention that it must be calibrated relative to something, which is true. But this is why we rely on fundamental constants to calibrate our SI base units to and assume these quantities stay constant as time progresses. Physics has to be useful for predicting outcomes of experiments, enabling technologies. It is not concerned with the "true" nature of reality as long as it is not measurable. For all we know we could as well just be a boltzmann brain. Philosophy is an interesting field in and of itself, concerned with questions like "the ship of theseus" and the like. It has been helpful to physics and technology by posing new questions to ask. But you'll have to remind yourself that physics is mostly just concerned with frameworks for technologically useful predictions, everything else is "extra". As Descartes once said "I think therefore I am" we have to make reasonable assumptions from which we can build our frameworks upon. There has to be some sort of value proposition to change views, albeit just an exploration out of curiosity.
From the beginning I was skeptical that if velocities are relative so should be acceleration. So am I feeling the force or the reference frame. is force even real?
This is a very interesting video, but I think you are missing one point. No object is really solid. All objects, even the most solid objects behave in a way like a spring. This means that when a force is applied to an object, it is usually not applied homogeneously on all its parts, but mostly on one part. So when a force is applied to an object, the acceleration is not applied instantly to all its parts, but applied first to one part, causing the object to shrink slightly. The shrinking increases the inner tension forces, which causes the rest of the object to accelerate at the same rate. Try imagining accelerating a spring by pushing it, and you will see what I mean. This shrinking is what activates your nerves, but you can technically measure it by other methods. You do not need the surroundings to know that a force is applied to you. In the example of the accelerometer in a closed room, you forgot that the person is going to feel a force due to the non-homogeneous acceleration applied to him. That is, his feet will accelerate before his head. Even if the non-homogeneous acceleration is too slight to detect by human senses, (like the example of the train) it is still measurable by more subtle devices. If you would apply the force homogeneously to all parts of the object, perhaps like with a charged body in an electric or magnetic field, theoretically you can create an accelerating frame of reference with real undetectable force.
You should be abke to still detect electrical forces by changing/transferring charge between the objects in your reference frame, which will cause force changes and acceleration changes in those objects. And for superficial charges, the force will be non homogeneous too.
Forces exist whether we feel them or not. Can you feel the magnetic lines of force acting on iron filings around a magnet or on a compass needle? Nevertheless, the force is empirically evident. No feelings required.
I love your video on the metric tensor so I arrived here. Quick side note: the fact that a force causes an acceleration is not metaphysics, that is physics as defined by Newton. 'Feel' is associated with the biological conception of sensation and perception. Biological organisms can 'feel' forces, but does an electron 'feel' the electromagnetic force from other charged objects? That is metaphysics. We know things 'experience' forces, thus things move.
Deduction of local orders of Kinematics means we assume our notion of space, velocity, acceleration etc have temporal-orders of 0,1,2,3,... Yet this is a local assumption. Imagine an observer that deduces what we observe as velocity (first-order) they observe as spacial coordinate (null-order). Would relativity be relative to this shift in order???
Wouldn't this make gravity and acceleration the same? Since gravity is the amount of energy which creates the force and accelerating is adding energy to move an object therefore making it the same as gravity but with a different kind of energy?
A single particle doesn't "feel" force, but multiparticle systems do. Same with entropy. It's an interesting perspective, but it seems to show we don't fully understand the physical world. Our best theories are know to be wrong, good mathematical models for our context, but still wrong.
A better question is that can you design a device that can detect if the current reference frame is accelerating or not and thus inertial or not, that way you can frame this video better than asking the question of whether we feel the acceleration of the frame of reference or not. The example with a pendulum inside an isolated room and the person deducing that there is a magnet behind the wall instead of deducing that the whole room is accelerating does not make a little bit of sense, I think you lost what you were trying to convey there as it is entirely possible to deduce from that event that the whole room is accelerating...
The very same kind of problem exists with mass actually, as you could always rescale it. In fact, in a world with only electric fields/interactions, there is no way to discriminate the (absolute) motions of similar q/m physical objects. This excess "freedom" should convince you that there is some kind of redundancy in these charges, or that there has to exist another fundamental interaction that would "split" those (as in, there should exist a physical experiment involving other interactions that are able to discriminate the motion of two similar q/m physical objects)
Please correct me. Inertial and non inertial frame is distinguished through pseudo forces irrespective of knowledge about the surroundings. And pseudo forces is "observed at most qualitatively", and this pseudo forces is stated in "experimental" !st law of Newton.
I am struggling to see the issue with thinking of acceleration as absolute. I don't think relative comparisons really need to come into it. It is a concept that stands on its own. You can add another object to your thought experiment if you like, and then see the implication for relative motion/position but it seems entirely optional and unnecessary for having a conception of acceleration. In my understanding acceleration is fundamental to the definition of mass. I define mass as energy that resists acceleration. Anything with mass, even the most fundamental things, are composite entities that involve interactions between parts such as the Higs field or the quarks and gluons in hadrons. Without such interactions, the unencumbered energy would travel at c and have no mass. In the photon box analogy, there is a pressure on the back side of the box when a force accelerates the object, which could just as well be a neutron, a quark, a person or a cheese burger. In all cases where there is mass there is something confining energy, preventing it from traveling at c creating an asymmetry in pressure when it is accelerated that resists said acceleration. I could be wrong but I have listened to your arguments multiple times and I have failed to be convinced that we need to think of acceleration as a purely relative phenomenon.
p.s. you can't directly measure velocity, energy etc. either. Physics is bootstrapping and we could "easily" replace it with others, whilst not having the foggiest about what's "really" going on. There is no escape from this
when you think about it, the only thing we ever really measure is distance and how it changes. Time? A clock has something that moves a certain distance at a regular time interval. Velocity? We measure the change in distance per unit time. Acceleration? We measure the change in velocity per unit time. Mass? A scale measures the distance a spring is compressed.
From what I understand proper acceleration is simply a mechanical force applied on the object, it may not even have a coordinate component. Just stress on the mass. Say in the context of general relativity when a vessel accelerates just to stay in one position in an orbital free fall.
A fun series that has allowed me to better clarify these issues in my mind - thanks. I do believe there is a simple solution to this and the Twin Paradox and will continue my work hoping to arrive at such a solution.
The "co moving" reference frame in which the CMB is the same temp in all directions really seems like a universal reference frame. Even though it's not. But, if you wanted to compare a point in space with literally everything else, it's about as good as it gets. Im probably wrong.😊
If something breaks due to internal stress during acceleration in a rocket ship, an observer in the ship would not necessarily be able to explain it. We can't assume a frame of reference to have any knowledge, only to be able to observe motion. To a "dumb" observer on the ship, the fracture would be spontaneous and spooky. To an "intelligent" observer the fracture is explained by an increase in internal stress, a concept invented by man. Therefore, concepts such as force and stress are relative, observer-dependent concepts and can't be real, objective truths about nature. I THINK that is is the point Dialect is making.
Force should be the first derivative of momentum, not simply acceleration, which does have an observable component namely it's conservation. This is to say inertia, or mass, itself. Inertia is the thing we 'feel' as we accelerate. For example when a proton colides with an object the majority of its mass is do to the internal 'collisions' of its quarks or rather to the exchange of gluons , creation and annellation of quarks and so on. Exactly the strong force. Also, for now most scientist assume gravity will be added to quantum mechanics not that strong, weak, and electromagnetic forces will be shown as illusory.
Defining force as the derivative of momentum is certainly no issues; but unfortunately it hardly resolves the problem. The conservation of momentum is a direct consequence of an inertial frame (i.e., in a non-inertial frame, momentum will not be conserved) so the definition only works so long as one is certain one's frame is inertial. To be certain one's frame is inertial means they have to be certain it is not accelerating... meaning they have to be certain the derivative of momentum is conserved... meaning they have to be certain their frame is inertial (repeat vicious cycle indefinitely...)
What about changing acceleration or accelerating changing acceleration? More philosophy…-ve speed or going slower than stopped. Accelerating into the stopped state. Figure it out!!!
