# Help with time and speed of motion please.

#### DeeBee

Can someone point me at an explanation of how time differs depending on how fast you travel. Preferably in very simple terms if possible, without the equations etc.

Or maybe someone here can help me?

Maybe some of you will view this post as some 'smart alec' trying to be clever. Some of you will pity me. Some(most?) even think I'm babbling. But I really want to understand this. (I've been to bed once tonight and can't sleep from trying to work it out!)

I've been trying to get my head round this for years.. I'll try and write how I understand it and maybe someone can help me. Note: Some or even all of what follows maybe complete drivel and half truths picked up from TV and radio.

Using the clock thought experiment.
If you travel away from the clock at the speed of light, the hands of the clock will appear stationary to you.

Correct?
Ok I understand that.

If you travel towards a clock at the speed of light, the hands will seem to be moving twice as fast.

Images arriving in your eyes at the speed of light, plus you are moving towards them at the speed of light equals twice the speed of light or time passing twice as quick.

Is this good or nonsense?

I have also heard that if you travel out into space at the speed of light for one year, many more years will have passed on the planet you came from.

I don't get this?
Surely, even if this is so, if you return to the starting point, time will 're-adjust' to be in sync again? It's all relative yes?

Say we have planet A millions of miles to your left.
And we have planet B millions of miles to your right.
You are somewhere in the middle.

There is a clock on each planet, and you can see both. You also have a clock with you.

If you move towards planet B at the speed of light. Time on planet A RELATIVE to you and your motion has frozen?

Time on planet B is passing twice as quick RELATIVE to your motion?

Time for you remains is constant?

Time itself does not change, just the way you perceive it? In different locations?

Is this what is meant by relativity? It just depends on how fast you travel and in what direction in relation to the second object?
But time really doesn't speed up or slow down in a given space? or do I miss understand?

To babble further.. to experience time, you need to have motion. All things in the universe are in motion, so they experience time. If they ceased to move, they would no longer experience time so cease to exist?

Also I read of an experiment with two atomic clocks. One was flown round the earth and was slightly out of sink with the other on returning.

Can someone explain what this has to do with time?
In my mind all it shows is that two counters go out of sink.
They are, to me, representations of what we presume time is, and not actually time itself.

So can someone explain how they came to the conclusion they did? I presume it has something to do with it being 'atomic' clocks and atomic is somehow linked to time?

Sorry to be dense, but I just can't work it out myself. I'm not well educated, so simple examples and "Imagine it like this..." type of stuff please if you can.

I'm going to sit here now and see if I get any replies. Might aswell I'm not going to get any sleep!

Thanks.

PS, where's the search option on these boards, I can't find it?

Related Special and General Relativity News on Phys.org

#### yogi

Hi Dee Bee - you have a lot of ideas sort of scrambled - all this stuff comes about from Einstein's explanation of the physical effect(s) that result when clocks are in relative motion. There are some things that are observational - how an observer in one frame would interpret the passage of time in another - and there are some things that involve real differences in the amount of time accumulated on relatively moving clocks. It is almost impossible to explain SR w/0 some math - although most folks on these forums tend to use a lot more than is called for in most cases. I suggest you Pick a simple single example - and rephrase your query so that the members have a target they can aim at. There are many helpful posters on this board, you just have to phrase your interrogatory(s) so they can be addressed.

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#### pervect

Staff Emeritus
DeeBee said:
Using the clock thought experiment.
If you travel away from the clock at the speed of light, the hands of the clock will appear stationary to you.

Correct?
You can't actually reach the speed of light, but in the limit as your velocity increases indefinitely, the clock appears stationary.

If you travel towards a clock at the speed of light, the hands will seem to be moving twice as fast.
Ooops. No. The hands will seem to move infinitely fast.

Let's put this in a formula, and let's stick to velocities v<c, because those are the velocities that material objects must have and where relativity theory makes sense.

The doppler shift 'k' is the rate at which the clocks hands appear to move when viewed through a telescope. It is also the ratio of

(frequency recieved)/ (frequency emitted)

k has the value of
$$\sqrt{\frac{c+v}{c-v}}$$

With this particular form of the formula

when v is positive, k>1, so v>0 is physically an object moving towards the you, and k>1 implies that incoming light is blueshifted, and that the hands on the clock move very quickly when watched through the telescope.

When v is negative, k<1, so v<0 is physically an object moving away from you, and k<1 implies that incoming light is redshifted and that the hands on the clock move very slowly when watched through a telescope.

Note that for any velocity v, k(v)*k(-v) = 1, i.e. the doppler shift when the object is moving away from you is 0.1, the doppler shfit when the object is moving towards you is 10.0.

#### DeeBee

Yeay! Thanks for the reply.

I've just been reading through some of the posts on this board and I think I can sum it up like this.

Quote from "Space-time" thread (started by BenGoodchild)
Quote from nwall.

"If this man, also being inquisitive about the universe, were to place one clock at the center of the disc and a clock of identical construction at the edge of the disc, he would find that the clock at the edge (in a stronger gravitational field) would tick slower than the clock at the center of the disc."

This is the bit I have a problem with.

Does the clock in the stronger gravitaional field REALLY tick slower (asuming the clock IS time itself), or does it just appear to tick slower if you observe it from the centre of the disc, due to the clock having greater speed?

But then if the clock is in a perfect orbit around the viewer, surely it would seem 'at rest' to the viewer as it has no away or toward motion?

I gather that the atomic clock experiment prooved that time runs faster under certain conditions, but like I said above. Doesn't this experiment only prove that clocks can run out of sync. The clocks aren't actually time are they?

---

I suppose my question is: How do we know time speeds up or slows down when the experiment that confirms it, to me is flawed. The clocks are NOT time. But mere represenations of time, Thus the experiment has little to do with ACTUAL time?

Like I said, I presume it's something to do with 'atoms' (from the atomic clock) actually being time...

Ouch, thinking is such hard work.

#### DeeBee

Thanks Pervect!

I think I follow your explanation, sort of. (I'm no physicist) I'm glad you took the time to try and explain why time seems to run infinatly fast rather than twice as fast. Thanks. This is all so fascinating. It's a shame I really don't have the education or mental ability to follow it all.

I think I'm going to have to settle for the fact that I'm never going to 'work it out'.
Think I would have been more at home a few hundred years ago, when the world was flat, we saw in black and white (before colour TV was invented D) and everything made sense!

Think I'm going to hang around abit though and read some of these amazing threads.

Thanks for your time.

Huh, should have read my horoscope today? ;-)

#### pervect

Staff Emeritus
There will always be a point at which a "why" question does not have an answer. I hope I've at least clarified what relativity says, if not "why" it says it.

As far as your thinking goes, it appears to be based on old ideas, basically you appear to be assuming that velocities add linearly. This viewpoint is not compatible with relativity, for the speed of light in relativity is the same relative to any observer, regardless of the motion of the source and receiver.

#### Physics101

DeeBee,

Don't lose sleep over this concept. Suffice to say that objects experiencing distortion of Space-Time will age less when compared to other relative objects that are not. Add to this that both Gravity and Acceleration/Deceleration (one and the same -- Principle of Equality) are distortions of Space-Time. Thus, if object A moves away from object B, later to be rejoined with object B, then object A must have undergone Deceleration/Acceleration at some point, thus having aged less than object B. The same can be said of object A being under some Gravitational influence when compared to object B that is not (relatively speaking, of course). Both these situations involved distortion of Space-Time, which is the root cause of it all.

Note: This is my understanding of how relativity works. If I'm mistaken, please correct me. Thanks.

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