If anti-matter would be found to be affected differently by gravity, then that would be a big problem for general relativity wouldn't it? If gravity is warped spacetime geometry, then it has to affect the path of all particles in the same way.
Is that correct? That sounds wrong. Why should the net gravitational "charge" of the gamma rays not be negative or zero?
Discussion: https://physics.stackexchange.com/questions/391/is-anti-matt...
Imagine throwing a ball up, and watching it fall back to your hand. Run the clock backwards and it looks exactly the same!
If you know some calculus, observe that Newtonian gravity is coupled to acceleration but NOT velocity:
d^2 x / dt^2 = a = g
To show this isn't just mathematical hand-waving: consider a magnetic field. This does couple to velocity. Reverse time, and the magnetic field pushes the other way. Electromagnetism is not time symmetric, it's time ANTI-symmetric. Gravity is different from EM in this respect.
I don't understand the difference with magnetism though. Let's say you have a positively charged item and you're standing on top of a large negatively charged surface. You "drop" the positively charged item. How does its velocity over time differ from the gravity example? And how is it different with time reversed? Seems like it would be the same to me... In forward motion, it would accelerate until it collided with the surface. With time reversed, it would start with a fast velocity and slow down over time.
Edit: I googled this. It sounds like the motion of magnetized bodies is time symmetric, but when the charge of the electric field produced by the motion is taken into account it has to be reversed. Is this correct?
If we isolate E, we get rest: electrostatics. Charged particles exert an attractive or repelling force in proportion to their charge, just like gravity, except charge instead of mass. So E is time-symmetric. This applies to your example.
Now consider just B. A positively charged particle is moving to the left, through a magnetic field that points inwards. By the right-hand rule, the force on the particle is up. Reverse time: now the particle is moving to the right, so you rotate your hand 180 degrees and the force is downwards. This magnetic field produced an upwards force forwards in time and a downwards force backwards in time: it's antisymmetric.
Put mathematically we have:
F = qv x B
So E is symmetric and B is anti-symmetric.
QM does so distinguish, for reasons way beyond me.
It would also have to be moving faster than c.
I'm basically arguing that anti-matter is anti-mass/anti-energy, which is not true but still fun to think about.
Relativity says:
E^2 = m^2 - p^2
Even in this view nothing moves faster than c. If we permit faster than c, then either E or M has to become imaginary to get a negative sign after squaring. Imaginary energy is even weirder than negative energy and is well above my pay grade; google "John Baez tachyon" for more.
Or they just respond to gravity in the usual way - a fact that suggests the outcome of the experiment is unlikely to surprise anyone.
If repulsion were time reversed attraction, then wouldn't you expect to see Earth launch into interstellar space?
The argument against this is you should end up with equal amounts of matter and anti-matter if that's the case - though I don't think it excludes the possibility we're just in a universe where the antimatter part of the loop was extremely rarefied.
And given that Feynman diagrams (and many other things based on them like Feynman path integrals, QED, and QCD) are the backbone of quantum theory, I think it's more than a safe bet that this view is widely accepted. One of the many predictions using them is the existence (and method of synthesis) of the Higgs boson.
The reason I said "maybe" is because it's important to have a distinction between models and reality (maybe bringing this up in this discussion was a bad idea). Physics is all about constructing models that approximate reality with strong predictive power, it's not really interested in what reality is but rather how it can be modeled so we can predict it. Obviously, the models have to be true on some level in order to be reliable, but many examples (like fields for instance) are not real objects.
So while saying "the particles are traveling backward in time" (theory == reality) is the most accurate statement you can make with modern physics, I would say "at the very least the particles act like particles that are traveling backwards in time" (theory >= model). Even if newer theories are discovered that invalidate the current theories the latter is still just as true of a statement -- otherwise Feynman's models wouldn't work and we know for a fact that they do.
Does that make more sense?
> a single particle oscillating through spacetime gives rise to all matter everywhere by doing the loop in indefinite cycles
This is the idea of the "one electron universe" (which attempted to explain why all electrons appear to be so identical that we've never detected discrepancies)[2], which was arguably one of the inspirations for Feynman's 1949 paper. To be clear, this is a non-standard interpretation of the implications of Feynman's ideas (though this was actually an idea pitched by John Weller on a phone call to Feynman very late at night). Like most interpretations of quantum mechanics, it doesn't make much of a difference to your life -- at the end of the day the maths is the same either way.
[1]: https://authors.library.caltech.edu/3520/1/FEYpr49b.pdf [2]: https://en.wikipedia.org/wiki/One-electron_universe
What are the implications for cause and effect then?
Stated differently: How does this affect entropy? Could a scrambled egg return to the state of an unbroken egg?
Now, this is a more general and complicated thing than entropy. Entropy is a statistical effect (and has very little to do with cooking eggs -- other than its link to thermodynamics). I can give you a short primer on entropy with an example:
You have 2 balls and push one so that it knocks the other one. Record it, then reverse the footage. In principle neither view of the event is more correct than the other (this is slightly untrue due to friction and the ball would appear to accelerate on the tabletop, but imagine it's a sufficiently frictionless surface for the purpose of this discussion). So motion is symmetric in time, right? As with most things in physics, the correct answer is "yes, but..."
But what happens when we add more balls? Imagine you have 100 lined up in a grid and you knock one -- you end up with a completely jumbled mess. Now reverse the footage, and a jumbled mess becomes 100 perfectly lined up balls. Which is the correct direction of the footage? Obviously there is a difference and you can tell what it is without me needing to explain it -- it's almost impossible that knocking a jumbled mess would just sort it! Why is it so improbable -- after all, isn't the motion just as time-symmetric as in the 2-ball example? Yes, but the problem is the statistics. What is the probability that a given jumbled set of balls will be sorted when you knock them? It's astronomically small.
In thermodynamics, these same statistics are the basis for what we consider entropy (imagine the 100 balls are countless numbers of particles). Effectively the idea is that because there are so many particles of matter involved in gas interactions, that the probability distributions are so skewed that you only ever see one macro-level result (that the ideal gas law is obeyed for instance). So while entropy may look like some fundamental force, it's a statistical effect that effectively guarantees that certain effects will happen.
Entropy is the thermodynamic arrow of time (and was the first thing that spawned the idea of an "arrow of time"), but there are other arrows of time. For instance, the universe is clearly expanding and thus there is a cosmological arrow (if you reversed the motion the CMBR would be different). There is a "weak" quantum arrow of time where some reactions violate CP-symmetry and thus by CPT theorem they must violate T-symmetry (the T in CPT being time).
I recommend (if you're interested) that you really study things more thoroughly, not just skim some material, as you plainly did. That way you avoid mixing up concepts like you are doing right now.
Not saying this to be rude, just found your comment rude.
As for what the parent said, it's quite clear. If it doesn't hold up, it's not because of lack of clarity:
"Repulsion is time reversed attraction so IIRC if antimatter is a time reversed normal matter then it should behave as a negative gravitational mass".
I.e. => seeing something being repulsed will be the same as seeing something between attracted played in reverse. (e.g. if we film a falling ball --attracted to Earth by gravity-- and play the video in reverse, it will look the like it's repulsed by the Earth and goes upwards).
Hence, the parent argues, if antimatter is "time reversed normal matter" (something often used as a description of antimatter, at least in layman terms), then it should behave like it is mass with inverse gravitational force.
Here lies the problem. If you play the video in reverse, the ball won't look like it's repulsed by the Earth. It will go upwards, yes, but only because of an upwards velocity. It will still look like it's trying to fall back. That is, it will still be accelerating downward.
Acceleration doesn't change under time reversal. It is velocity that switches signs:
https://en.wikipedia.org/wiki/T-symmetry#Effect_of_time_reve....
In this picture, what imparted the upwards velocity to it, given its starting point on the ground?
>> a falling ball [...] in reverse
> it will still be accelerating downward.
It seems strange to ask this of someone with your choice of username, but for the sake of anyone else who is reading along in this thread :
Which way does a freely-falling accelerometer point? [1]
(Ignoring the atmosphere.)
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(But if we don't ignore the atmosphere, in the limit where a smartwatch inside a dropped wall reaches terminal velocity, which way does the accelerometer point? If the smartwatch can move freely within the ball, where does one expect it at terminal velocity, and where does one expect it far from terminal velocity? cf. "vomit comet" [5]).
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[Now make the ball transparent to a broadcast of the accelerometer's reading. Who is "more right" about the interpretation broadcasted value: you standing below and observing it moving ever faster towards terminal velocity, or a skydiver who manages to tuck into a shape that allows her to remain alongside the ball-and-accelerometer for some time at the start of their descent? What do they make of your on-the-ground accelerometer as they are (a) far from terminal velocity, (b) as aerodynamics separates one from the other?]
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I agree that the value of the accelerometer on the ground with you does not change if we run a film of this experiment backwards; it's the only one that should remain effectively constant throughout.
As we take the anti-falling ball from resting on the ground -- can we agree its accelerometer will be at the ground-facing end of the ball, and that it will have the same reading as your always-on-the-ground accelerometer? -- to a drop balloon hovering kilometres overhead, what do you expect the direction of the accelerometer to be? How does the magnitude evolve, approximately? (In the human-time direction, experimenter lifts the ball off the floor of the gondola of the balloon, carries it to the door, extends her arm, and drops the ball. The ball falls and eventually crashes onto the ground right beside you. Let's pretend we absorb bounces and avoid bursts and breaks of the ball resp. accelerometer inside the ball.)
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> It is velocity that switches signs
(with the caveat that we are supposing that we are working with a family of systems of coordinates, and coordinate velocity, and a way for all parties to measure that; or that everything is moving slowly with respect to light and aren't aiming for maximal precision, so don't engage Pound-Rebka and similar results if we want to measure spectroscopically rather than kinematically (or astrometrically, per IAU [2]) [3]. More generally, velocity can get really complicated especially when we are dealing with a system in which matter's coupling to gravitation is what's under study.)
Taking a different view, Baez says, "In general relativity, we cannot even talk about relative velocities, except for two particles at the same point of spacetime -- that is, at the same place at the same instant. The reason is that in general relativity, we take very seriously the notion that a vector is a little arrow sitting at a particular point in spacetime. To compare vectors at different points of spacetime, we must carry one over to the other. The process of carrying a vector along a path without turning or stretching it is called `parallel transport'. When spacetime is curved, the result of parallel transport from one point to another depends on the path taken! In fact, this is the very definition of what it means for spacetime to be curved. Thus it is ambiguous to ask whether two particles have the same velocity vector unless they are at the same point of spacetime."[4] and I find it hard to disagree.
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[1] If you have an iPhone or iPad, I highly recommend https://phyphox.org/ and a case like https://griffintechnology.com/survivor and a visit to a grassy or snowy park where you can give your device a good hard throw (or e.g. climb a tall tree or form a human pyramid and drop it). Don't rely solely on gedanken experiments or search-engine results. Go measure! What a pity Einstein didn't have these tools for his jumping-off-a-roof, and that Newton didn't for his apple tree.
You'll want to consider https://phyphox.org/experiment/acceleration-with-g/#more-680
[2] Soffel et al. The IAU 2000 resolutions for astrometry, celestial mechanics and metrology in the relativistic framework: explanatory supplement. Astron. J. 126(2003), 2687–2706 ( https://arxiv.org/abs/astro-ph/0303376 ).
[3] Lindegren, Dravins. The fundamental definition of ‘radial velocity’. Astron. Astrophys. 401(2003), 1185–1202 ( https://arxiv.org/abs/astro-ph/0302522 ).
[4] http://math.ucr.edu/home/baez/einstein/node2.html
[5] For example https://www.youtube.com/watch?v=i2KrEH-8lJw https://www.youtube.com/watch?v=O9XtK6R1QAk
"spacetime" - math, [x,y,z,t]. "geometry" - math "warped" - math (transformation)
For example, OpenGL is used heavily to calculate pictures, which are looking close to reality, but we cannot use OpenGL to talk about reality.
Please, draw the line.
We can use math freely for predictions and to describe laws of nature. But we cannot use math to explain nature.
"Don't confuse the map with the territory".
His reasoning seems to be:
Math is used as a tool in physics to model reality.
But the mere use of math doesn't make such a model real. It could be still way off.
And his example is:
Similar to how we use math to model physical reality, we also use OpenGL (or 3D graphic APIs in general) to model real world objects (e.g. in movies).
Well, just like OpenGL doesn't correspond to some deeper reality (it's just a tool to make models, which could be totally non realistic), same is the case with math. It's just a tool we use to build physical models, it doesn't automatically lend legitimacy to those models (i.e. it's not like "this physics theory uses math so it must be correct representation of reality').
Yes, mathematical theories are merely a model of actual reality. But a model that produces accurate predictions is useful--even if we know it is not a complete model.
Even if we know it's not always accurate, we can model the inaccuracies of the model and constrain how we apply it. NASA knows Newtonian physics is less complete than general relativity, but they still use Newtonian to plot routes through the solar system because it's good enough.
So, sure, mathematical physics is not reality. But unless someone has a better tool than mathematical physics for making predictions, merely pointing out that fact is a pointless. Scientists already know it... that's why they do science!
Telling a scientist "your theory is not complete yet" is like telling a writer "you haven't written the perfect book yet." Yeah--they know.
EDIT to clarify that, while I'm replying to coldtea, my comments are really directed at what v_lisivka is saying in this thread.
v_lisivka, you're not wrong, but you're also not adding anything of substance to the conversation. "Physics is not a complete description of reality" is a critique can be dropped into any discussion of physics, which means it does not add any value to a discussion of a specific topic within physics.
It's like saying "you can't prove the sun will come up tomorrow" when someone is trying to plan a lunch date.
That's true, but that's not all the above says though.
It also says something else that _is_ actionable: don't just trust (or defend) a physics theory because it says it uses math or because it got its math right.
It could still be not just non corresponding with "real reality", but even get its predictions wrong.
You've got to fund somebody. So you fund the people with the math that looks good. Objecting to that is basically asking for unrestrained corruption. Crackpots certainly can't be relied upon to identify themselves.
However, I see no significant differences in position about QM between me and Mario Bunge, famous scientist. :-/
Everything is fiction at hypotheses generation phase. It will be science only when it will be confirmed by an experiment and reproduced by some one else and withstand critics. String theory requires extra-dimensions, which are never seen, while my theories are doesn't, so they are less fictional.
https://arxiv.org/abs/0902.2811
https://scholar.google.com/scholar?cites=9368943224139680400
https://academicjournals.org/journal/AJMCSR/article-full-tex...
https://scholar.google.com/scholar?cites=3459540491989485865
http://fs.gallup.unm.edu/PiP-2015-04.pdf#page=57
https://scholar.google.com/scholar?cites=2136327572232256000