# What fundamentally is acceleration?

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1 minute ago, swansont said:

It’s true that an interaction is required to have an acceleration, but that should come as no surprise, as this is described by Newton’s first law.

Remember when I said I've usually gone off the rails early...

I think my fundamental realization of this is just that - acceleration requires an interaction, which requires more than one reference, and thus involves proper "real" quantities and effects.

Thanks!

Scott

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2 hours ago, sgabc123 said:

I believe that if I accelerated every particle in the universe, including your beverage, just not your car, you're spilling your drink, and nobody else notices.  It's still all relative.  The only reason we would never assume such a thing is because we know it's wildly impractical to accelerate the entire universe.

This is an argument similar to Mach's principle. I think scientific theories neither support nor refute it.

How would you even conceptually accelerate everything? You could for example use a uniform gravitational field throughout the universe. But then, you could detect gravitational time dilation between different points, where there currently is none.

On the other hand, could you use frame dragging to cancel out those effects? I don't know enough about frame dragging to say anything, except that it's conceivable to me that if you rotated the entire universe around your body, frame dragging might cause your arms to pull away from your body, ie. a possible way to make Mach's principle true.

I can think of two opposing ideas here. One is that physical quantities are typically defined according to things that can be measured. Eg. time is defined as what a clock measures. It might be okay to say proper acceleration is what an accelerometer measures? Another is that measuring devices generally are not perfect. For example if you apply force directly to the cantilever in an accelerometer, it says that the body itself is accelerating, which it isn't. A test for whether an effect is "real" might be, "Does this effect affect *every* measuring device, or just individuals?" For example, a pendulum clock on a ship might tick slowly, but that doesn't mean time is slowing down, because a cesium clock would not tick slowly. On a rocket traveling at relativistic speed, time really is slower than a stationary clock, because all clocks on the rocket tick slowly.

If you manipulated everything in the universe so that everything accelerated, but all possible things that can measure acceleration are adjusted so that they don't detect it, then practically I'd say that there is no difference between that and things not accelerating at all. Philosophically I think this is called empiricism. If there really is no possible way to detect a difference between two things, I'd say there's no difference.

2 hours ago, swansont said:

If I accelerate an electron - a fundamental particle - it will emit radiation. This does not happen with constant velocity. So no. This is fundamental.

Would an electron in freefall in a gravitational field radiate? I'm guessing it wouldn't, so it emits when it is properly accelerated? But that would still give you a way (eg. using specific gravitational fields) to accelerate things and have them not detect proper acceleration? Could you say that for practical purposes, proper acceleration is a locally measurable difference in acceleration (or force) applied to different nearby points?

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4 hours ago, md65536 said:

This is an argument similar to Mach's principle. I think scientific theories neither support nor refute it.

How would you even conceptually accelerate everything?

If that question was for me, I have no idea.  It was purely a thought experiment with no regard for how one could even theoretically accomplish such a thing.

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14 hours ago, sgabc123 said:

I feel like I'm challenging my betters here, but forgive me, I'm still hung up on this relativity thing.

I see that this thread is still going.

I gave you one just before you posted this, but you have not answered.

It really is very simple if you don't keep introducing all sorts of inappropriate ideas.

The free surface of a liquid in a container demonstrates the phenomenon of acceleration very simply.

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9 hours ago, md65536 said:

Would an electron in freefall in a gravitational field radiate? I'm guessing it wouldn't

This issue is notorious, and you will find large numbers of papers written about it, many of them arriving at diametrically opposed answers.

To make a long story short - yes, the electron in "free fall" does in fact radiate. The equivalence principle does not apply to electrically charged test particles, and when one does the actual maths for this (which turns out to be a surprisingly complex and subtle endeavour), one finds that the electron does not actually trace out a geodesic of spacetime (so it isn't truly in free fall), and is in fact surrounded by a radiation field.

Perhaps even more surprisingly, an electron at rest within a curved spacetime background (e.g. an electron confined in a vacuum tube at rest relative to earth) does not radiate, even though a comoving accelerometer would show non-zero proper acceleration. In some sense this is expected, since anything different would violate local conservation of energy; nonetheless, it is somewhat counterintuitive result.

This is one of those cases where common sense and intuition are at odds with GR, and one has no option but to work through the (extremely tedious, in this case) maths.

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4 hours ago, Markus Hanke said:

This issue is notorious, and you will find large numbers of papers written about it, many of them arriving at diametrically opposed answers.

I distinctly remember a thread where I was spectacularly wrong, but I can't find that one, just the thread about a charged ball in free fall from 2018.

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5 hours ago, Markus Hanke said:

Perhaps even more surprisingly, an electron at rest within a curved spacetime background (e.g. an electron confined in a vacuum tube at rest relative to earth) does not radiate, even though a comoving accelerometer would show non-zero proper acceleration. In some sense this is expected, since anything different would violate local conservation of energy; nonetheless, it is somewhat counterintuitive result.

This is one of those cases where common sense and intuition are at odds with GR, and one has no option but to work through the (extremely tedious, in this case) maths.

Actually, that does make intuitive sense in retrospect (cheating, OR if your common sense considers enough information).

The intuition is that an electron radiating EM energy, is associated with "change" rather than proper acceleration. An electron at rest relative to Earth isn't changing relative to an EM field. Instead it should be expected to radiate if you moved an EM field around it.

In freefall, the electron isn't changing in terms of inertia due to spacetime curvature, but it is (or can be) changing with respect to the EM field. So, no proper acceleration, but radiation is possible.

But I agree about the maths; intuition is useless if it doesn't reflect what the maths say.

Edited by md65536

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