# Perspective of Massless objects

1. Mar 21, 2006

### LnGrrrR

Hey all!

New to the forums here, and I must say, I've been greatly enjoying all the reading. Just three quick questions to ask...

If one were able to have the 'perspective' or viewpoint of light, then they would see time as non-existent correct? They could go anywhere whenever and time would never occur.

Additionally...what's the dropoff from light speed? For instance, massless objects can go at the speed of c, but what's the fastest known object that HAS mass?

And thirdly...how can something have no mass, but still interact with our universe? How does it travel, etc etc? If someone could provide an informative link I would much appreciate it.

Edit: Found a third question :)

2. Mar 21, 2006

### Staff: Mentor

No, in the framework of relativity this is not possible. There is no inertial reference frame in which a particular light pulse is stationary. A light pulse travels at speed $c$ in every inertial reference frame.

3. Mar 21, 2006

### LnGrrrR

JTBell,

Yes, light must always go at the speed of c. But would it SEEM, to light, like it was going it's own speed?

For instance, for things with mass, we see it as 'normal'...so if we were going 99% of the speed of light, it would seem the same as on earth to us, but to the person actually on earth that could look at my clock on the wall, it would be barely moving, correct?

So let us assume that (for some hypothetical reason) I can make my ship travel at light speed. Does time essentially 'stop' for me? Even though I can see myself traveling at the speed of light?

And furthermore, if time does 'stop' for me, how would I be able to tell how far it would take to travel, say, 1 light year? Would I have to use a measurement given by an 'outside' observer?

4. Mar 21, 2006

### DaveC426913

Not that you can (and you can't - so this is an invalid frame of reference, but...) what you would see is the entire universe aging and dying in an instant. If you extrapolate on this, and ask how long it might take to get anywhere, you can see that it takes zero time to get everywhere, meaning you are all places at once. Also, you see everythnig all at once - an entire universe of photons would impinge upon you in that one instant, meaning you are bombarded with an infinite amount of radiation...

You see how quickly it degenerates into a non-sensical viewpoint?

5. Mar 21, 2006

### LnGrrrR

Dave,

Thanks for the answer. It was a bit of a thought experiment, assuredly. And it's not like QM is very sensible either. ;) I was just wondering what the general theory had to say about a theoretical at-lightspeed travel (I knew that technically the person going light speed would not see time travelling...but wasn't sure what exactly that meant. Would they see everything at once? Be able to jump around in time? Be stuck forever in one time period? Maybe it's just a matter of no one knowing because it's impossible.)

6. Mar 21, 2006

### chroot

Staff Emeritus
It's a meaningless question, LnGrrrR, like "what would it feel like to step outside the Universe?" It's not that the answer would be wacky and weird -- it's that the question literally has no answer, at least according to existing physical theories.

And QM is quite sensible; perhaps you're still learning it.

- Warren

7. Mar 21, 2006

### DaveC426913

At .9999999999999999c the universe will live and die in a few short instants from our perspective (it will also be very short lengthwise!). While we cannot actually reach 1.0c, we can come arbitrarily close, thus we can still have a meaningful discussion about it.

8. Mar 21, 2006

### pervect

Staff Emeritus
We can talk about the limit, and whether or not it exists. We can't talk about going "at c" at all. The distinction is necessary - sometimes the limit of a specific physical quantity exists, and sometimes it doesn't.

The question makes a false assumption that the limit of necessity does exist. That's why it's a bad question, and that's what I more or less patiently try to explain to every person who keeps asking it.

Relative velocity is a good example of the case in which the limit does not necessarily exist.

See for example the sci.physics.faq

This is one of those questiosn where the answer is easy, but the people asking it tend to ignore the answer, and to ignore quoted references, too.

9. Mar 21, 2006

### LnGrrrR

Pervect,

Well, I honestly will say that I lean much more towards 'philosophical' topics than scientific ones. However, I do like being educated. I AM still learning about QM and am rather new to it, but sadly, I find that there are three levels of knowledge on the subject.

1) I know nothing.
2) I know the basics.
3) I'm a physicist.

I understand that we can not reach light speed if we have mass. I did not know whether or not there was a theory that would explain what would happen if we theoretically could.

I see now that there isn't because it's impossible, so it's pretty much a, "What if, who cares" type thing. Right?

Edit: I read your link, and it seems to say that, yes, those questions are pretty useless...unless you're drunk at 2 in the morning sitting around with a bunch of bored friends. :) Thanks for the link, quite informative and interesting.

Last edited: Mar 21, 2006
10. Mar 21, 2006