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# Can we stop time of any body by stopping it's movement through space? Answered

Hello!

I was searching on the internet on "Is time passing by around us or are we moving through time?". I found an article, it said that after the bigbang, the four dimensions were created instantly, three of space and the fourth one was said to be time. It said that the bigbang provided energy to everything in the universe so everything in this universe is constantly moving outward from the point of bigbang. So as it is moving through space, it is also moving through time.

Now I don't know if all of that was true or not, but if it were true, than if the movement of anything through space is stopped then it should also stop moving through time as well. And also from this explanation I don't understand that how would "time not exist" for bodies moving with the speed of light through space, whereas they would also be moving through time as well with a faster rate and should "age" more quickly.

So can you clarify this please, thanks.

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## 7 Replies

There are a bunch of misconceptions here, which are probably the result of trying to extrapolate your knowledge starting from something you read on the Internet :-)

The simplest is that the "Big Bang" was not an explosion. Rather, in the Big Bang the whole of spacetime had a very high energy density (not "infinite", but much, much higher than anything we can create in particle accelerators), and at the Big Bang space started stretching.

This stretching (the cosmological expansion) applies everywhere in the Universe, and can be described very simply in terms of a scale factor a. If you imaging laying a coordinate system out in the Universe, the scale factor just tells you how far apart the tick marks are.

Since the universe is expanding (we know that from direct observation), the scale factor is changing with time, a(t). If you laid out tick marks 1 km apart a billion years ago, then today those tick marks would all be about 12 km apart today (I'm using a Hubble constant value of 80 km/s/Mpc, and assuming linear redshift, for that calculation.

So, I said that space is stretching. That's an important point: as part of the Big Bang, or the cosmological expansion, nothing is moving. Objects certainly have local motion -- you walk around, the Earth revolves around the Sun, the galaxy rotates, and so on. But all of that motion is small scale, and local to some region.

With the Big Bang, you can just as easily consider all of the galaxies and clusters as simply at rest, stuck to a particular (x,y,z) location in that coordinate system you laid out above. As space gets stretched, those objects get farther apart (use the scale factor a(t) to compute that), but the objects are not moving. They are at the same location (x,y,z coordinate) as they were a billion, or two billion, or 13 billion, years ago.

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You also have brought in a completely separate, and unrelated concept. You wrote "how could 'time not exist' for objects moving at the speed of light?" This is a limiting case of special relativity, unrelated to Big Bang cosmology. As velocity increases, lengths measured in that moving frame get shorter, and time intervals get longer. If you plug a velocity of 'c' into those equations, you discover that lengths appear to become zero, and time intervals become infinite (i.e., "time stops").

So pretend you're in a spaceship, watching the Universe go by as you go faster and faster. To you, it's the Universe going faster and faster. So you look out the window and measure how far apart two stars are: the faster you go, the closer together they seem. And you look out the window watching a Cepheid variable pulsating: the faster you go, the slower it oscillates. If you could get your spaceship up to 'c', that Cepheid variable would appear to stop pulsating entirely, and everything you see would be squished to a single point (zero length).

Wisaam (author)2013-02-27

So what you'r saying is that, the galaxies are not moving instead it is the space that is stretching and pulling those galaxies apart, but I still don't understand that how things are constantly moving through time if they are stationary in space.

kelseymh (author)2013-02-27

Right.

As for time, nobody knows what "time" actually "is." We perceive time as a sequence of events which unfold. In special relativity, everything is static: objects consist of worldlines, which are trajectories through spacetime. How do you reconcile those two views? And why do we perceive time unfolding in a particular order, rather than the opposite order? Right now, these are questions for philosophy (which means they don't have any concrete or testable answer, just half-assed opinions), not physics.

In cosmology, we go back to an approximately Newtonian view: space is expanding, while time "unfolds" independently as we observe it. Of course, that's not true when we describe cosmology in general relativistic terms, and construct static models of curved spacetime, with singularities (like the Big Bang, or black holes). In those situations, there's no "time unfolding," just spacetime with slices that are small and slices that are large.

Wisaam (author)2013-02-27

Thanks for the explanation.
Just one more thing, it is not related to the question that I asked, as you said, but still, if anything is moving with a close to light speed, than why does it appear to move slowly or "age" slowly to the stationary observer. The events that take place on that moving object appear more delayed to the stationary observer.

Can you explain to me why it is and can you also do it in an easy enough way for an amateur like me to understand. And also if this does happen than wouldn't it mean that time actually is there and is slowed down for the moving object due to which it "ages" slowly?

kelseymh (author)2013-02-27

The best thing you could do for yourself at this point would be to read a good introduction to relativity. If you're still in school, ask your physics teacher for a recommendation.

Special relativity starts with two basic assumptions: (1) the laws of physics are the same for every observer, whether they are moving uniformly or at rest; and (2) the speed of light is the same for every observer, whether they or the light source are moving or not.

The first assumption is pretty easy -- if you do an experiment in a room which is moving uniformly (let's say, the back of a truck which is not accelerating or turning), then you'll get the same result as you would if the truck were stopped. This may not be true if the truck speeds up, slows down, or turns while you're working, which is why the moving uniformly restriction is there.

The second assumption seems kind of weird. Einstein put it in for two reasons, one theoretical and one experimental. First, if you work through the math which describes light (Maxwell's equations), you'll discover that it appears to predict that light always travels at the same speed, even if the light source is already moving, and even if an observer is moving relative to the light beam. Second, very precise experiments were done in the 1880's to try to observe an effect on the speed of light changing because of the motion of the earth, and those experiments "failed." They showed that no matter what direction you shone a light beam (with the Earth rotating and revolving around the Sun), it travelled with exactly the same measured speed.

So why is that a problem? Well, before Einstein, physics assumed that "Galilean relativity" was correct. It also says that the laws of physics are the same for moving vs. at-rest observers, but it also has a specific rule about how to deal with motion: velocity add up. Suppose I watch you zip past me on a flatbed truck at 60 miles per hour, and you throw a baseball toward the front of the truck at 60 mph. If I pull out my radar gun, I should see that baseball moving (relative to me, on the ground) at 120 mph (60+60). No problem, right?

Well, the problem comes if you replace your baseball with a flashlight (or better, a laser pointer). Sitting there on your truck, you turn on your laser pointer and measure the speed of light (how long it takes the beam to get from you up to the cab of the truck). You measure 186,282 miles per second. I use my measuring devices to watch your laser pointer; Galilean relativity says I should measure 186,282.02 miles per second (186,282 mps + 60 mph).

But the Michelson-Morley experiment (see above) says that's wrong! What I will actually measure is the same exact value that you measured. Even though your light source is moving past me. So either Maxwell's equations, and the Michelson-Morley experiment, are wrong, or else Galileo's rule for adding velocities is wrong.

Einstein showed us the latter. In order to get Maxwell's equations to work out properly in every frame of reference (at rest or moving), Galilean relativity must be replaced by more complex rules. Those rules require that, in a moving reference frame, lengths get shorter, and time intervals get longer. The really interesting trick is that those two effects are precisely enough to "balance out", and lead you to measure light travelling at exactly the same speed in every frame.

As for whether time is "actually slowed down for the moving object," that's a reasonable question. One aspect of that question is the twin paradox, which I leave for you as a homework problem.

We can also observe time dilation with short lived subatomic particles. When one kind of these particles (a muon) is produced in a lab, nearly at rest, they will live for about two microseconds, and then decay into other particles which we can measure. Muons are also produced by cosmic rays up at the top of the atmosphere (say, about 50 km up). Those muons are produced with very high energies, which means they're travelling close to the speed of light. With a lifetime of 2 us, they should travel about 600 meters before decaying.

But they don't! Those muons travel all the way through the atmosphere, can be detected on the ground, and have enough energy to penetrate hundreds of meters into rock. How could they live long enough to travel all that distance? Time dilation. Their clocks are slowed down so much that, relative to us at-rest observers, they can travel 50 to 100 km before decaying.

Are their clocks "really" slowed down? Well, to some extent that depends on your perspective. If you were "riding along" with one of those cosmic ray muons, it would appear to you that it lived for about 2 us, and travelled about 600 m, before decaying. But that apparent 600 m would be the Lorentz contracted distance from the top of the atmosphere down to the ground.

Wisaam (author)2013-03-02

Here is another question that keeps me wondering.

Wisaam (author)2013-02-27

thanks, my concept is very much clear now.