What Happens to Laser Light Inside a Spaceship Traveling at Light Speed?

[IamVector]

so I was recently studying some ray optics and then suddenly a weird question came across my mind I just don't know if it's the correct thread to ask but let's continue anyway
So the question is : suppose there is a spaceship traveling at light speed and someone fire a laser inside it what will be it's nature and speed and suppose spaceship is traveling at half the speed of light will the speed of laser be half times the original?

 

[jedishrfu]

No, Relativity postulates that the speed of light is the same for all observers. So folks inside the spaceship te laser light traveling at light speed and those outside looking in as the spaceship travels past will see the same that the laser light is traveling at the same speed.

This notion first came up to explain the Michelson Morley experiment where they could find no difference in the speed of light no matter which direction they angled their test equipment. Physicists at the time believed that light like so many other wave phenomena traveled within some medium. They didn't know what's light's medium and so called it the ether. If light travels through the ether and the Earth does to then they expected to measure it and found nothing.

Einstein then said there is no ether and that light travels at the same speed for all observers. This then introduced the world to the notions of time dilation and length contraction. You read more about it here in the book on Special Relativity:

http://lightandmatter.com/

 

[member 656954]

So the question is : suppose there is a spaceship traveling at light speed

Technically, if the ship were traveling at light speed, time would stop for the ship and its crew so they could not actually fire the laser. (Of course, anything with mass cannot accelerate to c, hence the 'technically'.)

There are quite a few posts on PF about why the speed of light is constant, such as this one, that might help your understanding, but if you search the forum you will find lots of others, @IamVector.

 

[phinds]

Technically, if the ship were traveling at light speed, time would stop for the ship and its crew

No, it would not. That's such a common misconception that there's a FAQ entry somewhere on the forum that explains it.

EDIT: also, another misconception that you likely have is that in a spaceship traveling close to the speed of light, time slows down for the occupants. That's not true either. The proper time on the ship, in the ship's frame of reference, remains the same irrespective of the ships velocity relative to other objects.

 

[member 656954]

No, it would not. That's such a common misconception that there's a FAQ entry somewhere on the forum that explains it.

I'll need to hunt that out @phinds, thanks. But does that mean a photon does experience time?

EDIT: also, another misconception that you likely have is that in a spaceship traveling close to the speed of light, time slows down for the occupants

Nope! I understand that within the ships frame of reference, they experience time as they always have. It's only a different frame of reference where the time discrepancy is seen.

 

[phinds]

I'll need to hunt that out @phinds, thanks. But does that mean a photon does experience time?

No, it means the opposite. The concept of time for a photon is meaningless. Proper time is measured in a rest frame. Photons don't have one.

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

 

[member 656954]

Okay thanks. And does that mean the OP's hypothetical spaceship traveling at the speed of light also has no rest frame?

 

[phinds]

Okay thanks. And does that mean the OP's hypothetical spaceship traveling at the speed of light also has no rest frame?

Right. Which is part of why it is impossible.

 

[member 656954]

Which is part of why it is impossible.

I write sci fi, using physics wherever I can but throwing it out the window for the sake of the story if need be, @phinds, and my misconception of time stopping at speed c really threw me. So, I've been slugging my way through various posts on PF and reddit and other places, not because I will learn enough math to truly understand it, but to at least twist the concept into shape and somewhat come to terms with it.

I won't claim it has entirely jelled, but I already see that shape of it and you are right that it's common. I've even found posts on Uni sites that state it. I've no doubt I'll get there on the 'why', but I wanted to thank you for the heads up. I expect to be baffled in most forums on PF when experts come out to play, but to find a long held assertion that seems to make so much sense was flat out wrong, well, it's a bit of a shock!

 

[phinds]

... to find a long held assertion that seems to make so much sense was flat out wrong, well, it's a bit of a shock!

Welcome to cosmology. Here's a statement I used to make. Haven't used it in a while:

Don't feel bad, we all go through this. Things in cosmology (the very large) and Quantum Mechanics (the very small) are WAY outside of human's evolutionary experience and not anything that there was any survival value in knowing (because it was all unknowable for all but the tiniest portion of human evolution) so "common sense" and "intuition" are often not only not helpful in those arenas, they are downright counter-productive. When I was first learning both, I used to run around screaming "THAT CAN'T BE RIGHT !" and tearing my hair out.

 

[DennisN]

I used to run around screaming "THAT CAN'T BE RIGHT !" and tearing my hair out.

I remember that for me, relativity was the most difficult thing to accept, but nowadays I don't remember why. It might have been that I wanted to hold on to the concept of Newtonian absolute time.

Nowadays I only scream "that can't be right" when I have hardware/software problems with my computers. .

 

[phyzguy]

For what it's worth, I never had the, "That can't be right!" feeling when learning relativity, because it isn't that counter to intuition, at least for me. What causes that feeling for me is quantum mechanics!

 

[DennisN]

What causes that feeling for me is quantum mechanics!

I know the feeling. My relation to quantum mechanics is based on one-sided acceptance on my part and no mutual real understanding at all.

 

[jedishrfu]

For me the confusion was the twin paradox and the deceleration turn around explanation as the reason for aging of the stationary Earth twin and not the traveling twin.

 

[kent davidge]

Among the advanced theories, Special/General Relativity seems to be the most easy to understand and make sense, both in terms of the math and intuition. It is all about geometry as people say. I have been studying it seriously for a couple of months now and so far I haven't come across a concept I couldn't find an example to show myself it working.

I don't know if Quantum Mechanics and Quantum Field Theories work the same way. I would be very disappointed if I ever start studying them and find that I can't work on examples, because at least for me, examples are a essential part in understanding a thing.

 

[DennisN]

Among the advanced theories, Special/General Relativity seems to be the most easy to understand and make sense, both in terms of the math and intuition. It is all about geometry as people say. I have been studying it seriously for a couple of months now and so far I haven't come across a concept I couldn't find an example to show myself it working.

I don't know if Quantum Mechanics and Quantum Field Theories work the same way.

They don't.
I predict that when you encounter quantum mechanics your intuition will be thoroughly challenged.
And I also predict that in that fight quantum mechanics will win and your intuition will lose.