# Question about time and particle interactions

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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

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Mentor
because my teacher stated that everything around an individual approaching the speed of light speeds up.
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.

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.
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.

stoomart
MrDickinson
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.
?

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..

Gold Member
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.

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

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 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?

MrDickinson
MrDickinson
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?

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?

sweet springs
Hello.

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.

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.

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

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.

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

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

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

MrDickinson
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.

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)?

sweet springs
Hi. First ten pages of this old book https://archive.org/details/TheClassicalTheoryOfFields would be helpful. Compact but not easy. Best.

MrDickinson
Mentor
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)?
Spacetime Physics by Taylor and Wheeler.
https://www.amazon.com/dp/0716723271/?tag=pfamazon01-20

MrDickinson
MrDickinson
Hi. First ten pages of this old book https://archive.org/details/TheClassicalTheoryOfFields would be helpful. Compact but not easy. Best.
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.

MrDickinson
Spacetime Physics by Taylor and Wheeler.
https://www.amazon.com/dp/0716723271/?tag=pfamazon01-20

Thank you. I will purchase this book sometime in August when I have some free time to explore this topic. At the moment, my daylight is being consumed by the black hole that is academia.

2022 Award
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 traveled 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 traveled in space. And having traveled 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?

Gold Member
2022 Award
I think I understand. Both particles experience the passage of time similarly, correct?
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.

MrDickinson
2022 Award
I may be beating to death something you now understand but just to be absolutely clear, that is a poor statement.
Are you saying that flogging a dead horse does not make it go faster in a moving reference frame either?

MrDickinson
Gold Member
2022 Award
Are you saying that flogging a dead horse does not make it go faster in a moving reference frame either?
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.

MrDickinson and Ibix
MrDickinson
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.

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?

Gold Member
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?

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.

MrDickinson
Gold Member
2022 Award
So, time only appears different relative to observer.
correct

Everything that is moving at normal speeds here on Earth might appear to move very fast relative to particle A,
correct

...but in its absolute sense, time is experienced uniformly from particle to particle?
I have no idea what you mean by this

But, I have read on this forum that photons do not experience time because such particles travel at the speed of light
correct

(perhaps my wording is poor)... if this is the case, does this change in how time is experienced happen instantaneously?
I have no idea what scenario you are describing
A particle moving at .99c still experiences time as a particle would moving at 1000 m/s?
yes, EVERYTHING is stationary in its own reference frame and experiences time at one second per second.

But what happens at the instant the particle achieves the the speed of light?
Impossible, so irrelevant

Gold Member
2022 Award
But photons do move at the speed of light.

So, how is it possible for photons to interact with us when we experience time, but a photon does not experience time? It does not seem appropriate to say that they are frozen in time, rather, that their speed is so great that we cannot perceive their movement.

Is time discrete for a photon?

Thanks
You are applying an observational ability to a photon, a thing that is clearly impossible. Time simply is not defined for a photon. Leave it at that. A photon HAS NO reference frame.

MrDickinson
You are applying an observational ability to a photon, a thing that is clearly impossible. Time simply is not defined for a photon. Leave it at that. A photon HAS NO reference frame.

I see. But why is it impossible for a photon to have a reference frame?

Gold Member
2022 Award
I see. But why is it impossible for a photon to have a reference frame?
How could it possibly be possible? A reference frame is one in which the object is at rest. Photons do not exist "at rest" they are always moving in EVERY reference frame. So ... contradiction in possible circumstances.

MrDickinson
MrDickinson
How could it possibly be possible? A reference frame is one in which the object is at rest. Photons do not exist "at rest" they are always moving in EVERY reference frame. So ... contradiction in possible circumstances.

But are there not such things as non-inertial reference frames? Can't we consider a reference frame in which the object is not at rest... ? Or even a reference frame of one photon to the other?

I mean... is it possible to establish a reference frame in which a particle is accelerating or in motion of some sorts?

I think I understand where this is going. A photon lacks observational capacity. And nothing capable of observing can travel at the speed of light (which a photon travels); thus, it is impossible to establish a frame of reference for a photon??

Gold Member
2022 Award
Let's say you define a frame, call it whatever kind you like, in which a photon is itself at rest. Velocity is a relative thing. That means that in the frame of reference of the photon, other things, massive things, are traveling at the speed of light relative to the photon. Impossible.

MrDickinson
MrDickinson
Let's say you define a frame, call it whatever kind you like, in which a photon is itself at rest. Velocity is a relative thing. That means that in the frame of reference of the photon, other things, massive things, are traveling at the speed of light relative to the photon. Impossible.

I understand this. But why not do one of two things...

1. Establish a new frame of reference that is accelerating?

or

2. Simply acknowledge that, similar to time dilation, the speeds of these massive objects only appears to be traveling at the speed of light, however, the speeds that these massive objects are actually experiencing is something different?

Gold Member
2022 Award
You are trying to set up new definitions in physics (and ones that don't work well at that). Good luck with that.

MrDickinson