Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 01/03/2018 20:12:59
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I found this MIT technology review article and wondered if the conclusions were actually valid.
https://www.technologyreview.com/s/419367/new-quantum-theory-separates-gravitational-and-inertial-mass/
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Interesting read.
Any particular aspect you wonder about, or is it the whole idea of questioning the equivalence principle?
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It will be interesting to see what comes out of it.
Some effects eg tunneling are only noticable at quantum level, this might be same.
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Interesting read.
Any particular aspect you wonder about, or is it the whole idea of questioning the equivalence principle?
I am just looking for opinions. I am interested in inertia in the quantum mechanical domain. This article caught my eye.
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If the relevance of the equivalence principle to QM is to be tested, wouldn't that require a theory of quantum gravity?
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If you can show functions that differentiate between inertial and gravitational mass then there may be experiments that can be performed to verify it without a quantum theory of gravity.
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If I recall correctly, we did find a way to differentiate between acceleration and gravity in another thread.
I've no idea how to find that now, but could find the distinguishing factors.
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I'd be interested to review that if you can find it.
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I'll give it a go, but "duty calls" at the moment, so it may take a while.
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If I recall correctly, we did find a way to differentiate between acceleration and gravity in another thread.
I've no idea how to find that now, but could find the distinguishing factors.
The usual way is that in a uniform gravitational field there are no tidal forces, but i think @jeffreyH knows that already. Would there be another way you know of?
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I vaguely remember the conversation. It wasn't about tidal forces. I can't remember if the idea was debunked.
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The quickest thing was to find the original notes from which I quoted in the other thread. What I posted then was probably an extract from this.
In closed windowless box we should be unable to tell the difference between acceleration and gravity, there should be no experiment we could perform that would give us that information.
Suppose you have with you in your box two marbles. Can you use these in an experiment to discover if you are being mechanically accelerated, or are stationary on the surface of the Earth? Surely you can, if you have a sufficiently sensitive measuring instrument.
Release your marbles simultaneously from the top of the box. They will fall to the bottom. If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, because gravity operates as though the entire mass of the gravitating object were at its centre; so the marbles will move towards the centre of the planet; thus they will converge as they fall.
Another experiment you could try would be to release one from the top of the box and one from waist height: under acceleration, they will maintain that separation until the first one hits the floor. Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase. This happens because the lower marble is closer to the centre of gravity than is the upper one, and as gravity varies according to the inverse square law the force on the marbles is different.
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@Bill S the experiment you suggest is the tidal forces one I mentioned. Jeff thinks there might have been a different one.
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Jeff thinks there might have been a different one.
Thanks Colin. I wish Jeffrey every success finding the other, I'd certainly be interested.
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I may not be remembering this properly. I made no notes. Was the other thread about the equivalence principle?
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A slightly related thread is this.
https://www.thenakedscientists.com/forum/index.php?topic=71619.msg526317#msg526317
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The quickest thing was to find the original notes from which I quoted in the other thread. What I posted then was probably an extract from this.
In closed windowless box we should be unable to tell the difference between acceleration and gravity, there should be no experiment we could perform that would give us that information.
Suppose you have with you in your box two marbles. Can you use these in an experiment to discover if you are being mechanically accelerated, or are stationary on the surface of the Earth? Surely you can, if you have a sufficiently sensitive measuring instrument.
Release your marbles simultaneously from the top of the box. They will fall to the bottom. If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, because gravity operates as though the entire mass of the gravitating object were at its centre; so the marbles will move towards the centre of the planet; thus they will converge as they fall.
Another experiment you could try would be to release one from the top of the box and one from waist height: under acceleration, they will maintain that separation until the first one hits the floor. Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase. This happens because the lower marble is closer to the centre of gravity than is the upper one, and as gravity varies according to the inverse square law the force on the marbles is different.
The problem with these types of tests is that they are being perform over a region (such as between the top and bottom of the box), But the equivalence principle only requires that they be indistinguishable locally (over a small enough region.)
Or let's put it another way. You have your box sitting on the Earth. As you pointed out, there will be differences when dropping objects from top bottom in it. But let's move this box to a planet with 4 times the mass and twice the radius of the Earth. You still have 1g of gravity at the bottom of the box. But now the difference between the results you get vs. an accelerating box are less apparent. (for example, the angle between objects falling on different sides of the box will be smaller.) We can keep moving our box further and further from our gravity source while increases the source mass to compensate. As we do so, the falling objects in the box behave more and more like those in an accelerating box.
This is the fundamental idea of the equivalence principle, that as you move your box further away from the gravity source, the behavior of the falling object approach the a "limit" in their behavior equivalent to that of an accelerating box.
Gravity and Acceleration are equivalent "in principle".
As far as the article goes, it fully depends upon when we can actually perform the experiments that will confirm the predictions one way or another. The exciting part is that it will finally gives us an idea of what needs to "tweaked" in order to get GR and GM into agreement.
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@Janus sums it up nicely. I did find the other thread. https://www.thenakedscientists.com/forum/index.php?topic=71799.msg528928#msg528928
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Some background reading.
https://en.m.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_experiment
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Two things that makes me wonder.
1. The article is about ten years old, where are the experiments?
2. What it refers to, the ' atom trampoline ', anyone found this 'experiment' done and described?
I can't find it?
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This?
https://physicsworld.com/a/quantum-trampoline-measures-gravity/
https://arxiv.org/abs/0911.0203
And btw :)
You're good Janus, and perfectly correct.
Still, I fail to connect those to " atom trampoline, in which a matter wave falls under the influence of gravity but is bounced by an electromagnetic force. " the article I found refer to " The technique involves firing vertical laser pulses at a collection of free-falling atoms, which bounces some atoms higher than others. " to then study " an interference pattern " created by them recombining.
What 'matter wave? And what " electromagnetic force." ?
I suppose you could call a 'photon' a electromagnetic 'wave packet' possibly?
And then to further obfuscate the subject define a atom as a 'matter wave', but why?
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Ok, we start with https://en.wikipedia.org/wiki/A_Dynamical_Theory_of_the_Electromagnetic_Field aka Maxwells equations, in where he deduce that light can be described as a 'electromagnetic wave' although without any charged 'particles' contained in it (light has no charge classically). From there we wander towards https://en.wikipedia.org/wiki/Matter_wave which describe the de Broglie hypothesis " De Broglie, in his 1924 PhD thesis, proposed that just as light has both wave-like and particle-like properties, electrons also have wave-like properties. By rearranging the momentum equation stated in the above section, we find a relationship between the wavelength, λ associated with an electron and its momentum, p, through the Planck constant, h" which then also includes atoms. And from that to my personal favorite John D. Norton's description of it https://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/quantum_theory_waves/index.html
So ok, call it 'matter waves' and 'electromagnetic force' but it still becomes obfuscating.
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To put it simply. If I'm a collection of matter waves over (in) time, what keeps me coherent? The 'particles' I consist of is a more understandable description to me, as in 'we consist of stars dust', and '(most) atoms lasts forever'. It's not that I find it wrong, just not the whole truth.
Ps: and yes, the first is a slight word game.
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Now, if you're an adventurous person you could connect this to 'many worlds' hypothesis :)
Just by allowing the Copenhagen interpretation to be valid, then assuming that a 'particle' won't 'exist' until 'measured', also allowing the matter wave to extend " Take a particle that we localize to just one place, so its matter wave is a spatially localized pulse. Left to itself, that pulse will spread out in all directions as propagating waves. It is just like what happens when a pebble hits the surface of the pond. The localized splash immediately spreads out in broadening ripples. That type of behavior is called "Schroedinger evolution," because it is governed by Schroedinger's wave equation.That equation just says that matter waves propagate like waves."
Just one thing missing here, the 'observer', as the 'wave' is nonlocalised until measured extending all through whatever there is, and as I suggest we ignore 'dimensions' for this, we can assume that the 'observer' is what makes it 'exist', and there is nothing stating that there must be one single observer, is there? Alternatively you might want to include, if adding 'dimensions', that to avoid a breakdown of 'internal logic' belonging to any sort of dimensional 'reality' you will need a 'many worlds' scenario in where the observer force a bifurcation when observing. You could make it even more intriguing by allowing observations to be what defines 'time', After all, without them, where is time?
But as I said, only one thing missing, the 'observer'. From where will you get it?
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If I recall correctly, we did find a way to differentiate between acceleration and gravity in another thread.
I've no idea how to find that now, but could find the distinguishing factors.
We discussed this topic in:
https://www.thenakedscientists.com/forum/index.php?topic=71799.0
Is that the one you recall?