Help me settle an argument about gravity and spacetime

[Thenatdude]

So, my father has recently taken a (very skeptical) interest in science, specifically physics and cosmology. Based on what he has seen from the various documentaries and reading he has done, he cannot grasp how Newtonian time can be incorrect in the sense that bending space with gravity affects the passage of time. In his mind, time is necessarily constant and unchanging, and since time itself is not a particle (which it might be; you know gravitons and all that) it absolutely cannot be acted upon by particles which exert gravity. He cites his own logical deductions such as "100 years before the big bang was still 100 years before the big bang, whether or not there was anyone there to measure it" and "The difference in time of two observers such as Earth compared to the ISS is meaningless because they are simply observing it differently and time itself has not actually been affected" He also considers time and space being inter-related to be nonsense, and that if you destroyed the universe, the passage of time would still take place in the resulting nothingness. Can someone clear this up for me? Does he actually have a point here? Is there an easy to grasp explanation if not? Thanks for any help in advance.

[ajb]

"The difference in time of two observers such as Earth compared to the ISS is meaningless because they are simply observing it differently and time itself has not actually been affected"

The time an observer measures is the (local) proper time. Your father seems to understand that different observers measure different durations of events (in general). We have good experimental evidence that this is the case.

 

You must ask him to clarify what the observers are measuring and what he means by 'time itself'?

 

We also have coordinate time, which sometimes can be understood as a proper time of some special observers. But in general the time as measured on a clock is (almost by definition) the proper time.

Edited April 20, 2016 by ajb

[Strange]

I think your dad just has to get over the idea that if something "makes sense" it must be right. Until he does that I don't think he is going to find any explanations acceptable.

 

But the thing is, it doesn't matter what our common sense tells us. When we make measurements, we find that time and space are related and that time varies depending on speed and gravity. This allows technology like GPS to work.

 

(Sorry, that wasn't very helpful was it...)

[EdEarl]

Perhaps doing one of Einsteins mind experiments will help; although, it depends on whether he accepts that the speed of light is finite or not.

 

Given the speed of light is finite, have him think about traveling away from a large clock, visible from afar, at or near the speed of light. He looks back at the clock to check the time (ignore redshift). If he is traveling the speed of light, he and the light from the clock are traveling the same speed. Thus, when he looks to see the time, the hands on the clock will not appear to move. In other words, if he started moving at 9am, the light showing the hands to be 9am would travel at the speed of light, along with your father, so looking back would always show the time to be 9am. To see the clock move, he would have to slow down or stop. The slower he travels away from the clock, the faster time changes.

 

The next hurdle, provided he understands the thought experiment, is that the effect is not an illusion.

[bluescience]

The reason why Einstein deemed time as not absolute is because according to the laws of physics, time is always constant no matter what. This did not make sense with Newtonian physics, since in Newtonian physics considers space as relative and time as a separate phenomena. Also Newtonian physics relates space and time through the phrase,"space at a given moment in time" So in simple words, newtonian physics goes with your dad's line of thought. However what einstein reasoned was that if space is not absolute, yet the speed of light is always constant then time has to be factor that changes. He even made an equation for the difference in time based on velocity called time dilation.

This is a link to a good explanation of the time dilation equation:

 

The reason why einstein hypothesized time is variant is because if you imagine an observer at 'rest' and he sees light pass by him and he measures it to be 186000 miles/sec and then if you see a second observer moving very fast in the opposite direction of light or travelling toward the light, than the speed of light for that observer should be faster, however it remains the same, 186000 miles/sec. So if we know that the distance was variant than the only thing to keep the velocity or speed the same, would be to vary time.

 

And when it comes to gravity, It makes perfect logical sense when you think of it this way:

A geodesic is the shortest distance between two points. If you draw a geodesic between two points on a sphere and then lay the outermost layer out as a 2-d surface, then that line would be curved right? yes. Say you had a 2-d creature observer living on that outer 2-d surface, for him, the geodesic would be a straight line, but since that 2d layer is part of a 3d sphere, anything traveling on the sphere would seem to take a curved path for the 2-d creature. You probably know where i am taking this now. It is the same for us. In the 4d coorinate system called spacetime, when an object is moving, it is going in straight line, and we, 3d observers, see it as a straight line. But the moment our object comes close to another object, it changes coordinates in the 4th dimension and continues to travel in a straight line in the fourth dimension, however For us 3-d observers, it tends to curve or become stuck in a orbit, pretty much any conic section. Thus gravity according to General Relativity is simply geometric distortion between dimensions.

Edited April 20, 2016 by bluescience

[swansont]

The reason why Einstein deemed time as not absolute is because according to the laws of physics, time is always constant no matter what. This did not make sense with Newtonian physics, since in Newtonian physics considers space as relative and time as a separate phenomena. Also Newtonian physics relates space and time through the phrase,"space at a given moment in time" So in simple words, newtonian physics goes with your dad's line of thought. However what einstein reasoned was that if space is not absolute, yet the speed of light is always constant then time has to be factor that changes. He even made an equation for the difference in time based on velocity called time dilation.

 

 

Newtonian physics had uniform time. You seem to imply the opposite here.

 

But you're right — once one acknowledges that c must be invariant (which is required for electromagnetic waves to work, and I think people can agree that radio waves exist) then we get interesting implications for length and time no longer being absolute.

[bluescience]

 

 

Newtonian physics had uniform time. You seem to imply the opposite here.

 

But you're right — once one acknowledges that c must be invariant (which is required for electromagnetic waves to work, and I think people can agree that radio waves exist) then we get interesting implications for length and time no longer being absolute.

Oh sorry, i kind of said,"space at a given moment in time" because i couldn't figure out how to show the relation between space and time in newtonian physics, so i just quoted from Carroll's SR lecture notes. He was trying to explain the difference of how space and time was integrated in a coordinate system in newtonian physics versus that of SR. Carroll seemed to reason that newton did say there was uniform time, but the only way time was tied in with space 3-d coordinates was that: at this moment of time, this is how a certain space was. do you get what i mean? It is hard to explain so just left it at the quote.

 

Sorry for confusing anyone...I am not a scientist...yet...nor an expert at this topic...so thanks for letting me know any mistakes.

[ajb]

In Newtonian mechanics we have a space-time that is globally and canonically the Cartesian product of time and space. This is the global and unchanging time in Newtonian mechanics. It is also something that is independent of the observer.

 

In special relativity and general relativity we do not have a canonical splitting of space-time into space and time. The notion of time is more subtle here and we usually have to take care with proper time and coordinate time.

Edited April 20, 2016 by ajb

[elfmotat]

You should point out that:

 

1) Human brains evolved to intuitively understand slow moving (compared to c), macroscopic, gravitationally weak phenomena. It should not be surprising that we fail to intuitively understand fast moving, cosmic scale (or micro scale), gravitationally strong phenomena. Common sense is not useful in these regimes, and will lead you down the wrong path more often than not.

 

2) That time dilation's existence is not under debate. Relativity is one of the best tested fields in science. As an everyday example, GPS systems must account for time dilation due to both motion and gravitation. They would drift off-mark significantly without relativity.

[elfmotat]

Also, what exactly is the difference between "two observers observing time differently," and "time 'actually' being effected"? What test should we contuct to determine if time has actually been effected if measuring differences with clocks is not sufficient? All 'clocks' are effected in the same way, including biological processes. If it were otherwise, we'd be able to measure time differences between different types of nearby comoving clocks, which is not what we observe. (Also, this would provide a way to measure 'absolute' velocity, violating the Principle of Relativity!) In what way does this not constitute time being effected?

 

I'm not sure how to convince your father that time and space are interwoven other than to point him to the math. The math interweaves them, and the math agrees with observation and experiment.

Edited April 21, 2016 by elfmotat

[Delta1212]

Also, what exactly is the difference between "two observers observing time differently," and "time 'actually' being effected"? What test should we contuct to determine if time has actually been effected if measuring differences with clocks is not sufficient? All 'clocks' are effected in the same way, including biological processes. If it were otherwise, we'd be able to measure time differences between different types of nearby comoving clocks, which is not what we observe. (Also, this would provide a way to measure 'absolute' velocity, violating the Principle of Relativity!) In what way does this not constitute time being effected?

 

I'm not sure how to convince your father that time and space are interwoven other than to point him to the math. The math interweaves them, and the math agrees with observation and experiment.

 

I mean, really, he doesn't have to acknowledge that time is "actually" doing anything, so long as he acknowledges that it behaves as if it's following the rules laid out in relativity.

[elfmotat]

 

I mean, really, he doesn't have to acknowledge that time is "actually" doing anything, so long as he acknowledges that it behaves as if it's following the rules laid out in relativity.

 

I agree. My point was that "time 'actually' being effected" is a vague and ill-defined concept.

[Thenatdude]

So he still won't take the bait. His only explanation is "Clocks are made of matter, and those can be affected by gravity, but time is not made of matter and can't be affected by matter"

I think your dad just has to get over the idea that if something "makes sense" it must be right. Until he does that I don't think he is going to find any explanations acceptable.

 

But the thing is, it doesn't matter what our common sense tells us. When we make measurements, we find that time and space are related and that time varies depending on speed and gravity. This allows technology like GPS to work.

 

(Sorry, that wasn't very helpful was it...)

This is unfortunately his go-to method of understanding the universe. He often tends to argue with mathematical scientific data based on the dictations of his own common sense, which comes from a significant grounding in electrical theory and not much else, causing him to have gotten on board with Walt Thornhill and the Electric Universe guys on occasion.

Also thank everyone very much for your contributions, it has certainly taught me quite a bit that I wasn't even aware that I didn't know. Even if my father may not listen, I still plan to.

[Janus]

So he still won't take the bait. His only explanation is "Clocks are made of matter, and those can be affected by gravity, but time is not made of matter and can't be affected by matter"

 

This is unfortunately his go-to method of understanding the universe. He often tends to argue with mathematical scientific data based on the dictations of his own common sense, which comes from a significant grounding in electrical theory and not much else, causing him to have gotten on board with Walt Thornhill and the Electric Universe guys on occasion.

 

Also thank everyone very much for your contributions, it has certainly taught me quite a bit that I wasn't even aware that I didn't know. Even if my father may not listen, I still plan to.

The problem is that the mathematical scientific data doesn't jive with his common sense. Take his argument on gravity's affect on clocks. It is based on the idea that a clock sitting on a mountain top runs faster than one at sea level because gravity is weaker on the mountain top and this effects the clock less than it does at sea level.

The upshot is that even if there were no difference in the gravity felt by the clocks and thus no difference in the affect gravity has on the matter of the clocks, the higher clock runs faster.

However, the math of Relativity does not connect the clocks' respective tick rates on the local gravity they experience, but on their difference in gravitational potential. The mountain clock runs faster because it is higher than the sea level clock and would still do so even if there were no difference in the gravitational strength experienced by the clocks.( If fact, if gravity strength were constant between the two altitudes, the difference in the clock rates would be even more.)

Relativity also predicts that it is possible to have a situation where the clock feeling a lower gravity pull will run slower than one feeling a higher pull.

 

All the experimental data collected confirms this. The formula they use to adjust the GPS clocks is based on this. If the clock rate was instead determined by local gravity affecting the matter of the clock. If instead, the clock rate were affected by a difference in the gravity felt by the clock, then you would need to use a different equation to adjust the clocks. But we don't; the equation based on the concept that gravitational time dilation is not tied to local gravity strength and thus not due to gravity physically altering the operation of the clock, is the one we have to use to get GPS to work properly.

 

All the scientific data is incompatible with the contention that gravitational time dilation is due to local gravity strength affecting the matter of the clock and thus its operation.

[swansont]

The effect Janus describes was initially confirmed by looking at photon emission and absorption at two different heights in a tower. Photons are not matter, and the emitter/absorber (Fe-57) did not comprise a mechanical system that would be affected by changes in gravity.

 

For systems that are moving time also changes. The clocks are not mechanically affected, since you can't tell if you're moving. They operate the same way as if they are at rest.