Einstein’s principle of particular relativity predicts some loopy phenomena, none extra nonintuitive than the concept that transferring clocks tick extra slowly than stationary ones. As clocks strategy the velocity of sunshine, they tick ever extra slowly, getting nearer and nearer to not ticking in any respect.
So, this raises an fascinating query: Since fast-moving objects expertise time extra slowly and the velocity of sunshine is the final word velocity restrict, does gentle “expertise” time? On on-line physics chat boards, many solutions are given. However what’s the reality?
On the face of it, the concept that gentle doesn’t expertise time appears sort of foolish. In any case, we see gentle move from the Solar to the Earth. We are able to even time how lengthy it takes. (About eight minutes.) So, it appears somewhat apparent that gentle experiences time. However that’s the time we expertise. What does gentle expertise?
Answering this query is a bit tough. Physics is an experimental science, and the definitive method to reply questions is to do experiments. We may design an experiment through which a clock is connected to a photon. The one drawback with that concept is that it’s fully inconceivable. In any case, solely objects with out mass (like photons of sunshine) can journey on the velocity of sunshine, and objects with mass should journey slower. Clocks most actually have mass, so no clock can journey alongside gentle to permit us to do the experiment.
The facility of limits
As we’re forbidden from doing the definitive experiment, we should flip to theoretical concerns. What do Einstein’s equations inform us?
Right here, the story will get a bit of extra difficult. Einstein’s time-related equations apply for objects touring with zero velocity as much as, however not together with, the velocity of sunshine. On the precise velocity of sunshine, they break down. Thus, these equations don’t apply for gentle itself — just for objects touring slower than gentle.
If we are able to’t do an experiment and our equations don’t apply for the velocity of sunshine, are we caught? Effectively, to a level, sure. Then again, whereas Einstein’s equations don’t apply for 100% the velocity of sunshine, there’s nothing stopping us from asking the identical query for objects touring at 99.999999% the velocity of sunshine. And if you wish to toss some extra 9’s in there, go forward; the equations work simply tremendous.
So, let’s use the strategy of limits, usually utilized in calculus class. Should you can’t clear up an issue precisely for a selected worth of some parameter, you should use different values of that parameter and ask what occurs as you get nearer to the worth you need. Fairly often, the development you see tells you what is going to occur while you get to the forbidden worth.
We are able to use that strategy right here. What occurs in the event you take an object with mass and transfer it sooner and sooner? How does that object expertise time?
Approaching the velocity of sunshine
Right here, we’re on a lot firmer footing. Scientists have been doing this experiment for many years. We are able to take subatomic particles and speed up them to very excessive speeds — speeds very close to the velocity of sunshine. Moreover, these particles have their very own clock. We are able to use these tiny clocks to look at what occurs as we get them to go sooner and sooner.
How does this work? For instance, let’s take into account a subatomic particle known as a pion. Pions are sort of like low-mass protons. In addition they are unstable, decaying in 28 × 10-9 seconds. This lifetime has been measured to unbelievable precision. Should you had a pion and hypothetically accelerated it to the velocity of sunshine, which is roughly 300,000 km/sec (186,000 mi/sec), it ought to journey simply over 8 meters (27 ft) earlier than it decayed. However that’s in a Universe through which all clocks tick equally — that’s, a stationary human clock and a transferring “pion clock” tick on the identical fee. They don’t, although.
When scientists create pions touring at 99.99% the velocity of sunshine, they discover that they journey about 600 meters (1920 ft) earlier than decaying. That may solely occur if fast-moving pions expertise time extra slowly than stationary ones.
By the way in which, 99.99% the velocity of sunshine will not be the report for particle accelerators. Scientists can speed up subatomic particles to a lot increased speeds. The report was achieved in a particle accelerator situated in Europe at which electrons have been accelerated to the jaw-dropping velocity of 99.9999999987% the velocity of sunshine. In that unbelievable atmosphere, Einstein’s equations nonetheless labored completely. At these speeds, a hypothetical clock accompanying the electrons would tick a bit of over 200,000 instances slower than a clock close to a stationary electron.
Given the effectiveness of Einstein’s equations and the truth that the one restrict to the velocity of an electron is the velocity of sunshine, we are able to see that the nearer we speed up a clock to the velocity of sunshine, the slower it ticks. If it may obtain the velocity of sunshine, the clock would cease.
No time or area
So, what does that imply? From a photon’s perspective, it may move by means of the whole Universe with out experiencing time in any respect. Billions and billions of light-years can fly by, in far lower than the blink of an eye fixed.
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There’s extra. Whereas the topic of this text is the passage of time skilled by a photon of sunshine, the speculation of relativity additionally tells us how area is skilled. As objects go sooner, the Universe shrinks within the course they’re touring. Utilizing the identical methods described right here, we are able to additionally see that for a photon, the Universe is shrunk to zero measurement. Billions of light-years disappear, which means that, from the photon’s standpoint, it concurrently exists in every single place alongside its path of journey.
Relativity is actually a nonintuitive principle, and it makes some very weird predictions. Nonetheless, maybe probably the most weird of all is that gentle experiences neither time nor area, present in every single place and all instances without delay. This crazy-sounding consequence reminds us that the legal guidelines that govern the Universe are bizarre — and it offers us a lot to ponder.
