Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Why believe relativity?

Learn more physics!

Most recent answer: 10/08/2014
When we observe time dilation, such as on the Space Shuttle or GPS satellites, how can we assert with any certainty that time is actually changing for those observers, as opposed to just the rate of matter acceleration changing? Every means we have to try and measure time involves the acceleration of matter. Atomic clocks use the rate of radioactive decay, digital clocks use electronic oscillators or crystals, and grandfather clocks use pendulums. All of these clocks require a fixed, predictable rate of matter acceleration in order to measure time accurately. It seems like this creates an artificial relationship between motion/acceleration and time, leading us to conclude that since matter slows down in certain situations, time must also be slowing down. But is time really slowing down for that observer, or is it just an illusion for that observer caused by the slowing of matter acceleration, which is the only means we have for measuring time? If we recall that the faster something is going, the more energy it takes to accelerate or decelerate, it makes perfect sense why the acceleration of matter is affected by velocity, and does not require a redefinition of the rate of time for that observer. A Lorentz transformation still correctly predicts the rate of observed acceleration dilation without asserting that time itself has dilated. This artificial relationship between time and motion/acceleration leads to thought experiments like the Andromeda paradox, and the strange assertion that it's impossible for two events to happen simultaneously in the universe. In fact, it goes so far as to say Event 1 happens before Event 2, yet at the same time Event 2 happens before Event 1, because two different observers moving at different velocities relative to the events and/or each other would *observe* the events as occurring in a different order. But this phenomenon is because light cannot move fast enough to carry the event information to both observers instantly, not because the events actually occurred in the opposite order than they occurred. If someone bounces a basketball on the far side of a park, that basketball hits the ground at the exact moment it hits the ground. If an observer hears the sound 0.5 seconds later, and sees the light 0.00000000000001 seconds later, that doesn't mean the event actually occurred 0.5 seconds or 0.00000000000001 after it occurred. It just means that's when the observer learned about the event. Since the speed of light is constant for all observers in all situations, it should be possible to define a fixed point in space. Otherwise, what is the speed of light (300,000 kps) relative to? A rate of particle acceleration at this fixed point in space should be considered absolute time, and all other observed rates of time due to velocity or gravity should be considered offsets of the absolute time, as opposed to entirely different dimensions of time with their own unique planes of events. Have there been any observations in the universe that would specifically make thinking of time in this manner impossible?
- Mike (age 39)
Woodbridge, VA, USA

You've written a nice summary of people's first intuitive reactions to hearing about special relativity.

In your description of time, you've used the word "acceleration" in a very broad sense, to refer to rates of any physical change. So really you're just asking how we would notice if all local rates changed in exactly the same way. We wouldn't, of course, unless we looked at something else that hadn't changed the same way. That's exactly what relativity says, except that it drops your idea that there's some abstract time, a clock outside the physical universe, against which to measure these rates.

It's tempting to wish that the apparent breakdown of absolute simultaneity is just due to people being too stupid to correct observation times for the light transmission times in order to figure out event times. That correction, however, has been routine since Romer applied it to the moons of Jupiter in about 1670. The breakdown of absolute simultaneity and absolute time-order occurs for the calculated event times, already including that obvious correction.

You ask whether we've seen anything that contradicts the possibility that there's just one true time, for example the one shown on your watch. All the other times would be wrong, in exactly the way predicted by General Relativity. No, haven't seen anything to contradict that and we never will. The reason is that a metaphysical assertion like that has no implications for what we'll see. If I say that the only true time is what's shown on my watch, and yours is wrong, there are no data of any sort that help us tell who's right and who's wrong. We couldn't even find any evidence against a far-fetched theory like that Lee has the right time.

Mike W.

(published on 10/08/2014)

Follow-up on this answer.