5 ms·

If anti-matter is regular matter moving backwards through time, it seems it should 'fall' upward.

by snarfy 8y ago

jetrink 8y ago
I'm just thinking through the implications of this... General relativity says that mass and energy are equivalent; an electron produces as much gravitational attraction in another particle as an equivalent amount of energy does. If matter and antimatter gravitationally repulse each other, then an electron and positron in close proximity will produce a near-zero amount of acceleration in a nearby particle, since the attraction and repulsion cancel. However, if they annihilate, producing gamma rays, then the gravitational acceleration experienced by that nearby particle will suddenly increase, since the energy of both particles will occupy the region previously occupied by the matter and antimatter. Is that correct? That sounds wrong. Why should the net gravitational "charge" of the gamma rays not be negative or zero?
- stcredzero 8y ago
  What you're thinking of is negative mass, not Antimatter. Antimatter has positive mass-energy, as far as we know, including empirical measurements. We've already operated Penning traps and stored minuscule amounts of antimatter.
gotocake 8y ago
It’s important to note the distinction between being able to model antiparticles as the time-reverses versions of their partners, and then actually “moving backwards through time” as it were. It’s not as though a sample of anti-hydrogen experiences a reverse or cause and effect. Discussion: https://physics.stackexchange.com/questions/391/is-anti-matter-matter-going-backwards-in-time https://physics.stackexchange.com/questions/391/is-anti-matt...
ridiculous_fish 8y ago
No, gravity is time symmetric! Imagine throwing a ball up, and watching it fall back to your hand. Run the clock backwards and it looks exactly the same!
- zeven7 8y ago
  But if you just hold out a ball and drop it, it won't look the same in reverse.
  - ridiculous_fish 8y ago
    It will! Imagine the ball right before it hits the ground: it will have a large downward velocity. Reverse time and the ball acquires an upwards velocity, that slows as it reaches your hand, exactly as if it's accelerating towards the earth. If you know some calculus, observe that Newtonian gravity is coupled to acceleration but NOT velocity: d^2 x / dt^2 = a = g Now run time backwards: let dt be negative. This doesn't change a, because dt is squared and squaring a negative is still positive. The acceleration is independent of the direction of time. 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.
    - zeven7 8y ago
      Ok, interesting. The gravity part makes sense. 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?
      - ridiculous_fish 8y ago
        In classical electromagnetism, we separate the electric field E and the magnetic field B. 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 where q is the charge, v is the velocity, B is the field. Reverse time and the velocity gets a minus sign, so F reverses. So E is symmetric and B is anti-symmetric.
zentropia 8y ago
But some particles are they own antiparticle. In which direction would they fall?
- TheOtherHobbes 8y ago
  They hover? 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 anti-matter is regular matter moving backwards through time, it seems it should 'fall' upward.

by snarfy 8y ago

If anti-matter is regular matter moving backwards through time, it seems it should 'fall' upward.

I'm just thinking through the implications of this... General relativity says that mass and energy are equivalent; an electron produces as much gravitational attraction in another particle as an equivalent amount of energy does. If matter and antimatter gravitationally repulse each other, then an electron and positron in close proximity will produce a near-zero amount of acceleration in a nearby particle, since the attraction and repulsion cancel. However, if they annihilate, producing gamma rays, then the gravitational acceleration experienced by that nearby particle will suddenly increase, since the energy of both particles will occupy the region previously occupied by the matter and antimatter. Is that correct? That sounds wrong. Why should the net gravitational "charge" of the gamma rays not be negative or zero?

stcredzero 8y ago
What you're thinking of is negative mass, not Antimatter. Antimatter has positive mass-energy, as far as we know, including empirical measurements. We've already operated Penning traps and stored minuscule amounts of antimatter.

gotocake 8y ago

It’s important to note the distinction between being able to model antiparticles as the time-reverses versions of their partners, and then actually “moving backwards through time” as it were. It’s not as though a sample of anti-hydrogen experiences a reverse or cause and effect. Discussion: https://physics.stackexchange.com/questions/391/is-anti-matter-matter-going-backwards-in-time https://physics.stackexchange.com/questions/391/is-anti-matt...

ridiculous_fish 8y ago

No, gravity is time symmetric! Imagine throwing a ball up, and watching it fall back to your hand. Run the clock backwards and it looks exactly the same!

zeven7 8y ago
But if you just hold out a ball and drop it, it won't look the same in reverse.
- ridiculous_fish 8y ago
  It will! Imagine the ball right before it hits the ground: it will have a large downward velocity. Reverse time and the ball acquires an upwards velocity, that slows as it reaches your hand, exactly as if it's accelerating towards the earth. If you know some calculus, observe that Newtonian gravity is coupled to acceleration but NOT velocity: d^2 x / dt^2 = a = g Now run time backwards: let dt be negative. This doesn't change a, because dt is squared and squaring a negative is still positive. The acceleration is independent of the direction of time. 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.
  - zeven7 8y ago
    Ok, interesting. The gravity part makes sense. 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?
    - ridiculous_fish 8y ago
      In classical electromagnetism, we separate the electric field E and the magnetic field B. 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 where q is the charge, v is the velocity, B is the field. Reverse time and the velocity gets a minus sign, so F reverses. So E is symmetric and B is anti-symmetric.

zentropia 8y ago

But some particles are they own antiparticle. In which direction would they fall?

TheOtherHobbes 8y ago
They hover? Or they just respond to gravity in the usual way - a fact that suggests the outcome of the experiment is unlikely to surprise anyone.