# B Question about time and particle interactions

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1. Jun 29, 2017

### MrDickinson

First, I am new to physics and only taking my first course in calculus based classical mechanics with topics covering thermodynamics and an introduction to general and special relativity.

Everything here is pretty much a question, even if periods exist and not question marks.

My teacher had made a statement about two particles interacting with each other while approaching the speed of light, but this really confused me because my teacher stated that everything around an individual approaching the speed of light speeds up.

Here is my dilemma:

Supposing that we have two particles, A and B. Supposing that A moves at .99c. If this is true, then both particles experience time differently; that is, the particles exist in different states of time.

It seems impossible for the present state of particle A to interact with the present state of particle B as both exist in different dimensions of time or different states of time.

If any interaction between B and A is possible, such interaction can only occur such that effects of particle A on B or the effects of particle B on A can only change a future or past state, but neither particle can cause a change in the present state of the other particle?

How is it possible A to measure any real quality of a particle B or vice versa, when A and B exist in different states of time or different dimensions of time. Isn't it only possible to measure the present-real quality of an object when both A and B are in the same state of time?

I had thought that perhaps the following had some relationship to my current inquiry:

If we look at a star that is some light years away from us on earth, x light years away. We observe the star not in its current state, but in its past state. If we could somehow grasp what we see, the light from the star, and cause some change or measure some quantity, any outcome we force upon the star is an outcome forced on a past state of the star, nots its present.

Remember, I am not saying that we can reach out and grab the star, only that we can reach out and grab what we see, which is the star in its past state. If two particles are experiencing time differently, then the real-current state of either particle can be altered. Only present or past states can be altered.

Thanks

Last edited: Jun 29, 2017
2. Jun 29, 2017

### Staff: Mentor

Either you misunderstood what he said or he was expressing himself rather badly. That's not how time dilation works... and to see this you might consider that if I am moving at close to the speed of life get relative to you, we could just as easily say that you are moving at close to the speed of light relative to me, while I am at rest.

As long as the two particles are at the same place at the same time, they can interact.

You are right that when you see a star $x$ light-years from earth, the light that is forming an image on the retina of your eye left the star $x$ years ago, and therefore the image is that of the star as it looked then, when the light left the star and not now when the light reaches your eye. There are two interactions here: first, one in which the star emits light; and then much later one in which the light interacts with our eyes. Nothing we do when the light reaches our eyes can affect the star in any way, any more than anything I do when I open a letter could affect the post office from which it was mailed.

3. Jun 29, 2017

### MrDickinson

The problem that I see is that they can never be at the same place at the same time because time is experienced differently for each particle? What I mean is... how can two particles be in the same place at the same time if both particles experience time differently?

The only interaction that seems possible is interaction that alters the past or future state of a Particle B or A, but not their real-present state....

This is quite confusing..

4. Jun 29, 2017

### Mister T

Are A and B moving relative to each other, or relative to you?

If you look at your hand from about 3 feet away you don't see it, you see what it looked like 3 nanoseconds ago. If you blink you prevent that light from entering your eyes, but that doesn't make your hand disappear and it certainly doesn't have any effect on what happened to your hand 3 nanoseconds ago.

If your teacher tells you things that make no sense, try to find out how we came to know those things. Physics is a study of phenomena, so to make sense of it you have to connect what you learn to the actual phenomena. So, for example, when we say time flows more slowly what are some of the things that actually happen to make us think that's so?

5. Jun 29, 2017

### MrDickinson

If A and B are moving relative to each other, we might say that their interactions are similar to A being on a train and B being on the ground. I wanted to understand the motion of A and B relative to each other.... what I mean is I wanted to understand how A and B can interact with each other, given how they experience time differently.

Is time distorted when an object moves near the speed of light or at the speed of light?

6. Jun 29, 2017

### sweet springs

Hello.

You should check world interval or distance defined by
$$d=\sqrt{c^2 (t_A-t_B)^2-(x_A-x_B)^2-(y_A-y_B)^2-(z_A-z_B)^2}$$
it is scalar, i.e. the same amount for any inertia frame of reference. The event for A and the event for B interact when d is real. Here You see simultaneous events in different places time of which are $$t_A=t_B$$ have no interaction with each other.

Last edited: Jun 29, 2017
7. Jun 29, 2017

### Mister T

It happens at any speed, it's just that the effect increases beyond all bounds the closer the speed gets to the speed of light. Objects with mass cannot move at the speed of light but they can get arbitrarily close.

There are lots of books written about the special theory of relativity. You could spend a few minutes browsing in a library.

8. Jun 29, 2017

### Staff: Mentor

They don't experience time differently. You've misunderstood something somewhere.

9. Jun 30, 2017

### MrDickinson

I think I understand. Both particles experience the passage of time similarly, correct?

10. Jun 30, 2017

### MrDickinson

Is there a particular source you might recommend to explain this more deeply (conceptually) for someone who is very new to physics and only has a background in single variable calculus with an introduction to integration (real variables)?

11. Jun 30, 2017

### sweet springs

12. Jun 30, 2017

### Staff: Mentor

Spacetime Physics by Taylor and Wheeler.
https://www.amazon.com/Spacetime-Physics-Edwin-F-Taylor/dp/0716723271/

13. Jun 30, 2017

### MrDickinson

Thanks.

I have begun to read those first ten pages.

I have an exam this weekend, but I will endeavor to understand those ten pages to the extent that my faculties permit.

14. Jun 30, 2017

### MrDickinson

15. Jul 1, 2017

### Ibix

Hermann Bondi says that there are "public" and "private" quantities in physics. A "public" quantity is something like height gained climbing a mountain from a car park at the bottom. The route doesn't matter; when you're up, you're up and when you're down, you're down. Private quantities are things like the distance travelled to get to the top - you could go straight up the hill while I spiral round and round as I climb. The route does matter in this case.

In Newtonian physics, time is a public quantity - everyone agrees on it. In relativity, it's a private quantity - your route (through spacetime, not just space) matters. Regarding your collision question, your personal elapsed time is the spacetime analogue to distance travelled in space. And having travelled a different distance to climb the hill doesn't stop us meeting at the top. Why should having a different elapsed time stop us colliding?

16. Jul 1, 2017

### phinds

I may be beating to death something you now understand but just to be absolutely clear, that is a poor statement. They don't experience time "similarly", they experience time identically. All "experiences" of time, that is the measurement of time in an object's rest frame, are the same; one second per second. It is our perceptions of what's happening to something else when it is moving relative to us, or is at a different height in a gravity well, that we perceived its time as being different than ours.

17. Jul 1, 2017

### Ibix

Are you saying that flogging a dead horse does not make it go faster in a moving reference frame either?

18. Jul 1, 2017

### phinds

Well, it SEEMS to go faster, but the odd thing is that beating it harder doesn't change how fast it seems to be going.

19. Jul 1, 2017

### MrDickinson

So, time only appears different relative to observer. Everything that is moving at normal speeds here on earth might appear to move very fast relative to particle A, but in its absolute sense, time is experienced uniformly from particle to particle? But, I have read on this forum that photons do not experience time because such particles travel at the speed of light (perhaps my wording is poor).... if this is the case, does this change in how time is experienced happen instantaneously? A particle moving at .99c still experiences time as a particle would moving at 1000 m/s? But what happens at the instant the particle achieves the the speed of light?

20. Jul 1, 2017

### Mister T

That can never happen. Particles with mass always have speeds less than $c$, particles without mass always have a speed of $c$. Thus it's not possible for either of them to make that transition.