# Questions on Time Dialation

1. Dec 9, 2015

### Daniel K

Hi. Alright, so I'm not extremely informed with matters regarding special relatively so I have some basic questions that I'd like to ask you guys.

My first question is that for an individual approaching near light speeds, would slowly moving objects appear to be moving much faster? The person approaching light speed's time has slowed down, and so does that mean that they'll observe our time to be going much faster and so therefore us moving quicker as well?

2. Dec 9, 2015

### PeroK

You've packed a good number of common misconceptions into such a short post! You ought to find a good resource on SR and study it. Anyway:

First, there is no such thing as absolute velocity, so nothing can be said to be travelling at near light speed. Only at near light speed relative to something else.

Second, something can be moving at near light speed relative to the Earth, but then the Earth is moving at near light speed relative to that thing. Neither experiences any relativistic effects themselves (because all motion is relative), but they do measure relativistic effects in the things moving relative to them.

Third, time does not slow down for something travelling fast relative to something else. But, if A is travelling fast relative to B, then A's time is slower than B's (as measured by B). And, by the symmetry of the situation, B's time is slower than A's (as measured by A). They both measure the other's time as running slow.

In short, nothing is moving absolutely and no one's time is running slowly absolutely. But, everyone's time is measured as running slowly by anyone moving relative to them.

3. Dec 9, 2015

### Daniel K

Ah, okay. Thanks for the explanations!
I do have a follow up question although.

I've often been told that if someone were to fall into a black hole, then they would be able to observe the entire universe unfolding before them. I don't see how this is possible under relatively? Thanks!

4. Dec 9, 2015

### Janus

Staff Emeritus
This is a bit different than the relative velocity scenario and deals with gravitational time dilation. The deeper you are in a gravity field, the slower your clock runs as compared to a clock higher in the same field. Unlike time dilation due to motion, this is not reciprocal, so the higher clock sees your clock running slow and you see the higher clock running fast. As you get closer and closer to a black hole, you are getting deeper and deeper into a very strong gravity field and someone watching you fall in would see your clock run slower and slower. Conversely, you would see the clock for someone far removed from the black hole as running faster and faster.

5. Dec 9, 2015

### rootone

I've heard that sort of thing said as well, and it doesn't make sense to me either since I've also been told. by what I believe are reliable sources, that from the infaller's point of view, once they have crossed the event horizon it would be a matter of only seconds before they arrive at 'the singularity', (whatever that is).
Putting aside for a moment that nothing would be observed in fact because the observer will be ripped to shreds, it's hard to conceive of witnessing the evolution of the entire Universe in a few seconds.
Then again though, there are a lot of hard to conceive things I guess.

Last edited: Dec 9, 2015
6. Dec 9, 2015

### Daniel K

It seems to me that the individual that is falling into the black hole would observe events taking place at faster than light speeds.

7. Dec 9, 2015

### jbriggs444

We have had some other threads on the black hole situation recently. You may be able to search them up. What someone falling into a black hole will see (i.e. with light hitting his eyes) will not include the entire future of the universe. Instead, there will be a cutoff beyond which signals from the future can not catch up to him before his worldline terminates at the singularity.

If you use notion of simultaneity from Schwarzschild coordinates then there will be a time on the victim's worldline before he crosses the event horizon that is "simultaneous" with any given future event. But that doesn't help him actually see the light from such an event before he crosses the horizon. Or even before he is spaghettified.

I think (but lack the background knowledge to perform the requisite math) that a hovering observer could, in principle, follow a trajectory that approaches the event horizon asymptotically in a fashion that would allow him to actually see the entire future of the universe in finite time as recorded on his own wristwatch. He could hover closer and closer to the horizon, seeing signals from the universe more and more blueshifted so that an infinite amount of future history would be seen in a finite span. But that's not a free-falling observer.