Q&A - The Story of Spacetime - with Fay Dowker
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- Опубліковано 2 лип 2024
- If gravitational forces don’t exist, how can you explain acceleration? Fay Dowker answers questions from the audience following her talk on general relativity and the nature of spacetime.
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Fay Dowker is Professor of Theoretical Physics at Imperial College London and works on the problem of quantum gravity. Her research is based on the hypothesis that spacetime is fundamentally granular or atomic, and she has done numerous public lectures.
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lovely answers; if truth is beauty, then this is it.
In a sufficiently small region of space, over a sufficiently small period of time, there exists a frame of reference in which objects which have no forces acting on them will appear to be either stationary or moving at a constant velocity. This is known as an inertial frame of reference. In any other frame of reference, objects which have no forces acting on them can appear to accelerate without a cause. Near the surface of the Earth, from the perspective of someone in an elevator that is in free fall, over a short period of time, objects in the elevator will appear to behave as they would in deep space away from any gravitational influence.
Objects on the surface of the Earth appear to accelerate downwards when let go because we are not in an inertial frame of reference. This is similar to how if an object were let go in a spaceship that was accelerating in deep space, we would perceive the object to accelerate towards the bottom of the spaceship.
In the neighbourhood of a massive body like the Earth, due to it's gravitational influence on space and time, there is no universal inertial frame of reference in which objects with no forces on it would behave as they would in deep space. In each local region of space for a short duration, there exists a frame of reference which approximates to an inertial frame. This is analogous to the fact that when you draw geometric figures in a small region on the surface of a sphere, the properties of the figures are the same as if the figures were drawn on a flat surface.
The lady's phrase that the earth is the one which move or pushed toward the object with acceleration 9.8 m/s bears some weirdness, because she said the object is free no force exert on it, but she did not explain what is that force which drive the earth with acceleration toward the object ?
On the surface of a massive body, such as the Earth, the matter content of the body (the electromagnetic interaction between the body and ourselves) prevents us from undergoing an inertial motion (free fall). This means, we are in a non-inertial frame of reference by virtue of the fact that we are not in free fall. Each location on the surface of the Earth is in a different non-inertial frame of reference.
Imagine you're in a rocket accelerating in deep space. If you drop an object, it will appear to fall towards the ground. This is because you are in a non-inertial frame of reference while the rocket is accelerating. On the surface of the Earth, dropped objects appear to fall towards the ground because you are in a non-inertial frame of reference.
"All the time excists all the time", but practically we can not go back in time and change the past events, for example we can not go back in time to stop the process of trapping electrons to nuclei that composed helium and hydrogen as was the beginning of formation of atoms of lightest and heaviest elements in the universe.
Dr. Dowker's explanations of the microphone "falling" through spacetime, and her answer of how "everything else is accelerating up rather than the 2 objects accelerating down toward earth" are not good answers. There are unexplained issues in her answers, that make them incomplete.
For example, in the first question about the microphone falling, her answer said that the microphone would move through time, but would ALSO move through 3D space (following the shape of space - which would be curved due to earth's gravity). So if the microphone were stationary in spacetime (not near earth - so not in any curved space), then her answer again says it would "move" through both spacetime -- thus moving through time (yes, normal) but it would also move through 3D space. It does not make sense why it would move through 3D un-curved (non-gravitational) space.
In a sufficiently small region of space, over a sufficiently small period of time, there exists a frame of reference in which objects which have no forces acting on them will appear to be either stationary or moving at a constant velocity. This is known as an inertial frame of reference. In any other frame of reference, objects which have no forces acting on them can appear to accelerate without a cause. Near the surface of the Earth, from the perspective of someone in an elevator that is in free fall, over a short period of time, objects in the elevator will appear to behave as they would in deep space away from any gravitational influence.
Objects on the surface of the Earth appear to accelerate downwards when let go because we are not in an inertial frame of reference. This is similar to how if an object were let go in a spaceship that was accelerating in deep space, we would perceive the object to accelerate towards the bottom of the spaceship.
In the neighbourhood of a massive body like the Earth, due to it's gravitational influence on space and time, there is no universal inertial frame of reference in which objects with no forces on it would behave as they would in deep space. In each local region of space for a short duration, there exists a frame of reference which approximates to an inertial frame. This is analogous to the fact that when you draw geometric figures in a small region on the surface of a sphere, the properties of the figures are the same as if the figures were drawn on a flat surface.
Maybe I am late to the party, but your observation is right. The answer to the first question was not good. The last answer, however, was the right one for both questions and spot on. It was very important to explain that everything we deem to be still standing (we, the floor, the room we are in, all the furniture) is actually accelerating all the time upwards at 9.81 m/s2. So if we hold an apple (less expensive than dropping a microphone ;-), the apple experiences the same acceleration as we do and we can feel it by the weight of the apple, which is the force we excerpt on it. Once we release it, it is not accelerated any more, but we still are and we (as is the floor) are accelerating away from it upwards. This makes the apple look like it is falling down to the ground. Imagine we would jump off the tower of Pisa (just as an Gedankenexperiment of course) with the apple in hand and then release it. Now we and the apple would travel without acceleration and there is no relative movement and none of us would feel a force.
There is a next question, however, if we are accelerating all the time, why don't we get anywhere? E.g. If we are accelerating with 9.8 m/s2 up wards and the people on the opposite side of the world too, then we would travel apart with an acceleration of 19.6 m/s2. We do not see that. The distance between us remains the same.
That's is where the spacetime curvature comes in. Spacetime curvature means that space is not static in time. Here on earth it is constantly 'flowing' inwards to the center of the earth. So, when the apple is released it follows this flow of space, we 'resist' it because the floor pushes us constantly up. On the opposite side of the world space if flowing in from the other direction. This is the reason that although we are accelerated in opposite directions, the distance between us doesn't change.
All this sounds strange and incredible, yet it makes perfect sense in general relativity and the many predictions of GR (Gravitational waves, time dilation in gravitational fields, gravitational lensing,. etc) shows that it is indeed a more complete description of gravity then Newton's theory. But it is not the end point as the question of quantum-gravity is still unresolved.
please add subtitles
+UniQuE TV well, for many people it's easiest read in english than hear to somebody speaking.
+Alexis Gamboa We would absolutely love to but with such a small team we just don't have the time to transcribe all of our videos. This is something we'd very much like to do and we have attempted to provide them for all of our short videos but currently there's no ETA on when the longer ones get transcribed!
Well, now if the thing the reality is that the 2 objects falling from the leaning tower of Pisa have no forces on them & that they're absolutely free and that every thing else is in fact accelerating upwards,..... then why don't we start teaching GR to kids of 2nd and 3rd grade??!! why are we still teaching 'em that Newtonian gravity is the way things are??!!!
Education is a learning process taken in steps. Once you have a grasp of the basics you can move on for advancement. Were you so precocious at such an early age?
The math required to actually use GR is way too complex; requires several years university mathematics and physics. Newtonian mechanics describes and predicts motions extremely well, generally (even if premised on incorrect assumptions) and so may be used as the basis for studying/predicting motions under many circumstances. I assume that most new physics texts at the high school level have some explaination outlining the very most basic theory of GR and the existence of spacetime curvature but explain that for the purposes of courses at this level the Newtonian laws/equations of gravity/mechanics are perfectly adequate.
I think we are fixated on a historical framework to education. Where we are able it always seems our teaching of concepts has a timeline imposed on it. Though we do need the math from earlier concepts to build on there is no reason that is or must be the only way. Philosophical approaches do not work that way but we are less happy with them as scientists because they are not inherently testable or necessarily falsifiable. That doesn’t mean the historical process is the only one applicable, we just do not know or all agree on an alternative. The presenter’s use of meditation to temporarily unlearn or suspend our experiential reality alludes to this quite wonderfully.
We need a new sense of reality and it seems this includes the embracing the discontinuous nature of spacetime.
I'll just add to some of the replies here, that classical Newtonian mechanics is still necessary to learn b/c the theory is still quite useful and applicable in many engineering fields. In the approximation of flat spacetime and motion with velocity much less than speed of light, Newton's Laws work extremely well and offer accurate results. In fact SR and GR reduces to Newtonian equations in the low energy approximation.
I hope science teachers are NOT teaching children about the "force of gravity" but explaining that Newtonian calculations can still be used in approximation even though gravity constitutes curved spacetime in reality. (Dot)
So the proposal that the Earth is accelerating upwards at 9.8m/s2 - that presumably extends to our solar system too? And our galaxy? And most everything else we can observe in space? And it's only when objects are temporarily freed from this upsurge, by being put into an independent or free state, that we appear to see them as 'falling down' or subject to 'gravity'? That is one hell of a mind flip!
+3enjoy3 literally insane. i can't believe she and probably many other scientists believe that rubbish.
+viermidebutura so if a plane is flying parallel to the earth's surface, why doesn't the earth accelerate into the plane?!
viermidebutura so i think i understand the distinction between 3D space and 4D space-time a tiny bit better, but if the earth is still accelerating towards the plane in 3D space, the must then be constantly flying upwards? this to me is ludicous lol
+3enjoy3 In special relativity you can convert between non-accelerating (inertial) frames by a Lorentz transform. In general relativity, where spacetime is not flat, the closest you can have to the inertial frames of special relativity are the frame of a geodesic observer, i.e. the objects in free-fall. So frames of bodies that are NOT in free-fall are not good analogues to non-accelerating frames in flat space.
Locally gravity looks like acceleration, so an object that is fixed at some (not very large) distance from some (spherically symmetric) matter density must be accelerating outwards.
+Jason93609 Excuse my non-scientific mind by asking this but does it mean then that all celestial bodies are subject to the same upwards acceleration? I'm trying to picture the wider effect. Is 'upwards' not a concept then? You say accelerating outwards.
E = mc2 - interesting if c equals zero.
UniQuE TV
Google: *Scientists stop light completely*
At this point E = mc2 becomes 0 = m
The end of space-time and the beginning of something amazing which will be later known as singularity.
In the future science and religion will merge. Science will explore the technological usability, while religion will explore the cohere of consciousness and the singularity.
This will lead to beaming, a complete new type of space-traveling (through a singularity), relativize death, allow to see the world through the eyes of any living being (most probably any being ever lived) and new weapons of course.
.
UniQuE TV Yes, c is a variable for the speed of light. If the speed of light becomes 0, e.g. at absolute zero, then there is no movement at all, no movement = no time = no space. At which all matter fits into a infinite small point - a singularity.
Also see: *Bose-Einstein condensate*
That's the nature of our universe - infinite relative infinities. The question is - if there is no matter at all - what the heck do see then?
+UniQuE TV i think he's serious lol
+UniQuE TV c = cat
c by definition is the speed of light in a vacuum - thus c is always the same universal constant. Of course light traveling through other mediums can slow down to speed less than c. In the energy-mass equivalence E=mc^2, you cannot set c to zero, that makes no sense b/c the equation is meant as an expression of variables. The relationship that Einstein derived means that a particle with rest mass m also contains energy E, where their relationship is given by E=mc^2.
So.....when objects are dropped, the tower, the ground and everything else is accelerating upwards at 9.8m/s2: and if two objects are simultaneously dropped from another tower on the other side of the planet, everything is also accelerating upwards - but in the opposite direction. Really?
+horatiobottomley Oh, how I would love to have heard her explain that!
+horatiobottomley Acceleration and gravity are interchangeable (*locally*), that's just the equivalence principle.
Jason93609 Can you elaborate?
Je Suis Ce Que Je Suis If you are standing on the surface of a spherical gravitating object the direction that you call up-down will change depending on your position on said sphere.
So at any point on that sphere the frame is locally equivalent to an accelerating frame, one that is accelerating outwards.
The fact that you cannot distinguish gravity and acceleration locally is the equivalence principle.
Jason93609 Thanks, but not at all sure I follow. If the dropped object doesn't fall then surely according to the scientist the Earth accelerates towards it. And being a sphere (sorry flat earthers) that would require it to expand at the rate of 9.8 m/sec^2, which is .. I think the technical term is ... bonkers. If the Earth is simply exerting a gravitational force upon the object then the object falls towards it, contradicting what she said. What am I missing?
👍👍👍👍👍
everything touching the ground is accelerating upwards at 9.8m/s squared? wow. that's literally insane.
+tonyfalca Forget relativity for a second, and just think of good old Newtonian mechanics for the following. Let's imagine two observers, A and B, in otherwise empty space. Now there is a force that acts locally on A but not on B.
In B's rest frame A will accelerate as a result of that force. However, you can always mathematically describe B's trajectory in A's rest frame. In that case, B will be the one accelerating. However, bearing in mind that the force in question did physically act only on A, which one of the two frames, ((1) the one where A is accelerating or (2) the one where B is accelerating) provides a 'natural' description of the situation?
Jason93609 i would say (A) provides 'a "natural" description'
Well, in that case, the situation with gravity is the lame. The frame in which the ground is at rest is the one where B is accelerating (even though A would be the one pushed by the force in question) and the rest frame of falling objects is analogous to the one where A is accelerated and B is at rest.
So the frame of the surface of the earth looks (within a sufficiently small region) like an accelerated frame - a manifestation of the equivalence principle.
When we think of falling bodies as, well, falling bodies we are implicitly assuming the frame in which the surface of the earth is at rest. But observers standing on the earth are the ones to feel an upward force, whereas freely falling observers do not feel any force (before they impact the ground).
Of course all this does not imply that there exists a useful frame in which the surface of the earth accelerates in all directions (the effect only makes sense locally after all), what happens is that the mass of the earth is the source of the earth's gravitational field and that field is described in terms of the geometry of spacetime.
Jason93609 yeah, well said. i get that much, which makes sense because motion is relative, right? even though the earth's surface might be accelerating, from one frame it can be said that the falling object is accelerating or moving if it is said that the earth's surface is at rest. or am i completely off?
tonyfalca Essentially the point is that unlike inertial frames, accelerating frames may not all be equivalent. An observer standing on the earth may be taken to be at rest but at the same time there is a force acting on the feet of such an observer (so in order to make the observer appear at rest means that the frame itself is accelerating).
earth is rotating around sun because of spacetime curvature. Then it should eventually fall at the centre of curve but its not happening. what could be the possible explanation for this?
+Anurag Joshi because there is essentially nothing in the way, which takes away energy from the earth... so why should it fall into the sun? the thing is... in a circular motion, you may fall all the time, without hitting anything... falling doesn't mean you have to hit something... just try to seperate the problem into small pieces of time... without forces, objects move in a straight line... as the earth would do, if there was no sun... which would mean, it would fly away in a tangantial way (the speed it has in a certain moment on it's way around the sun)... but since there is the gravitational force of the sun on the earth, it pulls the earth a little bit towards it, while the earth actually wants to fly away in a straight way along its current direction of movement... this force is directed perpendicular towards the sun... which forces the direction of the earth's movement a little bit into the direction of the sun... but not straightly towards it... it's like the string example she had... the earth is forced towards the sun all the time a little bit, but actually it just wants to fly in a straight way.... so it's not flying in a straight way and not directly into the sun, but it orbits the sun in a circular way...
so, again, why does the microphone fall downwards every time it's dropped? why doesn't it sometimes fall upwards? because that's its natural path? that doesn't explain anything...
+tonyfalca As Dowker tried to explain in the last 60 seconds it doesn't fall downwards at all - the earth is _falling upwards_. That won't make sense I know. Einstein's thought experiment using elevators (lifts if you're english) goes a long way to explaining it - have you come across that before?
***** No, I haven't. Can you provide me a link to an accurate account?
I have, however, been discussing this with a couple of other people, and this is what they also have been explaining.
+tonyfalca Having thought about this a bit, you're best bet is Einstein's space rocket rather than elevators.
1/ Imagine an empty universe with just you in a space rocket, engines off not moving. You're just floating around inside the rocket which is pointing upwards.
2/ Now switch on the engines. Pretty quickly you'll find yourself on the floor of the rocket as it accelerates upwards into you.
3/ With the engines still running you'll find that you can actually stand up. If the rocket is accelerating up at 9.81 m/s^2 it will feel exactly like you're standing on earth. If you were to stand on some bathrooms scales you'd find that you'd weigh just the same.
4/ Einstein's 'genius' idea: if it walks, swims and quacks like a duck - it's a duck. If it feels like gravity then that's what gravity is.
Does that make sense?
***** that's a great analogy. really simple. easy to understand. but it begs another question that somebody else posed elsewhere in this comment section: if the earth is accelerating 'upwards' (or in one direction), why would two people on earth, each placed at positions that are polar in relation to one another, both have the exact same experience of gravity? or is the earth also 'expanding' (like a balloon being inflated) and accelerating, similar to how the universe is supposedly expanding?
+tonyfalca Now we get to the difficult bit!
1/ Any object that has mass has inertia. It will continue as it is unless some force acts on it.
2/ Let's put you back in space, only this time without your rocket. You're not moving (actually not moving is a very interesting discussion but let's stay on topic)
3/ Maybe you're not moving through space but you _are_ moving through time. So now we need to think about space and time being one and the same thing. Think of your position at any given moment in 4 coordinates x, y, z and t. At rest x, y and z don't change but t - time marches on and so t always increases.
4/ If we think about 3D space for a moment rather than 4D spacetime, if any one of the coordinates x, y or z is changing and the others aren't we are moving in a straight line - right? So in 4D spacetime, with only t changing, we must be travelling in a straight line too.
5/ Einstein guessed that massive objects bend spacetime. The more mass, the more the object bends spacetime.
6/ If now we send you at a constant speed through space so that you pass close to earth, because you have inertia, you're going to continue in a straight line though space time in a straight line.
7/ But because spacetime is curved as the earth is pretty heavy your straight line through spacetime will also be bent. That's what we see when comets pass by.
8/ But, and here's the clutch, if you're not on a fly-by but rather floating a mile above the earth's surface, at rest with respect to the earth, _you are still moving in a straight line through space time_.
9/ Spacetime is curved, not only in x, y and z but also in t. Your straight line through spacetime will be bent towards the earth and down you go.
10/ Finally we need to change my original statement a little. Both the earth and you are massive objects, so you're both warping space time. When you're standing at rest on the earth's surface both you and the earth are still moving through spacetime - a spacetime that you are both warping. You are trying to accelerate into each other.
Does that make sense? It's certainly not an easy concept to grasp. If not, let me know which step you're getting stuck on and I can drill down in a bit more detail.
that last one was a ... funny non-answer
No, it's actually the right answer. The ground is really accelerating upward at 9,81 m.s-2
She didn't explain Einstein's Equivalence Principle or the observers and frameworks parts that underly it.
It would take a much longer show. The Planck University has a nice explanation in their Special Relativity pages.
Bottom line, gravity and acceleration are equivalent at least locally.
In a sufficiently small region of space, over a sufficiently small period of time, there exists a frame of reference in which objects which have no forces acting on them will appear to be either stationary or moving at a constant velocity. This is known as an inertial frame of reference. In any other frame of reference, objects which have no forces acting on them can appear to accelerate without a cause. Near the surface of the Earth, from the perspective of someone in an elevator that is in free fall, over a short period of time, objects in the elevator will appear to behave as they would in deep space away from any gravitational influence.
Objects on the surface of the Earth appear to accelerate downwards when let go because we are not in an inertial frame of reference. This is similar to how if an object were let go in a spaceship that was accelerating in deep space, we would perceive the object to accelerate towards the bottom of the spaceship.
In the neighbourhood of a massive body like the Earth, due to it's gravitational influence on space and time, there is no universal inertial frame of reference in which objects with no forces on it would behave as they would in deep space. In each local region of space for a short duration, there exists a frame of reference which approximates to an inertial frame. This is analogous to the fact that when you draw geometric figures in a small region on the surface of a sphere, the properties of the figures are the same as if the figures were drawn on a flat surface.
@@clemja28 It is?
So, if I and a group of my friends around the globe all arrange to drop balls simultaneously, they are all floating while the Earth is expanding at 9.8m/s/s in all directions to meet them?
Makes no sense.
@@atallguynh In a way yes. The earth is expanding in a contracting space over time.
funny how everyone is laughing at the answers because that’s exactly what we should be doing
There are two kinds of reference frames, inertial and non-inertial. In an inertial frame, you experience no fictitious forces, whereas in a non-inertial frame you will observe fictitious forces acting on local things. When you are in a non-inertial frame, you are accelerating with respect to a local inertial observer. There is no way to know your velocity using any local measurement other than relative to some particular local object, but it is possible to distinguish between whether you are in an inertial or non-inertial frame of reference.
Each local patch of the Earth (considered separately from the rest of the Earth) is accelerating upwards with respect to someone who is in free-fall within that patch of the Earth. With respect to someone on the surface of that patch of Earth, there is a fictitious downwards force acting on everything around them including themselves. However, unlike the person standing on the surface, the person in free-fall does not experience any of the effects of acceleration. It only makes sense to talk about the magnitude and direction of acceleration of each local patch of the Earth with respect to a local observer; just like it only makes sense to talk about "up" and "down" with reference to each local patch of the Earth.
If you got on a spacecraft in deep space that accelerates at 9.8 ms⁻² with respect to someone floating just outside the spacecraft, then everything you experience within the spacecraft will be indistinguishable from what you experience on the surface of the Earth, including the fictitious forces that makes everything appear to fall towards the bottom of the spacecraft.
In the neighbourhood of a massive body like the Earth, due to it's gravitational influence on space and time, there is no universal inertial frame relative to which objects with no forces on it would behave as they would in deep space. Rather, in each local region of space for a short duration, there exists a frame of reference which approximates to an inertial frame and this corresponds to a local observer in free-fall. Every small chunk of the Earth and everything on it is accelerating upwards (relative to a temporary local inertial frame), due to the forces acting on it from the chunk of the Earth underneath it.
That last theory was such a load of bull..
There are two kinds of reference frames, inertial and non-inertial. In an inertial frame, you experience no fictitious forces, whereas in a non-inertial frame you will observe fictitious forces acting on local things. When you are in a non-inertial frame, you are accelerating with respect to a local inertial observer. There is no way to know your velocity using any local measurement other than relative to some particular local object, but it is possible to distinguish between whether you are in an inertial or non-inertial frame of reference.
Each local patch of the Earth (considered separately from the rest of the Earth) is accelerating upwards with respect to someone who is in free-fall within that patch of the Earth. With respect to someone on the surface of that patch of Earth, there is a fictitious downwards force acting on everything around them including themselves. However, unlike the person standing on the surface, the person in free-fall does not experience any of the effects of acceleration. It only makes sense to talk about the magnitude and direction of acceleration of each local patch of the Earth with respect to a local observer; just like it only makes sense to talk about "up" and "down" with reference to each local patch of the Earth.
If you got on a spacecraft in deep space that accelerates at 9.8 ms⁻² with respect to someone floating just outside the spacecraft, then everything you experience within the spacecraft will be indistinguishable from what you experience on the surface of the Earth, including the fictitious forces that makes everything appear to fall towards the bottom of the spacecraft.
In the neighbourhood of a massive body like the Earth, due to it's gravitational influence on space and time, there is no universal inertial frame relative to which objects with no forces on it would behave as they would in deep space. Rather, in each local region of space for a short duration, there exists a frame of reference which approximates to an inertial frame and this corresponds to a local observer in free-fall. Every small chunk of the Earth and everything on it is accelerating upwards (relative to a temporary local inertial frame), due to the forces acting on it from the chunk of the Earth underneath it.